MPG gauge not working

[Kev_Ush]

The mpg gauge on my touring isn't working. Are there any common things that go wrong with them? Is it likely to be my clocks or something else? How does it work?

Thanks
Kev

[Brianmoooore]

It works by calculating the instantaneous MPG from speed data from the speedo, and fuel consumption data derived from the injector pulse length from the engine ECU.
The fault is usually the cluster itself. On the only one I've ever tried to repair, rather than just replace, I found a burnt out resistor on the speedo PCB, but replacing this didn't cure it, so the cluster was replaced. I later found a cracked transistor on the SI PC board from the same cluster, and I've since seen this same cracked transistor on another board since.

[DanThe]

Have you managed to get one working accuratley with the M50 Brian?

Mine is a compulsive liar

[Brianmoooore]

Have you managed to get one working accuratley with the M50 Brian?

Mine is a compulsive liar

The one under the speedo? The figures are pretty meaningless in themselves however accurate they may be, since you don't drive along with a constant reading for very long in normal driving - it's more of a "oh dear, is it really using that much fuel? I'd better lift off a bit" type of gauge.
Mine certainly works on the M50, but I suppose, using injectors with a different flow rate, it does read differently. Never really thought about it.

[Kev_Ush]

It works by calculating the instantaneous MPG from speed data from the speedo, and fuel consumption data derived from the injector pulse length from the engine ECU.
The fault is usually the cluster itself. On the only one I've ever tried to repair, rather than just replace, I found a burnt out resistor on the speedo PCB, but replacing this didn't cure it, so the cluster was replaced. I later found a cracked transistor on the SI PC board from the same cluster, and I've since seen this same cracked transistor on another board since.

Thanks for that. So to save myself a load of hassle I should just replace the cluster...I'm not very electronically minded, I'm alright with wires but when it comes to resistors and transistors I'm lost.

While I'm on an electrical note...how do the headlight wipers work? Do they come on with the window wipers (mine don't) or is there another switch? I've tried pushing everything on the dash but I can't get them to work. Is it quite common for them to stop working?

[Brianmoooore]

As I said in my reply, I've never successfully repaired a MPG meter, although replacing the speedo assembly and the SI board should do it.
Headlamp wipers have been comprehensively discussed on a recent thread, as they have many times before.
There is no seperate control; they should operate every fifth or sixth time the screen washers are used with the lights on.
There is just one common problem with them, which will occur every eight to ten years or so. No great problem to sort, and then they should be good for another ten years. Use the forum search feature.