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RWD - Charging Mystery Continues: 1993 Volvo 940

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Charging Mystery Continues: 1993 Volvo 940 900

I think the key here is the intermittent lights before starting the engine.

The alternator circuit is laid out such that there is a wire that goes from the alternator to the instrument cluster, and then either to ground or back to the alt. This is the "excited wire". Often, people have a burned out bulb in the cluster for the battery light, leading to an intermittent or constant no-charging situation.

Since your dash lights come on differently some times than others, I would figure that out. I think it will lead you to the issue.
I can think of two ways to do this:

1) Put the key in the ignition, and see if the lights come on. Hook up a meter to the alternator b+ and ground when the battery light does not illuminate. Start the car and watch the voltage. Do not rev the engine at this time.
2) Get the car into a situation where it is not charging, then turn it off, turn the key back to run (engine off) and see if the battery light is on.

I think that you have an intermittent connection to your cluster. Maybe a bundle of wires gets yanked on or moves while driving. I think that if you pull your cluster, the problem may be pretty obvious - somewhere between a burned trace and corrosion on a connection. You want to be very gentle with the plastic film circuit board connections.

The other thing I've seen baffle someone is that their alternator was missing its ground strap. This matters because on RWD Volvos, the rubber bushings actually can isolate the alternator body from the bracket.

Have you checked for voltage between the ends of your battery cables? This would be from the battery post to the alternator b+ terminal, not on the actual cable ends. I've had corrosion accumulate *inside* of a terminal lug, and cause issues. You should see around 0.1-0.2V end to end on the cables. Resistance should be (far) less than 1 ohm.

Good luck!






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New Charging Mystery Continues: 1993 Volvo 940 [900]
posted by  onlyVolvo  on Mon May 13 23:04 CST 2019 >

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