Reverse light wiring head scratcher

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GreenLove

GreenLove

New Member
Posts
430
Location
Carmarthenshire, South Wales
The Haynes manual doesn't appear to provide a wiring diagram for the reverse light, which is a pain because I'm currently trying to diagnose why the reverse light isn't working.

Can someone please confirm if I've guessed the circuit setup correctly:


  • 'Ignition on' sends 12V to reverse switch (screwed into top of gearbox).
  • With reverse gear engaged, reverse switch connects the circuit, allowing current to flow to the fusebox slot (providing ignition is on).
  • Current flows through fuse, then goes straight to reverse light at rear.
  • Current goes through reverse light, then to earth.

Not sure if there's relay there somewhere.

FYI: at the moment I've established that the wire from the reverse light to the engine compartment is fine (zero resistance), but only has 0.03V flowing through it when the ignition is on and reverse gear is engaged.


Thanks.
 
Last edited:
Not exactly. Not unless someone has made some horrible modifications to your wiring :)

The battery feeds the engine fuse box (on the bulkhead at the back of the engine bay). Fuse 2 then feeds the ignition switch via a brown wire.

When ignition is on, power goes via a white wire to fuse 17 of the passenger fuse box. This fuse then feeds the reverse light switch (screwed into gearbox) and the brake light switch in parrellel via green wires.

When reverse is selected, power flows to the reverse light via a green\brown trace wire. The light is earthed via a black wire.

If it were me, I'd start by seeing if (when ignition is on) you have power to the reverse light switch. Whip out your centre seat/cubby box and the panel underneath so you're look at the top of the bell housing. Those couple of wire there are the reverse light switch wires.

Not sure what Landy you have, but mine's a 300TDi. The first 8 pics or so on this page may be of use.

Landy random pics

With mine I found that the switch was gunked up (for which, I took it out and squirted a bit of contact cleaner and work it in with some rapid press/releasing), also I found I have a break in the green/brown trace wire somewhere inside the chassis rails. So I ran my own wire from the switch to the back and by-passed that.
 
check the earth. most of the time its a bad earth.
Use a volt meter to work ur way back towards the light to find the fault.
i had a similar issue and found the previous owner had wired it in to the wrong feed under the bonnet.
 
Jonteon said:
check the earth. most of the time its a bad earth.
Use a volt meter to work ur way back towards the light to find the fault.
i had a similar issue and found the previous owner had wired it in to the wrong feed under the bonnet.
Click to expand...

Well, I've already used a multimeter to test the reverse light wire coming from the engine compartment to the reverse light. Results as follows:


  1. Resistance was fine i.e. zero resistance.
  2. Only 0.03V flowing when reverse gear engaged and ignition on! If the earth (near the reverse light) is bad and I correct it, does this mean the volts would increase because the circuit is now able to complete?

Jonteon said:
check the earth
Click to expand...

Is it just the reverse light earth I need to check, or is there something else I should consider?

Sorry for the stupid questions, I'm a novice noobie.
 
GreenLove said:
Well, I've already used a multimeter to test the reverse light wire coming from the engine compartment to the reverse light. Results as follows:


  1. Resistance was fine i.e. zero resistance.
  2. Only 0.03V flowing when reverse gear engaged and ignition on! If the earth (near the reverse light) is bad and I correct it, does this mean the volts would increase because the circuit is now able to complete?


Is it just the reverse light earth I need to check, or is there something else I should consider?

Sorry for the stupid questions, I'm a novice noobie.
Click to expand...

Do you have 12 volt (approx) at the under bonnet end?
 
How exactly did you measure your 0.03v?

It doesn't really make much sense to say that 0.03v was flowing as such.

If you ever measure voltage, you need to connect your meter in parrellel with what you are measuring. That means that the red probe goes to the wire/terminal you want to measure, and the black probe to a vehicle earth (e.g. the chassis, or an original black wire).

If you ever measure current, the meter must be in series with what you are measuring. That means you must break the circuit and re-link it through the meter.

It sounds like you've measured your 0.03v by connecting your voltmeter with a probe at each end of the same piece of wire.

The bulb in your reverse light itself should sink all of the voltage when it is on. So when ignition is on and you are in reverse, all of the coloured wires I mentioned above should be at battery voltage (or extremely close to it). Only the black wire will be earth.

One thing I didn't ask. You have checked the fuse and bulb haven't you?
 
mundungus said:
Not sure what Landy you have, but mine's a 300TDi. The first 8 pics or so on this page may be of use.
Click to expand...

