1993 Volvo 940 Alternator/parasitic draw issue

jw051288 · Post by **jw051288** » 20 Oct 2009, 21:05

Hi.
I had a problem where my 940 died going down the road.
I figured it was the alternator since i had the battery replaced not more than 6-8 months ago.

I removed the alternator and was looking for a new one. I went by an alternator shop i knew and they tested it for me and even opened it up to check the brushes etc. He said the alternator was fine and the brushes were still good for another 15-20,000km.

So I went home and charged the battery fully. then I vat tested the alternator and found out it was not charging.

There is one wire from the back of the alternator that I was told was a reference wire that has a 12v supply. so i checked if there was 12 volts going to it and there was not.

I decided to by pass the wire straight to battery voltage just to check.
When i did this and tested the alternator again, it was charging the system.

I left the car overnight and in the morning tested it and the car was dead again. I checked it out and checked for a parasitic draw and found there was a 12v draw coming from somewhere. I did the checks of pulling fuses and relays to see if the voltage would drop, but none of these did the trick.

Just asking around if anybody has any input or tips to give.
Was I wrong in bypassing the "reference wire" @ the back of the alternator?

any help would be greatly appreciated.

thanks!

DeRail · Post by **DeRail** » 20 Oct 2009, 22:45

I've read that the voltage regulator, which is a bosch unit (if I recall correctly) is a common failure on the alternators. Might be something to look into...

As long as you're not having any other weird problems, you probably didn't damage anything using the bypass wire for a short period, then again anything is possible. Most likely though, you're probably okay.

-Darrell

jw051288 · Post by **jw051288** » 21 Oct 2009, 00:36

the alternator is a nippon denso. Not sure if they have the same issues.

Post by **billofdurham** » 21 Oct 2009, 15:35

When checking for a parasitic draw you are looking for amps not volts. If you have a multimeter switch it to the ammeter. Using the voltmeter should always return about 12 volts.

This is courtesy of wikiHow:

1. Remove the negative side battery cable from the negative battery terminal.
2. Attach an ammeter(this measures amperage) between the negative cable and the negative battery post. Wait a few seconds to several minutes for the car to go into sleep mode - i.e. when you make the contact with the ammeter, the cars computer systems "wake up". After a bit of time they will go back to "sleep".
3. If the ammeter is reading over 25-50 milliamps, something is using too much battery power.
4. Go to the fuse panel(s) and remove fuses, one at a time. Pull the main fuses (higher amp ratings)last. Be sure to observe the ammeter after pulling each fuse.
5. Watch for the ammeter to drop to acceptable drain. The fuse that reduces the drain is the draw. Consult the owners' manual or service manual to find what circuits are on that fuse.
6.Check each device (circuit) on that fuse. Stop each lamp, heater, etc. to find the drain.
7. Repeat steps 1&2 to test your repair. The ammeter will tell you exact numbers.

The attached is the wiring diagram for the alternator on a 940. It may help you to solve the puzzle of the 'reference wire'.

940 Alternator circuit.pdf: (26.21 KiB) Downloaded 951 times

Bill.

jw051288 · Post by **jw051288** » 25 Oct 2009, 22:05

thanks. the diagram will be a great help