TheBeefChief

Hi all,

I'm in the UK and my 2005 Executive failed its MOT today on two things, both related to the rear fog light. Basically neither the light nor the dash display is coming on when they should be. The front are fine, but the rear one is essentially dead. Even though I NEVER use it, it has to work to pass the MOT.

I've checked the bulb and the fuse, both are fine. I dismantled the stalk where the switch resides and all wires look fine and seem properly connected at the base of the unit, although I had no idea how to get inside the actual stalk itself to make sure everything was connected up correctly in there.

I tested with my multimeter and there is no power getting to the bulb socket. I don't really have much of a clue what I'm doing with the multimeter, but I tested the other sockets at the back and was getting voltage through them.

I've tested the fuse in the under-dash fusebox and it's getting power, too.

Any suggestions for what it might be would be hugely appreciated. I'm wondering if it's a faulty switch, as the fact the dash light isn't coming on suggests the switch isn't doing much of anything, but I'm worried there's a relay in the ECU or something else expensive that might have gone. All other lights are functioning fine.

All advice welcome!

UK-Kev

Hello,

Sounds to me like the switch is broken.

To confirm if the switch is faulty you could bridge out the switch and if the fog lights come on this would confirm the switch is the problem.

How - Trace the wire colour to find out what wire feeds to switch and then from the switch to the fog lights and make a bridge wire between them.

Rgs,

Kev

TheBeefChief

Thanks for the reply, UK-Kev. I got a new switch, but it made no difference. The rear fog light goes through the Multiplex Control Unit, so my next guess was that maybe that had gone. I found a way of testing it, though - here: http://www.crvclub.ru/manuals/CR-V-2...p/group31c.htm - and according to the test the MCU is doing its thing. When I turn the rear fogs on in MCU test mode, the ceiling light flashes to acknowledge that the signal is being received.

I've had a look and can't see any damaged or loose wires anywhere, so I'm pretty much stumped at this point. Anyone else have any ideas of things to check? I'm hoping the ECU isn't involved, although nothing else seems to be wrong electronically.

TheBeefChief

Another quick update - I hooked up the rear fog light to the tail lights and the bulb lit up when the tail lights were on. That also illuminated the symbol on the dash to show the rear fog was on.

So, everything is fine at the bulb end, and everything seems good up to the MCU/fusebox. The problem presumably lies somewhere between those two points.

UK-Kev

Hi,
From the Multiplex Control Unit; I suggest you find the wire that should be feeding the rear fog lights and with the aid of a long piece of wire connect to the Multiplex Control Unit and then to the rear fog light connection. Basically this is bypassing the vehicle wiring and will be testing the circuit down to the F/light.

Rgs,

Kev

TheBeefChief

Cheers, Kev. That makes sense. The only problem is there are hundreds of bloody wires all tucked up there so finding it isn't going to be easy.

I was able to trace the wire back from the rear fog up to next to the driver's seat, then it goes under the carpet, into the floor and out somewhere in that spaghetti junction of wires.

I did bridge the wires down by the driver's seat and the light came on, so I know the wiring is good from there backwards.

So basically I know:

The switch is working.
The fuse is intact, with correct voltage at the fusebox.
The switch is sending a signal to the MCU.
The MCU - according to the diagnostic mode - is acknowledging the fog light switch.
The wiring from the driver's seat to the fog light is intact and the light is earthed correctly.
The bulb is working.

So either the MCU diagnostic mode is lying to me and there's a problem in there, or it's a problem with the wiring somewhere between the MCU and where I tested beside the driver's seat.

If I could find something that told me which one of the wires from the MCU was the rear fog that would make life much easier.

UK-Kev

So I guess you’ve had the plastic plate off by the driver’s seat / door – I had to remove that plate on my last CRV because I had a problem with the rear wiper washer draining back and just cut washer pipe and inserted a one way valve which fixed the problem.

Okay getting back to your problem – From all the items you have checked and said are okay it all points back to the Multiplex Control Unit being the problem but the diagnostics said its okay!!!! It is just a simple case of a loose plug? The only other thing I can suggest can you obtain / borrow a Multiplex Control Unit to eliminate this as the problem.

Rgs,

Kev

TheBeefChief

Yeah, I took that plate off, which is round about where the wiring vanishes under the driver side carpet.

I've unplugged most of the plugs into the MCU and checked them and they seem fine, but I think today I'm going to take the whole thing out and have a look at it with everything unplugged. Maybe I'll find something there, or at least figure out where the fog light wires are.

I'll update when it's done.

UK-Kev

Hiya,

Thought about something else you can check when you have the unit out – Where the plug connect into MCU check for damaged or bent pins.

Rgs,

Kev

TheBeefChief

OK, I took the MCU out and checked everything I could - no bent pins, no corrosion, no loose wires, and everything looking pretty much as it should.

I've found a breaker online selling one for £60 delivered, so I've ordered that and will swap them out to see if that fixes it. If it does, great. If not I'm going to have to give it to someone who knows what they're doing to investigate further.