This is a very interesting set of problems. It hasn't helped that I haven't been able to get the time to focus on the issues.

Earlier this year I started the 454 again, and noted the voltage gauge was at roughly 12V, not the 13.5 or so I want to see. I question the integrity of the alternator, go to the battery, and remove the ground. The engine stops.

OK, time for an alternator replacement. I put in the old one I was using before I did all the work a couple years ago. Again, I noted the problem of the voltage gauge. I turn the key to the off position, and remove it. Odd, the engine is still running! I go to the battery, and remove the ground. The engine stops.

So, I thought the key felt loose in the switch, I didn't think the ignition switch had a good feel to it when going from position to position. I swap it out, and the new ignition switch feels great to turn. And it starts the engine great, and when switched to the off position, the engine remains running.

Google tells me that other guys with trucks and RVs from the 1986 to 1990 timeframe have seen similar things, but no one has an answer that I have found.

So now I am wondering if there is some odd path, or more, such that power is applied to the distributor through a path other than the ignition switch. I will have to check for splices, relays, and so forth. I'm wondering if this situation is killing the alternators, I will have to take them to be tested, if I can't figure it out with the alternator in place.

I have the wiring manuals from GM. I am trying to imagine how a failure of the magnetic starter switch combined with a failure of the starter solenoid (while still starting the engine) might combine to somehow drive a voltage on circuit 3 of the wiring diagram.

I've found earlier documentation from GM, in reference to the older 10SI alternator, indicating that the alternator might "back feed", and keep an engine running with the key in the off position, and that a diode from Radio Shack is the answer.

Maybe there are grounding issues at the root of this. I am going to start tracing wires within the cable sheaths, I guess.

And just to be clear, the engine is not dieseling, it's running great as a gasoline powered engine should!

Happy Wrenching,
Matt

I had a 72 Chevelle once that done that when I was a teenager, and it was the coil wire that had got hot in the loom and melted and shorted out with another wire. Oh and my neighbor had one that the horn was stuck on. The horn didnt work so he had no idea that the horn relay was on all the time. He discovered it by pulling fuses one at a time. I discovered mine when I was rewiring due to hungry mice after I got home from the navy. Guess it could also be a shorted starter wire holding the circuit closed. Hope you find your gremlin and bend him to your will. lol

Stop stop stop! Take a breath, you started with with questioning alternator output and went to the ignition switch and starter relay. First things first, do not ever disconnect the battery while the engine is running, that’s back from the days of generators. Since the alternator is off take it to O’reilys or your any other parts house that has a test stand and let them test it.

Secondly, I subscribe to “whenever something screws up, look at the last thing you did”. Remove the ignition switch again double-check your wiring. If it is a molded connector then skip this step. Plug the harness into the original switch and see if the problem goes away.

Quit throwing parts and money at an unknown problem, diagnose, diagnose and prove the fault first.

Years ago I uploaded the entire GM Motorhome Chassis Manual to this site, seek it out if you need to.

Good luck,Billy T

Hi Paul and Jaime! Isn't it fascinating when electrons go where they want, and not where we want? You got the Chevelle working, didn't you?

Another clue: when there is no background noise, and I first go to start the engine, when the key is in the "run" position, prior to going to the "start" position, I seem to recall that I could hear the "whrrrrr" sound from the electric fuel pump in the fuel tank. I am not hearing that.

There is a "electronics module" shown on the wiring diagram, that I am aware of. It shows up as part 12040947 on the wiring diagram. It is taking the TACH signal from the distributor as an input. The main function of this module is to determine when to energize the diverter valves for the AIR pumps. I believe that this module is also feeding an input to an "electronics module" 12010015 for control of power to the in-tank fuel pump.

There's a bit of logic that is implemented in these circuits:
- key was in off position, turned to run position, turn pump on, for a bit of time
- key is in run position, turned to start position, keep pump on
- key is in run position, engine is running, keep pump on (disable timer that was keeping pump going)
- key is in run position, engine is stopped, turn pump off
- key is in off position, keep pump off

The idea of this, is if for some reason, the engine stops running, like because a fuel line disconnected or there is an accident, that fuel is no longer pumped forward in the vehicle. I like that. I don't know what sequence of events resets the "startup pump timer".

So....

My first level of debug will be to remove the modules, and see if the engine turns off when I want it to. I'm wondering if between the path to the distributer via TACH, and to the ignition circuits, and to the power circuits, that there is a failure mode of one or both of the modules which provides a power path to the distributor.

