Truck dies when the alternator is plugged in

HELEN BAILEY

MEMBER
1988 CHEVROLET S-10
2.5L
4 CYL
2WD
MANUAL
100,200 MILES

I was traveling home one night when all of a sudden my truck started sputtering slight backfiring then shut off. I waited a short bit of time and it was hard to start but it did start idles really rough then sputtered again and died. Could not get it going had to leave it had a mechanic come and look at it and said he thought I needed the Electronic Module and a distributor cap, rotor, plugs and wires. Installed all of that and made no change. It did start it was very hard to start and I limped it home. It was acting like it was not getting fuel so another mechanic misread the fuel pressure specs and thought I needed a fuel pump. So I put in a fuel pump filter and fuel pump relay made no change. Tried to get it home it kept dying we started a couple of times I could drive about a mile and it would die. Until it finally would not start had to leave it again on the side of the road and towed it home the next day. I have had five mechanics look at this truck and only one found this problem online and told me to unplug the alternator and see if it will start. So I can now unplug the alternator the truck will start and run and drive. As soon as I plug the alternator back in the truck immediately dies. I have also replaced at the ECM no change I replace the alternator no change. I am now seventeen days down five mechanics looking at it one of them being a twenty year GM mechanic veteran. All left scratching their heads one of them even saying that the gremlin in my truck is smarter than he is. So seventeen days later I am still broken down. I have now lost my job because I have no transportation and I am very frustrated. I know a fair bit about cars and I have done tons of research over the past couple of weeks. We have taken a test light and a voltmeter and checked every stinking sensor in that truck and they all are functioning normally. The injector is functioning normally I am getting spark I just do not know what to do next. The common denominator is the plug on the back of the alternator two wires one red one Brown one goes to the light in the dash and the other to the ignition. I have read that it could be an ETS problem with the brown wire with a black stripe to be grounded or open or it could be the purple and white wire. I just don't know what to do next I have replaced several things on this truck over $300.00 worth of parts and I get no change. Can anyone please help? There is very little online about this problem. I found one blog or a guy had the same truck as I do had the same problem was communicating with a mechanic asked if he can run a new wire from the plug of the alternator that goes to the ignition in other words just run a new wire from point A to point B and if that would fix it then the blog stop but there was no answer. I am so frustrated I do not know what to do several mechanics $700.00 and I still have no vehicle has anyone heard of this problem and how to fix it.

Saturday, August 13th, 2016 AT 4:06 PM

4 Replies

CARADIODOC

MECHANIC
34,330 POSTS

The first thing to do is replace the battery. Second thing is to have the charging system professionally tested. You can start the testing yourself with an inexpensive digital voltmeter. With the engine running, measure the battery voltage. It must be between 13.75 and 14.75 volts. If it is low, replace the generator again. If it is okay, have your mechanic finish the testing. He will measure full-load output current and "ripple" voltage. If the charging voltage is a little high, clean and tighten the battery cable connections.

Since GM redesigned their generators for the 1987 model year, they have had a real big problem with them developing harmful voltage spikes that can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. The battery dampens and absorbs those voltage spikes, but as it ages and the lead flakes off the plates, it loses its ability to do that. It is real common to go through four to six replacement generators in the life of the vehicle. To reduce the number of repeat failures, replace the battery at the same time that you replace the generator, unless it is less than about two years old.

If the voltage spikes destroy one of the generator's internal diodes, the most output current you will be able to get is exactly one third of its rated current. 30 amps from the common 90-amp generator is not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference as it slowly runs down over days or weeks. That low voltage can cause running problems by itself, but a failed diode also cause ripple voltage to be very high, and the resulting pulsing current magnetically induces voltage spikes into adjacent wires in the harness, and if those wires are for sensors, the Engine Computer will interpret those voltage spikes as coming from those sensors. It will try to run the engine based on those "noisy" signals. This is where unplugging the generator stops the production of those voltage spikes, stops the interference with sensors signals, and the engine runs better as long as the battery voltage does not drop too much.

If testing shows the new generator is only producing one third of what it should, and ripple voltage is high, it has already been damaged. Replace it and the battery.

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Saturday, August 13th, 2016 AT 11:07 PM

HELEN BAILEY

MEMBER
3 POSTS

I already have replaced the alternator and the battery at the same time. It will start but not run for more than a few seconds. Sometimes it will produce a code 42 after doing so, but not always. The battery and alt are producing the right amount of voltage also. I have had the charging system checked, all normal. I have been reading that the EST wire may be grounding causing the stall, no start problem? But I am wondering if the wires coming from the alt plug could be a problem? One goes to the idiot light that exits the alternator, the light is working normally. The other wire? This has become so frustrating. The days following the initial break down it would start (always a very hard start) and run. At least for a while. Now its worse! With all the new parts and everything hooked up like it is supposed to, now it will start but will no longer stay running.

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Sunday, August 14th, 2016 AT 8:19 AM

HELEN BAILEY

MEMBER
3 POSTS

Well I got it to start and run with the alternator plugged in! That is a step in the right direction. And the engine light went off! Yay! I think all its acting like now is its out of time, maybe? I took a video of me starting it and tried it a couple of ways. Gave it gas while turning the key I know w/fuel injection you do not need to do they (that is the only way it would start since it broke down) it wouldn't start and I smelled fuel. Let it sit for a few turned the key started up w/out giving it gas. Idled really rough, gave it just a little gas, it tried to smooth out but stalled. I just had to take a break from it for a while. So what I have done so far today? I unplugged the ECM and with a voltmeter checked the wires that were causing the Code 42. They all tested fine like they should lightly cleaned the pins little bit on the connectors for the ECM and plugged it all back in make sure everything was tight and plugged in the truck even the alternator. Then proceeded to do what I mentioned above and started the truck. But it is still not right, still have a ways to go. Any ideas?

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Sunday, August 14th, 2016 AT 12:50 PM

KEN L

MASTER CERTIFIED MECHANIC
54,142 POSTS

This sounds like the ECM is bad and once you increase the voltage in the system the computer overloads and fails. The computer has an internal voltage regulator that sounds like it is not working.

Please let me know,

Best, Ken

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Monday, August 15th, 2016 AT 2:58 PM