I have wondered for some time how much current the starters on our cars actually draw during the start up process.

I went out today and measured the whole start up sequence of my 69. Here are the results:

http://i274.photobucket.com/albums/jj264/bikeron_2008/Starter%20Current/tek00076S-1.jpg

I have expanded the first 20 milliseconds so you can see better what happens.
http://i274.photobucket.com/albums/jj264/bikeron_2008/Starter%20Current/tek00078A.jpg

This is why you need good battery cables. Now that I have this data I know what I need in terms of wire in order to mount my battery in the rear.

This is exactly the reason why I ran a relay directly from the ignition switch to the battery and back to the starter.

This is exactly the reason why I ran a relay directly from the ignition switch to the battery and back to the starter.

How is it wired and what does it do?

In reference to your expanded view, there is a point labeled "current from battery solenoid engagement", a roughly 200A peak after which the current drops back to zero. Why does the current drop almost directly to zero if this is the current that operates the battery solenoid? What's happening in the 3 ms between that point and the next rise in current?

I have wondered for some time how much current the starters on our cars actually draw during the start up process.

I went out today and measured the whole start up sequence of my 69. Here are the results:

http://i274.photobucket.com/albums/jj264/bikeron_2008/Starter%20Current/tek00076S-1.jpg

I have expanded the first 20 milliseconds so you can see better what happens.
http://i274.photobucket.com/albums/jj264/bikeron_2008/Starter%20Current/tek00078A.jpg

This is why you need good battery cables. Now that I have this data I know what I need in terms of wire in order to mount my battery in the rear.

Ok you convinced me......the other day I was a skeptic of the numbers you quoted. Can you pass me some "Humble Pie" .....looks like it is time for me to eat some. I must say that I knew there could be disasterous results when a battery cable wears down the insulation and there is a direct short! That said I would NOT have suspected numbers like you display in the above post under normal starting conditions. WOW!!!

In reference to your expanded view, there is a point labeled "current from battery solenoid engagement", a roughly 200A peak after which the current drops back to zero. Why does the current drop almost directly to zero if this is the current that operates the battery solenoid? What's happening in the 3 ms between that point and the next rise in current?

All of that current that you see (200A) is going to the starter motor not the solenoid just to be clear..

There is "contact bounce" in the solenoid. This bounce is caused from a mass (the armature of the solenoid) being moved forward at high speed and then slamming into a stop. First, to engage the gear into the flywheel, then after the gears are engaged, the contacts (which is a separate mass as one set of contacts float on the armature shaft) in the switch section of the solenoid allow current to flow to the motor. The high speed causes the armature to slam into a stop and the contacts are pushed by a spring into each other. The mass and the spring causes the contacts to bounce (oscillate).

If you can envision a car running into a wall then bouncing backward off the wall you can see that same phenomena occurring with the contacts as you slam them into each other; they bounce. You will notice that there are actually four of these large jagged "spikes" of current, this is the bounce decaying (the contacts bounce more than once, but each bounce is less in distance). That is why on the first bounce the contacts completely disconnect from each other (the current goes to zero) while on the next spike the current goes down but not to zero. Finally, after the four bounces (it may be more than four bounces, notice that there are small spikes too which are most likely harmonic frequencies of the major bounces), we get all the energy in the bounce used up (it was stored in the spring) and we have consistent contact. Only after we have consistent contact does the starter motor get full voltage and start to turn.

Notice that the voltage at the starter goes up when these "spikes" go down in magnitude.

If all the current in that pulse is going to the starter, when is the solenoid coil engaged? If the contacts have a 3ms period for the bounce, how long does it take for the solenoid to travel from its resting position to the point where it connects the battery to the motor? I would expect to see a drop in the voltage at the battery some period of milliseconds before the start of the current to the starter motor, i.e. the point at which you turn the key to 'start'. The pull-in coil of the solenoid also draws several amps, and I don't see where this is reflected in your measurements.

If all the current in that pulse is going to the starter, when is the solenoid coil engaged? If the contacts have a 3ms period for the bounce, how long does it take for the solenoid to travel from its resting position to the point where it connects the battery to the motor? I would expect to see a drop in the voltage at the battery some period of milliseconds before the start of the current to the starter motor, i.e. the point at which you turn the key to 'start'. The pull-in coil of the solenoid also draws several amps, and I don't see where this is reflected in your measurements.

The current clamp is placed in the big wire going to the starter only while the current for the solenoid comes from the ignition (off the battery through the fusible link). That is why you don't see the solenoid current in the yellow trace, it is going through the fusible link and the current probe is inserted after the link.

