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Discussion Starter #1
The professor and I were working on the wiring of my 72 SS 454, trying to get the dash gauges working and somehow we lost spark to the plugs. The car ran fine before we disconnected and reconnected a couple of wires. This car has a MSD 6AL amd Mallery coil. The MSD does have power going to it. It seems that the coil is working ok but still no spark to the plugs. Does the MSD have a circuit breaker or built in fuse that may have popped?

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Tom (Wildman704)
72 Chevelle SS454
67 Camaro Super Street
runs 10.3's @ 130 mph
01 Vette
98 Ram V10 (tow truck)
 

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What do you mean by "the coil is working OK"?

If you have spark at the coil, but not at the plugs, I'd be suspecting a defective distributor cap, coil wire, plug wires, in that order.
 

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Discussion Starter #3
There is some spark coming from the coil to the distributor but nothing going out of the distributor. There was nothing done to disturb the distributor but the MSD circuit out could have been disturbed. Should the distributor work ok with the coil putting out spark independent from the MSD 6AL box?

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Tom (Wildman704)
72 Chevelle SS454
67 Camaro Super Street
runs 10.3's @ 130 mph
01 Vette
98 Ram V10 (tow truck)
 

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I see by the installation instruction for the MSD that the unit has an internal fuse. How would that be checked.

Also, in a related issue. We have been fighting with an on-going electrical problem. When we started we had a short condition that resulted in the need to remove the gage fuse from the fuse block. If the fuse was in place the starter would engage as soon as you moved the key to the on position. I found that the purple wires under the dash that would have gone to a nuetral safety switch were disconnected. Just laying loose. I used a jumper and a multi meter to make sure that I had a good connection to the s side of the starter through to the saftey switch plug in block. To begin with the starter worked normally with the fuse removed and the neutral safety switch unplugged. Obviously the starter curcuit was back feeding some how to the starter curcuit, most likely through the gage curcuit. The starter could be spun with the transmission in any position a situation we need to address. I removed the power feed wire to the MSD "coil" to see if the feed back power was coming through that curcuit. We did not remove the power or do anything to the MSD box. This resulted in the loss of the starter until we jumped a wire accross the transmission safety switch. We thought that we had found our problem and ran a new wire from the fuse box to the coil. This resulted in a loss of spark at the coil and we have not been able to start the car since. To back track we put everything back as it orginally was. This did not give us back our spark nor did it return the prior situation with the starter. We find that we now have to have the safety jumper in place to spin the starter. And that we can install the fuse in the gage postion without it spinning the starter. Unfortunately we still don't have any gages with the fuse installed. We have not been able to recreate our original situation.

Now a couple of observations. We have not removed the fuse block but I can see where the previous owner had removed a great deal of the original firewall insulation resulting in a slight gap between the fuse block and the firewall. I can also see a purple wire sandwiched between the fuse block and the firewall. I am almost certain that there is no such wire on my 72, and my 70 has all of its orginal firewall insulation so I can't see behind the fuse block. We also found that the fuse for the interior lights was removed and when we attempted to install a fuse it would blow. The prior owner spliced into the drivers side door switch with a wire and connected it to one of the battery spade connectors in the fuse block to power the dome light at one time but that battery position is now dead. I am fairly sure that we have a dead short to the body behind the fuse panel. I can get a ohm reading from the fuse holder barbs to ground under the dash.

Any suggestions? I wondering if there is not a dead short behind the fuse block that at one point allowed feed back through a curcuit to the starter. Why we can't recreate that situation is beyond me. The MSD ignition thing has me completely stumped unless it is a ground problem.

any advice you can offer would be appreciated.

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70 and 72 Chevelle
ACES # 4051 MCC # 448
TC # 1549 81/70 Cruisers
 

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Not an MSD expert. Do a search on past posts. One guy last year accidentally keep shorting the MSD box input because he had an ignition switch problem. The box died. Maybe this is similar. He did say that MSD has a flat rate repair charge.

"To begin with the starter worked normally with the fuse removed and the neutral safety switch unplugged. Obviously the starter curcuit was back feeding some how to the starter curcuit, most likely through the gage curcuit."