I should probably also identify all components on all electrical nodes of each of the modules, by reviewing the wiring diagram.

So, work in progress.

Happy wrenching,
Matt

quote:

diagnose, diagnose and prove the fault first.

BillyT! How right you are. The division of my employer gave me the "road warrior" award for the past year. I'd been through around 2 dozen airports, countless trips...

I diagnose faults in industrial electronics! And the boss and the customers are happy!

So right now I am home from a 2 week trip away for work and getting ready for another trip. I'm hoping on this interlude to get the right diagnosis done, so if a parts order is needed, I can do so before I fly out again. And mow the lawn.

My present situation reminds me that there are no shortcuts for electrical and electronic systems diagnosis. The army saying "the easy way is always mined" is also coming to mind.

Happy Motoring!
Matt

Another clue: when I have the engine running, and the parking brake on, the parking brake light is bright on the dashboard. When I turn the key to the off position, the parking brake light goes to about 75% brightness. There is a bit of a skip in the idle speed when the key is switched off. The parking brake light is on circuit 3. So it seems something has failed, for unswitched power to be delivered to circuit 3. I hope to get the time this weekend to investigate.

Matt

I'll say it is solved when everything is back together. Here is the intermediate status: the fuel pump relay module is out of the circuit, the check engine light relay module is out of the circuit, and the AIR diverter valve relay module is out of the circuit. The engine still keeps running when the key is turned off. There is still one last semiconductor module in the system at this point, the HEI module.

I'd been speculating that perhaps there was a path by which the tach signal output was feeding power to the HEI module. I removed the combined tach/power connector from the distributor, jumpered the power connector from the wire harness to the power input of the distributor. The engine starts and stops under key control, now, as it should.

Right now I am speculating that there is a fault in the HEI module, and/or a fault in the tach circuit, going to the VDO tachometer in the dash. That has to be diagnosed. I will look at the wiring in the region, and perhaps replace that HEI module and see what happens.

Looking at the circuit boards on the relay modules is like a jump back in time in the semiconductor world: Fairchild, Motorola, TI (still around), RCA (the Radio Corporation of America!), SGS... No burn marks are evident, no leaky capacitors....

And somehow it seems this issue has cost me two alternators as well....

So, onwards, towards having electrons obey all our commands!

Matt

With a GM D1906 HEI module newly installed, the engine starts with the key, and shuts off with the key.

The VDO tachometer seems to be stuck at 0 RPM during the time the engine runs.

I am planning on replacement of the relay control modules as part of "putting it back together". Then I can diagnose the alternator issue.

happy motoring!
Matt

Geez Matt, that is a weirdo for sure. Really surprised that a fault is in the HEI always told that thing is indestructible!!
Good thing you have the skills to determine the fault!

Good luck!!!

It gets worse.

I thought I had it understood. I found what I thought to be a burned diode in one of the relay control modules, and I replaced all 3 of the modules.

I had the fuses out for most of the circuits and under key control, the engine starts and stops as it should.

I added the fuses back. I started again, and I see that the gauges aren't working. The gauges fuse had blown. I also found that the heater blower motor fuse had blown.

I tried to start again. More smoke than I've ever seen from a circuit came from the starter and I was afraid I was going to lose it all to fire and I grabbed for the extinguisher. The smoke cleared and I went to disconnect the battery.

The ground lug of the battery had melted and the ground cable was freed from contact. I've never heard of anything like this.

I'm guessing that system grounding needs attention. And also that there is something going on in the gauges circuit.

oh darn!
Matt

Geez Matt wish I could help but no idea what to do, except wish you good luck!!!

quote:

wish you good luck!!!

Duane! I'll take it!

I got the battery cable off, to the starter. The cable had been "fixed" by a prior owner, the Commonwealth of Massachusetts, and the fix worked for 20 years. I never liked the cable end, when I had to replace the solenoid, but "if it ain't broke, don't fix it".

It felt like there were two cables joined with a rat tail splice under the red electrical tape. The joint was stiff, and the lug connector was at a bad angle. It gave out, at least it was in my yard.

Perhaps the battery had issues too, that was 7 years old.

I'm going to start rebuilding first the starter circuit, and go on from there. This isn't the only electrical issue there. I'm going to give lots of attention to the grounding integrity of all the circuits and try to see what is up.

It's no wonder that "the pros" hate working on these things, a customer brings in something for a little fix, and a cascade of other issues become evident.