You make a good point here. The total current from the battery is not shown in either of these oscillographs; only the starter motor current is shown. The total current from the battery has to include the ignition, solenoid, EFI, interior lights, brake lights etc. Now you can see why there is an accessory circuit in our cars; the circuit goes dead during the starting period so as to make as much power as possible available for the starter and ignition.

Therefore the total current from the battery is greater than I have shown here. I was only trying to figure out the stresses on the starter circuit so I can design the proper wire size and contactor (starter solenoid) for the trunk mounted battery.

If you look at the first oscillograph, the whole starting cycle, however you can see the point where the solenoid first gets current. It is where the battery voltage first drops, the point called "ignition is on battery voltage drops". It looks to be around 50mS for the solenoid to get the point where the contacts engage.

The other point to make about the test set up is that the voltages were measured with differential probes, one at the center of each of the battery posts and one probe at the + terminal of the starter solenoid with the - lead of the probe attached to the starter body so the voltage losses you see include the ground path and the positive path (big red wire and big black wire from the battery).

Also keep in mind this test was done at 20 degrees C. If it were colder the current would be higher, but the copper losses in the cables and the starter would be less, so it seems that the system is some what temperature compensating.

The test has also been done on only one car (mine). I would like to look at a few more vehicles. I believe big blocks would be worse in peak current terms.
Perhaps the Nor Cal Chevelle's will allow me to hook up to their cars at one of our meetings. If it's OK and I get some data then I will post some more results.

Ron

Ron

This is good stuff. Thanks for collecting this info and sharing it.

Is it correct to say that the current quickly drops below 300A, looks to me to be less than 50ms. Total start cycle lasted right at one second? Looks like the current drops below 100A at around 800ms, would you agree that under sustained cranking the current would likely stay around 100A?

Ron

This is good stuff. Thanks for collecting this info and sharing it.

Is it correct to say that the current quickly drops below 300A, looks to me to be less than 50ms. Total start cycle lasted right at one second? Looks like the current drops below 100A at around 800ms, would you agree that under sustained cranking the current would likely stay around 100A?

Elree, Yes, I think your right. It looks like 150A or so at the end there but it is still declining so it may settle out at 100A or so.

It seems if you want to do a relative compression test you disconnect the ignition (so the plugs won't fire) and you compare the current peaks (as each cylinder goes into compression the current goes up) to each other as the currents are relative to compression. It does not tell you if your compression is low, just that all the cylinders are the same or not.

This technique is only valid though after the current settles out as you describe.

Ron

Two things of note:

1. Electric motors like the starter motor use MORE current as they turn SLOWER. When the motor is spinning merrily, it produces "Back EMF" that acts similar to higher resistance. It limits the current the motor draws.

So it's NO surprise that instantaneous inrush current to the starter motor is extremely high; and as the motor gathers speed, the amperage comes WAY down. I can't see any reason to buy cables rated for 700 amps of current draw when that 700 amps is essentially a "burst" of such short duration that any reasonable cable will not heat enough to cause problems.

2. In a previous life (billions and billions of years ago), I'd tested hundreds of vehicles for starter amperage draw. This was using a Sun VAT-40 amperage tester--so there was no fancy graph readout. Just a pair of big ol' analog meters with a needle swinging for amperage and another for voltage. During steady-state cranking (engine cranking with ignition disabled) of ordinary "whatever drove through the door" vehicles using stock starters (no mini-starters) a GM big block may draw as much as 225 amps; generally a bit less than 200 amps. (Our shop got into trouble one time; we were using 200 amps as a cutoff; a customer came in with an Olds 455 that showed something like 220 amps on our test. We sold him a new starter. He did some research and discovered the GM spec for his vehicle was 225 amps...and we had to refund his money.) We always saw more than 100 amps for anything with the "traditional" GM starter motor--even Vegas and V-6s.

I'm therefore curious as to what your graph would look like if you disabled the ignition and let the starter motor spin--throw out the first one second of operation and show me what the rest of the graph looks like.

Ever seen a starter motor so fried that the main battery cable goes "THUNK!" against the sheet metal when you turn the key? The current draw is so high that the cable becomes an electromagnet with enough force to "stick" to any nearby iron or sheet steel.

Two things of note:

1. Electric motors like the starter motor use MORE current as they turn SLOWER. When the motor is spinning merrily, it produces "Back EMF" that acts similar to higher resistance. It limits the current the motor draws.

