91 Volvo 940 Not Exciting Alternator or Staying Excited, time for cocaine?

OK so I am at my wit's end with this, alternator doesn't excite. Started like this every now and then (for about a week or 2) it would start up with a bunch of dash lights on (classic bad alternator) drive it like 50' car was good. It would only do it on cold starts (this is still true at least for start up). Once it did it I was good.

Next we have a new symptom that started the other day, it will un excite while driving, drop it out of od into 3rd, revs hit 4k no issue. Did that about 4 times in 60 miles the other day. Symptoms of not exciting when cold.

Today we have a new symptom I haven't seen before alternator stopped charging, rev to 4k; won't excite. Rev it to 5k; no go. Rev to 6k, still no go. Pull over turn car off back on, alternator is charging, no lights. WTF.

What i've tried so far:

1. reving it (usually works see above),
2. I checked to see if there is voltage at the exciter wire at the alt, there is half of a volt, jump it to the battery, all lights go out, remove jumper, they all come back on. Once the car warmed up, all lights went out on their own. Charging was taking place at 14.37 v as indicated at the alt AND the battery.
3. Wiggling the wiring harness in the engine bay, seems to make no difference
4. Wiggling the wiring on the back of the cluster. I never had this issue before I had the cluster out, but nothing seems loose? I've tried pulling it out again and then

The alt was rebuild about 15 months ago when I blew out the rectifier, battery is 8 months old tops.

Other thoughts are maybe it's a bad ignition switch? But this seems unlikely as I haven't noticed anything else not working.

Ideas? I'm kinda stumped. I am debating pulling new wire from the cluster to the alt but that sounds like a huge pain. Also, anyone have the harness book for a 91 volvo 940? I seem to be missing it. I am about to take a long trip in this car and would like to get it sorted.

I usually stick in a JY brushes pill from some random euro car.

It sounds like the reg or brushes. If it was brushes it *should* re-excite as soon as the lamps on the dash come on though.
The only other thing I can think of is if the exciter wire is shorting to gnd somewhere. Do you have a volt meter in the car?

Check brush pack.
Re-solder printed circuit board. Common on the 91+ 7/9 clusters.

"Sell" (give away?) grandpa car if you can?

Had the same problem on my 91. Had to re solder on the board,you could actually see the break.

oldnfast said:

Had the same problem on my 91. Had to re solder on the board,you could actually see the break.

Click to expand...

I had a very similar issue with the alternator not exciting consistently on my '89 760. Smacked the dash out of frustration one morning and the damn thing excited. Pulled the cluster, resoldered, no more problemo.

Cluster sounds suspect and it would explain why hitting a bump has fixed it a few times. I have an alt on another car I can pull and put on. Are the cracks that obvious? Were they cracked in the middle of the trace?

I should clarify the alternator in it is a 100 amp denso.

More so the traces will be lifted, or peeled back in most of the cases I've seen. I had to use pretty low heat to keep from dicking it up further.

I'll echo the comments about the '91 clusters being pretty crappy.
Check your alternator ground.

Ground should be good, there is the stock one run to the manifold and i have a 4 gauge ground from from the mounting to the block to the chassis

I have never seen a trace that lifted. That doesn't mean they don't. What I have seen many times is a cracked solder joint causing exactly what you are experiencing. They are obvious when you look at the solder joint with a magnifying glass. Read $500740's article about fixing the clusters. The whole process takes 1/2 hour.

Anyone know what's the incoming and outgoing wire positions? I am. Just going to bypass the cluster for now

www.volvowiringdiagrams.com is your friend. It is SO MUCH easier/quicker to just pull the cluster and fix the actual issue.

Bypass the cluster? Seems like the long way around the block, but OK. If you are going to do that, might as well just run a wire with ign switched power to the d+ on the alt thru a light bulb holder?

I have a 91-92 cluster thats "good" (speedo/odo works...for now, though not sure if it was ever correctly repaired). It's an N/A one, so you would need to swap the tach with the boost gauge...if you care...or not. The boost gauge is fairly useless. Re-solder yours or that one?

Pm me with a price. I have reflowed several joints without much success. Still having the same issue. Almost looks like the cluster isn't pushing 12v through there. I will try taking the cluster apart and seeing what's up. Looks like all the diodes work

IDK, cheap because I don't trust those 91-92 speedometers (though you can swap yours in and have a spare, probably not a bad iea).
Can't go flat rate (too big), but you aren't that far away if you are still bay area-ish...$30+actual shipping?

Sounds good. Send me a pm for paypal

So I spent 3 hours today and no luck.

I desoldered and soldered almost every single solder joint on the cluster (with 63 37) (I reflowed them first didn't work so I removed it all and added new)

I checked the wire from the cluster to the alt, .01 ohms so it's good to go it seems (didn't check it against ground guess I should have but I suspect it's fine)

I replaced the ignition switch, no change. What do the diodes specifically do on that cluster? Maybe I have a bad one?

Any other ideas?

The lack of it self exciting has me wondering a little....
Internal short when it warms up and breaks in the alt?

I've had some where they won't self excite sufficiently to charge the battery when the cluster or wire breaks connection though.

I guess i should try removing the exciter wire and then see if it'll self excite?

For now using a test lead between the output on the alt and the exciter input is working fine; just annoying.