Alternator or voltage regulator? How do I test?

[FritzE30]

On Thursday, dash indicator lights started flickering while I was braking. On Friday, as I was driving home, the radio started going in and out, blinkers were having a hard time; the car became plagued by a host of electrical issues in a short time. I had just pulled off the highway in a residential street and it died suddenly, no sputtering, no jerking, the engine just shut off.

I came back two hours later and – surprise, surprise – I was able to start the car and drive it the few remaining blocks (without headlights) to my garage where it died again.

Considering that the car shut off while driving (when the alternator should supply all required power) and that it started the second time on battery alone, I dare say this is likely an alternator problem, rather than a battery problem.

Now the question… how can I tell that this is caused a broken alternator rather than a broken voltage regulator? I wouldn’t want to buy an alternator if the problem can be solved cheaper by replacing the VR. The Bentley only says that a broken VR will result in higher voltage at the battery. I don’t think this applies here.

Whatever tests I could do now, I’d have to do them while jumping the car from my other car. Does it even make sense to test voltage while jumping?
How do you test the voltage on the output pins of the alternator? What should it read?

Thanks
Fritz

[strad]

What I would do is pull the vr off the back of the alternator and inspect the brushes. If they are worn out it'll be pretty obvious. And if they are worn out it would create the symptoms you are experiencing.

Don't play with the alternator unless you disconnect the battery first.

[ScottinSoCal]

How do you test the voltage on the output pins of the alternator? What should it read?

It doesn't really tell you much to test the voltage on an alternator, because that doesn't tell you if it's putting out the amps you need. It needs to be load tested, and that can't be done with a voltmeter.

[graycat]

Agree with Mr. Strad... discon the battery before fooling around with the alty/vr wiring. Only takes one slip of the wrench to make things sparky.

My experience; Replaced VR while on a road trip. Like you, blinky dash lights, then it quit. Let sit a while, got it to a garage.
Was okay for a while, then noticed "pulsatile" headlamps at idle. Changed out entire alty with one from Advance Auto with a lifetime warranty. Make a photocopy of receipt, the store original will fade within a couple of years.

I still carry a spare VR and "pulse" or timing sensor, just in case, for my 90i.

Good Luck...
graycat

[grantorino62]

The VR is probably the easiest thing to remove/ install on the whole car. Disconnect battery, remove 2 screws holding it on to the back of the alternator. Look at it, are the brushes worn more than half the length of their holder? If so, replace VR. Then have the alternator load tested for amp output.

[Trevor M]

Worn brushes on the voltage regulator will cause exactly what you described.

To test an alternator you do this....

If the battery is dead now, charge the battery fully....use a charger, not by boosting or jumping.
Start the car and place a voltmeter across the battery.
You should read 14v plus or minus 0.5v
Switch on some heavy loads like headlights and heater blower
Watch the voltage. If it drops, rev the engine a bit and it should come right back up.

If you read something like 12v or 12.5v (or lower) during these tests, your alternator isn't putting out.

If your still not sure, remove the alternator and take it in to be load tested on a bench.

Used alternators from a wrecker are cheap & plentiful.

[thejlevie]

+1, though I'll say that only see about 13.5v at idle but you should see 14v at 2500rpm. The system voltage may dip slightly under load (HVAC blower on max and headlights on) at idle, but should still be 14v at 2500rpm.

Most of the time the only problem with the alternator will be worn brushes or a bad voltage regulator, which can be fixed by brush or VR replacement. Replacement of the entire alternator is only called for in the case of bad bearings, a bad diode, or shorted windings.

[FritzE30]

Thanks guys.

I'll check the brushes first then I'll probably replace the VR anyway; it's only $20 on pelicaparts, and considering the history of this car I wouldn't be surprised if it were the original part from 22 years ago (do they even last that long?).

I may as well invest in a battery charger to diagnose the alternator; it'll be useful in other situations - more than once I forgot the glove box compartment open, with its light on, only to wake up to a car with a dead battery in the morning.

[thejlevie]

I can't imagine having a car and not having a good battery charger (or a set of heavy duty jumper cables).

[FritzE30]

I never really needed a battery charger; in the few cases where I accidentally drained my battery by leaving a light on I solved the problem by jumping it from my other car. It's a useful backup tool though.

[strad]

Thanks guys.

I'll check the brushes first then I'll probably replace the VR anyway; it's only $20 on pelicaparts, and considering the history of this car I wouldn't be surprised if it were the original part from 22 years ago (do they even last that long?).

