Engine idles rough

JOHNMOSQUEDA

MEMBER
1995 CHEVROLET SILVERADO
4.3L
6 CYL
2WD
AUTOMATIC
200,000 MILES

Truck has irratic idle when altnator is charging. Recent work done fuel pump replaced, rebuilt throttle body including new injectors tps and iac motor, replaced map sensor, replaced o2 sensor, replaced coolant temp sensor. Truck runs great and idles perfect if system voltage is 12.5 ( running on bat only) as soon as altnator is pluged back in system voltage goesd to 14 volts and truck run bad poor throttle response and timing is irratic. Have tried new altnator same issue. Have a know good ecm from another truck and same issue with either ecm. Good truck runs with either ecm. Replaced distributer with new. Also tried several ign modules. Set timing at 0 as per instruction on alldata. Unplugged alt. And test drove truck all work great. Stationary with battery charger and truck runs like junk. Have tested all power and ground circuits to ecm including voltage drop test and found no issues. Check all 5 volt sensor supplies all good. Check at sensors as well all good. Prefomed voltage drop test on battery cables and body + frame grounds all good. Checked voltage at fuel pump good as well. Fuel pressure 13 psi. Also have access to gm tech 2 scanner have no codes and data all looks good data does not change when charging voltage is applied. I have tried just about every thing I can think of including disconnecting harness and checking for shorts to ground and or power no isses found. I have a schematic and have tested every thing on ecm side I can find. Any thoughts? At a complete loss!

5 Replies

CARADIODOC

MECHANIC
33,916 POSTS

Replace the battery, and most likely, replace the generator again. You're way ahead in testing compared to what most people know to do, but you have to understand that since these were redesigned for the '87 model year, GM's generators went from one of the better designs in the world to by far the worst. Due to their design, they develop huge voltage spikes that can destroy the internal diodes and voltage regulator, and interfere with computer sensor signals. The battery is the key component in damping and absorbing those harmful spikes, but as they age and the lead flakes off the plates, they lose their ability to do that.

I'll be back in a minute with part 2.

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Monday, June 29th, 2020 AT 6:55 PM

CARADIODOC

MECHANIC
33,916 POSTS

Okay, to finish my sad story, when the battery gets to be more than about two years old and the lead has been flaking off the plates, it doesn't do a good job of damping the voltage spikes created by the generator. The most common thing to fail is one of the six internal diodes. AC generators put out three phase output that is rectified and turned into DC current by those diodes. Rectified three phase output voltage is real steady and pretty smooth. When one diode fails, you lose one of those phases AND you lose two thirds of the generator's maximum current capacity. You can still find the output voltage, (as measured with a digital voltmeter), to be a nice 14.0 volts, but if you looked at the actual current flow with an oscilloscope, you'd see a nice pulse of current, a nice pulse of overlapping current, then a gap of missing current. The output voltage will drop a lot but too briefly to be seen by the voltmeter.

This guide will help

https://www.2carpros.com/articles/how-to-check-a-car-alternator

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Monday, June 29th, 2020 AT 6:55 PM

THIS IS MIKE

MECHANIC
686 POSTS

Johnmosqueda
September 10, 2015.

I have tested battery it test good at 800 cca also have identified that it happen any time system voltage is above 13.2 volts. I ran truck today using an adjustable battery charger instead of alternator and found voltage is a factor as stated at 13.2 volts.

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Monday, June 29th, 2020 AT 6:55 PM

JOHNMOSQUEDA

MEMBER
2 POSTS

Ok lets see today I tried a new altnator new battery new battery cables both including replacing altnator feed to battery. Checked all switched power and rechecked grounds + added new ones from body to battery and from grounds on intake to battery. Used external wires to bypass and run truck without ign switch. Also used a pig tail directly from ign module to computer. Have tried new computer but eprom chip has to be moved from one computer to another. My question is what does the eprom chip control are the five volt reference voltages generated from there? Is it possible that the eprom is the whole issue?

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Monday, June 29th, 2020 AT 6:55 PM

CARADIODOC

MECHANIC
33,916 POSTS

Every battery that causes these problems checks good because they're only a few years old. Also, load testers only draw about half the cca rating for the test. Regardless, it's not the battery's ability to crank the engine that is in question. It's its ability to absorb voltage spikes to prevent them from becoming a factor.