86 740T Speedo Bounces /car not moving

Hey everyone,, I thought Id pop a question in here,
Has anyone ever seen thier speedo gauge bounce up and down like 0 to 20kmh
with just the engine running and the car not moving. Mine does and if i rev
the engine up the speedo bounces higher, sometimes as much as 60kmh. Also my
odometer slowly ticks over (cause it thinks the car is moving). I bought
this car a couple years ago and the previous owner mentioned it too me, so
its been doing this for years. I have checked all the connections on the
back of the instrument cluster and everything was tight fitting.

Thanks

Ken
86 740turbo 256000km (indicated) running a 1991 940 engine with 90000km on
and the original Garrett Turbo system.

 

Since it is electronic and mysteriously cares about the engine speed, I'd
start by looking at the alternator and battery. A good place to start would
be to measure the AC voltage across the battery with the engine running.
This requires a digital volt meter as opposed to the analog type, since the
analog meters are confused by the DC across the battery. Anyway, les than
0.1 VAC means the problem is elsewhere. Voltages around 0.2 VAC usually
indicate a bad battery or the battery connections, but the real suspect here
is if the voltage is 0.5 VAC or more. That indicates bad diodes in the
alternator.

Mike

 

The ignition pulse for the tachometer if installed is not being blocked
in the circuit board for the instrument cluster and bleeding into the
speedo circuit. Or if one is not installed then the speedometer is
acting as a receiver and the tach wire from the coil is providing the
signal. Relocate the unconnected tack wire to eliminate the second
option.

Bob

 

Unconnected tach wire? 700 series cars came standard with a tach, and the
turbos came with all the guage spots full. I fixed this problem on a similar
740 by resoldering a bunch of nasty solder joints on the speedo circuit
board.