I may as well invest in a battery charger to diagnose the alternator; it'll be useful in other situations - more than once I forgot the glove box compartment open, with its light on, only to wake up to a car with a dead battery in the morning.

Yeah definitely get a battery charger. While you're at it get one that will hold a battery at float voltage (fully charged but without overcharging it).

As for how long a vr/brush set lasts, I replaced one on my 318i that I think was original and it had 200k miles on it. Brushes were worn down far enough that they were no longer contacting the commutator (I think that's what it's called anyways). New vr/brushes and that alternator worked fine again.

[reelizmpro]

The Bentley should have specs for the brush length of the voltage regulator.

[T77911S]

I can't imagine having a car and not having a good battery charger (or a set of heavy duty jumper cables).

i cant imagine owning a car that does not need work

[FritzE30]

The Bentley should have specs for the brush length of the voltage regulator.

Yes, I missed that paragraph the first time I looked for the alternator troubleshooting section.

The Bentley says the new brushes are 12mm long and the VR should be replaced when the brushes have eroded to 5mm. I took mine out and ... the longer brush is 4mm long, at its longest point (1 mm shorter in the middle). The other brush is 3mm and the contact surface is covered in black soot.

[FritzE30]

Update:

It was indeed the voltage regulator that had failed. I replaced it with a new one ($20 + shipping on Pelican Parts) and the car works just fine.

Here's a pic of the old and new parts side-by-side.

[85-318i]

Update:

It was indeed the voltage regulator that had failed. I replaced it with a new one ($20 + shipping on Pelican Parts) and the car works just fine.

Here's a pic of the old and new parts side-by-side.

Wow! good to know

[Brewski29]

I'm having the same problem now but my car doesn't die when it's running I got the bl-inky dash once today when I was braking but when I got the car home i shut it off and it started right back up with no problems. But a few days ago I stop somewhere for 3 mins and when I came back out and went to start it everything just shut off. I had to get it jumped, so would that be the ALT or the VR Anyone got any ideals?

[failedattempt]

I'm having the same problem now but my car doesn't die when it's running I got the bl-inky dash once today when I was braking but when I got the car home i shut it off and it started right back up with no problems. But a few days ago I stop somewhere for 3 mins and when I came back out and went to start it everything just shut off. I had to get it jumped, so would that be the ALT or the VR Anyone got any ideals?

Sounds like a battery issue.

[mountainman141]

Sounds like a battery issue.

It more than likely is. My best advice is to get it replaced especially if its old and hasn't been changed for a long while. Costco batteries from my experience have always been darn near bulletproof and instead of paying for a new Costco battery when it dies, just take it in before the warranty ends, tell them it's giving you issues and a hard time, and they'll replace it FREE OF CHARGE since it was under the warranty. *Knock on wood*

[grantorino62]

First, Check alternator belt. Can you rotate the alt. pulley by hand? If so, belt's too loose. Second, remove voltage regulator, compare your brushes with the prior posted picture. Most likely it needs a regulator. Regulators are cheaper than batteries!

[Brewski29]

I tested it I got 11.3v with the car off, at idle it was 13.9v, then the 1st time under a load it read 0.0v so I did it again and it showed 13.8-13.9v any ideals?

[strad]

I would still pull the VR and have a look at the brushes. Also you may want to check the condition of your battery connectors and check all grounds for condition and tightness. I have also seen a bad starter cause issues that presented a bit like a bad alt or bad battery.

[Quinthirty]

A simple multimeter will show you the voltage charge rate of the battery. Before removing the VR, you can just start the car, leave it on idle, and stick a multimeter (put on 12v) to the terminals (red to positive, black to negative, just in case you need to know), and read the voltage readings. Now, switch on lights, A/C, and everything else electronic to create load, and check again. Voltage should be around 12.6v according to the Bentley when engine is off, and should be around 13v++ at idle, and go up to 14.5v when you rev it (try 1500RPM and above). Readings should not go beyond 16.5v. Below 13.5v is indicative of a worn or weakening VR (carbon brushes). You can re-use the VR and solder in new carbon brushes for cheap if you're trying to save money.

[Brewski29]

Ok so I tested it I got 11.3v off 13.9v @ idle and the 1st time under a load was 0.0v then I did It again and I got 13.8v-13.9v any ideals on that. It's been fine the last few days beside my headlight that i already replaced went and then came back on two days later.

[Brewski29]

I used my multimeter I got 11.3v off 13.9v @ idle and then the 1st time under a load I got 0.0v so I did it again and I got 13.8-13.9v any ideals. It'd been fine for the last few days beside the headlight i already replaced went out and then came back n two days later.