Rear Fog Light Switch

[sparky135]

With my MoT booked for Thursday afternoon I thought I'd take the opportunity to check all my lights today. All fine except rear fog lights. When I depress the switch, no rear fog lights and no indication on dashboard. I have checked the fuses and bulbs and all appear to be good.

I haven't got my multimeter at home so can't check the switch electrically until I get to work tomorrow but I think there may be a mechanical issue with the switch. When I operate the front fog light switch I feel a definite click as the switch engages but when I operate the suspect rear fog light switch there is no such feeling of the switch engaging and it just comes straight back out once the pressure is released. I'm assuming the switches are the same so, should I be able to feel a click when depressing the switch to indicate it has functioned correctly?

Obviously, I can't swap the switches over to confirm it is the switch at fault as they are keyed so can not be interchanged but there isn't a lot left to check.

The car is a TF on a 54 plate so has the newer style switches.

[bgunn]

The rear fog light switch isn't supposed to be a latching type like the front switch is, so will just push in and spring out..

[adrianclifford]

Asking a few daft questions John.

Did they work before ?
Do you have your headlights on too ?

If so, pop another switch in, you'll get one from the breakers for next to nothing. I can post you one but you won't get it for a few days if that helps.

[sparky135]

The rear fog light switch isn't supposed to be a latching type like the front switch is, so will just push in and spring out..

Thanks for that. I guess I'll have to test it electrically when I'm reunited with my meter tomorrow then.

Asking a few daft questions John.

Did they work before ?
Do you have your headlights on too ?

If so, pop another switch in, you'll get one from the breakers for next to nothing. I can post you one but you won't get it for a few days if that helps.

No such thing as a daft question, Adrian.

They worked at the last MoT. Haven't had occasion to use them since.
I did have my headlights in when I tested them.

I'll have to see if I can source a replacement tomorrow if it does prove to be the switch. Failing that I'll have to cancel my MoT until such time as I've fixed it.

Typical isn't it. Never use the damn things but as soon as I need them for the MoT they pack up on me.

Thanks for the prompt replies folks.

[sparky135]

I've been reading my MG TF Workshop Manual (available from the MGF Register Online Shop ) trying to get to the bottom of this and, even though I haven't been able to test the switch electrically yet, following on from bgunn's reply I'm no longer 100% sure it may be the switch after all.

The manual uses rather expensive sounding words like multi function ECU. I believe the relay is housed within this unit as opposed to the relay panel adjacent to the passenger compartment like the front fog lights. I'm assuming this is a sealed unit and, should the fault lie within, will need to be replaced as a whole and not just the defective relay.

Is this ECU the one situated in the vicinity of the heater matrix behind the console? The Workshop Manual is not clear on this.

[sparky135]

Is this ECU the one situated in the vicinity of the heater matrix behind the console? The Workshop Manual is not clear on this.

No!!! That's the alarm ECU. I should have known that after what happened a few weeks ago.

I believe the multi function ECU the manual refers to is situated behind the passenger compartment fuse panel. Getting a bit dark to continue work this evening so I'll put the tools away and continue work in the morning.

[sparky135]

OK. Realisation is finally dawning. If the switch is electrically sound when I test it tomorrow I'm looking at a rather large bill. There is no multi function ECU behind the fuse panel and my original deduction that the ECU behind the centre console is indeed responsible for the rear fog lights, along with a number of other functions including the alarm.

So, it looks like I am back where I was about a month ago. Facing a bill of several hundred pounds to replace the Alarm ECU. Don't think putting the ECU on a radiator overnight is going to get me out of this one.

Can't blow a fuse or a bulb like normal people, me.

Fingers crossed for a defective switch when I test it tomorrow.

[Keith Williams]

Things are never easy are they? Hope this one is.
The older cars were so much simpler. Power, fuse, relay, cable, switch and lights. Why are they so much more complicated these days?

[Matt Parker]

If you have not used the rear fog lights in a while then the switch internal contacts can stick creating the effect of a faulty(duff) switch. Try repeat-idly pressing the switch quickly to see if it brings it back to life, this is a fairly common fault on rear fog light and hazard light switches that are hardly used throughout the year in-between MoT's, so I always have to give the switches a good working to get them to function before I think about failing them

[sparky135]

Good tip, thanks for that Matt.

