Mk7 heater and headlights

[Needs1975]

Hi all,

I have a 2007 Mk 7 Tipper, my heater blower and headlights dont work. The side/tail lights work ok and so does the headlight flasher. This has only been happening since i replaced both batteries (twin battery set up). Is there anything common i should be looking for, like a relay or common earth? Or ignition switch live feed, fuse, relay etc? Seems a bit of a coincidence that both have stopped working at the same time. Any help greatly appreciated. Many thanks

[Needs1975]

Update :- i have had a go at the headlight circuit, found the relay in fuse box behind glove compartment, swapped it for the main beam relay as they are identical. Neither main beam or dipped would work, so im assuming a duff relay, however the headlights still wouldn’t work with the “good” relay. Ive tested the supply to the fuse box, the supply is present to the headlights. Using a link wire i shorted out pins 3&5 ( to bypass the relay) and the headlights came on. However the supply to the coil is permanently live, even with the ignition and headlights switch off. Could this be a bad connection in the fuse box? Is it difficult to remove the fuse box to check this out? Ive been scratching my head for ages so any help greatly appreciated

[v8dave]

The supply to the coils are live because the bcm switches the earth instead of the power.

Check fuse 71 on the bcm. It supplies the power to the light switch.

[Needs1975]

Thanks, whats the BCM and where do i find it?

[v8dave]

Body Control Module.

You found the fuse box behind the glovebox, behind that is the bcm. Sometimes refered to as the second fuse box or the passenger junction box if you have the owners manual.

viewtopic.php?f=40&t=49206

[Needs1975]

Thanks, couldnt find that fuse on the bcm, not sure if its on the back of it but i couldnt see it from the front. But what has happened today has confused me even more. I have used spade connectors and cable to test the headlight supply at the fuse box, i have 12v directly supplying the relay at the fuse box, but the relay wouldnt click to bring the headlights on, however if i connected the relay directly to the battery it worked fine, but it still refuses to work at the fuse box. Im at my wits end with this, especially as the adjacent relay for the main beam tests out exactly the same. I have managed to get the headlights working by running a flying lead from the headlight switch directly to the relay and using an earth from the passenger footwell, this will do for now but i still need to get to the bottom of it. Ive also noticed that i couldnt find a switched live from the headlight switch for dipped beam, only a supply for side and tail lights, which i am now using to get the headlights to work. Could a duff switch be causing all this? Also does this bcm unit also have something to do with my blower fan not working? As im not getting 12v at the switch?

[v8dave]

Body Control Module.

You found the fuse box behind the glovebox, behind that is the bcm. Sometimes refered to as the second fuse box or the passenger junction box if you have the owners manual.

viewtopic.php?f=40&t=49206

Go and read the topic linked so you can check fuse 71.

IMPORTANT
The light switch does not switch the sidelights or headlights on and off.
It tells the computer in the bcm what to do.

Do you have fog lights at front or rear?
Do they work?

[Jim Archer]

Just as importantly, as v8dave says, the relay is ground switched, so is permenantly live. The BCM sends a ground return signal to the relay to operate it.

Find this connection on the relay and drop to ground/Neg/chassis earth somewhere, this will confirm if it's the relay itself, or the BCM not switching.

Jim

[Needs1975]

V8dave, yes it had rear fog lights, and yes they do work

[v8dave]

Right, working fog lights strongly suggests that fuses F69, F80 and F81 are sound.

Does the headlamp flash work ?

The power to the light switch to enable signals to the bcm is seperated across three sources.

The foglights are powered from F69.

The flash is powered from F42.

The sidelights and headlights are sourced from F71, it's only 5 amp because it only does signalling to the computer.

[Needs1975]

The headlamp flasher works but it doesnt stay on when i release the stalk, ie is doesnt switch the dipped beam off and main beam on. Its having an mot at the moment, not sure if it will fail on this. When i get it back ill check F71

[v8dave]

Does the headlamp flash work ?

Sorry, you did answer that one right at the start....

[Jim Archer]

Headlamp flash is a separate function to dip/main switching.

It suggests a fuse, as the BCM is still able to operate the bulbs.

Jim

[Needs1975]

I have checked F71, and its sound. I have also tried a new headlight switch, no joy with that either. As Jim suggested earlier, i earthed the relay and it clicked in fine, so the relay is ok. What do i check next?

[cheglemegers]

I have checked F71, and its sound. I have also tried a new headlight switch, no joy with that either. As Jim suggested earlier, i earthed the relay and it clicked in fine, so the relay is ok. What do i check next?

Had exactly the same problem recently.
No headlights but have flash.
Heater blower not working.
Heated screen switch stuck on.
Headlight relay and fuses checked out ok.
New light switch no effect.

It was relay 21 that had failed (found on the first fuse box behind the glovebox).
You can swap it with relay 19 (as is the same) to check it.
It sits upstream of the circuits that are not working (hence being a heavy duty relay).

Hope this solves your problem.