Defender 110, 1992, LT77 gearbox

mundungus said:
If it were me, I'd start by seeing if (when ignition is on) you have power to the reverse light switch. Whip out your centre seat/cubby box and the panel underneath so you're look at the top of the bell housing. Those couple of wire there are the reverse light switch wires.
Click to expand...

I discovered that the switch was in fact broken (someone didn't use a locking nut), so replaced the switch and tested it using a multimeter to ensure it had zero resistance and that current flowed through when reverse was engaged (12V passed through). The strange thing is that there is power going into the switch with reverse engaged but ignition off!

mundungus said:
also I found I have a break in the green/brown trace wire somewhere inside the chassis rails.
Click to expand...

I resistance tested the chassis wire going from the engine compartment to the reverse light and got a reading of zero, which I believe means that the wire must be ok?
 
mundungus said:
How exactly did you measure your 0.03v
Click to expand...

I have a piercing probe that is able to pierce the wire insulation using a very sharp point. So, I pierced the wire with one probe, then earthed to a nut on the engine with the other probe, all with ignition on and reverse gear engaged. Double checked and settled on a reading of 0.03V.

mundungus said:
One thing I didn't ask. You have checked the fuse and bulb haven't you?
Click to expand...

Yes. Double checked the bulb and fuse.

I'm guessing that 0.03V is too low a voltage to be powering the reverse light?

BTW: thanks for your help so far mundungus, always appreciated.
 
GreenLove said:
Rear wire reads 0.03V when in reverse. I know, it's a bugger :)
Click to expand...


No, it's not a bugger. 0.03volt is the sort of reading that I'd expect to get just wafting the probe around in the open air. In otherwords it's nothing.

Get back to the gearbox switch. Do you have 12 volt going into it? If no then your problem is backward from there. If yes, then if all you have is this 0.03 coming out, the the switch is knackered.
 
dogsbody said:
Do you have 12 volt (approx) at the under bonnet end?
Click to expand...

I have 0.03V under the bonnet.

In other words, the wire (green/brown) that comes from behind the dash, through the engine compartment, into the chassis rails and to the reverse light, when reverse is engaged and ignition is on, reads 0.03V.

Where does the wire go once it enters the dash from the engine compartment? To the fuse box I guess?

This is what is confusing me.
 
Follow these instructions and you should be able to sort it!

1. Ignition ON!

2. Pull the electrical plug off the gearbox reverse switch and bridge the wires, go and look to see if the reverse lights up?

If Yes then the switch is gone in the gearbox, if No go and take the lens of the reverse light.

3. Remove the bulb check for 12v live (Multimeter set on DC 20 V), and then check the earth terminal of light bulb holder to earth on the body with the multimeter set on continuity. (Remember to leave the reverse switch bridged while carrying out these tests)
If live and earth are ok, change the bulb!

4. If No live feed is found go and check the fuse! If fuse is ok check for ignition supply of 12v at fuse.

5. If No earth is found check the earth from the light to the switch and from the the switch to the body with the meter set on continuity.

:)

p,s Continuity is the setting that beeps when you hold the two electrodes together.
 
you have 12v at switch ,which presumably puts 12v at other side of switch when reverse selected ,wire then goes to dash loom to go to rear up into rh rear tub cover then back out to other side
 
Tunelessblues said:
Follow these instructions and you should be able to sort it!
Click to expand...

Cheers for the info Tuneless.

Have done most of that, apart from:

Tunelessblues said:
4. ...if fuse is ok check for ignition supply of 12v at fuse.
Click to expand...

How would I check for ignition supply of 12v at fuse exactly?

Tunelessblues said:
5. If No earth is found check the earth from the light to the switch and from the the switch to the body with the meter set on continuity.
Click to expand...

Oh, I didn't know there was an earth between the light and the switch, only thought there was a feed for the light (with the light earthed to a rear screw)?
 
dogsbody said:
No, it's not a bugger. 0.03volt is the sort of reading that I'd expect to get just wafting the probe around in the open air. In otherwords it's nothing.

Get back to the gearbox switch. Do you have 12 volt going into it? If no then your problem is backward from there. If yes, then if all you have is this 0.03 coming out, the the switch is knackered.
Click to expand...

Have recently replaced the switch on top of the gearbox and tested it (tested both resistance and volts.) When reverse is engaged I have zero resistance and 12v flowing in and out of the switch, so I assume this is working as it should.
 
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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