Centre diff lock - warning light circuit advice needed

[Northwards]

Hi folks,

It’s a 1994 300Tdi. More or less standard, with a recently rebuilt Lt230 transfer box. Diff lock switch looks new, as I’d expect from a reputable rebuilder.

I’ve just adjusted the diff lock and hi/lo linkages and, earlier this evening, checked that the diff lock lever and diff lock actually work (handbrake on, one front wheel jacked up. Diff lock off - it turns ok; engaged and it doesn’t).

But, the diff lock warning light doesn’t work. I swapped the bulb (led) for a known good one, still nothing.

I was assuming the circuit would be fairly simple, with power supplied directly to the switch via the blue black wire. I’ve come in and checked my workshop manual though, and found that it might not be that simple. Although my manual is for the 96/98my and my car is a ‘94 vehicle, though probably a ‘95my.

Anyway, if the diagrams are relevant, I think power is supplied from the ignition switch to one side of the warning lamp by a white wire, and then the lamp is earthed via the blue black wire, and the switch itself.  I’m not convinced that the blue black wire is in such good nick so would think about replacing it. But my manual doesn’t say how it’s routed and doesn’t show any connectors between the dash light and the switch.

Apologies if I haven’t explained that too clearly, but if anyone else has had to work on this circuit, can you offer advice on the routing of this blue black wire to the switch? I could just order a couple of metres of blue black wire and replace the whole run, but I’d prefer to keep the wiring as original as possible if I can.

Thank you.
 

 

[chadler]

Have you driven it any distance? The diff lock engages mechanically but quite often the light doesn’t illuminate until the wheels have turned a revolution or two.

[TSD]

This is probably down to Land Rovers wacky way of presenting wiring diagrams. I suspect you only have half the manuals and so are misreading the schematic. My copy of the 97 manual shows the black/blue wire passing through C448 pin 7, C162 pin 7, but it's only shown as a small mark on the wire representing that circuit, not the whole connector. That's the "engine harness to main harness" connector pair at the centre exit of the engine bulkhead.

If you hunt out the 'Defender Electrical Library', available at all good search engines, it shows pictures of each connector, identifies pin numbers and wire colours.

EDIT: Forgot to say, there should only be small differences from 94 to 97. I traced out a 95 wiring loom using the 97 manual a few weeks ago and I don't think I spotted any changes.

EDIT 2: ... and if the switch is new, does it have an earth lead on its second terminal?

Edited July 4, 2022 by TSD
cant type

[miketomcat]

Have you tried un doing the lock nut on the switch and winding it down a bit with the diff locked. A multi meter across the two pins or across pin and switch body (if it only has one) set to continuity. Once the switch is made unlock the diff and check it's unmade. Lock the lock nut and try with the warning light connected.

Mike

[western]

Black/blue wire to one terminal & a plain black earth wire on other terminal to a ring terminal under one of the hi/lo lever housing bolts. 

Difflock will only light up when it's fully engaged & go out when fully disengaged, this could be after a few years of driving. 

[Northwards]

Thanks very much guys, 

I didn't know about the 'drive it a few yards' thing. I haven't done that. One for this evening - though I do now suspect it's more electrical.

The wiring colours are correct - blue/black leading back up to the bulkhead connector; plain black for earth. Both connected to male spade terminals on the switch. The earth doesn't go to a ring connector, but back into the loom - however when doing some voltage tests yesterday it seemed to provide a good earth, so unless there's a good reason to change I'll stick with what's there. 

Electric stuff isn't my favourite, and I hadn't thought to use the continuity section of my meter to check the switch function. Another one for this evening.

When I started 'mechanicy' stuff it was on Minis, using the Haynes manual wiring diagrams. Dead simple. The information in the WSM is more complex, but I spent more time studying those last night and came up with the C448/C162 connector. Once I'd identified that, and been mucking about with the dash warning lights it occurred to me that I don't have a handbrake warning light (despite the wire being connected to the lever!). I'm sure both of those cables are bound together in my car, so I'm now wondering if there's an additional plug in my loom, between the switches and C448; or if the loom is simply damaged. 

But I now have a couple of other things to try, and I understand more about how the circuit operates, which will help narrow it down. Thanks for your help. I'll come back and say how I got on.... or ask more questions! 

[TSD]

Random thought - you replaced the warning light with a known good one, was that also LED? Some (maybe all) of the LED warning light replacements are polarity sensitive (they only work one way around) where the original filament bulbs are not. It's just possible you've been really unlucky putting the bulbs in?

[Northwards]

^^ I did know this.... I'd just forgotten to consider it....!

I would have been unlucky twice I suppose. Not only did I swap the bulbs, but I wanted to double check the fitted one, so gripped it lightly in some mole grips for stability, then ran power across it from a battery on the bench - lit up like a beauty. I did not think to connect the wires to the opposite contacts as a polarity check. Easy to confirm later though. 

[FridgeFreezer]

As hinted at above - the switches need to be adjusted in the hole so the plunger is at the right height to be hit by the selector inside the box.

You can prove the light by removing the wires from the switch and shorting them together / shorting the warning light wire to ground.

I've had polarity-sensitive LED lamps, I've also had some that were so badly soldered they just fell apart inside during fitting.

[Northwards]

Thanks again.

A mixture of frustrating and successful last night. Short and happy story is that I have a working circuit, and a lamp that lights - but that the new-ish switch is U/S. 

^^Eventually I did just as suggested above and made up a short jumper cable to go from the blue/black to the earth, bypassing the switch, and got the light to work. I then removed the switch to check its continuity/operation, and even with the plunger pressed way back into the housing, farther than it would go in operation, I couldn't get continuity. I know you can't assume new stuff is working, and this just demonstrates the principle again. 

Along the way, I did check the LED polarity (they are polarity sensitive) and replaced it with the original bulb. It struck me that LEDs might be great if all's working, but not much use if you're troubleshooting. To check the circuit, you need to be sure the bulb is working/making the circuit.... but if you're not sure the circuit itself is good, then you have two unknowns and can't progress. 

I was doing all sorts of other circuit and power tests too, trying to narrow down the problem, and I think that possibly the flexible printed circuit that sits behind the dash bulbs is a bit iffy. For example, I can see a worn track on the printed copper for the diff lock light, and I think that sometimes I was getting the bulb contacts to meet copper, other times not. Luckily I have a new old stock one to hand if I need it.

So thanks for the assistance. Next question is the handbrake lamp circuit, but I'll post another thread to keep things neat.