AlgaeHater

The part that was replaced was a relay called a boost signal interpreter. I guess the dealer mis-identified the part

Metal Guru

Where is said part located?

AlgaeHater

Under the drivers seat I believe.

fritz k.

part 96564138100 would be conceivable, a kind of interpreter relay for the OBC. its located in the trunk.
Fritz

AlgaeHater

Yes I believe that was the relay that was replaced. All I know it was crazy expensive for a relay!

heliolps2

That unit/relay sets behind the fuel gauge, which you can't access from the trunk. blue in color

fritz k.

for sure, available as original part from Porsche for 640 € / 750 $. And helio is right, sits behind the dash.

Fritz

jd964t

Hi all, I installed a new clutch (Sachs K70284-05 kit) and Patrick Motorsport lightweight flywheel and car started up immediately and runs fine. But my tach / rpm gauge has now stopped working and the boost pressure reading on the on-board computer is stuck at 0.7 bar.

From reading this thread and heliolps thread https://rennlist.com/forums/964-turb...ease-help.html it appears the control unit (965 641 381 00) controls both tach and boost signal to the gauge.

From the wiring diagram I’ve ascertained the following:

Interface Electronics
HKZ - bk/gn - to TCU #2 signal td
P1 - br/gn - to Charge Air Pressure Sender on intercooler
Term 15 - orange - to Speedometer
Td - bk/vi - to Diagnosis plug socket #14
Td - bk/vi - to Speed Sensor w/ OBC #21 td
Ld out - br/wt - to Speed Sensor w/ OBC # 11 ld
Term 31 - br - to Ground Point XXI

How can this unit be tested to confirm if it’s faulty or not?


Control unit for on-board computer

fritz k.

You should first check whether the supply voltage is present at terminal 15 (switched plus) and terminal 31 (ground). If that's ok, try to check the input and output signals.
The input signals come e.g. from the HKZ (speed signal) and from the pressure sensor (boost gauge), the output signals go to the tachometer.
To measure the signals you will probably need an oscilloscope; a multimeter with a frequency measurement function should also be sufficient.

Fritz

jd964t

Thanks for the reply Fritz, I hope you’re well.

Ok, I will check supply voltage at terminal 15 and 31. As for the input signals, I assume the speed signal should be measured at approx 15 Hz and the pressure sensor signal is measured in ohms? Is there a specific range?

If the correct signals are present at the control unit base then the control unit itself must be faulty because it’s not passing them on to the tachometer and OBC - correct?

Cheers

fritz k.

Hi JD,
yes right, at least if directly at the output of the interface no signal can be measured. Unfortunately it is not documented what type of signals are generated by the interface and send to the tachometer.
For the input signals, I would expect a resistance of between 10 and 184 ohms from the pressure sensor, which is the range on how most VDO transmitters were designed. The speed signal should be around 45 Hz at idle, based on 2700 ignition pulses per minute at 900 rpm.Please let us know what you found out.

Fritz

jd964t

Quick recap: 1991, 3.3 Turbo. Disconnected battery, removed starter & slave etc, dropped gearbox, installed new flywheel, clutch etc, reinstalled gearbox and reconnected battery. No other grounds touched. Car started immediately and drives well. But tachometer and boost pressure are no longer working (were fine before) and all other functions of OBC are good.

Electrics not my strong point and a bit tricky getting the multimeter probes in there but here's what I found:

Control Unit base
Term 15 (+ve) & Term 31 (-ve) 12.25v ignition switched ON
Term 15 (+ve) & Term 31 (-ve) 13.85v at idle

Hz (+ve) & Term 31 (-ve) no reading at idle (same with probes reversed)

Term 15 (+ve) & Hz (-ve) no reading at idle (same with probes reversed)

P1 (+ve) & Term 31 (-ve) no reading at idle (same with probes reversed)

I did a quick test of Td & Ld outputs but got nothing as expected. Assume the Control Unit needs to be in place and working for output signals.

So other than power I'm not reading any other signals. It occurred to me later to check pin #2 of the TCU for tach signal to the Control Unit base so I will check that next. Otherwise I'm still unsure whether signals are getting to Control Unit base - and if not why not. Or they are and I just need to replace the expensive Control Unit.

One other observation, the OBC is stuck at 0.7 bar with the Control Unit in place and 0.0 bar when removed.

Cheers

peterpullin

let us know if you can solve that. my car 1991 has same issue since rebuilt. all obc functions present - 0.7 bar stuck.

fritz k.

JD, did you remove only the transmission or also the engine? Has the intercooler been removed? Is it possible that the connections for the boost pressure sensor and the overboost switch were swapped?

Fritz

jd964t

Hi Fritz, dropped transmission only. Intercooler wasn’t touched. I’ve checked connection at boost pressure sensor and put a multimeter on its brown wire and sensor terminal, got a reading of 5.5k ohms at idle. If I swapped wires between it and overboost switch pretty sure it wouldn’t start at all?

Cheers