Idle is rough but only when alternator is charging

Tiny
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!
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Wednesday, September 9th, 2015 AT 6:32 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
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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Wednesday, September 9th, 2015 AT 7:56 PM
Tiny
CARADIODOC
  • EXPERT
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.

Also, those pulses of current induce voltages into any wires running parallel to the output wire running back to the battery. If one of those is for an engine sensor, that induced voltage might only be a few tenths of a volt, but that's huge to the Engine Computer. That's where your elusive running problems come from. It's why we always start out by suggesting unplugging the generator as a test when everything else has been tried without success.

When you found the same problem with the battery charger connected, it was caused for the same reason. Its output is rectified single phase voltage. That has a peak of about 18 volts, then it goes down to 0.0 volts, then back up again. That is 18 volts of "ripple" voltage. When the generator has a bad diode, and that circuit is putting out 0.0 volts, the other two phases are still putting out something, so ripple voltage won't be quite as high. What this boils down to is if the battery can't smooth out the voltage, the engine is likely to run worse with the battery charger operating than with the defective generator operating.

You did the first part of the charging system test already by measuring battery voltage with the engine running. That must be between 13.75 and 14.75 volts. Since that is okay, it is okay to do the second half of the tests, but that requires a professional load tester. Suppose your generator is listed as a 90 amp unit, which is a fairly common size. Under a brief full load, you're either going to get very close to 90 amps, or you're going to get very close to 30 amps along with very high ripple voltage, (which most load testers measure).

To add to the misery, because the battery is so important, it is common to go through four to six replacement generators in the life of a GM vehicle. That's when people come here for help and we explain the need to replace the battery, unless it's less than about two years old, to reduce the number of repeat failures.

Even if your generator appears to be okay, a new battery should solve the running problem by holding system voltage steady. In the rare event the battery doesn't help, that's when we'll have to dig deeper to figure out what's happening. It sounds like you already covered the likely suspects with the wiring.
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Wednesday, September 9th, 2015 AT 10:21 PM
Tiny
2CARPROS MIKE
  • ADMIN
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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Thursday, September 10th, 2015 AT 10:18 AM
Tiny
JOHNMOSQUEDA
  • MEMBER
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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Thursday, September 10th, 2015 AT 6:21 PM
Tiny
CARADIODOC
  • EXPERT
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.

The eprom is just GM's way of storing software specific to that vehicle's engine size and emissions system. The basic computer is the same for all vehicles, then just the eprom is different as needed. That way only a few computer part numbers had to be kept in inventory.

GM did have a lot of computer trouble in the late '80s and early '90s, at least that is what I'm led to believe by how many people replaced them and solved problems. I have a feeling they developed bad solder connections on the connector terminals, but I only took a couple apart to try to fix them. Never was successful.

If a new battery doesn't solve this, the next thing I would pursue is looking at the generator's output voltage waveform with a scope to see if there's a lot of spikes, or "noise" on that wire. It's not going to look perfectly smooth but it will be obvious if there's garbage on there that shouldn't be there. Related to that, look for any wires, particularly computer sensor wires that are not routed as they were originally. Anytime two wires are running alongside each other, they can magnetically induce voltages from one to the other.
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Thursday, September 10th, 2015 AT 10:17 PM

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