Something tells me it’s time for a new alternator.
 

A while ago I installed a dash mounted volt meter. Recently while driving I noticed some abnormal voltages. I was hoping the brain trust could give me some insight.

The car is a 1975 S with a late 1983 SC motor.

At first everything starts out fine. Engine starts up, generator warning light goes on then off. No noticeable trouble at first. But after a while, what I noticed while driving on the freeway is at a constant speed (2,200 RPM) is the volt meter would drop below 12 volts. Reving the motor didn’t improve the indicated volts. After a while the volts go back to normal by itself, then after some more time it drops back to below 12 volts.

The battery is about 10 years old. Never been drained dead, it looks good, no leaks or bulges.

The next day I ran the engine in the garage with a volt meter for more info.

*Engine off, volt meter indicates battery at 12.13 volts
*Engine running- 2 min after starting volt meter indicates 13.51 v. 800 RPM - no gen light
*Engine running- 10 min after starting volt meter indicates 13.63 v. 1000 RPM-no gen light
*Engine running- 15 min after starting volt meter indicates 13.64 v. 1050 RPM-Gen light dim
*Engine running- 18 min after starting volt meter indicates 12.05 v. 1200 RPM-Gen light brighter. Reving engine makes Gen light less bright but still noticeable.

Shut off engine Gen light goes out completely.

What do you guys think? Voltage regulator and/or alternator?

Thanks

Holy smokes! Buy a dang battery for $100! Ten years???

The typical battery is only good for approximately 6 years so I would say you are due for a battery change. If you have the external voltage regulator, I would suggest you buy a replacement and as a worst case keep it as a spare. I have had similar alternator issues as you are reporting and the problem in my case was one of the exciter diodes in the alternator failed. I have submitted a post on replacing the exciter diodes at a cost of less than $5.00 if you have the DIY skills.

Battery likely not at fault but I would replace it. After 18 min. you have lower voltage than before you started it. Reason being alternator is not keeping up with load requirements. After initial start your voltage looks good then time and rotten connections become concerns to me. First check the ground wire from alternator to engine case behind the fan. If there isn’t one then install one. Next check all ground points. They are grouped together in several places on the car. Main grounds on chassis and fuse box area. Also check main alt. feed wire to starter. If all connections are good then pull the alternator and check brushes/slip rings. High mileage will wear the the slip rings to alarming depths. The slip rings can be changed but you may consider alt. replacement if wear is a concern.
Side story: We did a 1400 mile Hershey trip two years ago and this past March I pulled the engine (‘87 3.2) to do the clutch. I was amazed at how dirty/rotten/loose a number of connections were. Enough so I am now amazed the car went with absolute reliability. Make haste slowly...http://forums.pelicanparts.com/uploa...1531399703.jpg

A ‘75 has an external regulator, so why not lean the contacts and purchase a new voltage regulator before tearing into the alternator removal? You can buy a transistorized regulator for not much money and that may solve your issues (don’t understand why the regulator and alternator are supposed to be the same manufacturer as long as it has adequate capacity). And I’d agree that it’s probably time to consider a new battery.

Thanks for the replies guys. . .

I’ve got a new battery on the workbench ready for installation, I wanted to address the likely culprit of a failing alternator first.

The motor is a late 1983 SC motor so I’m assuming I’ve got the internal voltage regulator and not the external one. Just to be sure, If it did have an external Voltage Regulator where would it be?

My battery connections look good, but I’ll double check grounds anyway.

Likely I will buy a new alternator and replace it.

Jack Staands, The car is 75 with an 83 3.0 engine so it is unknown what type alternator is installed.

if its an 83 it will be the newer alternator with the regulator built in .
the regulator is on the left while peering into your engine bay ( check pet for 75 ) in the old cars behind the plastic cover ( if you still have it )

I think your first predictions are correct , you are in for a new alternator/ regulator

Tea Tray shows the likely problem, dirty 14 pin connector. A drop below 13 volts while running shows that the alternator is no longer charging. Pin 11 on the 14 pin connector takes the instrument alternator light current and uses that to excite the alternator. If the connection is bad, you will have intermittent charging, as you see. Clean the connections in the 14 pin connector and see if this helps. After 10 years, you need a new battery. You guys who live in warm climates seem to get more life from a battery than I do.

got 12 years out of my interstate battery and I live in Canada !! I don't replace when I "should" I replace when I need to .

Though I am prepared to replace the alternator it is interesting to note the voltage anomalies only occur after the engine is hot. Maybe that affects the contacts on the 14 pin connector or just a coincidence

clean up the connectors and see

don't worry so much about heat but idle speed

does it idle lower when its hotter ? mine does . 800 vs 950 for example.

No it actually idles lower when cooler , but my car is different, ‘83 SC motor with no DME. My pal who did the engine swap did some modification to the entire system to eliminate the DME and O2 sensor. I guess he tweaked it to run optimal when warm. Car doesn’t like it when I start up cold and drive off, one minute of warm up then ready to go.

Not a big deal in warm So. Cal. weather.

UPDATE: I took the 14 pin connector off and cleaned the contacts.

Initial results were good. The indicated voltage read a little higher, 13.8v than before cleaning the contacts. Unfortunately, after 25 minutes issues began . . .

At the 25 minute mark the voltage dropped to 12 volts and the GEN light dimly glowed. The car’s idle increased by itself, voltage increased to 13.5 and the light went out.

At the 30 minute mark the voltage dropped down to 11.8 volts and the GEN light came on a little brighter. I let the engine run an additional 5 minutes and the voltage went down to 11.6 volts. I revved the engine voltage increased to 12.3 and the GEN light dimmed a bit but remained dimly lit.

I’m sure cleaning the contacts helped but I guess it’s time for a new alternator and voltage regulator.

Any alternator brand recommendations?

You should have an internal VR - built in to the alternator.
May be faulty. What is your tacho doing? - you haven't mentioned it. So I am guessing is normal. Normally a failing alternator/VR shows up in erratic tacho.
Alan

Tachometer normal. No erratic behavior.

When my alternator failed I ordered a Valeo replacement from Pelican. The alt light glowing while the car is running indicates to me that one of your diodes may have failed. This causes the other two to work overtime to compensate for the reverse current flow from the bad one, putting greater load and more heat through the alternator. Also explains why you never get over 14 volts while charging.

SCs don't have a DME .. lol

Mototronic? DME? Brain Fart. In any case don’t have either

Give him a break, probably misspoke and meant an O2 sensor and Lambda box, the under seat unit in the SC.