Porsche Enthusiasts Club Forum

Hi
I thought I had a successful day sorting stuff.
Cruise control now works (refebed module)
Intermittent wipe works (new wheel)
and rear windscreen heater works (new relay).
However the Speedo no longer works !
I guess I have either knocked something or it's something to do with the new cruise unit.
There is nothing obviously disconnected at the back of the pod.
Any ideas ? Does anyone know where the speedo feed connects to the pod ?
Regards Adrian

Hi Adrian,

Please can you post your model and year? The speed sensor for the 928 moved depending on whether it had the PSD (in other words digi-dash model) or not.

Hi

It is 928 S2 1986

Thanks Adrian.

In that case the speedo signal is taken off the diff at rear, and is plugged into loom at connector in spare wheel well (towards front of car I think).

May just be coincidence that speedo stopped working after your fixes, so worth unplugging that connector and plugging back in to check for firm connection.

Fine list of fixes! You haven't had pod off which would make potential cause more obvious but in working on the intermittent wipe rheostat you may have simply inadvertently tugged on one of the dash connectors (assuming the connector in spare wheel well is OK). There are 3 going to pod on S2 I think. Two are horizontal and one vertical. The speedo signal is fed via the vertical connector so with your head in the footwell you could try checking this is firmly on and/or unplugging and gently cleaning the connectors. I think that to get access to the vertical connector you have to loosen the pod but don't have to remove it.

Anyway at least your to-do list is quite a lot shorter. I find working on a 928 is very much two steps forward, one step back. the step back is always me ballsing something up when doing another job so giving me something else to fix!

Thanks, I will have ago tomorrow

OK Adrian. You'll find a wiring diagram for your year is essential for a lot of electrical fault-finding. I only have the diagram for my MY so if you can track down/buy the diagram for S2 that's a big help.

I had an interesting morning, got the pod out and back in again all connectors seem sound and
everything still works ( bit of a miracle really !) except the speedo.
On the off chance I disconnected the new cruise control ECU and hey presto the speedo works again.
So either there is a problem with the ECU or probably more lightly the impulse transmitter signal is not strong enough to
run them both, anyone got any ideas how to check this.
When pulling away the speedo does not move till you get to a bout 20 mph I don't know if this is normal
or connected to the problem.
Does anyone know which of the connectors in the spare wheel hole is the one
for the impulse transmitter ? Regards Adrian

Just wondering what you did to the cruise ecu ("tempostat") to restore function?

Have found wiring diagram for your car. I'm not as familiar with interpreting it as I am with S4 diagram (it's annotated differently) but looks like there is a feed to speedo from CE panel which is taken directly from input to CE panel from the cruise module. It's on page 7 of the MY 86 diagram (title 'engine - digital engine electronics')

So if there is a problem with cruise unit then you can have problems with the speedo signal. But then if you have disconnected cruise, how is the speedo signal getting to CE panel? Feel out of my depth here!

The speedo connector near battery box is a 4 pin connector.

Hi

That speedo sticking is a physical problem not connected to your signal problem, mine does it sometimes and so did my brothers, he cured it by EXTREMELY carefully removing the speedo needle and lubricating the shaft...which totally cured it.....on mine when it happens a clout on the dash does it lol....

If it works with cruise ecu not connected, I would guess its that cruise ecu.....go back to whomever reconned it....

All the best Brett

hi
The ecu is a reconditioned unit I bought. I've had a look at the schematics too.
There is a feed from the impulse transmitter in the gear box too the fuse box.
It splits there one feed going too the cruise ecu the other to the speedo.
If I disconnect the ecu the speedo works so its not mechanical.
I've been in contact with the ecu people.
Apparently there should be a diode in the feed line to the ECU if this is not working the ecu could
interfere with the speedo.
So I'm going to do some more tests tomorrow if I can find a thing called a diode !
Out of my depth too !
Failing that I will have to hand it over to an expert (expensive) or give up (less expensive).
Adrian

Job done ! The diode on the feed to the ECU sorted it. Speedo and
cruise control now working.
Many thanks to the guys at the ECU repair centre Holland
for their assistance and patience. Adrian

Nice one Adrian.

Can you point me to the page/grid reference on the wiring schematic as can't find the diode you mention and it's driving me mad looking for it!

I have the MY 86 schematic.

Hi

I'm not good enough at reading circuit diagrams myself to recognize something
like that, I don't even know what symbol they use !
The guys at the ECU repair place told me there usually is one, it's position varying between
cars/models, sometimes its it the line sometimes in the ECU itself.
All i know is putting one in it sorted the problem !

Adrian