DanielGr

My 89 C4 started to show a strange behavior, not charging at lower rpm.

Short history:

The car was driven rarely by the previous owner. When i got the car charging system was flapping, during drive sometime the voltage read 13,5 volts, sometime it dipped to 11,9 volts. Investigations revealed the alternator's power regulator is faulty, so i replaced the alternator with a refurbished one.

Drove now for about 2 months without any issue. Last week when i came home late, i noted that my dash illumination and lights dimmed and i could see the battery light on. Voltmeter reading was 11.6 volts.

Charged the battery fully next day to drive to my mechanic. When i started everything looked the same, but while driving i noted the voltage jumped from 11.8 volts to 13.8 volts and stayed stable at this level while driving on the highway. As soon i left the highway and rpm dropped to idle, the battery light came back on.

While testing with my mechanic we found out that the voltage will go up at around 1400rpm, that is also visible at the dash illumination which will become more bright immediately, dimming again when the engine drops below 1000rpm.

We believed it is the battery and tried another one which was proved to be good. But as well with this battery we could see the same problem.

Is the alternator faulty or do i have a different problem here?

Thanks
Daniel

WW7

You should be able to have the alternator tested at one of the auto parts stores....The rebuilt alternators of today are crap, I have seen guys go through 2 or 3 sometimes before getting one that works properly... You're better off having your alternator rebuilt by a reliable auto electronics shop..Just for reference, the alternator should never go below 13.5 ,or above 14.9 under normal circumstances with a fully changed battery......WW

playsdixie

my 85 with the stock alternator would do it with the a/c on at idle....a common problem according to advice from the forum...replaced it with a higher output alternator when I modded the engine and have had no problems since...if your battery's good and the belt is tight, it gets down to the alternator...

c4cruiser

It could be that the internal regulator on this one is going out. If the alternator is only 2 months old, you should be able to get it replaced under warranty.

Was the alternator a "remanufactured" or "rebuilt" one? A fully remanufactured unit should have all new parts inside. A rebuilt unit usually only gets internal replacement parts that are necessary.

You might also check to see if the serp belt is slipping. A bad belt tensioner could be causing belt slippage at idle resulting in the alternator not spinning enough to provide power.

DanielGr

Thanks all for replying.

@WW7: That's the strange issue here, once the rpm limit is reached, the alternator delivers a steady output between 13.5 and 14 Volts (depending if the cooling fan is running or not). The output only dips when rpm is falling below 1000.

@playsdixie: Thanks, that was already done. I had the choice between the normal and a so called heavy duty alternator, i ordered the heavy duty one because enough power is always good.

@c4cruiser: The parts store stated it as remanufactured. Warranty should not be the issue to get a replacement.
The belt was replaced at the same time. In fact i had a bad belt tensioner before, when the mechanic tried to tighten the belt the old one broke before he was able to get the belt to the required tension. So the tensioner was replaced as well.

Which part tells the alternator to deliver power? Is that all purely built into the alternator or are there any parts outside i have to look for (which may have failed).

Thanks again.
Daniel

c4cruiser

Did the mechanic check the voltage output at the large red wire on the alternator? And check the voltage at the battery positive terminal? It's possible there is a small variance in the dash display and what the voltage really is at those two points.

It could be there is a ground problem somewhere. Or this alternator has a bad internal regulator.

If you are operating a number of the car's electrical systems at the same time, the ability of the alternator to keep up at engine idle RPM is critical. But it should still be able to handle large current draws for some time. The fact that there is such a large voltage drop over a 400 RPM difference points to something being wrong.

Where did the replacement alternator come from? Brand?

DanielGr

Yes we did that. Yesterday we spent around one hour measuring from different points under different conditions (with existing battery, with new test battery). The voltage at the alternator directly was around 0,4V higher than on the dash display. We also tried to put an additional cable to see if better ground (assuming that some ground points may not be in good condition) improves the situation, but nothing changed.

During test we turned everything off, so no light, AC or interior light while measuring. Only the dash itself and the two lamps inside the hood where on.

The alternator is from a local store here who imports parts for different US car brands directly from the US. I don't remember the brand name, will need to check that with my mechanic when he is back on Tuesday.

So hopefully the problem is the alternator and not somewhere in the car.

WW7

The dash gauge reading lower then the "actual voltage" is normal, mine reads about the same difference as yours..WW