Disco 1 More Electrical Gremlins!

Rear fog lights being a pain now, thought it was the switch changed it but no, still not having it!

If you turn on the rear fog lights then want to turn them off they won't switch off, the light on the button goes out, but the lights on the rear stay on. Press the button again & the lights go out but the light on the switch stays on???? Its almost like you have to press the button twice to turn them on & off again & its driving me mad trying to find out the reason why As I said found another (used but tested) switch & it does exactly the same, got me really scratching my head, anyone any ideas???

Then swap the relay and see if the issue disappears, the relay is just standard 4pole, there's few others in the footwells so just borrow one.

I'll try that in the morning, thanks Discool

Well tried swapping the relays over, same result thought I was on to a winner there. Another weird gremlin that's living in my Discovery! If I slowly push the button in until I hear the 'click' the light on the switch comes on & the fogs come on, then if I push it again half way down till the 'click' they go off. But if I use it like its intended and push it all the way 'on' then all the way 'off' it takes 2 pushes to turn em off??? Puzzling????????

Ok so the two easy items to test, switch and relay have been done although the switch seems iffy, it should be the same as the switch above it.
Next is the expensive multi function unit MFU which fortunately has a diagnostic function, its the MFU that switches the relay after the rear fog lamp switch is pressed, which is ‘momentary’ so doesn’t latch, just press on, press off.
If the test below doesn’t complete satisfactory then its MFU replacement time.

Multifunction Unit Diagnostic Testing
Close all doors and the tailgate.
NOTE: If the oil pressure switch goes open circuit (engine is started) during the following procedure, the MFU will exit the diagnostic mode immediately.
Press and hold the heated rear screen switch and switch on the ignition.
Release the heated rear screen switch within 2 seconds and the MFU will beep to indicate that it's in diagnostic mode 1.

Press the heated rear screen switch; the MFU will beep twice. In turn, operate each of the following switches, heated front screen, rear fog guard lamps, side lights, headlights, front wiper, and rear wiper. The MFU will beep twice each time a switch is operated.
Pressing the heated rear screen again will cause the MFU to enter diagnostic mode 2.

The MFU will provide outputs to the following in order given below:
Heated rear screen relay energized for 2 seconds
Heated front screen energized for 2 seconds
Courtesy lights energized for 2 seconds
Dim dip relay energized for 2 seconds
Rear fog guard lamp energized for 2 seconds
Headlamp power wash energized for half a second
Front wiper energized for half a second
Rear wipe/wash energized for half a second
The MFU will then exit the diagnostic mode.

Thanks, will try this tomorrow. Is it possible to remove the rear fogs from the existing circuit and put them on one of their own or is that just asking for more trouble???

Jasonmichael said:

Thanks, will try this tomorrow. Is it possible to remove the rear fogs from the existing circuit and put them on one of their own or is that just asking for more trouble???

Click to expand...

Yes, it is possible and simples. So if the feed from the relay plus the fog lamps prove to be ok, then isolate the MFU feed at the relay and then install your own switched 12v feed to the relay.
Its a bit more involved if you wish to retain the function of existing switch, but is possible.

Right, now I tried to do the MFU test, it took about 3 attempts to get the thing to do it. In diag mode 1 - Nothing. Diag 2 Heated rear screen relay energized for 2 seconds
Heated front screen energized for 2 seconds - YES
Courtesy lights energized for 2 seconds - Yes
Dim dip relay energized for 2 seconds - NO
Rear fog guard lamp energized for 2 seconds - Yes
Headlamp power wash energized for half a second - NO
Front wiper energized for half a second - Yes
Rear wipe/wash energized for half a second - Yes

Then I did it again & got different results, but the troublesome rear fogs always worked but the side & headlights didn't work at all in the test however when I switch them on via the stalk they work???? I'm beginning to think that this car is possessed!!!

All things being equal the you should be able to enter the MFU diagnostic programme without any issues first time, and it will complete the test.
Diagnostic 1 not functional could indicate an issue, what that is the workshop manual doesn’t state, unfortunately.

If you don’t have the dim-dip function because say the relay has been removed, you will have a negative response to that part of the MFU test.
I removed the dim-dip function years ago, by simply unplugging the resistor in the engine bay because the light from the dim headlights looked a bit naff, with the bright LED sidelights bulbs switched on at the same time.

Does the headlamp wash operate normally? if not it could be as simple as a blocked pump, loose wire, otherwise Diag 2 was a success. So back to the Switch and check it with a meter.