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MG MGF Technical - Tacho limiting to 500RPM

My tacho has gone on the blink! It only comes up to about 500 rpm no matter what speed the engine is actually running.

Everything else seems ok - good response and no mis-firing etc, and since it imediately comes up to 500 and dies when I cut the engine, the electrical connections would seem to be okay, so my feelings are that it is the tacho itself.

I have the circuit diagrams and intend to trace it back through to the ECM, but before I start, does anyone know which input the ECM uses to derive the revs?

Also, what sort of wave should I expect? Square or pulse?

Tony

Tony

Well, actually, its 600 RPM

Tony
Tony

Actually it's reading 600 RPM.

Just been trying to put a 'scope on various connectors in the circuit, but without any positive conclussion, since I'm not seeing any signal at all - which of course proves nothing!

Onwards......................

Tony
Tony

The tach wire comes from the engine management ECU directly to the cluster, also the same wire goes to the EPAS, The wire is white with a black stipe, unplug the MEMS and stick a pin in the socket hole which corrosponds to the white/black wire and trace it from there.
Will Munns

I thought it was the speedo that was connected to the EPAS, not the rev counter.

Least when my speedo stopped working, I lost the EPAS, even though the rev counter was operational throughout.

paul weatherill

Although, there's the rev counter input, also for the "satety cut-out feature" I guess.
paul weatherill

>> what sort of wave should I expect? Square or pulse? <<

Good question. I'd guess a square wave, but I don't know: no info on this in the workshop manual that I recall.
Rob Bell

It's the crank sensor (magnetic coil sensor) which gives the input to the ECU for rev calculation.

Nothing mentioned in the manual about the output wave form from the ECU. Could be each and everything.

HTH
Dieter
Dieter K.

Just a thought - if your car has EPAS, why not try reving the engine for 30 seconds whilst stationary. If EPAS goes into fail-safe mode then presumably the ECU's engine speed input is fine and the fault must lie with the tacho itself.

FWIW!
Tim.
Tim Jenner

Okay, tracked down the necessary info - it's stashed away in some gubbins regarding the MY2000 update that espouses the benefits of EPAS...

Anyway - the engine speed signal is a square pulse wave, 1 - 12 Volts, with two pulses per rpm.

Hope this helps!
Rob Bell

Thanks to all for your input.

My initial test proved nothing, but I guess this was not helped by kneeling in the rain trying to find a test point under the dash and then being convinced that the scope was on the right point!

Since I'm not experiencing any other problems I'm convinced the fault is not prior to the ECU, so it will have to wait until the coming week-end.

I'll keep you posted.

regards
Tony
Tony

Hmmmmmm?

Not sure if what I'm seeing is good - certainly looks suspious! Accepting that the waveform below is crude, this represents the signal input to the tacho as seen at the connector block on the fuse panel under the dash.

The frequency looks right at approx 70ms positive peak to peak at tickover (guessing at about 800 rpm), and reduces with engine speed, but it's not a square wave - with or without tacho connected, it remains the same.

The fall off to nominal after the +ve peak is a sine wave and so is the rise after the -ve edge, but the range is about 20v overall


/ / +10v
/ /
/ /
-- ---- --- --- -----
/ /
/ /
/ / -10v

Scope ground was to chassis, for want of find an easier point at the time.


I'd appreciate comments.
Tony
Tony Thompson

Oh poo ! Diagram didn't work ! I'll have to try a better way of doing this !!!!

What it should have shown is a vertical rising edge 0v to 10v, followed by a slow sine drop off back to 0v. Then a vertical falling edge to -10v, followed by a slowing rising sine wave back to 0v. Then repeating...........etc

Tony
Tony Thompson

Sounds like you have the scope set to AC, try setting it to DC and try again.
Will Munns

Will,

No, I checked that - definitely set DC.

Actually, I was wrong to say it was a slow sine drop off - it's more like a typical RC discharge, initially fast & then slowing down as it approached 0v.

(My scope is quite old & hasn't been used for quite a while but I suspect it's ok. Unfortunately it doesn't have a test output. Going to try & find a way of checking it out today)

Tony
Tony

Will,

I've checked my scope and it's okay.

The waveform I was trying to draw can be seen at..

http://members.aol.com/apttony/tachowave.JPG

Since this is what I get with or without the tacho connected, it suggests to me that it's the output that is wrong, but I need to have it confirmed that it should be a true square wave.

Tony
Tony

The literature is quite clear about the specs of the tacho output - square wave, +1 to +12V. I guess this would be readily confirmed by trying another car with a functioning tacho...
Rob Bell

Tony,

If you are checking the signal from the MEMS unit without the actual tacho connected then this may be why you get a strange signal. Perhaps you need a resistor from the wire your are checking to ground or to 12v. Try something like a 4k7 ohms. The tacho may provide this load resistor normally.

Hope you understand this. If not give me a bell. And welcome back by the way!

Bruce
Bruce Caldwell

When the heavens stop throwing down all this wet stuff, I have another go!

Tony
Tony

bump
Tony

Bruce,

Tried a 4k7 resistor from the tacho feed wire on the fuse panel, both to ground and to 12v, to no avail. Had very little difference to the signal. And then the RAIN came again!!!!

Still none the wiser, I guess.

Tony
Tony

This thread was discussed between 15/11/2003 and 28/11/2003

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