Electrical Troubleshooting Problem

If you connect your "volt meter" to the ampere current range and connect it to the socket, you will blow the fuse. And you might even fuse the intermittent contact on the reverse switch, if indeed that is the problem.

To measure current in a circuit you must 'open' the circuit anywhere and place your meter set to 'current' 'across' this point...in other words, the meter 'must' be placed 'in series' with the rest of the circuit. BUT, you don't need to do this, the bulb has already proven that there's a high resistance in series with this circuit, you must get rid of this before you can go any further...

When you checked the voltage at the sockets, did you use your volt meter and did you touch one meter lead to the positive terminal in the socket and the other to a ground somewhere in the back of the car? Or did you touch the negative lead to the negative terminal in the socket?

When you say that the voltage drops to 0 when either lamp is inserted, what points are you measuring voltage between?

When you say that the voltage drops to 0, what points are you measuring voltage between?

When you say that the switch has very high resistance, did you unplug it and actually measure resistance between the switch terminals with the switch closed? If so, how much resistance did you measure? If you are getting less than 1 ohm, then the switch is good.

Reverse lights should not be drawing 10 amps. You're more likely to see somewhere under 3 amps. Also, check the amperage rating for your multi meter. Many will not handle more than 3 amps unless you have a higher end one or one with an inductive clamp.

The easiest way to check out your theory of a bad switch is to unplug the switch and apply 12 volts to the circuit on the harness side of the switch connector. If the bulbs light up, then the switch is bad. If the bulbs do not light up, then you have another problem. I think that you have a poor connection to ground.

That's correct. But if I connect my volt meter properly, I'd be using the ameter sockets and selection on the meter's dial ...

Yes to both. I also used the appropriate pins on the connector after I determined that the socket itself was getting its juice.

The supply wire and the ground wire. Substitute any of the available options for ground and source.

See above.

No, I haven't found the switch yet -- admitting that I haven't spent a great deal of effort looking for it. I am assuming that I have a great deal of resistance on the switch because of the results I'm finding as I try to figure out why the lights don't come on. I understand that less than 1 ohm is good, my assumption is that I have several orders of magnitude more ohms than that.

You're right, they shouldn't be drawing 10 amps, but the fuse is rated that high. I's guess the amperage of the lamps is closer to 3 amps. If I remember right, the bulbs are rated to 4W. 12 / 4 = 3.

That's the hard part. I happen to know that much of the driveline on the car is covered by the exhaust system and a heat shield -- both of which I hesitate to remove for such an insiginficant problem -- but it turns out the tail end of the transmissiion is not covered.

I considered the poor ground connection, but have dismissed it for now. The reason being that all of the tail lights (lights on the back of the car) share the same ground, and they all work fine.

Just because "Jeff Strickland" could, he/she/it opin'd thus:

I'm not entirely sure what you mean by that, but if your ammeter is connected correctly (in series with the circuit under test) you can't test how much current is available, only how much current is being drawn by whatever is in the circuit . . . .

-Don

You didn't mention what type of vehicle - is it the BMW?

BTW, the reverse switch may be in the shifter console instead of the shifter itself. You might want to check it out by pulling back the shift boot and looking from above.

Good luck!

One more time...

If you connect your amp meter to the lamp bulb socket and apply power you will blow the fuse. Or worse. Ampere measurements are made in series circuits only.

Yeah, it's the BMW. The switch is in the tail of the trans. I also had trouble with the Parking Brake indicator, and hoped that the parking brake switch and the reverse switch shared a connection that wasn't made, and that this lived inside the console somewhere. It turns out the parking brake switch was missing a return spring -- a purely mechanical issue, not electrical at all.

Thinking the reverse switch might be in the shifter itself, I asked on the BMW newsgroup. Somebody posted a pic of it on the tail of the trans, and I should be able to get at it with my floor jack.

Good luck with the repair. Next time, take an IS for a test drive ;-)