Back-up Lights

[devo230]

The back-up lights aren't coming on. The fuse box states that the back-up and the fog lights share the same fuse. The fog lights work. Is there a switch or something that I could/should replace?

[kang]

There is a switch on the tranny that can fail. It's easy to check - unplug the wires from the switch and touch them together. If the back-up lights come on, it's a bad switch.

See "reverse light switches" on this page: http://www.pelicanparts.com/catalog/SuperCat/0834/POR_0834_ELSWCH_pg4.htm#item17

[devo230]

I don't have a Bentley manual. I do have Adrain Streather's book (not very technical).

[kang]

On the side of the tranny. It's easy to see. It's the only thing on the tranny with wires going to it.

[devo230]

I unplugged the wires. The ignition was on and the trans was in neutral. They stayed on no matter if the trans was in reverse or not or the wires were touching or not, if I recall my scenarios correctly. Is it clear that the switch is the problem?

[kang]

Seems weird to me. Touching the wires together should simulate the closing of the switch caused by putting the trans in neutral. I would think the lights would be off with the wires not touching and on with the wires touching. With the wires pulled off the switch, it doesn’t matter what you do with the gear lever. I don’t know why they are always on. Perhaps there is a short someplace else in the line, but that wouldn’t explain why they are off with the wires connected to the switch.

But at least you’ve ruled out the fuse, bulbs, etc.

[Don Settergren]

I found my back-up light wires broken slightly "upstream" of the transmission switch where the wires were tied to a bracket. Both wires were broken leaving erratic on and off situations.

[benzyne]

I've touched the two wires together in my car, and still nothing. Fuse is good. Bulbs are good. Where do I look next?

[lucittm]

Benzyne,
You received your car?

Does your car have the rear fog light (non-USA model)? If so, does the rear fog light work? If it doesn't, you have a ground problem from the lights in the rear reflector to chassis ground. If it does work or you don't have this, read below.

The backup switch gets its power from relay #1 which is powered via fuse 14. Swap the relay with a known good one of the same type, like relay #21 (sunroof), or relay #23 (horns).

Good Luck,
Mark

[benzyne]

Hi Mark,

I got the 965 yesterday. Tearing into it now to get things sorted.

I checked the relay and it works fine. Fuse is OK too. Stumped.

Ben

PS. The radio is great thanks!

[lucittm]

Ben,
You are my brother in Vancouver, I have to help you.

I am looking at the schematic. The two wires on the side of the transmission connect to a simple SPST switch that routes +12V directly to the lights. Put the transmission in reverse.

With the key in the on position, through the contacts of the energized relay R1, there should be +12V on the violet wire (measured from the case of the transmission) and you should have some resistance (through the two back-up light bulbs) on the grey w/ brown stripe wire (measured from the case of the transmission). Touching the wires together simulates closing the switch and the back-up lights should come on.

If you don't have the +12v at the transmission violet wire, reconnect the transmission wires (either wire to either connector is OK). Then ensure relay #1 has a good input of +12V on terminal 5 (key on). If the relay is good, then the relay is not energizing or there is a broken wire between the relay and the transmission. The output of the relay is terminal 5 and you cannot measure the +12V with the relay removed. To eliminate all doubt, insert a spade to spade short jumper wire between terminal 3 and terminal 5, the numbers are small. If the back-up lights come on then the relay is bad or the relay coil is not energizing from the main +12V bus, terminal X and ground, terminal 31. The terminal X +12V bus is powered from the always hot terminal 30 bus via relay 33. Check this one too.

If you don't have some resistance (less than 10K ohms but not ground either) on the grey w/ brown stripe wire, then disconnect the connector behind the center reflector. Pin 1 measured from the car side is ground, pin 2 is for the back-up lights, and pin 3 is for the rear fog lights. On the reflector side of the connector, going across pin 1 and pin 2 should measure the resistance of the light bulbs, if not there is an open wire.

You can also jumper +12v to pin 2 of the center reflector and ground to pin 1 to see if the lights come on.

How do you know the bulbs are good? I have seen an internal short in a bulb that caused the whole circuit to be grounded and no lights, but the bulb looked and measured fine. You really have to illuminate them to be sure.

Let me know,
Mark

[benzyne]

Hey Mark,

Thanks for the indepth guide! I went back to look at the bulbs like you recommended ... illuminate them to be sure. As I was back there, tracing wires, I came to a connector that wasn't connected! The center reflector wasn't plugged in. Boy am I feeling stupid.

I've spent quite a bit of time under the car, and fiddling with relays, and all along it was a main connector that the previous owner had disconnected at some point.

Reverse lights work now.

Thanks Mark.