Check the main instrument cluster circuit board for cracks at the pins that the tach plugs into. The board itself may be cracked somewhere else too- they get very brittle when they're old.
Possibly another instrument cluster would work for you. If you don't need a working speedometer, one ought to be cheap. Any 86-92 unit ought to plug right in- the 93's have a couple differences in the speedo itself and would be hard to find anyway.
And also check the 2-pin grey plug near the coil- the white/red tach wire goes through this connector. Perhaps you're losing signal there. The spade lugs at the coil can crack too. Unplug and reconnect everything to help remove any corrosion.
Good luck with it.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 225K, 88 744GLE- 209K, 91 244 183K. Also responsible for the care and feeding of: 88 745GLE, 229K, 88 244GL, 146K, 87 244DL, 235K, 88 245DL, 236K

Check for the 12V source and see if it is still there. Also check the ground wire to make sure that it is still giving a good ground connection. A MULTI-METER would be great to check for continuity in the wire from the coil to the tach. When in doubt start at the begining and work you way forward. If you fear the tach is the problem take the tach out and try to hotwire it outside the car along the engine compartment. Get 12V from the battery, a ground to the frame/intake manifold, and get the signal from the coil. If the tach works then check the wiring. If the tach does not work I guess the tach is down for the count. I have not heard of many tachs dying unless you fried it but anything electrical can always just die. Good Luck!

Sincerely,
--
Julio Meza

1979 264GL B27F 123K

Features Added:

BW55 To M46, Central Locking System, 25mm IPD Front Sway Bar, 81+ Dash W/ Oil & Ambient Temp Gauges In C, Oil Pressure, Volts

Next Project:

+Cruise Control