If someone cut the purple wire off the neutral safety switch, they tied it someplace else. Starter won't work unless there is 12 volts to it. Need to find out where. If you can't see anything obvious under the hood, perhaps someone got behind the fuse block. Good question is why do this. If the N/S switch went bad, one could hack up the circuit by jumping the 2 purples together as you found out. One guy last year did have a short in a N/S switch on a 72. The switch was so bad that the 12 volt ignition lines were shorting to the grounded seat belt warning portion.

"I can also see a purple wire sandwiched between the fuse block and the firewall. I am almost certain that there is no such wire on my 72,"

Taking bets? The purple wire runs from the N/S switch, through the bulkhead connector, and down to the starter. At least on the California automatic models.

"The prior owner spliced into the drivers side door switch with a wire and connected it to one of the battery spade connectors in the fuse block to power the dome light"

Lord, this guy was a genius. The door switch provides ground to the dome light. The dome light has 12 volts to it all the time. Maybe this was once part of a home made alarm system and the guy wanted to switch 12 volts instead of ground.
 

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John, thanks for the reply. So your saying that seeing a purple wire sandwiched between the back of the fuse block and the metal firewall is normal. I'll admit I've never had the fuse block off the firewall on the inside but I would have thought that back of the fuse block and the portion that is actually on the engine compartment side were connected via some kind of straight through connector. I would not have thought that there would be wires connected to the back of the block to the back of another portion that acts as the "plug in" block on the engine side. It might be a bad switch... what is the best proceedure on checking that?

The transmission saftey harness was not cut off, in fact the harness on this thing looks to be in good shape with a couple of minor exceptions. The purple wire harness was simply unplugged. The tranny was changed out to a 350, "I think" and the stock floor shifter was re-used but we don't think the guy took teh time to sort out the neutral safety switch. The harness was just unplugged and that worked fine apparently, until we pulled and tugged on some things.

again thanks for the input.

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70 and 72 Chevelle
ACES # 4051 MCC # 448
TC # 1549 81/70 Cruisers
 

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I'm saying that somewhere the purple wire is/was getting power. I understand that the N/S switch was unplugged. What I don't see is why it was unplugged and where the purple (S) wire now gets it's power. Wonder if it was disconnected to form some anti-thief circuit much the same way as the door switches were altered. I guess if you can't see any wiring altered under the hood, maybe unplug the fuse block. It's been tied to something, somewhere.
 

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Discussion Starter #8
John, If the problem is in my MSD box couldn't I bypass the darn thing and go directly from the coil to the distributor and take the MSD box out of the loop? The distributor is breakerless and has a special connector on it that is hooked to the MSD box. What would I do with the special plug- in connector on the distributor if I wanted to go directly from the coil?

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Tom (Wildman704)
72 Chevelle SS454
67 Camaro Super Street
runs 10.3's @ 130 mph
01 Vette
98 Ram V10 (tow truck)
 

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I have no experience with MSD boxes and said that quite a few times. I have tried to read up and learn what I could. Still run points in both my Chevelles and probably will for quite a while. What you are asking makes sense. I can't answer what to do about the connector. Cutting it off is not an option. Is this a Pro-billet distributor? Maybe check on MSD's website for a hook-up diagram that uses only the distributor and not the box.
Another suggestion is what a couple of guys did in the past and it might be an option for you. Go back to the basics. Slip a used GM HEI distributor in there until you straighten out things. You have at least 2-3 other problems with your wiring starting with what somebody did to the starter wiring, the door wiring and heaven knows what else. My opinion is to get back to where the schematic starts making sense without working around other things. It's far easier to long distance troubleshoot when the car is stock. The HEI wire 12 volt wire change being the only difference.
MSD does make a good product and it does have a purpose. However, it does cause some people little problems at times.
 

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Discussion Starter #10
John, Thanks. Will let you know how things go tonight after we pull the fuse block and see what is going on there. I like going back to basics. This car will not be raced as long as I own it so the MSD is not a "biggie" to keep.

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Tom (Wildman704)
72 Chevelle SS454
67 Camaro Super Street
runs 10.3's @ 130 mph
01 Vette
98 Ram V10 (tow truck)
 

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Tom, I have a HEI stting out here in the garage that you're welcome to borrow until you order one or until we can get the MSD back together. Want me to bring it along tonight?

see you at 5.

BTW.. the guy didn't call as promissed on my 72... I'm not happy to say the least.



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70 and 72 Chevelle
ACES # 4051 MCC # 448
TC # 1549 81/70 Cruisers
 
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