Reverse light wiring head scratcher

bustersbus said:

Ah'm glad mines was easy!
http://www.landyzone.co.uk/lz/f41/reverse-light-switch-renewal-63810.html

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Ha, I followed that tutorial hoping it would sort the problem out. The switch is working great bustersbus, just no light...yet

Wiring gremlins in behind the dash or a dodgy earth seems to be the main contenders.

jamesmartin said:

so somewhere between switch and rear end there is a poor connection ,fuse is on feed to switch,so not there ,or at least shouldnt be ,wire goes through loom at dash to rear rh then back out to lh

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Yes. Because the switch is working as it should, and the wire from the reverse light to the engine compartment has zero resistence, we might be right in thinking that the problem begins with the wire running from the engine compartment somewhere into the dash.

So somewhere in the dash I reckon there's a broken connection.

Do you concur, Spock?

just put in reverse and check wire for 12v at each connection start at rear loom to dash loom near pedal box ,

Ah'd be really tempted tae start at the back like james says... might just be a crudded up connector

jamesmartin said:

just put in reverse and check wire for 12v at each connection start at rear loom to dash loom near pedal box ,

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I need to take the dash off at the weekend anyway (never done that before either ) I presume the wire coming from the engine compartment into/behind the dash is supposed to go straight to the fusebox?

bustersbus said:

Ah'd be really tempted tae start at the back like james says... might just be a crudded up connector

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Checked everything from the back to the engine compartment (just this moment double checked rear earth again - resistance and continuity fine), so it's just from the engine compartment into the dash and to wherever that I need to check, I think.

I know there's the reverse switch in the gearbox, but is there another switch I'm missing here, buster?

Now unscrewed some of the dash in order to trace the green+brown coloured reverse light wire going from the engine compartment through into the dash, but not sure where to go next. Hard to see anything in the tangled mess of wires behind the instrument panel.

Was wondering, where exactly should this wire go to? Does it feed straight to the ignition (then to the fuse box) or somewhere else?

Any help would be appreciated on this as I'm starting to lose my temper with this now, grrrrr :doh:

Is there power coming from the gearbox switch?

If so - and accepting you've checked the lamp holder itself - would run a new wire from that to the rear lamp rather than trace the break

GreenLove said:

Now unscrewed some of the dash in order to trace the green+brown coloured reverse light wire going from the engine compartment through into the dash, but not sure where to go next. Hard to see anything in the tangled mess of wires behind the instrument panel.

Was wondering, where exactly should this wire go to? Does it feed straight to the ignition (then to the fuse box) or somewhere else?

Any help would be appreciated on this as I'm starting to lose my temper with this now, grrrrr :doh:

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Just followed the circuit in those two documents I posted on your other thread. They indicate that the wire from the reverse light switch doesn't re-enter the cab. So you're looking in the wrong place.

At the back of the engine bay, mounted on the bulkhead there are a number of connectors. Connector 162 is the larger 13 pin connector that connects the main harness to the engine harness. Reverse light is pin 6.

This block of wires runs up to the top of the bulk head, which also tails off back down to three connectors mounted next to each other kind of on the side of the transmission tunnel. The reverse light circuit comes straight back down to the middle of these three connectors (the white one, aka connector 378, reverse light is pin 1).

You could check for voltage at both of these points when in reverse and ign on.

From this middle connector, the brown/green wire goes straight to the back through the chassis rail.


Failing all that, just run your own wire from the output of the reverse light switch to the light itself. (That's actually what I have done temporarily, as mine has exactly the same break in that circuit).


Also, update your profile to give a rough location. Someone local might be able to give you a hand.

mundungus said:

Just followed the circuit in those two documents I posted on your other thread. They indicate that the wire from the reverse light switch doesn't re-enter the cab. So you're looking in the wrong place

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Ah, I was referring to the wire coming from the rear. From the rear bulb holder there is a wire that runs through the chassis rail, up through the engine compartment and through a hole in the bulkhead into behind the dash somewhere (resistance tested this and all was fine.) This is what has mainly led to my confusion over the circuit setup.

Station House said:

Is there power coming from the gearbox switch?

If so - and accepting you've checked the lamp holder itself - would run a new wire from that to the rear lamp rather than trace the break

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Hi Station,

Yeah, I'm considering running a new wire. It's just that this is my first electrical fix and I really wanted to find this particular fault.

Only spent 3 solid days on it so far Still, I'm learning something along the way, hopefully.

If that wire is on its own and not part of the loom, it sounds like someone has already butchered that circuit, In which case, following the wiring diagrams is probably pointless.

I'd definitely say run your own wire now.

If you have battery voltage at the output of the reverse light switch when in reverse, then whip out fuse 17 to make sure that voltage goes away. That way you know your circuit is protected against shorts.

Then simply run a wire from the output of the reverse light switch to the light.

When I did mine I just threaded it through the gaps in the pressed sections of the underside of the rear floor, and tied it around the handbrake cable.

One day I'll re-run it through a piece of plastic convoluted conduit to better protect it. Its on my never ending todo list.

mundungus said:

If that wire is on its own and not part of the loom, it sounds like someone has already butchered that circuit, In which case, following the wiring diagrams is probably pointless.

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I've taken the following pics to illustrate the setup I've discovered so far:

if you have power to switch then you dont need to worry about this wire its one from switch to rear at fault

jamesmartin said:

if you have power to switch then you dont need to worry about this wire its one from switch to rear at fault

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This is the wire going to the rear bulb holder of the reverse light. When the ignition is on and reverse gear engaged the voltage of this wire changes from 0V to 0.03V, which I would have thought indicated that there's a break in the circuit. Correct me if I'm not understanding this correctly, but this is the wire that the bulb holder relies on for it's power, right?

green wire comes from fuse box to switch,then green/brown wire takes it to rear ,are you sure you have right wire and not those a bit lower one with single connector and one under it

jamesmartin said:

green wire comes from fuse box to switch,then green/brown wire takes it to rear ,are you sure you have right wire and not those a bit lower one with single connector and one under it

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The wire in question has been resistance tested several times. It's definitely connected straight to the reverse light bulb holder.

In the new pic below I've shown the two wires that actually go to and from the reverse light switch on top of the gearbox:

how do you mean resistance tested ,continuity test between light holder pin with bulb removed so not going across earth to other lights and the 2 wires from switch

jamesmartin said:

how do you mean resistance tested ,continuity test between light holder pin with bulb removed so not going across earth to other lights and the 2 wires from switch

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1) Multimeter was set to ohms, defaulting at 1.
2) Attached crocodile clip to wire going into back of bulb holder (with bulb removed).
3) Attached sharp probe to various wires in engine compartment that were part of the bunch coming from inside the chassis rail.
4) Ohms dropped to zero when correct wire was probed.

The wire labelled with yellow lines in the images above definitely provides the power to the reverse bulb holder. Question is, what route does it take to the point where it exits the dash, entering the engine compartment, making it's journey to the rear.

Good spot Mr Martin!

You need to be checking continuity not resistance - meter should have a setting for this - touching the 2 probes together should display a reading of '1' and possibly make an audible alert sound as well.