What would cause an alternator to become a *load* on the electrical system at idle?
Some backstory:
When I bought my Lumina at the end of 2019, I also purchased a 220A alternator for it. On a cold start, and at or above 1500 RPM it appears to work fine. Once everything warms up, at idle I see my voltage drop below battery voltage. I've seen it drop as low as 10V, and unplugging the terminal from the field winding gives me batter voltage as expected, the alternator is becoming an inductive load at idle when hot.
Voltages measured as follows:
I sent it back once and it was returned to me with a new pulley and a suggestion to replace my belt and tensiopner (both of which were brand new). I didn't bother with this, as a slipping belt would result in reduced voltage no lower than battery voltage -- and because the parts were new, and the stock alternator did not (and to this day does not) exhibit this issue. I got them to agree to an RMA but they wanted me to pay $50 in shipping on the *chance* that they'd accept it for a refund, they refused to guarantee a refund on receipt. So, I threw the stock alt back in and boxed up the 220A to come back to at a later date.
That later date was yesterday.
Suspecting perhaps a failing voltage regulator, I swapped to one from the stock alternator before driving home from the dealership I work at. Same result, after about 15 miles of driving I had 0.2V less than battery voltage at idle, 14.2V above 1500 RPM. Pulling the field terminal gives battery voltage.
Both when I originally installed it and yesterday, I put a scope across the terminals to check for presence of AC current (one or more failed diodes) and found a clean DC signal with only a couple milivolts of ripple, probably one of the cleanest outputs I've seen from an alternator. It's not bad diodes or a failed winding. Given that failed diodes and windings don't magically fix themselves under load, I expected to see what I measured, but it's always best to verify.
It's not a bad voltage regulator, as I've got the stock alternator back in now with no issues, with the voltage regulator that was originally on the 220A unit; both voltage regulators are now known to be good.
At this point, I'm planning to take it to a local rebuilder and have them work out and fix whatever is wrong with the thing, but I figured I'd check here to see if anyone's familiar with GM CS130 alternators and might have a simple fix. My in-depth training is MOPAR-centric and I've already looked over all of the generic stuff.
All I can think is that it's the LED I've got in the idiot light, but I would expect that to be a problem for the stock CS130 as well, if that were the issue. I'm willing to further investigate this as a potential cause, as I do know the alternator is expecting a certain amount of current to pass through it and into the field winding. I started down this road, but everything I'm reading on the subject is conflicting as to whether an LED would be passing too much or too little current, thus whether I would need to add a resistor in series or in parallel. I guess I could drop a 194 bulb back in there to test, but I'd really rather not have to take the dash apart twice if that doesn't help. I guess this is another question I would need a GM guy to answer.
Also potentially of note, the VOLTS light does illuminate when the engine is not running and when I run the engine with the belt removed, it is functioning as intended. It does *not* come on when the output of this alternator drops below battery voltage.
Any help I can get is appreciated.
Some backstory:
When I bought my Lumina at the end of 2019, I also purchased a 220A alternator for it. On a cold start, and at or above 1500 RPM it appears to work fine. Once everything warms up, at idle I see my voltage drop below battery voltage. I've seen it drop as low as 10V, and unplugging the terminal from the field winding gives me batter voltage as expected, the alternator is becoming an inductive load at idle when hot.
Voltages measured as follows:
- ALT + to case
- ALT + to BATT -
- ALT + to engine block
- ALT + to body
- BATT + to BATT -
- As reported by PCM (ignition voltage)
- As reported by my OBD-II scanner (voltage at DLC)
- As reported by my stereo
I sent it back once and it was returned to me with a new pulley and a suggestion to replace my belt and tensiopner (both of which were brand new). I didn't bother with this, as a slipping belt would result in reduced voltage no lower than battery voltage -- and because the parts were new, and the stock alternator did not (and to this day does not) exhibit this issue. I got them to agree to an RMA but they wanted me to pay $50 in shipping on the *chance* that they'd accept it for a refund, they refused to guarantee a refund on receipt. So, I threw the stock alt back in and boxed up the 220A to come back to at a later date.
That later date was yesterday.
Suspecting perhaps a failing voltage regulator, I swapped to one from the stock alternator before driving home from the dealership I work at. Same result, after about 15 miles of driving I had 0.2V less than battery voltage at idle, 14.2V above 1500 RPM. Pulling the field terminal gives battery voltage.
Both when I originally installed it and yesterday, I put a scope across the terminals to check for presence of AC current (one or more failed diodes) and found a clean DC signal with only a couple milivolts of ripple, probably one of the cleanest outputs I've seen from an alternator. It's not bad diodes or a failed winding. Given that failed diodes and windings don't magically fix themselves under load, I expected to see what I measured, but it's always best to verify.
It's not a bad voltage regulator, as I've got the stock alternator back in now with no issues, with the voltage regulator that was originally on the 220A unit; both voltage regulators are now known to be good.
At this point, I'm planning to take it to a local rebuilder and have them work out and fix whatever is wrong with the thing, but I figured I'd check here to see if anyone's familiar with GM CS130 alternators and might have a simple fix. My in-depth training is MOPAR-centric and I've already looked over all of the generic stuff.
All I can think is that it's the LED I've got in the idiot light, but I would expect that to be a problem for the stock CS130 as well, if that were the issue. I'm willing to further investigate this as a potential cause, as I do know the alternator is expecting a certain amount of current to pass through it and into the field winding. I started down this road, but everything I'm reading on the subject is conflicting as to whether an LED would be passing too much or too little current, thus whether I would need to add a resistor in series or in parallel. I guess I could drop a 194 bulb back in there to test, but I'd really rather not have to take the dash apart twice if that doesn't help. I guess this is another question I would need a GM guy to answer.
Also potentially of note, the VOLTS light does illuminate when the engine is not running and when I run the engine with the belt removed, it is functioning as intended. It does *not* come on when the output of this alternator drops below battery voltage.
Any help I can get is appreciated.
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