Rebuilt Tach Not Working - What to Troubleshoot Next? (3.2)
 

Well, I sent the tach out of my '84 off to North Hollywood Speedo and had it repaired and calibrated. I plugged it back into the car and still no luck. I'm measuring 14.2V at pin 5 (I believe).

What should I troubleshoot next? I have access to an oscilloscope, could someone tell me what kind of signal I should see and at which pin?

Thanks in advance Pelicans! SmileWavy

Pin 2 of the tach (black with violet tracer) should have a 0-12 volt pulses coincident with the spark pulses per the Bentley.

That's correct and use of a voltmeter will verify the signal, i.e. volts DC should show
a varying voltage with the engine idling.

The tach itself can be tested for basic functionality by rapidly touching pin 2 to 12 volts
with pin 6 at ground and pin 5 at 12 volts. The tach needle will move a little. If one
connects a file to 12 volts and drags the pin 2 input on the file, the tach can be made
to reach about 1K RPMs. The pins count from the rear left to right; 1-H on top row,
4-6 on the bottom row (tach upright).

After quick review of DME internal schematic you should see a nice square wave on the tach signal line. 0volts for a period of time, while it's 0vdc the coil is charging 'dwell time' then it should go to 12vdc while coil fires and waits for next charge to start. It looks to me like the wave will vary in duty cycle with RPMs.

The tach is driven from pin 21 on the DME so you can also check at the DME if need be.

But with a good scope you should see decent square wave at idle but it won't be 50% off and 50% on at idle. At 1000RPMs it should be off (0vdc) for about 3ms followed by 12vdc for about 17ms. At 1000RPMs you have coil firing at about every 20ms and the coil dwells (charges) for about 3ms so 3ms charge + 17ms = 20ms.

That's my educated guess from looking at the schematic.

This also means you can bench test the tach with a simple 0-12vdc square wave generator.

The tach output on pin 21 replicates the timing waveform of the coil signal on pin 1
of the DME ECM, i.e. the phase and duty cycle. With a resistive load vs a coil connected
to pin 1 and 12 volts, the signals are basically identical.