Help with Spoiler + Speedo outage

[thesmithsUK]

Hi all,

The speedometer and rear spoiler on my 91MY 964 C4 suddenly stopped working today. (The rear spoiler light says on.)

I understand that 'On the 1991 model, the signal comes in from the ABS unit as fed from the LF wheel sensor. It goes to R52, the Frequency Control Unit and also to R24 if you have a Carrera 4, then to the speedometer. The purpose of the FCU is to produce a square wave output using a device such as an ITT UAF 2115. The output of this integrated circuit is used to clean up the pulses from the ABS sensor'.

My guess is that the Frequency Modulator control unit on the relay board may need replacing.

Or could the fault be with the speed sensor?

Is there a way to diagnose the fault, either way?

 

[Alex]

If the wheel sensor emits a voltage or resistance signal then you can measure that at the point it plugs in to the socket.

 

[911UK]

Speedo issues, try https://www.reapautomotivedesign.com/contact/

the part you need is this

https://www.design911.co.uk/p/frequency-modulator-control-unit-porsche-964-carrera-4-1991/

 

[thesmithsUK]

Cleaned up the reference ring and tested the left-hand front speed sensor with a multimeter - all good.

I also tried a new Frequency Convertor Control unit in the relay board - no difference.

What next?!

 

[thesmithsUK]

Have also cleaned up all the connections on the back of the speedo...

 

[thesmithsUK]

Just tested the connections to the back of the speedo with my probe...

Pin 1: negative
Pin 2: positive
Pin 3: negative
Pin 4: positive
Pin 7: negative.

Am I getting somewhere?!

 

[thesmithsUK]

When the ignition is ON, Pin 1 becomes positive...

Assuming that means that power is getting to the speedo.

 

[thesmithsUK]

[All fuses checked.]