Reverse light wiring head scratcher

one provides power too the other from

Station House said:

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.

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Hi Station,

Continuity is fine too. Whilst the probes were attached I switched from the ohms setting to the audible continuity setting. All fine (beeeeeeeeep.)

That's a relief then.

Only other thing to suggest checking in terms of ensuring no wire breaks is to use a light bulb and battery to test - have encountered continuity test meter indicating all is well, but in fact only a few strands of wire actually still joined which couldn't carry enough power to light the lamp

From your photo (great pic by the way) it does look like the wiring has been played with by previous owner - I'd be doing the bulb test and running a new wire (defeatist I know) assuming there is switched live from the reverse switch itself

Station House said:

That's a relief then.

Only other thing to suggest checking in terms of ensuring no wire breaks is to use a light bulb and battery to test - have encountered continuity test meter indicating all is well, but in fact only a few strands of wire actually still joined which couldn't carry enough power to light the lamp

From your photo (great pic by the way) it does look like the wiring has been played with by previous owner - I'd be doing the bulb test and running a new wire (defeatist I know) assuming there is switched live from the reverse switch itself

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I would run a new wire, but am unable to at the moment because I haven't been able to confirm certain things:

1) The wire in question (pointed to by the yellow lines in the pic) only has 0.03V running through it when ignition is on and reverse is engaged. So if I run a new wire from there I'm faced with the same problem i.e. no power (there's no problem with the wire going from the engine compartment to the rear, the problem is somewhere prior to that.)

2) There is power going to and from the gearbox reverse switch but I don't know whether the return has passed through the fusebox or not, so I could wiring up a 12V supply directly to the bulb holder, which, of course, would be bad.

3) Haven't yet figured out where in the circuit the ignition comes into play, and I'd still like the reverse light to only operate when ignition is on.

Thanks for the pics kudos. Every little helps in this battle of wiring and messy spools.

Sorry, these things can be really hard to diagnose by email

1) With no wires attached to the reverse switch, is there a 12v output (from the switch itself) when in reverse?

2) If yes, and you then turn the ignition off (but still in reverse), does the voltage full to zero?

3) If you cut/disconnect the wire that goes to the reverse light in the engine bay, and run 12v straight through it, does the reverse lamp light?

if yes to all the above, just run a new wire from the reverse switch to the wire in the engine bay - job done and whatever is wrong has been bypassed

Station House said:

Sorry, these things can be really hard to diagnose by email

1) With no wires attached to the reverse switch, is there a 12v output (from the switch itself) when in reverse?

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To clarify point 1: how would there be any power attached to the reverse switch if I disconnected both wires?

Not got wiring diags with me, but thought one wire was the feed (+ve in) and the other the supply (+ve out) to the lamp

Was thinking if you disconnect the +ve out wire, turn ignition on and put in reverse and then get a good 12v reading from the terminal with no wire on it you know:

1) Feed works

2) Ignition Switching works

3) Switch works

If you can then put 12v directlly on the reverse lamp holder and the reverse lamp lights, fault HAS to be somewhere between the switch and lamp.

I have wiring diagram in front of me.

There should only be two wires from your reverse switch Green and green/brown.

Are you absolutely sure that the wire you have picked out in your piccies isn't in fact green/red

You've better eyes than me BD! Thought the arrow was pointing to the wire with that large black bullet connector - wire coming out of that look green/brown to me as it loops around and goes into the trunking with one other green wire:doh:

Thinking about it, believe you've cracked it BD

Those two wires going into the trunking on their own look green and green/brown - looks like the trunking could be disappearing down towards the gearbox.

it was mentioned earlier

Station House said:

You've better eyes than me BD! Thought the arrow was pointing to the wire with that large black bullet connector - wire coming out of that look green/brown to me as it loops around and goes into the trunking with one other green wire:doh:

Thinking about it, believe you've cracked it BD

Those two wires going into the trunking on their own look green and green/brown - looks like the trunking could be disappearing down towards the gearbox.

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Only nearly cracked it I believe

Think mr greenlove been testin wrong wires from not havin electrickety diagram to hand.

Think Greenlove you may find the problem is wire chaffing in the chassis.

Just outta interest what happens if you put Left indicators on an stick it in reverse??

jamesmartin said:

it was mentioned earlier

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It was. That's what got me looking harder at the problem.

