Hi Peter,

To me, the news that the two functions die together is good news. It means the cause is likely to be one we've seen before, not exotic.

To isolate it while it is steady broken, one would use an ohmmeter at the L-plug looking back toward the speed sensor, measuring between the black and blue wires on the '87. That you have a green/white wire tells me you're looking at a later version wiring harness, for 1990 (airbag) cars and later. Bentley says you should see the resistance of a coil of wire in the range of 2950 ohms.

Intermittent as it is, fixing it by beating on the dash, you'd think this was a problem in the cluster, so I expect you will verify that speed sensor continuity. Given you've already been down the road with a substitute signal according to my diagnostic pictorial, I'd guess it isn't the L-shaped connector either, but one more lousy solder joint somewhere in the gauge itself. Try reflowing them all?

If you decide to run it on the bench, remember you need to add a jumper into the pin sockets to get the unit to power up. In the following picture, a plug like the one you have is being used on an earlier unit.

The black jumper wire completes the power path, but +12 is supplied to the blue wire on the L-plug, and ground to the black wire on the L-plug.

Disregard the red clip and resistor on the end pin-socket -- that was a means of checking the taximeter output for the cruise or LH2.4 VSS.

The signal substitution is applied to the green/white wire on this L-plug. What do you have to provide a signal on the bench? Without a purpose-made signal generator, you can use an AC-output wall wart transformer for 60Hz, or a tone generator app on a picturephone.



You might just jump to the chase scene skipping the science project by re-heating all the solder connections one by one, and then giving it a few hundred miles of real-world driving experience to test it.

--
Art Benstein near Baltimore

Don't find fault, find a remedy. (Henry Ford)