Find the Wire

[RBBailey]

After having confirmed that the truck is usable, I'm now working on buttoning up all the wiring and systems so that I can actually drive it.

Everything works, except for the reverse lamp, the diff lock warning lamp, and rear defrost.

I think I can get the diff lock warning to work. I have got the light to turn on, it's just a matter of getting the switch properly grounded and wired permanently.

But the rear defrost is baffling me. It is a 1988 LHD with factory A/C. The hazard, interior dome, and rear defrost switches were all in the lower dash panel next to the door. This is one of the spots that doesn't seem to match any wiring diagram I can find; but then, I'm not sure exactly how to find one, or even if I could recognize it!

The fuse is easily located, and the brown/white + wire, going to a white/black + that runs back to the window is obvious. And the plug that went to the switch... with three different white/green wires... is hanging ready under the dash. One of the white/green wires is live with the run position on the key, the other two are not ever live. When I jump these wires, in any combination, nothing happens. And I get no power at the fuse, or at the rear window defrost. So.... Not sure what to do.

Secondly, I can't even find a wire, or two, or three to connect to the reverse light switch on the transmission. I have no clue what I'm looking for.

Part of the problem is that when I pulled the truck apart, I didn't intend to use this harness again... so I didn't take nearly as many pics as I would have otherwise. It has been a long, long guessing game. Any help would be appreciated, even if you could simply show me links to what could be the right wiring diagram.

 

[javelinadave]

The reverse light is a single green w/ yellow trace wire. I just messed with this only to have to run a new wire since I had a break somewhere.

 

[Airbalancer]

I believe the rear defroster is fed from a relay and a voltage sensitive switch. It should only come on when the car is charging. Around 13.5 volts.

 

[RBBailey]

The reverse light is a single green w/ yellow trace wire. I just messed with this only to have to run a new wire since I had a break somewhere.

Just to be sure, is this the one that is on the RH side, and for some reason it goes into the frame? I had always thought it must be the reverse light wire, but when I gave it a charge, the diff lock light came on.

 

[RBBailey]

I believe the rear defroster is fed from a relay and a voltage sensitive switch. It should only come on when the car is charging. Around 13.5 volts.

Sweet! I really hope you're right. Thanks for the tip. I'll test this out tonight.

 

[javelinadave]

Just to be sure, is this the one that is on the RH side, and for some reason it goes into the frame? I had always thought it must be the reverse light wire, but when I gave it a charge, the diff lock light came on.

According to the schematics, yes that is the wire. The diff lock light should be black /w a blue tracer. You may have a yellow/green wire that grounds the diff lock switch to the frame.

From a Google Search:

Diff Lock Light Wiring for 83-89 -- Modern T Case Switch Answered
To wire a newer style Diff Lock Warning Switch to an older truck

The 83-86 Defender

Black w blue trace goes back into the instrument cluster.

New Switch
Green w yellow trace
Green with brown trace

Connect the green w brown trace to the existing Blue w black trace wire. This will lead back into the cluster.

Add a ring terminal to the green w yellow trace wire and ground to any screw on the transfer case.

Your diff lock light should now work.

 

[RBBailey]

So I pulled the diff lock switch out and tested it, it works fine. Lights up the dash as it should. The problem seems to be that the bit inside the differential that pops up to trigger the switch doesn't seem to move enough to make it trigger. Even when I pull the adjustment nut off the switch threads, and tighten it almost all the way down, moving the handle back and forth either does nothing to the switch (no light) or the light is on whether it is in diff lock or not.

Not sure what to do. Not sure what this might mean.

 

[RBBailey]

I tried the defrost with the engine running to get the full voltage, nothing.

 

[Airbalancer]

I tried the defrost with the engine running to get the full voltage, nothing.

I have the same year car as you and couldn't get my rear defroster to work either. Although I didn't put much effort into it either. Just last weekend while driving it just came on, I realized the switch was on and probably has been for some time. The most likely culprit is the voltage sensitive switch. I think it's behind the gauges. There was a yellow relay looking thing back there if I remember correctly.
If you google " Land Rover Defender rear defroster wiring diagram"
One comes up from another forum.

 

[Airbalancer]

 

[Airbalancer]

Just looked on lrdirect, that switch (PRC4427) is 225 bucks!! You buy it first and let me know if it fixes it

 

[Uncle Douglas]

Just looked on lrdirect, that switch (PRC4427) is 225 bucks!! You buy it first and let me know if it fixes it

That is just an ignorant price for a 12v relay.

 

[RBBailey]

Thanks for the diagram... where does #6 connect to the battery in real life?

 

[Airbalancer]

Thanks for the diagram... where does #6 connect to the battery in real life?

I would start at the fuse and trace the brown/white wire to the relay. It's one of the metal relays below the fuse panel or behind the gauges. The 12v feed should be a brown off the harness.

 

[RBBailey]

Yeah. It disappears into the spaghetti. I would have to remove the vent and lower dash to get a view....

 

[RBBailey]

So I'm going to leave the defrost for now and try to find the connection for the reverse light. Does anyone happen to know what this should look like? Specifically, I need the wires from the harness to the switch, which should be two wires. I have no idea where they come out of to reach the switch on the transmission.

 

[RBBailey]

UPDATE: I Found the part of the wire that goes to the light itself! It was hiding in the bundle of wires that comes out of the chassis and up the bulkhead to the gaggle of plugs there. It was plugged into a spot that looked like a matching wire, but wasn't right.

Now I need the harness that goes back down from there to the switch, and I need the live wire that brings power to the switch.... both of these bits are still missing.... I may have to start pulling apart the dash, but now that I know what I'm looking for, I might get lucky once I pull the truck out into the full sunlight, and remove the air cleaner and stuff to get a clear view.

 

[RBBailey]

OK. Reverse light done.

Now for some luck on the defrost, or I'm just skipping it.

 

[Airbalancer]

I looked at my defroster this morning and it turned out to just be a bad switch. I took some pic to help you out a bit.

If you don't already have one I cant recommend the "Power Probe III" highly enough. It make troubleshooting stuff like this so easy.

Cheers

 

[Airbalancer]

I ended up just popping the switch open and hitting the contacts with some scotchbrite and back in business