Hi around 6 months ago or so, my Generator Light came on when running. But I could only run the car for 1/2 minute because the water pump was shot.

I just replaced the water pump and ran it today with no leaks (knock on wood) and while at it, I installed a brand new 55 AMP Alternator, but the red GENERATOR light did not go out. Here is what happens when i use a voltmeter on the battery:

Engine Off : 12.8 V
Engine Running with no Lights: 12.75 V
Engine Running with Lights: 12.44 V

Any ideas please of what it could be? I am afraid to take it out of the driveway as I am afraird of running out of battery charge. When I did the same thing for my GMC Yukon, the goes to 14.2 V when running.

 

Check your voltage regulator.

 

It should be about 13.8V with the engine running. Is this still the external regulator system?

 

If this is still an external VR, bypass it by jumping terminals F & 3 at the regulator plug, leave the plug disconnected and retest voltage at the battery with it running. If no change there is no alternator output or a wire problem. If there is voltage between approximately 13.8-14.2 with the jumper in place the regulator is bad.

 

Hi I have an update - please let me know what you think. I have a black voltage regulator I believe mounted on the firewall. For kicks and giggles, I removed the wire connection to the voltage regulator, scraped a little bit of the inside of the connector and then re-installed the wire connection to the voltage regulator. I started the car at 12.8 V and watched it this time climb to around 13.4 V so I braced myself and drove around the block around 3 times (around 3 miles total) and after letting the car run in the driveway aroud 5 to 10 minutes, I watched the battery go from around 13.8V to around 14.5V (no lights on). Still the Red Generator Light was ON the whole time. I don't know if it is a coincidence or not, but before I removed and reinstalled the wire, I think i didn't notice the voltage climb, but now I see it climb but slowly.

When I put the lights on, the battery immediatley drop to reads around 12.8V and when I turn the lights off, the battery slowly climbs back - probably would take around 8 to 10 minutes to get back to over 14V.

I didn't play with anything else and didn't re-remove or any connections. For the record, I have never seen the GEN light on since we had the car. With my Yukon when when I start the vehicle it immediately jumps to 14.2 V from 12.8 V.

I quite frankly never had the GEN light on before, so I don't know how the charging system normally operates because never checked it. In general, with the car running, no lights, it will go from 12.8 V to 14.5 volts in around 10 minutes or so - it slowly climbs , it is not immediate.

 

Well I started the car again just now (let it cool down after 5 hours ) and it did not charge past 12.8 Volts this time, it just stayed at 12.8 Volts - this time it did not climb.

So I took the voltmeter cap off and tapped each field contact very lightly to see if it was stuck - didn't notice anything funny. Put the cover back on. I went ahead and put a voltmeter on #2 (it read 7.8 volts and just for fun I put a voltmeter on F, 3, and 4 just to see what it said (I don't think this mattered, just mentioned just in case putting a voltmeter probe inside each connector could cause something to change)

I went to the battery and with the engine running, it now says 15.35 V and checked the GEN light and it went out. With the lights on, the battery reads 15.65 Volts. Glad the GEN light is out but that seems high. Anybody run a car that high? Should I still get a new voltage regulator just in case? Weird that just putting a voltmeter inside the regulator caused the voltage to go up so much and for the GEN light to go out.

Sugar Bear - by the way, how would jumping F and 3 cause it to run at 13.8 V. And by the way, you mean just take the connector off and a connect a wire to just F and 3?

 

You probably have a bad VR if the voltage is jumping to over 15v.

 

You need to verify battery health with a load test. How are the battery cables? 50-year-old cables likely need replacing WITH OEM exact-fit cables.
Clean the grounds to the Alt, VR, battery & engine.
Verify proper Alt belt tension.
THEN test the alternator & VR.
If the Alt is original have it rebuilt or at min don't turn it in for a core charge and get it rebuilt down the road.
If you need a VR get the solid state unit. The Delco cap can be installed on it to look like stock if that's a concern. At minimum keep it for the cap.

Attached is how I test the two components.

 

Yes F and 3 with it unplugged and it will likely exceed 13.8 VDC. I agree with oldcutlass that the VR is bad, since you've seen 15 plus volts the alternator works, replace the VR.

 

If the voltage regulator swap doesn’t work, consider pulling the gauge (did they still call it a tell tale?) and see if the bulb isolating gasket is still sound. I.e. not disintegrated.

I had a similar problem long ago and it was actually just light bleed over fron the the GEN-adjacent bulb. The gasket was dust.

Chris