Jumpy Speedo - MSD at fault?

[gritbox]

I have noticed recently that when I rev my engine, the speedo jumps round to 180+ mph, it is almost mirroring the tacho. It never used to do this until I had to replace my MSD and Dizzy. Any ideas where the fault lies? I never touched any speedo wires, and only refitted the tacho wire out of the MSD. The speedo does work normally and reads accurately, it only happens when you rev the engine when the car is out of gear or clutch depressed.

[Steve_D]

Cross talk on things like this can quite often be earth faults when one device tried to find a route to earth via another device due to its own earth being faulty.
A check on all the connections is the only way forward. Not easy to do I know but I can't think of any other way the two can be related.

Steve

[tjb]

I had something similar. 180mph when doing 60 and 25mph showing when stationary! It was a dodgy connection on the speedo sensor.

[gritbox]

Thanks guys, next question, where is the speedo sensor? on the gearbox?

[Steve_D]

There is a Porsche sensor in the side of the box but I don't know of anyone using it so you will most likely find yours pointing at a trigger wheel mounted behind the nearside inboard drive shaft joint.
Make sure it is clean, undamaged, square too and aligned with the trigger wheel and finally positioned about 1mm from the trigger wheel.

Steve

[gritbox]

thanks Steve i'll have a look tonight and check/clean it.