Will 6V or Ground on MSD 6AL ignition wire burn it out? - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

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post #1 of 7 (permalink) Old Nov 9th, 01, 4:25 PM Thread Starter
Join Date: Nov 2001
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The shop that installed my 6AL ignition didn't put in the diode on the gen lamp wire for my '66 Malibu. After removing the dash to fix a speedo problem, I took the opportunity to put in some missing bulbs, the Gen lamp being one of them. I didn't realize at the time that the shop had taken that lamp out to remove the run on problem. When I put the lamp back in, I got the run on problem but it really didn't connect with me that the run on problem was because of my putting in the lamp and the lack of the diode.

Unfortunately, the car died yesterday while debugging the problem. I thought I was out of gas because I was low, but after filling the tank and getting plenty of gas at the carb, still a no go.

I went about debugging the ignition and I think the ignition is dead. I get .7 ohms between the two coil terminals (MSD Blaster 2) and 10k ohms between the top post and either of the two terminals which seems about right. When I trigger the ignition with the mag pickup or by grounding the white/points wire, I can measure a very small AC current across the coil terminals, around .01V (I realize that this isn't going to be accurate because the AC duration is probably much smaller than the Vmeter can accurately read). If I measure the AC Voltage from the coil top terminal, I still only get a very small voltage, and when holding the coil to dist. wire near ground, I don't get a spark no matter which trigger I use. I have 12V at both the large and small red wires on the ignition and good ground. The fact that the ohms on the coil appear to be right leads me to believe that the coil is still ok and that something in the outputs to the coil are not working right.

The msdignition.com web site says that the 6 series ignition will work down to a 5 volt supply, but will 6.5-7.5 volts on the ignition wire (small red one) cause the 6AL to fail? This is the only thing that was occuring that I can think would cause the ignition to fail. I guess the one other thing I was doing was very briefly grounding the ignition wire at the ignition switch to stop the motor. I suppose it might have cause a brief back current through the MSD ignition but the unit was operating and stopped on its own when it failed, not after doing any grounding. I'm overnighting a new one from Jegs on Monday but in the mean time, I'm wondering if there is anything simple that could be done to fix the unit if the low ignition voltage is what caused the problem, such as replacing an internal fuse or something of that nature. If so, I'll cancel my Jegs order. The unit is out of warranty at this point and I know they can't warranty every situation because cars parts can be put through a beating, but I feel a bit let down at the moment both by the shop that improperly installed the thing in the first place and by the fact that the ignition gave out without, what I would call, a stressful scenario.

Has anyone else seen an MSD 6 series ignition give out from this sort of scenario?

rcollette is offline  
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post #2 of 7 (permalink) Old Nov 9th, 01, 6:51 PM
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Not from me. I'm not a MSD expert. Only learned what I know from other members and reading all the MSD tech literature I could find. If you get no other replies, I would suggest you contact MSD tech support.
BTW, glad you fixed or understand your run on problem. If it makes you feel any better at least you did it by yourself. The guy I use when I get lost starts at $65.00.
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post #3 of 7 (permalink) Old Nov 11th, 01, 1:29 PM
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guys, quick question.
Does it matter which way the diode is mounted on the voltage regulator.
I know I need to use the #4 connection on the regulator, but is there a backwards way to install the diode?
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post #4 of 7 (permalink) Old Nov 11th, 01, 2:54 PM Thread Starter
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Yes diodes are directional. They prevent current from flowing in one direction, up until a breakdown voltage.

There is a band around the diode cylinder. The band should be placed on the side towards the regulator (or in other words,facing away from the gen bulb). On my '66, this would be the brown wire that feeds up to the gen lamp.

The gen lamp will light when there is voltage on the pink ignition wire and something less than that voltage on the brown wire that goes to the gen lamp. During normal operation, there is equal voltage on the two wires causing no current to go through the lamp. In a run-on scenario, there would be no current going to pink from the red wire at the ignition switch, but the inertia of the motor keeps the alternator turning briefly, causing there to be voltage on brown which causes current to flow in the opposite direction through the lamp onto the pink wire and therefore continuing to make the ignition control think that the key is turned. The diode prevents current from traveling through the brown wire in the reverse direction which is what causes run-on. I kind of wonder why they make the ignition this way, why not have it expect a higher cut off voltage of like 10V rather than down to 5V? Maybe it's so you can get home when your alt. quits and the battery is dying down.

Someone on another post mentioned that the brown wire is also resistive so technically between the resistance of the bulb and the resistance of the wire, there would have to be more than a minor voltage drop on the regulator for the lamp to begin to light up. Diodes also have a .7V drop across them. So with a diode installed, I think the Gen lamp won't actually begin to light up until the regulator is putting out closer to 11V (12V Battery = .7V drop across diode + V drop across brown wire + V drop across bulb). This is why I really think a V meter in the car is really handy. When the alternator is putting out less than 13 or 14V you really want to know. It's a little too late if you've been running it for a while at something less than that because the battery isn't going to charge and by the time the gen lamp starts lighting up, you may not get your car started depending on how much oomph your motor requires to turn it over. Even worse, lets say the alt/regulator is putting out 11.5V. With the diode, your gen lamp won't light. Now the battery drains down to 11.5V. Maybe over time the alt puts out even less voltage, say 11V, but now the voltage difference between the battery and the alt is 11.5-11=.5V. This is still smaller than the .7V required by the diode to let current through so your gen lamp still won't light up. Small chance but under perfect conditions, your gen lamp will never light before you can't start your car.

Because I have a Vmeter in my car, obsoleting the need for a gen lamp, that may be why the mechanic ripped out my bulb rather than putting in the diode.

'66 Convertible
Tremac 3550 5 speed

[This message has been edited by rcollette (edited 11-11-2001).]
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post #5 of 7 (permalink) Old Nov 11th, 01, 4:58 PM
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Thanks for all that info, it is great.
I also have a Vmeter in the car, but I will go ahead and use the idiot light as well, assuming the diode does the trick.
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post #6 of 7 (permalink) Old Nov 16th, 01, 7:24 AM Thread Starter
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Well, I never did get a straight answer from MSD tech on exactly what this would do. I'm guessing that they aren't using any kind of isolation circuit on the ignition lead(like an optical isolator or even a standard buffer). So in the same tone as Egon from Ghostbusters, DO NOT ground the ignition lead on an MSD 6AL.

I also found out too late.. MSD has a $55 max on ignition repair. So if you can find a busted unit out there for nothing, it's a bit cheaper to send it in for repair than to buy a new one. I went out and bought a new one thinking that the labor would quickly add up to the cost of a new one. At least its running now and I learned a LOT about the ignition system.

If anyone here from Connecticut wants a free busted 6AL that can be sent in for the repair mentioned above, let me know and you can stop by and get it. I also have a brand new blaster 2 coil in the box, turns out I didn't need it because my current one still works. But that I'll have to get $20 for.
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post #7 of 7 (permalink) Old Nov 16th, 01, 8:58 AM
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I would send it in for repair for the $55.00 and you have a known good to use for troubleshooting or for an emergency spare.

I have a Crane Hi6 which is similar to the 6AL. Instead of a standard diode, I used a bridge rectifier (this one had spade connectors). You just hook to two terminals instead of all four. A full wave bridge is simply four diodes.

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