So it's NO surprise that instantaneous inrush current to the starter motor is extremely high; and as the motor gathers speed, the amperage comes WAY down. I can't see any reason to buy cables rated for 700 amps of current draw when that 700 amps is essentially a "burst" of such short duration that any reasonable cable will not heat enough to cause problems.

2. In a previous life (billions and billions of years ago), I'd tested hundreds of vehicles for starter amperage draw. This was using a Sun VAT-40 amperage tester--so there was no fancy graph readout. Just a pair of big ol' analog meters with a needle swinging for amperage and another for voltage. During steady-state cranking (engine cranking with ignition disabled) of ordinary "whatever drove through the door" vehicles using stock starters (no mini-starters) a GM big block may draw as much as 225 amps; generally a bit less than 200 amps. (Our shop got into trouble one time; we were using 200 amps as a cutoff; a customer came in with an Olds 455 that showed something like 220 amps on our test. We sold him a new starter. He did some research and discovered the GM spec for his vehicle was 225 amps...and we had to refund his money.) We always saw more than 100 amps for anything with the "traditional" GM starter motor--even Vegas and V-6s.

I'm therefore curious as to what your graph would look like if you disabled the ignition and let the starter motor spin--throw out the first one second of operation and show me what the rest of the graph looks like.

Ever seen a starter motor so fried that the main battery cable goes "THUNK!" against the sheet metal when you turn the key? The current draw is so high that the cable becomes an electromagnet with enough force to "stick" to any nearby iron or sheet steel.

I agree with what you are saying. You don't need cables that are rated to carry 700 amps. You do need a system that is similar to what GM intended to make the starter work as intended. That is why I'm looking for the total resistance of the system, in this case 3.3 milliohms, so that I can reproduce that same resistance value in the cables from the trunk. That is the first key parameter I was trying to find.

I have had some comments, questions, on what the current would be at steady state. In order to answer the question I would like to understand the set up you are thinking about. Did you mean with the starter on the bench with the starter running freely (no engine attached) or still in the car turning the engine over with no ignition active (the electronic compression test)?

I will have to do some more testing as I am trying to figure out what voltage losses occur across the solenoid switch. If I put another solenoid in the back with the battery I would have two solenoids in series with the battery current. I believe this will lower the voltage to the starter enough that it compromise the cold starting performance. I don't know for sure so I want to find out. I think I have a way around this problem too.

Wish I had a dyno for starters!

Another test I will try is to put a DVM across the red cable from the battery to the starter and the black cable from the battery to the engine and measure the voltage that the DVM shows during starting so there is a way that most on this forum can measure and get an idea of what they should see for a reading when the car is started. I understand (from people that have been doing this type of work for a lot more years than I have) that 300mV is a common number to use as a typical limit. I will see if I can confirm that and compare my results to the Lab scope and current probes that I used.

Anyone else have any tests they would like to see done while I am set up to do them?

Ron

I agree with what you are saying. You don't need cables that are rated to carry 700 amps. You do need a system that is similar to what GM intended to make the starter work as intended.
Agreed.
That is why I'm looking for the total resistance of the system, in this case 3.3 milliohms, so that I can reproduce that same resistance value in the cables from the trunk. That is the first key parameter I was trying to find.
If this were in MY driveway, I'd be doing a voltage drop test instead. If you have less than 1/2 volt difference between the battery end of the cable and the starter end WHILE THE STARTER IS CRANKING THE ENGINE--you're fine even if the battery is in the trunk. For that matter, the battery could be on the MOON, as long as the voltage drop is less than 1/2 volt on both the positive side and the negative side.

http://www.chevelles.com/showroom/data/500/medium/Starter_Voltage_Drop_Testing.JPG

I have had some comments, questions, on what the current would be at steady state. In order to answer the question I would like to understand the set up you are thinking about. Did you mean with the starter on the bench with the starter running freely (no engine attached) or still in the car turning the engine over with no ignition active (the electronic compression test)?
Still in the vehicle, cranking the engine with the ignition disabled so it wouldn't actually fire and run.

I will have to do some more testing as I am trying to figure out what voltage losses occur across the solenoid switch. If I put another solenoid in the back with the battery I would have two solenoids in series with the battery current. I believe this will lower the voltage to the starter enough that it compromise the cold starting performance. I don't know for sure so I want to find out. I think I have a way around this problem too.
If you use a remote "Ford" solenoid in series with the main battery cable, those solenoid contacts will also be in series with your voltage drop test. Under 1/2 volt of voltage drop--and you're golden. If you attach the voltmeter lead to the long copper tube at the starter instead of where the main battery cable connects, you'll have the plain old Delco solenoid contacts in series, too.