I'm going to get the multimeter on the switch today. I've managed to locate a spare switch which I should hopefully be able to get my hands on this afternoon just to prove that but I'm preparing for the worse.

I'll probably cancel my MoT appointment for tomorrow anyway as it's all getting a bit close now. I'll reschedule it for when I have a clearer way ahead on this.

I'll let you know how I get on.

Thanks again for the help everyone.

[Geoff.F]

Why not just replace the switch with a bent paper clip to see if the light works ??.
Geoff F.

[sparky135]

No need Geoff. I've managed to test it with a known good switch and the lights still don't work. The only thing left is the relay which is an integral part of the Alarm ECU. I've changed the bulbs even though I know they are good - more out of hope than anything. The whole rear fog light system is dead.

I guess I've got no choice now but to get the Alarm ECU replaced (which in turn will add the cost of a couple of key fobs) - which if I'd have had done when I had my problems a few weeks ago would have saved me the extra labour costs I'm about to incur and the hassle of being without the car again.

I find it a little bit annoying that a safety item such as fog lights are allowed to have a point of failure that cannot be fixed "by the roadside". Who thought that one up? Oddly enough the front fog lights which are not part of the MoT as they are not a legal requirement have their relay in the passenger compartment fuse box.

I'm going for a little lie down now.

[Rob Bell]

Are you sure about that??? Pretty sure that the fogs have absolutely nothing to do with the alarm ECU (I presume you mean the immobiliser ECU?) Much more likely is a fault with the Multi-function unit (MFU). Because the rear fog switch is momentary, it needs latching relays and a couple of logic gates to hold the on and off signal. Assuming that all else is well - and I'd go through everything again just to be sure - then you could attempt swapping the MFU with one from another car.

Hmm, a 54 plate TF. What type of alarm fob do you have? If the rectagular Lucas type, then you've got MGF MY2000 type wiring loom - for which I have diagrams. If the round version, then it's going to be different, and I am sorry to say, I don't have access to those schematics.

[bgunn]

It's a MY04 on car, so will have the all in one Pektron Body Control Unit; which has the alarm, and all of the old MFU functions in one. And sadly, their failure record is very high - the quality of the units is absolutely - to put it bluntly - shite.

[Rob Bell]

Oh goody. Sorry John.

New body control unit likely then.

Anyone got MY2004 wiring diagrams BTW?

[sparky135]

Yep. That's how it's looking right now, Rob. It's going to be an expensive business.

Thanks for the assistance once again everyone.

[Geoff.F]

Next silly question. Do you have headlights AND Ignition On ??
It may be possible to Jury Rig a feed from the Heated Rear Window connection although this would require the engine to be running as it is switched by a low oil pressure switch.
Geoff F.

[Rob Bell]

Geoff has a good point there - if the fog is the only non-functioning component of the BCU, why not fit a separate relay and re-wire the rear fog? You'll need to change the switch to the latching variety - the one that controls re-circulation on aircon equipped cars is ideal (part number YUG102600PMP) - and then pop off the switch cover from your rear fog switch and transfer it to the new one.

One last task would be to remove the key-ing of the harness plug, but that's simple.

Job done!

Just a thought...

[sparky135]

Just to bring closure to this thread, this has now been fixed.

Thanks for the help and the suggestions. I thought about rewiring the fog lights as suggested by Geoff and Rob but I have since discovered an intermittent fault on the intermittent windscreen wiper which caused a strange fizzing electrical sparking sound occasionally coming from beneath the console and the low coolant level alarm light to flicker on and off momentarily. It even on one occasion wound down the drivers side electric window. All of these functions are controlled by the Alarm SCU.

It seems like the water ingress in the Alarm SCU suffered earlier in the year did more damage that was first thought. With this in mind I decided the best cause of action was to bite the bullet and get the Alarm SCU replaced. I think any other cause of action would only be delaying the inevitable.

So I now have a working Alarm SCU and a very empty wallet.

On the other hand I now have nice green piece of paper signed by the very nice man from VOSA and the sun is shining.

Thanks once again for the assistance folks.

[Rob Bell]

Glad you got it fixed, sorry that it cost a mint to replace the alarm SCU.

Still, the sun is shining, and therefore perfect roof-down weather!