RPM and MPG gauges not working

[E30starkie]

Hi guys, new to the forum and to E30's so please go easy on me


On Monday i finally purchased my perfect E30, Glacier blue in immaculate condition with an M50B25 already fitted. Some of you may know it, it was previously Shanes from Big Perm Customs.

Anyway, the RPM and MPG gauges don't won't, the guy i bought it from said they worked very briefly once and never again. After hours of research i went out today and tested everything i could.

I thought seeing as the engine had been swapped, hopefully it was a simple wiring problem where the two looms meet. However i've tested the cable from the ecu to the clocks with no problems.

Next up i thought id check the SI batterys, but again these tested fine. My clocks have the 3v lithium batteries, both tested perfect with my multimeter.

Ive also checked all fuses and found none blown.

The only thing i can't think of now is that the tach coding plug may not have been swapped for a 6 cylinder version? I'm not sure if this would completely stop the two gauges working? Everything i find online just says it prevents the RPM gauge showing the correct RPM.

Anyone got any ideas? Would be very much appreciated!

Thanks, Oli

[Brianmoooore]

What age is your E30?

[E30starkie]

Its 1989

[Brianmoooore]

Although the MPG gauge and tacho. share the same instrument face, they are nothing to do with each other, and are both simple meters, with all their associated electronics on the SI board.
The tacho. relies on the engine speed signal from the engine ECU for it to operate, and the MPG gauge uses the fuel rate signal from the ECU, along with the road speed signal, derived from the speedo.
The engine speed signal is a black wire through pin 9 of the C101 20 pin engine loom plug and socket, and the fuel rate signal is a white/black wire on pin 8 of the same plug.
On E30s a couple of years earlier than yours, for a short while, these two particular wires were routed through a separate 3 pin plug and socket behind the instrument cluster area, and this is often overlooked in conversions, resulting in what you describe.
Of course, this would result in the tacho. and MPG gauge never working, which is at odds with what you say the previous owner said, but I would take that with a large pinch of salt.
Your car should be far too late to have the older plug and socket, but it would be worth checking either side of the C101 to make sure these wires are present, and also that the C101 locking ring is fully engaged, holding the plug and socket together.
An incorrect tacho. coding plug would just make the tacho. misread, and wouldn't affect the MPG gauge.

[E30starkie]

Thanks for the reply!
I have already traced the wire all the way through from the ecu to the c101 plug and from the c101 plug into the blue plug on the rear of the clocks. I have continuity from the ecu to the blue plug so assume it can't be that?
My next plan when I get home from work was to check power to the blue plug, from what I can figure out from wiring drawings, it's pins 2 and 23 on the blue plug?

[Brianmoooore]

Pins 2 and 23 on the blue plug, and pins 6 and 13 on the white plug are all power feeds to the instrument panel, with B6 possibly being the most important for the tacho. and MPG gauge.
Does the MPG gauge do anything, or is it simply dead?
Check fuse 10 very carefully - soft blown fuses are not always obviously blown.
The tacho./ MPG gauge assembly is connected to the main circuit board by a plug and socket, and I suppose it's conceivable that some ham fisted person has damaged that at some point in the past, if they've had it apart.

[E30starkie]

Ill check them all when I get home. Sorry for my stupidity, but what is B6?
The MPG gauge is just completely dead!
Ill recheck the fuses when I get home too.
My next ideal after checking all else was to check the circuit board, so ill make sure to check that
Thanks again for the help

[Brianmoooore]

Sorry, B is the designation for the white socket (A = blue, C = yellow). B6 is pin 6 of the white socket.
Never seen a completely dead MPG gauge before. The common faults all result in it swinging from one extreme to the other, depending on how the car is being driven at the time, so it looks like you have something unusual.

[E30starkie]

Ahh okay, thanks for clearing that up! Checked all 4 pins and have 12v on all.
Maybe my RPM and MPG unit/gauge is knackered then!
My last option tonight is to strip the cluster down again, plug it in and check everything on the board with my multimeter.
Thanks for all the help!

[Brianmoooore]

It would be unlikely that both displays have failed at the same time, so I'd look at the plug/socket to the main board. Unscrew the three small self tappers on the back of the case, and the tacho./MPG gauge just pulls out.

[E30starkie]

Ill give that a go tonight.
I might be pulling ideas out of the blue now, but by any chance could it be related to a faulty crank position sensor? Not sure if the CPS is what gives the ECU its engine speed?
Every so often i get a slightly lumpy idle, which could be related?

[Brianmoooore]

A duff CPS would definitely stop your tacho. reading, because the engine wouldn't work.
The prime purpose of the CPS is to provide pulses which are processed to drive the spark plugs, and without them, the engine won't be running.
Lumpy idle (possibly a cylinder dropping out because of a weak fuel mixture) is something with a different cause altogether.