Wish I had a dyno for starters!
You do. It's the rest of your engine.

Another test I will try is to put a DVM across the red cable from the battery to the starter and the black cable from the battery to the engine and measure the voltage that the DVM shows during starting so there is a way that most on this forum can measure and get an idea of what they should see for a reading when the car is started. I understand (from people that have been doing this type of work for a lot more years than I have) that 300mV is a common number to use as a typical limit. I will see if I can confirm that and compare my results to the Lab scope and current probes that I used.
I think you're describing the voltage drop test I've illustrated above. If you use 300 millivolts, you have an even tighter spec than what I've used.

How is it wired and what does it do?

It has a wire coming directly from the hot on the battery, a ground, one going to the starter, and one coming directly from the ignition switch. If your circuit is losing any current between the switch and starter causing a lower than normal voltage at the starter this will eliminate the voltage drop and give you full batter power when starting.

I would like to see two things.

1- The current draw of the solenoid circuit.

2- An extended run of the starter, without spark/firing helping turn the engine. Say 2-3 seconds.

Ron, you da Batman! Where do you get those toys? ;-)

You can figure out exactly what the startup current is for any electric motor... just by measuring the resistance of the windings. What does that predict for the starter motor?

That value is likely going to be very similiar for all starters of a particular configuration, for example all direct drive starters, no matter what engine they are intended to be mounted on. Of course the operating current will vary and you can see the bumps in current draw above for each compression stroke.

I did some of this analysis on my electric fans where I found a 70A startup inrush and a 15A operating current. Which is also how I came to the conclusion that the circuit labelled "low speed" was really a "startup" circuit (an inline resistor cut startup current in half). This phenomenom of large startup/low run current is also why one of the indications of a bad heater blower motor is a blown fuse.

Cool stuff here. :thumbsup:

Ron, you da Batman! Where do you get those toys? ;-)

You can figure out exactly what the startup current is for any electric motor... just by measuring the resistance of the windings. What does that predict for the starter motor?

That value is likely going to be very similiar for all starters of a particular configuration, for example all direct drive starters, no matter what engine they are intended to be mounted on. Of course the operating current will vary and you can see the bumps in current draw above for each compression stroke.

I did some of this analysis on my electric fans where I found a 70A startup inrush and a 15A operating current. Which is also how I came to the conclusion that the circuit labelled "low speed" was really a "startup" circuit (an inline resistor cut startup current in half). This phenomenom of large startup/low run current is also why one of the indications of a bad heater blower motor is a blown fuse.

Cool stuff here. :thumbsup:

Don't know what the resistance of the starter motor is but I can find out to see if it matches your hypothesis. I believe you are correct too.

Elree. got your request too. I will get a test plan set and I should be able to get the testing done on Saturday. I have to change my thermostat and maybe fuel pump relay(s) as the fuel system has been doing strange things...

No Goodguys for me...but I am going to Sacto on the 4th!

Really great!

What instrument did you use to get this?

How much?

I would love one! Well, after Chevelle repair and an O2 tuner.

Any cheap ways to do this?

Ron, you da Batman! Where do you get those toys? ;-)

You can figure out exactly what the startup current is for any electric motor... just by measuring the resistance of the windings. What does that predict for the starter motor?

That value is likely going to be very similiar for all starters of a particular configuration, for example all direct drive starters, no matter what engine they are intended to be mounted on. Of course the operating current will vary and you can see the bumps in current draw above for each compression stroke.

I did some of this analysis on my electric fans where I found a 70A startup inrush and a 15A operating current. Which is also how I came to the conclusion that the circuit labelled "low speed" was really a "startup" circuit (an inline resistor cut startup current in half). This phenomenom of large startup/low run current is also why one of the indications of a bad heater blower motor is a blown fuse.

Cool stuff here. :thumbsup:

Arrrggghh! I just realized, the answer to your question is right in the oscillograph! We have about 7V across the starter motor and the starter solenoid contacts so the resistance of the combination is 9.6 miilohms!

So your right! The initial starter resistance sets the start current!

Ron

Didn't get to do any measurments this Saturday. HAd to do some clean up on fuel pump wiring and alternator stuff, brake check etc. It was 9:30PM before we were done.

Next week end is the West Coast Chevelle thing at Sacto so if I can't get to it at night this week it will be two weeks from now. We will two cars to compare.

