Hello everyone. My name is chris i am fairly new here.

I have a question and need some help with an issue i am helping a friend with.

My friend has a 1982 monte carlo. He has complained about his car never starting and dying all the time. I had came into the project with the car having a new battery, and alternator installed. I got the car started by jumping it and measured 12.1 volts at the alternator post to ground. i checked fuses etc etc etc and verified grounds were in place. No corroded terminals at the alternator. I tried installing a 12 gauge wire from the positive terminal at the alternator to the positive terminal at the battery to maybe get voltage with no luck. I found the v-belt was loose and as i was doing this i had my multimeter attached to the battery posts watching voltage. As i tightened the belt i watched the voltage jump from 12.1ish to mid 13s.... This is great the car is now charging.

This is where i get confused. I tightened the belt and adjustment nuts. I let the engine idle and watched my multimeter voltage slowly drop from 13.5ish to about 12 volts over the course of about 5-10 minutes. This is perplexing me. I took the alternator out and took it to Autozone and had it tested 3 times with it passing every time.

This specific alternator has a positive post for im guessing the ECM and and connector with two wires that plugs into it. Can anybody help me with this issue on what to chase and or test next. I am a John Deere Mechanic but not to fancy with automotive applications and especially not to fancy diagnosing charging system issues with no schematics or testing proceedures.

Can anybody help me PLEASE!!!!!!

 

If you had to jump it to get it started that indicates insufficient amperage to activate the starter motor. Pull the battery and have it charged to full capacity, then reinstall. Check your output with the engine running and you should see approx. 13.5 to 14 volts steady at the battery. The reason you see a drop in voltage now is because the alternator cannot charge the battery. It is designed to maintain the voltage. It is not meant to recharge a battery - especially one that has been drained to the point of needing to jump the car to get it started.

 

Hey 03SSLE. You have a good point. However if at one point the system had a fully charged (NEW) battery im assuming it started up properly and should have taken an adequate charge. so my idea behind this is that the alternator itself should produce 13.8ish volts once the exciter circuit is energized. one would think regardless of battery voltage the output from the alternator would be supplying a charge to the battery regardless of its state.

Am i wrong here? Im just having a my heart set on that this might be short to ground or wiring issue from the connector (Red & Black wires) at the alternator to the voltage regulator. I will try your idea 03SSLE to see if potentially this might fix the issue.

Thank you for your response.

 

Tried the fully charged battery idea and no luck. Voltage still slowly drops and before you know it the alternator isnt producing a charge. I disconnect the exciter circuit connector and watch the voltage drop from 12.9 to 12.4. i then plug it back in and hear the field engage and produce voltage only for a second or two. i watch the voltage jump from 12.3ish to 13.5 then boom back to 12.9.... ive traced wires to the starter. Tried jumping ground at the starter and jumping source voltage. NO LUCK. HELP!!!!

 

Even though the alternator is new/reman it's possible that the rectifier bridge may have a bad diode. The alternator would still have output, but it would be less than optimal. The initial output may be within spec, but once the diode gets hot it may fail reducing the output. The starter motor requires a lot of amperage to engage. I don't think the alternator puts out enough amperage to turn the starter motor.

 

I totally agree with Mike (03SSLE)

A remanufactured alternator isn't a guarantee that its working properly..
I would replace it with a new one... (Get a refund for the reman alt) If that's what you replaced the original with... also new ones are no Guarantee there good... at the time of manufacturing.. (Building)

Since the auto parts checked it 3 times doesn't mean its good...I would guess it didn't get hot and or warm enough to fail as mike mentioned.

The belt tension.. when its starts losing voltage..is a tell tail sign the alternator has an issue..(Like a failing diode) as the added pressure could also indicate a bad failing bearing thus causing miss alignment of the poles.

Have you possibly also checked the starter?
GM's are notorious for failing bendix's.. (Usually causes a battery drain on over night parking)