1970 442 automatic with AC. When I bought this car 4 years ago the tach did not work. After removing the tach, I could see that it had been previously serviced. I took the tach to Redline Gauge and Clock Repair for restoration. When I picked up the restored tach John did a bench test showing that the tach worked. Still, however, it did not work in the in the car. During the restoration of the dash for my 65 442 I took the 70 tach back to John and he again showed me it worked on the bench. Still no luck in the car to this day. I've replaced the tach lead wire and the ignition condenser. The car does not have a tach filter. Recently I started working on this issue again. Automotive electrical is not my strong suit. I know the tach works on the bench. I just did an omh test with my multimeter between the tach input stud and the tach housing. I got 11.68 ohms with this test. Shouldn't this test show OL or no resistance? Is the tach input stud grounding to the case? The red insulators look good. With the tach removed, I then did and an AC test with the multimeter between the brown tach lead wire from the negative side of the coil and the dash ground (the tach lead was tested inside the car mimicking the actual tach connection). With the car running, I get an active RMP response on the multimeter. This test suggests to me that the wiring at the tach is good.

Can anyone help me determine why a tach that bench tests good with good connections does not work when installed in the car? Should I send the tach back to the repair shop and request a new circuit board be installed? Should I sell the car for scrap value and cut my loss while I still have a shred of sanity?

Here are a couple of pictures of the back of the tach.

 

Is the tach wire correctly hooked to the NEG terminal on the coil? What type of ignition system do you have? Have you verified power and ground at the tach when installed in the dash? Have you tried connecting the tach to the car with jumper wires to verify that it works that way?