Ron

[QUOTE=Schurkey;2273483]Two things of note:

1. Electric motors like the starter motor use MORE current as they turn SLOWER. When the motor is spinning merrily, it produces "Back EMF" that acts similar to higher resistance. It limits the current the motor draws.

So it's NO surprise that instantaneous inrush current to the starter motor is extremely high; and as the motor gathers speed, the amperage comes WAY down. I can't see any reason to buy cables rated for 700 amps of current draw when that 700 amps is essentially a "burst" of such short duration that any reasonable cable will not heat enough to cause problems. Quote

Yessir! Until that engine starts to turn the starter is in "dead short" condition. No surprise at all. :-)

I was redirected back to this thread from another Bikeron post on another thread.

Just thought I'd mention that with an appropriate oscilloscope, by viewing the amperage waveform of an electric motor, you can view the current drawn by each commutator bar. By carefully observing the pattern (each commutator bar's amperage waveform has it's own unique shape, sorta like a "fingerprint"), and counting the number of amperage peaks before the pattern repeats--you can even tell how many commutator bars the motor has.

Further, if you have a modern 'scope, you can then pick out one full revolution of the motor armature based on the repeating pattern of the commutator bars. The 'scope tells you (in milliseconds) how long it takes for one revolution; simple math gives you the motor RPM.

If one revolution of the commutator takes 11 milliseconds, 60,000 divided by 11 = 5454.5 rpm.

'Scopes (with the appropriate ignition and amperage adapters) are FRIGGIN' WONDERFUL tools.

The current clamp is placed in the big wire going to the starter only while the current for the solenoid comes from the ignition (off the battery through the fusible link). Ron


Unless I am badly mistaken the current for the solenoid pull-in winding is coming through the ignition switch, to the neutral safety switch, to the purple "S" wire, to the solenoid winding.

If the current was coming thru the fusible link, the solenoid pull-in winding would be energized all the time.

When you release the ignition key to the "run" position the purple "S" wire is de-energized as is the solenoid pull-in winding.

Unless I am badly mistaken the current for the solenoid pull-in winding is coming through the ignition switch, to the neutral safety switch, to the purple "S" wire, to the solenoid winding. I believe he is talking about the fusible link off the battery, (separate wire) which isn't being measured, because he has the current clamp on the big wire.

If the current was coming thru the fusible link, the solenoid pull-in winding would be energized all the time.

If he had a 72 or later, the test would be showing all current (including the solenoid) as the main fusible link is right off the starter BAT terminal.

Really awesome data!!!!!!!

How did you do this and what tools/meter setup does this take? And how much?

I believe he is talking about the fusible link off the battery, (separate wire) which isn't being measured, because he has the current clamp on the big wire.


If he had a 72 or later, the test would be showing all current (including the solenoid) as the main fusible link is right off the starter BAT terminal.

Yes, I was using the fusible link off the battery. Good point about the later cars.

Ron

Really awesome data!!!!!!!

How did you do this and what tools/meter setup does this take? And how much?

Instruments used:
Tektronix DPO4104
Fluke i1010 Current Probe (starter current)
Fluke 80i-1105 Current Probe (solenoid current)
Differential Probe ADF 15A (Battery voltage)

Cost? About $16K.

I have this stuff because I use it in my engineering work.

Ron

what tools/meter setup does this take? And how much?
I use an ancient Snap-On Counselor II (MT3000 or MT3000A) with the optional Snap-On high amps probe and low amps probe.

About $12,000 new; about $1200 (give or take $400 or so) used on eBay.

http://reviews.ebay.com/Snap-On-Counselor-oscilloscopes-A-Primer_W0QQugidZ10000000001691487

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=280431726003&viewitem=&sspagename=mem_guide%3A3&category=43989&ih=018

(but this one doesn't seem to have the amperage probes; and may not have other important probes/adapters/accessories--so BE CAREFUL)

O.K., I had asked is anyone wanted more data taken and I did get a request or two. I finally got around to doing it.

Elree and one or two others asked for current draw of the solenoid:
http://i274.photobucket.com/albums/jj264/bikeron_2008/Starter%20Current/tek00002text.jpg


http://i274.photobucket.com/albums/jj264/bikeron_2008/Starter%20Current/tek00003text.jpg


http://i274.photobucket.com/albums/jj264/bikeron_2008/Starter%20Current/tek00004text.jpg

Schurkey mentioned that the pattern of the commutator in the starter was clear, I can't see the commutator but I can see each cylinders compression (peak current) as the starter turns the engine. This may be due to my use of a mini stater rather than a Chevrolet/Delco one.

