Edit: Sorry - Title should have read, "Voltage Regulator Diagnosis"

Having some trouble with my `91 940 turbo. Driving down the street today the airbag light came on, then the ABS light came on, then the car died in the middle of the street. Turning the key there was nothing - no crank, no clicks, no noise, just nothing.
I hooked it up to some jumper cables and it restarting no problem, ran for a couple minutes with the cables disconnected, then the same thing - airbag light, abs light, and dead.
I think all the problems are electrical - due to a bad alternator. I think that the lights are coming on in the dash because theyre not getting enough electricity from the bad alternator.
I've been reading up on this a lot and it seems like it could be one of two things - either a bad alternator or a bad voltage regulator. It sounds as though if it were a bad voltage regulator - I'd actually be getting too much voltage. This might manifest itself with blowing lightbulbs or something - but not the car dying like it is. Is that right?
So it must be the alternator causing my problems..? If i pull it and take it to autozone or something - can they check the alternator independantly of the voltage regulator?

 

Not really. Yes a bad regulator can cause a over-voltage condition but the assembly also contains the brushes that will wear to the point that it won't charge anymore. You should have ~14VDC on the battery during idle ... The motivation for replacing just the regulator assembly is because it costs less then a complete alternator. Make sure all your wire connections are clean.

http://www.fcpgroton.com/product-exe...ategory_id/112

 

Check the battery while you are at it, with cables fit and cleanliness. Most likely it is the alternator, which is the voltage regulator in most cases-the alternator itself is usually ok for a lot of miles... I'd get a hold of a multimeter-it tell you what you need to know.

 

So there's two ways a voltage regulator can fail -
1) over-voltage (>14 volts across battery terminals at idle) or
2) under-voltage due to worn brushes (<14 volts across battery terminals at idle)

I`m definitely getting an under-voltage condition (as judged by the symptoms). Is there anyway to test the voltage regulator prior to replacing? Or do people just typically start with the $50 fix and cross their fingers before dropping the $200 on a new alternator (which I assume comes with a new voltage regulator?)

tedv - I've checked battery terminals. They're both clean and tight.

I've never had a multimeter before. They've always looked complicated and intimidating, but I think it's time to bite the bullet with all this car's electric issues. Is there anything that makes one multi meter better than another multimeter? Any reputable brands to look for?

The other option I'm considering is a trip to the local junkyard. Aside from pulling the old unit and bringing it with me, is there any way to find out which years/makes/models used the same piece?

 

The pictures I've seen on the parts store sites ... the 940 unit looks like the 740/240 ones. The drive belts might be different. My 240 is currently using a 740 junkyard part that has a higher amp rating.

Volt meters are simple to use, I would get a cheap one from a parts store or borrow one from your neighbor.

 

You can pull the voltage regulator from the back of the alternator (2 screws) and look at the brushes-they should be minimum 1/2 inch but don't quote on me that measurement-they are cheap... Voltmeters are simple and readily available, they are good for a lot of jobs... I got mine for $20 from Sears.