I have a 1990 740 Turbo with a tachometer that stopped functioning a month ago. I removed the instrument panel to check the electrical connections and all of them seemed fine. The tachometer needle however refuses to move. The engine starts and runs without trouble, all other instrument displays are functioning properly, and a visual inspection of the engine speed sensor did not reveal any problems.

A few days ago the car began making a haunting, high-pitched, whining sound that increases in frequency and intensity with speed. Could the engine speed sensor have failed in such a way as to be rubbing against the flywheel? If so, would not the ECU stop functioning or revert to an emergency mode?

I recently completed a major overhaul of the car including: suspension, steering, and brake system rebuild; engine tune-up and fuel injector pressure testing and cleaning; replacement of water pump and thermostat valve (had failed shut); replacement of accessory drive belts and camshaft timing belt; and replacement of heater blower motor. The car was running well for some months after these repairs and before the above problems surfaced. It currently has 114000kms.

Any suggestions would be welcome. Thanks.

Noise could be a failed timing belt tensioner or belt idler roller.

The belt tensioner and timing belt were changed and the tension rechecked after 1000kms. Also, the noise seems related to driveshaft speed and not engine speed. Thanks for the input though.

My own 740 has a dead tach which I am about to replace. Suggest you get one from a boneyard. I also suspect it is unrelated to the noise.

The loud noise: is it related to engine speed or driveshaft speed? Heard when idling or only when driving? I doubt the speed sensor is touching anything: it would quickly self-destruct and the engine would stop. Try using a mechanic's stethoscope to pinpoint the noise.

The noise seems related to driveshaft speed and is not audible when idling, suggesting transmission, differential, or bearings. I will try the stethoscope. Thanks for the tip.