I have a voltmeter on the dash and noticed my voltage had started to be at the red line on the gauge. Turning on my headlights kept it in the black until the new regulator I ordered arrived. Having installed it, voltage read higher than I would expect, but out of the red. The next day it was reading just above the red line on the low end! I got no alternator warning, but I watched the voltage creep down as I drove home. I put a voltmeter across the battery after parking and it was down to 10.8!

I trickled charged it overnight and checked if I got the battery light before starting the car. It came on with the other lights, which rules out my suspicion that the excitation wire was bad/loose. Starting the car, voltage read just below the red line on the high side again. I hadn't done anything other than remove and reinstall the battery.

Does this match a known failure mode for something I'm missing? I've been getting some intermittent belt squeal and am about to order new bushings for the alternator, but my temperature never went beyond the dead center of the gauge, so I'm assuming the alternator is still spinning at the appropriate speed the whole time.

I am puzzled!

 

You have a problem. A bad belt would cause it, as would some bad connection/ground. Get a real voltmeter and measure across the posts with the engine running--alternator should be putting out about 13.8V. Those in dash gauges are iffy...

 

Thanks for the quick response!

Wouldn't a bad belt cause the dash lamp to come on? When it didn't, I suspected the excitation wire, which I then ruled out when I saw the lamp come on before starting the car. I've adjusted the tensioner, but the alternator appears to have some flex in the bushings.

I just compared the gauge voltage to a proper meter across the battery terminals:

with key in RUN before starting - dash gauge needle just past vertical and the voltmeter read 12.76 V

at idle - dash gauge just below red on the high end and voltmeter 14.48 V

with headlights on at idle - dash gauge moved a couple millimeters lower and the voltage across the battery was 14.32 V

After installing the new voltage regulator, I still got the high dash gauge reading, followed by two trips with a low dash gauge reading, corroborated by the measured voltage across the battery when I took it out to charge overnight. After reinstalling the charged battery, I've only had the high dash gauge readings and then the numbers above. I haven't touched excitation wire or grounds recently and I replaced the regulator with the alternator in place, only disconnecting the battery's ground wire.

 

alternator bushings are a regular wear item, especially the OEM style black rubber ones. I'd suggest replacing them with the blue poly ones you can get at The Volvo Parts, Accessories and Performance Specialists Since 1963, they'll last forever and not allow the alternator to sag and go out of alignment.

 

Hopefully the bushings will take care of it. Otherwise, a bad diode in the alternator is all I can think of that could cause the wide range of voltage outputs, though I would think that would still illuminate the dash light. Has anyone experienced that?

 

a bad diode would make the alternator put out A/C, if you put a decent meter in Volts AC and see more than a few 10ths of a volt at the battery, its fubar.


note, some cheap or older multimeters will get false AC readings from DC. test this by putting the meter in ACV, turning the car OFF, and reading the battery voltage both ways (+ to + and + to -), you should only see like 0.02V or something when it settles.

 

Balzac: So now you are reading 14.48V? That should be OK, on the high side but not that high as to cause you concern. I wouldn't worry too much about the erratic dash gauge readings. I wouldn't expect much change with new bushings regardless of the type...

 

Sorry to take so long to answer. I had my notification preferences misconfigured and didn't see your post.

I wound up replacing the alternator when I did the bushings the other day. I realized that the old one would start at 14.8 V and then drop once the engine warmed up. Starting with a warm engine would result in the system not charging. I corroborated the dash gauge readings by putting a quality meter across the battery, both with the engine running and with it off. Fortunately my commute wasn't long enough to drain the battery completely! I had been forced to trickle charge it overnight before putting the new alternator in.

 

I replaced the original alternator bushings after 20 years with the blue poly ones and they only lasted a couple of years. They literally disintegrated so a couple of months ago I went back to the black rubber ones. Maybe they were of a lower quality than other poly bushings but I wasn't impressed. The few times that I have gone aftermarket and not OEM Volvo I have ended up regretting it.

 

That is good to know! I stuck with the black rubber for my new ones.

And to clarify, I had thought that it may have been belt slip when the engine ran at higher RPM since it would put out enough juice idling in the driveway but not on the road. Once I made the connection between temperature and output, I decided to get a new alternator.