I am having a proplem with a 87 300D turbo. Tachometer only works when engine is cold. then tach quits when the engine warms up and so does A/C. I have replaced the Climate Control unit. That fixed some other problems with A/C-Heat control, but not that problem. A/C works fine when Tach works, but no A/C when the tach is inop.

I can rev the car up 3000+ and the tach will start working when engine hot

Anybody out there know what controls the tach. Is it possibly return signal in the Intrument cluster. Any help would be greatly appreciated.