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Might be a bit of a random question - but does someone happen to know what the signal is for the tacho in the dash, and how could I use a multimeter to test that the signal is correct?

My issue at the moment (1994 1.8 NA btw) is since the rebuild, the tacho has not worked. The car has been megasquirted and I've fiddled with TACHOUT settings to no avail, and I don't think the megasquirt has anything to do with the tacho anyway - according to the wiring diagram.

The wiring APPEARS to be correct. There is a wire from the coil plugs, that is split to the diagnostics connector, to the ECU and to the dash. When doing continuity checks, they're all properly connected.

So the only other thing I'm thinking it could be is the gauge itself is broken. So how do I test that the signal received by the dash binnacle when the car is running, is correct?

Does your multimeter have a RPM function? The tachometer in the dash measures pulses and that is what you are looking for. A oscilloscope would be better as it displays time based measurements.

Can your ECU output the RPM signal? I had to make a change in the PS1000 to get the tachometer to work. http://mx5cartalk.com/forum/viewtopic.php?f=55&t=61189&hilit=Negative+Tacho

I initially figured it was a pulse signal, and yes I have a multimeter with RPM. But the reading I get is nonsensical from both the diagnostic wire and the wire to the dash.
Bizarro...

Standard, the two 1993-1995 NA8 coils have four pins each, one of which from each coil provides the tacho signal. But from your post it sounds like you've removed the coilpack & converted to CoP?

No - still using the stock coil packs.
I might just have another go with RPM on the multimeter and report back. Stupid wiring.

NA8s require a rpm signal from the ECU so make sure your megasquirt is outputting a tach signal on the correct ecu pin.

bartmanftw wrote:NA8s require a rpm signal from the ECU so make sure your megasquirt is outputting a tach signal on the correct ecu pin.

When you say "require" - do you mean the tacho requires it?
I've read about differences in wiring between 94-95 1.8s and 95.5+ 1.8s - and that its the 95.5+ cars that required a signal from the ECU.

Everything I'm seeing both in the wiring diagrams for mine, and random information on the net suggests that the tacho isn't dríven from the ECU.

wiring diagram for mine;

I just built a DIYPNP for a 1994 mx-5 and the dash tach signal was dríven by the ecu. So the tach would not work unless the ecu was outputting the signal.

These links might help.

http://www.diyautotune.com/diypnp/apps/ ... bp-mt.html

viewtopic.php?f=76&t=38367

Hmmmm fair enough.

My megasquirt was supplied by MSLabs... so its literally just a box i plug into the standard harness. That being the case, presumably I need to open it all up to ensure the actual MS2 board is wired correctly to the right pin out on the harness.

In TunerStudio i can turn the tacho output on - and send it to these possible outputs;
IGN
PT7
PT6
PA0
FIDLE
TACHOUT
ALED
WLED

So far - I've set it to on and "TACHOUT" - to no avail...

Is there any signal coming out?

Check with whoever built it or a manual if you got one with it which output pin is used for the tachout signal as it might not be on the TACHOUT pin. Also as magpie suggests check if it's actually sending a signal out or not.

Rather (un)helpfully, the MSLabs guy isn't responding to my emails. At any rate - have extracted the little black box from under the carpet again and taken it apart. Maybe I've found the culprit...





Might be a bit tricky to see, but that group of 3 jumpers is what it looked like when opened up. The jumper between "CAS" and "TPS" on the board is labelled "TACH" and is disabled by the looks..
Just about to plug it all back in with the jumper enabled and see if I don't fry something.

SUCCESS!

Geez such a simple fix that I've been struggling for ages to figure out
Thanks for the help bartman + Magpie.


So I know its completely academic at this point, but if that's an output, then according to the wiring diagram, it is being sent to the coil packs. What is it for??
The car itself has been running fine (tuning and no tacho aside) since the MS2 install... without that signal. So why does it go to the coil packs in the first place? Anyone know?