MS3X with BMW E36 - tach stays at 3000rpm

[Laminar]

I'm running a Ford 302 in a '97 M3. I've wired the MS3X Tacho output to the black wire originally on pin 36 of the BMW ECU, the wiring diagram tells me this wire is "CRNKSHFT RPM" and goes from the ECU to the instrument cluster, and also tees into the BMW data link connector.

I installed the JS9 to S12C jumper on my mainboard and JP3 is in place on the MS3X card.

With the engine running and a good tach reading on my TS screen, the physical BMW tach sits steady at around 3000rpm no matter where my actual RPM is. When I shut the engine off, the BMW tach takes a few seconds to finally decay to zero.

I've tried testing the output via the Output Test Mode - I/O screen, but I am not sure what my tach should read per the input. It will show a couple of different values at different frequencies, but I'm not sure what I've proven.

Should I be looking at the Megasquirt hardware configuration, software configuration, or my wiring into the BMW system?

[Matt Cramer]

Try making a chart of what RPM is displayed at what frequency when you use test mode.

[Laminar]

Hi Matt and thanks for the reply.

Here's what I see:

Hz - RPM
11.1 - 0
13 - 0
15.6 - 0
19.5 - 0
26 - 2100
39 - 2500
78 - 2800

This is at 50% duty cycle. I wouldn't be surprised if this tach was faulty as the car is kind of a pile and I'm not sure it was fully functional when I bought it.

[Matt Cramer]

Yeah, I'd wonder about the tach too. It shouldn't make an abrupt jump like that.

[Laminar]

Follow up on this - I plugged in the gauge cluster from a 4-cyl E36 and found the tach working as expected in test mode and with the car running. I stole the needle servo from that cluster for my original cluster, but got the same result, so it would seem that the issue is with the gauge cluster circuit board itself.