voltmeter jumps to 16 with RPMs, CHG light flashes

[jeremy93ls]

I took my car out yesterday for a short drive and the first thing I noticed was the BRAKE light inside the speedometer on. I fiddled with the e-brake switch to no avail and checked to make sure the fluids were OK. I stopped at a store a few miles down the road when I got back in and started it, the light went out when the e-brake was disengaged, like normal.

Later on, the voltmeter gauge caught my eye as it was bouncing from 14v-16+v while driving, causing the CHG light to flash. Returning to neutral or easing along at low RPMs kept the needle about 14 but every time I gave it gas, it would max out and bounce back and forth.

I crept home watching it bounce around until at one point it just settled down and stayed about 14-15 where it usually sits for the last few miles home.

I'm on my 2nd rebuilt Hitachi alternator since acquiring the car a year ago and haven't had a bit of trouble out of this one until yesterday. The only thing that might have affected it was that I tightened the alt/fan belt the day before because it was a bit looser than I like. The battery is good and also a year old.

I took it out for a drive around the block this evening and it acted normal. I was getting a little over 14v at idle straight off the alternator. I then turned my headlights on bright and got a reading of 13.1v at the alternator. A minute or two later, I was getting 12.8 and slowly dropping. By this time the battery was reading 12.5.

Do I have yet another failing alternator? My 78 has the internal voltage regulator and I'm guessing this is failing. Do you think tightening the belt caused this, somehow? I should have gotten a reading from the alternator when it was surging yesterday but I was more concerned with getting home.

[Zed Head]

Check the T plug, specifically the S wire. Maybe it has lost contact at the alternator. While you're there, make sure it reads battery voltage with the engine off. Use your meter.

[SteveJ]

Another possibility is the failure of the internal voltage regulator in your alternator. If you are mechanically inclined, I believe replacements are available. Check the T plug first.

[jeremy93ls]

The voltage at the S wire in the T plug is identical to the voltage at the battery. The connectors are nice and clean, too. I wiggled the T plug around with the probe inside and the reading never changed so the connection inside there is solid.

[SteveJ]

Pull the alternator and take it to a shop to be tested. Sometimes their testing equipment will reveal a bad VR.

[Captain Obvious]

Do I have yet another failing alternator? My 78 has the internal voltage regulator and I'm guessing this is failing.

I'll put in a third vote for this. The internally regulated system either works, or it doesn't.

You mentioned your charge lamp flashing yesterday... Does it still work today? The CHG lamp should light up when you turn the key to "ON" before you crank the engine. Does it?

[jeremy93ls]

The charge lamp is working normally now. It lights up just before turning the engine over and then goes out. When you say it either works or it doesn't, I wonder what made it act up temporarily?

I'll drive around this evening and see if I can get it to fail again.

[Captain Obvious]

Wait a minute... Last I heard, you were driving around with your headlights on and your alternator had failed. Your battery voltage was dropping? "A minute or two later, I was getting 12.8 and slowly dropping. By this time the battery was reading 12.5."

When I said that it either works or it doesn't, I meant that there really isn't much else in the whole design that could be an issue with the internally regulated system. Other than a loose drive belt or a bad connection, about the only thing that can go wrong is inside the alternator itself. The only other thing I could come up with is if your CHG lamp is burned out, there's a chance that your alternator won't get itself up over the hump of producing current. But if your CHG lamp works normal, then that shouldn't be the issue.

Take it out and have it tested or borrow a known good one if you can and throw it in and see what happens?

[jeremy93ls]

I only turned the headlights on to see if I could get the gauge to jump around again so I could get a voltage reading at the alternator, but it never did. That's when I noticed the battery was slowly dropping voltage.

I'll get it tested tomorrow and see what happens. Thanks for the troubleshooting advice.

[SteveJ]

I should be home over the weekend if you need some help. Send me a PM.

[Captain Obvious]

Glad to (try to) help. My additional advice would be to take it to SteveJ's place and distract him with a Digi-Key catalog. Tell him the centerfold this month is excellent or something.

Then when he isn't looking, swap your alternator with his.

[SteveJ]

The 74 is still externally regulated. He'd have to dig the one out of the 73, and that one is surrounded by hazards that scare me. It's dodgy at best to attempt that strategy.