964 C-4 (1991) ABS Control Unit
 

I have a 1991 C-4. Speedo, rear spoiler and any other item requiring a speed input doesn't work. Have been told by my porsche mechanic that this is a common problem and thinks there may be a cheaper fix than replacing the control unit. Can anyone help me? Thanks.

Starting in 91, the speed is sensed from I think the left front wheel through the abs sensor so that is where you might start.
You might be able to move the right one to the left for testing purposes.

FCU relay
 

Hi, if you are missing the speed pulse, look in the fuse box for a relay described as FCU. This is a frequency conversion relay. It takes a signal from the ABS unit and translates it to an output that the speedo and the DME etc are looking for, 8 pulses per wheel rev.
Either that relay could be missing or failed. Last time I looked for one, it was blue in colour. Try starting there.
Geoff.