I am surprised at the peak current the solenoid uses. Almost 27A peak.

Ron

Schurkey mentioned that the pattern of the commutator in the starter was clear,
No, it is me that was unclear. I meant that you can pick out the commutator bars in a lower-amperage motor like an electric fuel pump or heater blower motor. In fact, picking out the commutator bar pattern on fuel pumps is an excellent diagnostic tool--if the waveform doesn't look right--the motor is bad and needs to be replaced. If the RPM is out-of-spec, the pump or the wiring has problems.

Seeing the bars on a (high current device like a) starter motor is probably beyond the resolution of any 'scope available to ordinary humans, I'm sorry I wasn't specific.

I can't see the commutator but I can see each cylinders compression (peak current) as the starter turns the engine.
That's another way to do a compression test. If the starter motor current isn't fairly even for all the cylinders, the current draw for the "uneven" cylinder either has too much compression (too much current) or not enough compression (not enough current.) If you have the ability to synchronize the starter current to the crank position; you can tell WHICH cylinders are suspect based on the firing order.

I am surprised at the peak current the solenoid uses. Almost 27A peak.
Inrush current is going to be huge because the instant you energize the solenoid, it acts as a dead short. Steady-state current is much, much less.

...That's another way to do a compression test. If the starter motor current isn't fairly even for all the cylinders, the current draw for the "uneven" cylinder either has too much compression (too much current) or not enough compression (not enough current.) If you have the ability to synchronize the starter current to the crank position; you can tell WHICH cylinders are suspect based on the firing order.
Interesting thought... wonder if you accomplish the same thing watching battery voltage during cranking? Wonder how reliable it might be?

A lot of sensors used in EFI get real interesting if you can start synchronizing the sampling with engine operation. But that is potentially a very different topic. But with many EFI controllers having a battery voltage sensor this might be feasible. In case you wonder, they monitor battery voltage so as to adjust injector pulse widths to compensate for voltage induced changes to opening times.

Inrush current is going to be huge because the instant you energize the solenoid, it acts as a dead short. Steady-state current is much, much less.
So I completely get this for an electric motor but not for a simple electromagnet. At the instant you energize you see merely the resistance of the wire. As current begins to flow the magnetic field begins to form. So far so good :) Now where I diverge is the influence of a stable magnetic field on the coil of wire. I was under the impression that the wire/field had to be moving to generate reverse EMF?

Are you saying that even a stable magnetic field generates reverse EMF?

Interesting thought... wonder if you accomplish the same thing watching battery voltage during cranking? Wonder how reliable it might be?
First Guess: Entirely reliable IF (big IF) you have the resolution to see changes/differences.

Second Guess: As the battery capacity increases, the resolution on your 'scope better increase as well. It'd be easier to see if the battery is smaller because the voltage might vary more from cylinder-to-cylinder.


So I completely get this for an electric motor but not for a simple electromagnet. At the instant you energize you see merely the resistance of the wire. As current begins to flow the magnetic field begins to form. So far so good :) Now where I diverge is the influence of a stable magnetic field on the coil of wire. I was under the impression that the wire/field had to be moving to generate reverse EMF?

Are you saying that even a stable magnetic field generates reverse EMF?
We're seeing via the 'scope trace that there's a big hump in the current to the solenoid. And I'm calling that "inrush current"--maybe my terminology is incorrect.

WHY is another matter. Perhaps the current reduces because the winding heat up and the resistance changes.

But there's visual proof that SOMETHING is affecting the current draw of the solenoid--the solenoid takes more current at first, and then levels off later.

As part of redoing Brian's cars electrical system in the DVT process I went back to check that we actually improved the start up characteristics from the "stock" version. Although it is 71, rather than a 69 like mine I think it is a reasonable comparison as they are both small blocks with similar compression ratios.

He draws even more current into the starter than my 69.

http://i274.photobucket.com/albums/jj264/bikeron_2008/Alternator%20tests/Completestartcycle-1.jpg

http://i274.photobucket.com/albums/jj264/bikeron_2008/Alternator%20tests/Start1-1.jpg

The key thing is that during the starting process the voltage at the fuse box never goes below 8V at all. If you look at mine at the beginning of the thread the voltage was down to a little more than 6V so using the starter solenoid as a distribution point is not a good idea.

I will post the rest of the DVT results in the other thread.

http://www.chevelles.com/forums/showthread.php?t=300744

Ron