1993 Chimaera Tachometer Troubleshooting

[TVRMJ]

I have a 93 4.0L Chimaera and the tach stopped working. I confirmed that there was continuity between the white and black wire of the Tach connector and the white and black wires connected to the coil on pin 1-. I checked and confirmed power at the green wire of the tach and continuity for the black wire of the tack to ground. If I want to confirm a signal to the tach with the engine running, am I looking for voltage or ohms, and what values?

Edited by TVRMJ on Saturday 6th August 19:28

[Belle427]

Probably a scope to do it properly, this is the only info I can find but not sure if it’s relavent to a coil negative connected tach.
https://www.digitalguarddawg.com/hrf_faq/how-do-i-...

[TVRMJ]

Belle427, Thank you for this. I now have a place to start.

[blitzracing]

The problem you have is the tacho picks up the back EMF from the coil primary as it discharges. Typically it's in the hundreds of volts region and it is a very fast spike that a test meter cant really sample so even on AC . The voltage the test meter will show is an approximate average of the coil off and on period mixed up with the sample rate of the test meter hence the odd values but it's a good place to start if you dont have an oscilloscope. Putting a scope on there has it's own hazards as they dont like hundred volt spikes without some form of resistance dropper to reduce the voltage.

[Polly Grigora]

TVRMJ said:

If I want to confirm a signal to the tach

You've already proven that the wiring isn't open circuit, as long as it isn't short circuit anywhere and the tacho is connected correctly, the tacho will have been proven faulty

You could always set meter to high AC voltage range and see what it shows at the coil and tacho white/black, they should be close to the same reading

[TVRMJ]

Blitzracing, at this point I am looking to establish whether there is a signal or if the tach has failed. I had changed the coil. At present, white and black wires are on 1- pin and the yellow and white wires are on the 15+ pin.

[blitzracing]

Unplug the ignition amp. This will make the coil negative go to 12v with the ignition on. Then measure the input to the tacho to see if it's the same.

[TVRMJ]

So, I hooked up a voltmeter reading ACV to the black and white wire for the tach. At idle it read 6.47 with some fluxuation. I reved the engine to between 3500 and 4000 RPM and got 12.14. So it appears that a signal is present for the tach. I am going to see if I can find a used tach from a breaker. It can be any one of the Griffith or Chimaera or posible a wedge car and I can swap the face if I can find one.

[TVRMJ]

[TVRMJ]

[keeling54]

Where are you based, I have a working tacho I took from mine, you can plug it in and try it.

[TVRMJ]

Sadly, I am in Louisiana USA

[Belle427]

May be repairable depending who made it, obviously not easy if your overseas but still doable.
https://www.smiths-instruments.co.uk/tvr/chimaera

[Steve_D]

I'm old school or just old. I have a meter designed to measure RPM and dwell. It connects to a battery supply and the signal wire connects to negative side of coil.
May be worth seeing if you can find/borrow a similar meter.

Steve

[TVRMJ]

Belle 427 and Steve D - I was on the Smiths website last night and made an inquiry. Thanks for your replies. I have a thread now on FB looking for a replacement tach. We shall see.

[Polly Grigora]

Steve_D said:

I'm old school or just old. I have a meter designed to measure RPM and dwell. It connects to a battery supply and the signal wire connects to negative side of coil.
May be worth seeing if you can find/borrow a similar meter.

Steve

Lucky you, no longer have one, great bit of kit

[TVRMJ]

Keeling 54, would you be interested in selling your tach?