Tach reading double?

[Chris.94GT]

Back story - car (96 Cobra) blew up its engine just after Christmas, pulled an engine out of a 98 MK8 from junkyard and installed in car and also converted to COP. Deleted complete air injection system, and EGR.

Finally got the car running this past week. Started putting all the new fluids in and running the car a bit for coolant. Plugged in SCT to watch coolant temps on datalog. While its been running I notice the tach is reading a bit high. This is verified on SCT as it sounded like it was idling correctly. According to SCT the factory tach in the dash is reading double actual value.

Any ideas as to what this could be? Only wiring that was changed was the factory coil wires were gutted from the harness and wiring in the new coils in series configuration as wasted spark.

 

[mwolson]

Never seen a tach read double. You can test the tach with a cheap battery charger: http://accutach.com/Documents/TestingTachSpeedoR1.pdf

 

[Chris.94GT]

I was hoping you would chime in. It is definitely reading double, because at idle the sct is showing 800 and tach indicating 1600. If I hold it at 2k on the sct it indicates 4K on the tach. I thought it was odd as well that the PCM is reading the correct RPMs but the tach is double.

 

[mwolson]

If you can, run the test I described in my write-up. If, for some reason, something broke in the tach, it will read 3600 RPM rather than the 1800 it should.

On the PCM side, I can't figure out how the PCM would put out a 2X tach signal, unless there is noise in it.

There is some adjustabillity in the tachometer calibration adjustment pot on the PCB of the tachometer circuit, but it is not enough to double it. However, there is a ground trace on the flex circuit on the back of V8 clusters that is cut on the V6 clusters. If that ground trace is not making contact for some reason, it would turn your tach into a V6 tach. If you add the adjustability of the pot to that, you might be able to get to a 2x difference. (Your tach is acting like a 4 cylinder tach for some reason.)

I assume you are running the Cobra PCM, not the Mk8.

If you run the test in my writeup, run the test for both the V8 and V6 tachs to see if one of those gives you the same results you see in the car.

 

[deadernie]

Take some friends out for a ride and say "hey watch this" and watch their eyes bug out their head as you rev it to 8K RPM and beyond:lol1:. Maybe good for a few LOLs.

 

[Chris.94GT]

Found my issue. I moved all the harnesses to outside of the engine bay, apparently one of the connectors on the passenger side that does something with the ccrm was loose because I accidentally tapped it while hitting the ccrm to see if fan relay was stuck and the car shut off. Reconnected fully and fan came on and tach reading properly again.

 

[mwolson]

It is very interesting that that caused your tach to read 2X. I would never have predicted that problem.