Tricky electrickery on the old pig

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resto_d1

Well-Known Member
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Location
Wiltshire
Hi all, got some very odd electrical gremlins on the 300 tdi D1. I think I’m going to book the auto electrician for a visit.

On starting, the wipers would jolt - other than that they won’t run.

The passenger window was opening on start up. After driving it for a while I then couldn’t get it back up.

The rear fog light switch blinks, when turning from ignition point one to ignition point two. Once in ignition point 2, it doesn’t switch on

The rear windscreen heater doesn’t feel like it clicked down. Haven’t tested it with multimeter yet.

When trying to activate the wiper, the green relay clicks. It’s like they want to work but are tied down.

The speedo didn’t work on the way to the garage - now does (for now!)

Courtesy lights don’t work when drivers door opened but do on others

I changed the relay with one out of my other one - didn’t notice any difference. I also fitted the MFU from my other disco. No change.

Any ideas?! I can’t run the diagnostics check on the MFU either as nothing happens when I click the rear heated screen.

Thanks all
 
Last edited:
Hi I'm not following this section of the forum, next time post in the Discovery section if you want me to see it from the beginning ;)
resto_d1 said:
Everything is pointing to MFU but can’t diagnose as the rear heater switch don’t click/work.
Click to expand...
Yes te MFU is very suspect. Pull the heater switch and bridge pins 1(black/orange wire) with pin 4(black/earth) see if you can run the MFU diagnostic this way...if nothing happens measure if you get live on the BO wire, if not the only way to go further is to replace the MFU with known good one
 
Dippypud said:
Might be a common 'joint' where many become one, earths are a bugger for that and it can cause live feed to 'leak' across, start at the relay / fuse boards
Click to expand...
Thanks - I was thinking along same lines and have printed out a sheet with the different connector blocks on those circuits, all the relays and all the earth points and blocks. There’s actually a fair few 😂😂🤦‍♂️. I actually hoped swapping that MFU would cure it but not so lucky!! It’s funny, you get one that’s solid and then it throws electrics as you instead of holes in the body and head gaskets 😂
 
Very confusing. Got live at the fog and rear heater switch but neither seem to do anything so I can’t do an MFU test, even though I’ve swapped from my other. The alarm box isn’t present - it’s just had some wires joined etc. I plugged another alarm box in and all it did was illuminate immobiliser light. It didn’t affect anything else. I only tried this as a purple and green goes from window ECu to alarm box.
I’ve still got every issue above. So despite having the go ahead mechanically, I can’t use it as wipers won’t function and windows don’t work properly 😞
 
sierrafery said:
Hi I'm not following this section of the forum, next time post in the Discovery section if you want me to see it from the beginning ;)

Yes te MFU is very suspect. Pull the heater switch and bridge pins 1(black/orange wire) with pin 4(black/earth) see if you can run the MFU diagnostic this way...if nothing happens measure if you get live on the BO wire, if not the only way to go further is to replace the MFU with known good one
Click to expand...
Thanks - sorry didn’t see this
 
sierrafery said:
Hi I'm not following this section of the forum, next time post in the Discovery section if you want me to see it from the beginning ;)

Yes te MFU is very suspect. Pull the heater switch and bridge pins 1(black/orange wire) with pin 4(black/earth) see if you can run the MFU diagnostic this way...if nothing happens measure if you get live on the BO wire, if not the only way to go further is to replace the MFU with known good one
Click to expand...
I’ve got this at the switch so tried it in both pin 3 and 4. Doesn’t seem to activate MFU and then I’ll struggle to follow the sequence as the rear fog seems to do nothing either. I’ve plugged in MFU from my other disco and as far as I can remember windows, wipers etc all worked fine before I took it off the road to rebuild.
IMG_5288.jpeg
 
Oh and I also tried it with another switch from my other one and it doesn’t switch in on. The switch does flash when I go from ignition 1 to 2 though…
 
On starting, the wipers would jolt - other than that they won’t run. Don’t know, earth issue maybe

The passenger window was opening on start up. After driving it for a while I then couldn’t get it back up.
As above

The rear fog light switch blinks, when turning from ignition point one to ignition point two. Once in ignition point 2, it doesn’t switch on. As above also the switch has the ‘momentary function’ plus the head lamps or front fogs also need to be on.

The rear windscreen heater doesn’t feel like it clicked down. Haven’t tested it with multimeter yet.
The switch has a ‘momentary function’ and then is illuminated, the engine need to be running and after 15 minutes of operation it switches of automatically,

When trying to activate the wiper, the green relay clicks. It’s like they want to work but are tied down.
As above earth issue may be.

The speedo didn’t work on the way to the garage - now does (for now!)
Iffy connection at the speedo transducer fitted on the transfer box. But an iffy connection at the dash, maybe.

Courtesy lights don’t work when drivers door opened but do on others.
Remove the switch and connect wire direct to earth to test… do the interior lights time out as they should, something which is controlled by the MFU.

So what is the relay you changed? As your problem is probably electrical you will need to test each circuit, starting with the easy one for you and if lucky may find a common fault.

You should have identified the issue before installing the replacement MFU, to save possibly destroying that unit, the wiring issue u have may is blown a component in your original unit… so if you can confirm it is faulty… by installing the original in another “working vehicle” first
 
Last edited:
Thank you - appreciated as always.

It was the green relay I changed which was wiper (I think). Whatever was in the diagrams. Got the windows working. Passenger switch shot and window reg and motor needed a thorough clean and re-grease. Rear fog I’ve not tried with the lights on (school boy 🤦‍♂️).

Wipers I’ve had a closer look at today. One spindle looked seized up to the outer and the wiper arm switch was a bit crappy so I cleaned up and re-assembled. It bulges at scuttle when I operate it and then I think it times out to protect motor so I’m going to strip that all apart. I only stripped one the other day 😂

Rear screen could well be where it needs a long drive and to get the battery charged up well. I’ve continuity across it.

Hopefully we are getting somewhere. That weird issue of the passenger window opening it’s self slightly on start up didn’t re-occur once re-assembled.
 
Oh, interior lights time out as they should yes. Door switch looks to be faulty on drivers side as they’re coming on on other two doors but not drivers 👍🏻
 
This is the diagnostic access for the mfu test I have used and it works perfectly if correctly followed.

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.
…Spooky…
 
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