See you later!
Matt

How bizarre it gets!

In my experience with old cars, coaches and repairs I have seen so many totally strange things that defy logic.

With our dinosaur coaches we have accumulated many "ghosts in the machines." Some are due to the factory patch jobs during the transition from carbs to fuel injection. Remember all the vacuum lines on the 70's cars? They added solenoids, EGR, air pumps on the exhausts, timing retard, etc. Our 80's coaches have numerous examples. In addition to the factory voodoo, they have often been modified in sometimes completely strange ways, by owners or "mechanics" who do not truly understand the mysteries of electrons. In your case you have the added layer: Command Center related wiring....

My 54 Chev had 3 separate wires feeding power from the key switch the coil. One had three splices and they all went different routes! I ripped them all out, installed one new wire. Poor workmanship and bad changes everywhere.

When I got my 86 Regal, I got a low compression 454 engine, with AIR pumps, vacuum retard, and EGR that was rated at 240hp! 240hp from a 454 cubic inch engine! Several magic relays for choke circuits, fuel pumps and AIR pumps. None working well.

Long ago, after wasting countless hours trying to troubleshoot factory emission cobble jobs and owner mods, I have ever since followed the mantra...Rip it out and make it right.

On my Regal, I removed all the defective emissions equipment, (pumps, belts, brackets and hoses) rewired the fuel pump circuit, adjusted timing and carb. The mileage went from 6.3 to about 7.

The final touch was the 454HO engine. Up from 8.2 compression to 9.5. Fuel injected, now about 400hp. Mileage went to 8+mpg. Ran great, fun to drive. Up to modern standards of performance.

In my case, I understand electricity well. I could easily wire a coach from scratch. I have never had to redo any electrical changes I have made. That said, many do not have this knowledge. My advice is Keep it simple, get rid of anything that is marginal (most OEM GM wiring is marginal, in size, at least. Consider headlight wires. Stock headlights get about 10 volts.... there are fixes here on the site, etc) Use quality materials and common sense to simplify when possible.

Good luck with your dinosaur!

It would have been nice to have spent "one session" on this, rather than spreading it across most of a year, and relying on memory of what was done by me.

TOP LESSON: GO THROUGH YOUR ENGINE BAY, THE DASHBOARD AREA, THE FUSE AND BULKHEAD AREA, AND THE "FRONT OF THE RADIATOR" AREA, AND COVER ANY UNCONNECTED WIRES WITH YELLOW ELECTRICAL TAPE.

Secondary lesson: don't try to fix everything, all at once, in parallel tasks.

With the Yellow Tape in place, then know, "be careful about connecting this one". I'd apparently plugged a loose wire, that was pointing at a particular terminal in the fuse box, and forgotten about it. It was supposed to be left dangling loose.

I finally and "for real this time" got the engine to shut off under key control. My diagnosis about the HEI module and the tachometer were probably wrong. I didn't need to install a new key switch.

The connector on the fuse box was "BAT". The wire was spliced to a circuit, front of the radiator, where there is a 5A ATC fuse, on the way to the field coil of the alternator. When the fuse was out, the engine would shut off under key control. Likewise, with the "wire removed from BAT".

The electric choke circuit was never hooked up. The manual says "use the blue wire" but there were no blue wires. I had a hunch at one time that this repair, with a different wire, is what led to my dismay.

In my issues, I think I may have damaged the electronics portions of 2 alternators. I did find that the original battery cable was shorting out against the frame, but that was a coincident problem. It was better it happens at home than on the road I guess. It looks like either fate or me killed a battery on the way.

I do have the paper copy of the wiring diagram from GM for the right year. My P30 has a 4 wire alternator, the diagram shows a 2 wire alternator. The manual shows a 4 wire alternator for a C10 on the same year, but with different color wires. The manual was good, but the translation and interpretation of the colors may have misled me. I guess I need a manual for the next year.

I'll install each module (AIR, "check engine", fuel pump control) and make sure the engine starts and turns off after each fix. I have to get an alternator back in and running.

There are probably a dozen loose wires hanging around inside. 2 police agencies and another government agency had owned this before me and had many radios in and out, and their mechanics have left many wires there. I will clean up the wiring associated with that, that did not help the situation. It looks like there are some loose Barth wires, that are numbered, I will ask about that at some point.

On the road again!
Matt

It is a real shame we love these old machines, even if the PO's made a mess out of them. Good luck Matt!