Think so too, but a damn good spot all the same

Think Mr Greenlove may have gone to kill himself - do feel for the guy

On the plus side, probably explains why the wire he's been testing only registered 0.03v

Station House said:

You've better eyes than me BD! Thought the arrow was pointing to the wire with that large black bullet connector - wire coming out of that look green/brown to me as it loops around and goes into the trunking with one other green wire:doh:

Thinking about it, believe you've cracked it BD

Those two wires going into the trunking on their own look green and green/brown - looks like the trunking could be disappearing down towards the gearbox.

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jamesmartin said:

it was mentioned earlier

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Ok, decided to bypass in order to save myself a further time-consuming headache and relieve your kind selves.

If someone could confirm the following then I'll get on with it:

Gearbox Reverse Light Switch - Multimeter Test Results

Ignition on, reverse gear disengaged:

1) Attached one probe to metal surface, the other to terminal A on the reverse switch. Result: 0V.

2) Attached one probe to metal surface, the other to terminal B on the reverse switch. Result: 12V.

Ignition on, reverse gear engaged:

1) Attached one probe to metal surface, the other to terminal A on the reverse switch. Result: 12V.

2) Attached one probe to metal surface, the other to terminal B on the reverse switch. Result: 12V.

Stupid Questions

1) Which terminal should I connect the new wire (that will go to the reverse light bulb holder) to? I'm guessing it's terminal A because that only has power when both the reverse and ignition were on.

2) What amp wire should I use. I only have 30A and 16.5A spare at the moment?

Bromsgrove Defender said:

Only nearly cracked it I believe

Think mr greenlove been testin wrong wires from not havin electrickety diagram to hand.

Think Greenlove you may find the problem is wire chaffing in the chassis.

Just outta interest what happens if you put Left indicators on an stick it in reverse??

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Out of everything, that proposal seems to make the most sense. Here's a run down of what, most likely, has happened, and why my multi-meter test results are probably pointing to the wrong answer:

The cross member was replaced just before I bought it but the chassis wires were not removed prior to welding. I think that the welding heat fused the left indicator and the reverse wires together. This would explain why A) I'm getting zero resistance (good continuity) from that mysterious wire in the image (indicated by yellow lines), which is probably the old left indicator wire, and B) why my mechanic recently had to bypass the left indicator because he couldn't understand why it was intermittently working.

Knowing what might be the problem makes me feel better about the bypass now. Am buggered if I'm gonna rethread the entire chassis rail spool just to repair one wire that could easily be bypassed.

Station House said:

Think Mr Greenlove may have gone to kill himself

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I was just about to kick away the chair, but then the 'new reply' sounded on the PC

Station House said:

probably explains why the wire he's been testing only registered 0.03v

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Yup, we seem to have found the most plausible reason i.e. chafing within chassis rail, probably cause by welding heat when cross member was replaced.

Excellent Pal - always good to know the why - and glad you're still with us

So the good news - fuse, ignition switch and reverse switch all good.

Bad news - wire from reverse switch to reverse lamp buggered - hopefully other wires are OK

Answer - as you suspect - new wire from what you refer to as Terminal A to the rear lamp.

Either thread it through the chassis where you can using stiff wire as a guide to pull it through, or just secure out of the way using electrical pull through nylon clasps - B&Q sell in different lengths and either black or white for pennies (in their electrical section, usually by the fuses and plugs), or Halfords sell for a small fortune

In terms of cable, current will be low, so 5 - 10 amp is all you need. 16.5 amp will do fine if that's what you have laying about.

Crack on now and cross another job off the list

Sorry, just got back to this. Slightly too late by the sounds of it.

That green/brown wire that goes into that hole on the bulkhead should go to the brake pedal switch. However by the looks of that wiring, someone may well have changed that. The reverse light and the brake light are powered from the same fuse and use the same colour wires. I.e. green from fuse to switch and green/brown from switch to light.

Looks like someone may have confused the two circuits somewhere during all that butchery.

Good that you've decided to just run your own wire in any case.

You brake lights do work though don't they?

mundungus said:

You brake lights do work though don't they?

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Nah - that's 'tune in next week....'

Wow! Cant believe this is still going on !

If i hadn't fixed your problem within half hour my boss would have me guts for garters!

I pulled all my wiring back through the chassis on my motor, before i started welding.

Now i ran it on top of the chassis rail in plastic conduit by the brake pipe so i can continue welding in the future, and trace any wiring easily, i suggest you do the same.

By the way:

0.00V - 0.03V = means nothing!