intermittent tachometer

[inaminit]

I cannot seem to get the tach to work on a regular basis. Sometimes it works for a whole ride, sometimes it stops, then after a bit-it might start up again. Readings are plausible. Car is a 76s, and engine is an 86 3.2. The tach is an 84. The purple/black and black wires have been routed to the tach from the correct pins in the DME.
The engine swap was completed about 5 yrs ago, the tach worked at that time, then the car was stored for about 3 years. Now it works intermittently.
So far I have
swapped in a different DME-no change
swapped in a different tach-no change
jumpered in a ground in place of the brown wire to the tach no change
jumpered in a 12v wire to the black/red terminal, no change
measured the voltage between the black and black/violet wires with the engine running-voltage varies between 1.5 at idle and 4 or 5 when you rev it.
I do not have an oscilliscope or access to one. Are there any other tests that I can perform with the usual enthusiast home shop tools? I am thinking to jumper the black and black/violet off the 14 pin plug to bypass all the car harness possible issues-but after that, I am out of ideas. Any help will be greatly appreciated!

[inaminit]

Anyone?-I am out of ideas on this problem!

[sdjaytee]

Mine is a 77 with a 3.0 liter so not quite the same but my tach was jumping all over the place. My issue ended up being the thick brown ground wires to the alternator were cooked (one was completely disconnected). And the voltage regulator was bad. Replaced the regulator and re-wired the engine harness and the issue was fixed.

-j

[TeeJayHoward]

Sounds stupid, but when it stops, does it start working again if you tap on the glass? If so...

The tacho glass is concave, not flat. If this is your issue, an easy fix is to remove the tachometer, pry off the bezel, flip the glass over, and crimp the bezel back down.

[sparkysvega]

I have a 76 with a 3.2 also that was doing the same thing then quit completely. Changed the tach , worked awhile and Quit again . Turned out it was a bad connection At a connector in the frunk.

[911obgyn]

Spread the pins and clean terminals on 14 pin connector and plugs under the dash.

[inaminit]

Still struggling with this

[ischmitz]

Given that this is a transplant let's hope someone didn't hook up the tachometer to pin #11 of the DME - this is sure way to eventually destroy the DME. Pin #11 puts out the injector pulse duration and that signal is used for the shift light in a stock 3.2. Using this signal as input for the RPM signal for a tachometer actually will work somewhat but in the process you'll destroy the DME over time. A very expensive repair since a custom BOSCH IC needs to be replaced.

Other than that about the only thing that you haven't replaced is the signal wire path from the DME to the tachometer. Internally inside the DME there is an electronic switch (transistor) that momentarily provides GND for each ignition event to pin #21. That is the tachometer signal. Someone mentioned the connector where the DME harness connects to the chassis harness. In your case that might be a custom part depending on how the transplant was done.

For testing the signal wire disconnect the DME and use a good DVM (digital voltmeter) and measure the resistance of the signal wire path. Locate pin #21 inside the 35-pin DME harness connector with one DVM lead and connect the other lead to the signal wire on the tachometer. Take that reading. Next hold the two leads of the DVM together and subtract that reading from the previous. The result should be very close to 0 Ohm. If not I'd suspect the 14-pin connector or the conversion harness.

[inaminit]

Ok- ran the DVM test as described. Result was 00.00 ohms. I should explain that in this DME harness and also in a spare DME harness that I have, there is a 4 wire bundle that was probably was intended to go forward to the trunk or instrument panel. That bundle exits the main harness bundle a short distance from from the DME female plug, and it contains the black/purple wire from pin 21. There is no role for the 14 pin plugs. So, I brought that bundle into the trunk and spliced on some extra to get to the back of the tach. Measurement was downstream of the splice at the female spade that goes to the back of the tach.

[ischmitz]

Maybe time to get one of these cheap oscilloscopes on eBay for like 30 bucks. It’s really hard to troubleshoot something like this without one.

Have you tried a 3.2 tachometer? I don’t know enough about whether the signal processing is very different in tachometers of various vintages.

Keep in mind that the early tachometers received a hard GND from points. So it’s internal pull-up resistor can be fairly low (driving more current, less susceptible to noise).

Starting with the 6-pin CDI the tach signal is “softer”. Depending on the year of your tach maybe that’s the issue?

With an oscilloscope you’d be able to see if the DME is able to fully pull the signal to GND. If not either the tachometer or the DME need to be modified.

[inaminit]

the tach is an 84-correct for 3.2, as is the substitute tach.
I looked on e bay-under "automotive oscilliscopes" there are several units, all new, by Hanteck and others in the $130-$200 range. Under "oscilliscopes" there are a bunch of handhelds that are cheaper and a few used ones. Various features-can you suggest one that would be suitable? Thanks!!!