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940 alternator won't charge battery until engine is revved Jump to Latest

hitman

New member
Joined
Dec 1, 2020
I've got a 1993 944T with a weird electrical glitch. The car starts up fine, but the car power is at ~11.5V, and will stall out eventually if left to idle. Revving the engine kicks the alternator on and the car power jumps to ~13.5V. The drive belts are all new and set to the correct tension. Until a few months ago, the alternator would kick on around 2000 rpm. On one really wet/snowy day, it did not kick on at all. I replaced the alternator with a known working recently rebuilt unit from another vehicle, and now the car requires around 4500 rpm to kick the alternator on. I have also replaced the regulator which made no difference. Any ideas?
 
Click on the HOME tab in the top left and go to the front page of the website and read the second article.
 
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Sounds exactly like a self-exciting alternator. Does your charging light come on with the key on, engine not running?
 
Thanks for the quick replies!

I figure you guys are referring to the article about the cracked solder joint in the instrument cluster. Yes, the fuel gauge works about 1% of the time. I have done similar repairs to speedometers on both my 760 and 960, but never got around to looking at the instrument cluster on the 940 yet. I also know that swapping the fuel gauge from a different cluster is unlikely to work, because it has to match the sending unit. Volvo iterated through a LOT of combinations in the 80's and 90's.

I have never seen a charge warning light in this car. I assumed Volvo omitted it on the 940 like they did with the low coolant light. Also the car came with an LED from the alternator to the battery, which I assumed the previous owner used to verify that the battery was charging.
 
It sounds like your car has a failed exciter circuit and somebody has done some "rigging"/tampering.

Got a test lamp? DISCONNECT the exciter wire and see if it illuminates the lamp, key on. If not, there's yer problem.
 
I have never seen a charge warning light in this car.
Is there an oil light just to the right of the "0" zero mark on your speedo?
The charge light should be just left of the oil light.

Maybe it's as simple as the little light bulb has burned out?

- Cluster back-lit with a flashlight, bulbs removed.

Photo1621.jpg
 
It sounds like your car has a failed exciter circuit and somebody has done some "rigging"/tampering.

Got a test lamp? DISCONNECT the exciter wire and see if it illuminates the lamp, key on. If not, there's yer problem.
And don't make the mistake I, and probably many others have done, and try to test that exciter wire with a multimeter. It will probably show 12-ish volts, but with a failure somewhere in the circuit the instrument panel can still put out a tiny bit of amperage. It's 12V, but not enough juice to fire off the alternator.

It could be the charging indicator light itself. In a mild bit of irony, the bulb that supposed to warn you about charging problems can *cause* charging problems if it fails. it's not an independent monitor of the charging system, it's integral to making it work properly.

PS: You could probably use a multimeter if you leave the wire hooked up to the alternator.
 
Thanks all the for suggestions and info! I am swamped with family life, and this happens to be yet another Volvo that needs attention, so I apologize for the delay in the turn around. I finally got around to firing up and tinkering with the 944, so I figure an update is in order.

After sitting for half a year, the alternator now no longer kicks on: even at red line. This alternator was recently rebuilt and has a new voltage regulator, so like you guys I am more suspicious of the wiring and control circuitry.

I confirmed that there are no dashboard warning lights lit at the key on engine off state. I am appalled that neither of the two previous owners nor their mechanics mentioned this when asking me for advise on the fuel gauge. I pulled the cluster and was impressed to find three obviously cracked solder joints which further confirmed that the cluster was not even checked. After resoldering and reassembly, the fuel gauge is operational. I suspect it no longer matches the fuel sender unit because the needle said 3/4 for what should be a full tank.

While the cluster was out of the car, I checked the charge and a few other warning light bulbs, and none were burnt out.

Unfortunately, the resoldering has neither enabled the warning lights nor the charging circuitry. I am now planning to cross-reference those two circuits and double check the connections and look for tampering. Sadly, this car has some custom cut and spliced wiring, but I hope it is limited to aftermarket audio equipment.

Yes, the alternator has 3 wires attached. However, I completely forgot to check if the exciter wire is getting signal. This alternator also has an "internal" exciter wire that can be attached instead of the car's, and is currently detached. I have tried swapping to the internal signal, but found no difference in the car's running voltage.
 
After sitting for half a year, the alternator now no longer kicks on: even at red line.

Yes, the alternator has 3 wires attached. However, I completely forgot to check if the exciter wire is getting signal. This alternator also has an "internal" exciter wire that can be attached instead of the car's, and is currently detached. I have tried swapping to the internal signal, but found no difference in the car's running voltage.
You can verify that the alternator itself is still functional by momentarily connecting a small jumper wire between battery + and the exciter wire while it's running. It should magically start charging, and continue (even with the jumper removed) until the car is shut off.
 
You can verify that the alternator itself is still functional by momentarily connecting a small jumper wire between battery + and the exciter wire while it's running. It should magically start charging, and continue (even with the jumper removed) until the car is shut off.

^^^

My 940 had the same issue.

PO has put in a wire like mentioned above to manually excite the alternator.
It ran from battery + to a push button in the dash and then spliced in to alt exciter terminal.
After starting the car, I had to push the button once to excite the alternator.

It was indeed a faulty solder joint and it tool a while to figure out where it was on the instrument PCB board. I studied the wiring diagram and did a lot of measuring with a multimeter to find a wonky trace on the board.

Here is the thread I made, maybe it helps a bit: https://forums.turbobricks.com/showthread.php?t=350999
 
I took a closer look at the cluster wiring, and found that all the solder joints were showing connectivity when checking them individually (i.e. between two neighboring joints), but my meter was finding too much resistance when tracing through multiple joints at a time. I resoldered all the jumpers and diodes on the back of the cluster with silver solder, and could now confirm connectivity between the charge warning bulb and connectors D4 and C16. I just plugged the cluster back in the car and now I am getting check engine and oil level warning lights tested at ignition on engine off state. Not what I expected, but progress nonetheless. :)

I actually have a "tool" I picked up years ago that was perfect for confirming the alternator: It's a well insulated push button with two long wires terminated with alligator clips. Unfortunately the alternator did not start charging. I do have a spare alternator and will test it later, but it might not be any time soon.

The other next steps are to confirm that D4 is actually connected to the ignition switch and that C16 is the exciter wire to the alternator. While checking the car voltage, I tried using a cigarette lighter meter and was intrigued to find the cigarette lighter still powered even when the key is out of the ignition. I haven't run into this on any other Volvo and assume it isn't a 940 thing.
 
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