[glennvdb]

Thanks both for replying!

It is indeed my strong suspicion that it is an electrical issue and I think there is a probability that it can be solved without any heavy work on the gearbox itself.

The specialist just replied to me that the only thing they can do is replace the entire gearbox, either by a new one or a revised one. Both interventions would cost me well over € 10k.

Well worth getting my hands dirty myself trying to find the real root cause before taking more drastic measures.

I'll keep you posted.

 

[glennvdb]

Today my car was delivered to a workshop where I will be allowed to investigate the issue together. Much to my surprise I could drive the car off the trailer and into the building without any messages in the dashboard, and without any remaining codes in the (non-Porsche) diagnostic computer. All 4 shift rods where indicating 3 mm as their position value with the engine turned off.

Everything seemed normal... Who would have thought!

Together with the enthusiastic young mechanic in this workshop otherwise focussing on classic car racing, I hope to find a bad connection in one of the connectors or a damaged cable ...

Sooner rather than later I hope to be able to give you all an update!

 

[glennvdb]

So, here we are, with a perfectly functioning car after absolutely no changes to the gearbox at all ...

Yesterday I went to the workshop I mentioned before to try and find the root cause of the issue I had.

• We connected the Diagnostics computer to the car and monitored the shifting rod positions while jiggling the wires around the connector on the transmission, trying to trigger an unreliable contact, but the values didn't budge
• A visual inspection of the connector on the gearbox showed clean, non-corroded contacts on both sides.
• The same was true for the connector on the Transmission Control Unit
• We measured the continuity of the ground through the cable from the connector on the TCU to the gearbox and also this value was perfect.
• Also the PWM frequencies of the signals coming into the TCU while the car is running, were perfectly fine (2 times 1 kHz and 2 times 850 Hz)

So we weren't able to identify any faults in the cabling and signalling with the car on the ramp.

The next step was to go and drive with the diagnostic computer logging the relevant data, and much to my surprise the 20 minute drive with the tow truck in standby passed without any significant events (except for me getting emotional on how much I missed driving the car in the past 3 months!). Also looking over the data, we didn't find any strange behaviour for the monitored signals.

So this is more or less the worst possible outcome: we didn't find any issues and the car worked fine during a 20 min test drive, while I know that there is a problem lurking somewhere in there to rear its ugly head at the worst possible time. I cannot trust the car and I don't know what to do to increase my confidence again.

This evening or tomorrow I will take the car back home to free up space in the small workshop where it was in the past few weeks, and then the winter season starts here in Italy, giving me some time to contemplate on what to do next.

What would you guys propose?

Below some graphs from the report of the test drive (low quality; I couldn't get hold of the raw data ...).

The only thing I observe is that the values that are shown, are slightly offset. The three nominal values appear to be -5, 3 and 10 mm, so not centered around 0 mm. Maybe this is due to the fact that this is not a PIWIS but a generic Diagnostics computer being used?

 

[glennvdb]

So, I finally got the courage to get out the oscilloscope and had some stressful, nerdy fun measuring the signals between the transmission controller and the sensor itself. Quite nerve-wracking to say the least, trying not to create shorts and bend pins on the connectors of the transmission controller.

Again, also here, I did not find any obvious problems. The signals look more or less ok and also the duty cycles look nominal. Below I'll post some details on how I measured and what I saw.

My next step is to ask my independent Porsche specialist to measure the signals using the official PIWIS system to validate whether the control unit indeed reports the 3 mm offset I saw in the data of the OEM diagnostic tool. If so, a full calibration of the gearbox might be able to solve the issue.

If that doesn't solve the issue - I will break down again and - I will take the plunge and replace the sensor inside the gearbox.

Any opinions or advice appreciated, especially from people who have measured these signals before!

Measurement setup

The trim of the trunk of my car was still out after the first diagnostics session. At that time we pulled out the 3 trim pieces lining the trunk. Required some awfully high force, but nothing seems to be damaged right now. This is where the TCU lives:

To get easier access, I removed the clip holding the cables to the support of the TCU (requires some prying, holds on using barbs). I then diconnected both connectors. Obviously this is done with the car off and the key out of the car.

Then you can remove the TCU from its tray pushing down the two latches holding it in place.

The next step is to disassemble the grey connector to be able to remove the hood, preventing access to the rear of the connector to insert the measuring wires. Taking the connector apart requires opening two latches and then the two subconnectors just slide out. Surprising to find a BMW labelled part in a Porsche, by the way!

Then I very carrfully inserted the individual subconnectors back in the TCU connectors, very careful not to bend any pins (which I actually did but was able to repair).

I prepared 5 wires that I stripped on both sides qnd carefully inserted them in the back of the connector. This is the signal map:

• Pin 4 - Brown wire with black stripe - Signal GND
• Pin 21 - Blue wire - Position signal 1
• Pin 32 - Blue wire with white strip - Position signal 2
• Pin 33 - Blue wire with green stripe - Position signal4
• Pin 34 - Blue wire with red stripe - Position signal 3

This is what that looks like:

Putting back the black connector and connecting up the oscilloscope, made my setup look like this (yes, very scary indeed ...):

With the setup physically installed, I carefully started the scope, checked for a good ground with the multimeter. Then I put the key in the car (no sparks, smoke or fire), turned on the ignition (all good, signal appear on the scope) and turned on the engine (no codes on the dashboard!).

I was good to perform some actual measurements, even if limited in the gears I could select, as indeed the car had to remain stationary.

The measurement results

It appears the calibration/tuning of my oscilloscope probes was off. correcting that and I'll replace the graphs below as soon as I have better data!

So, indeed, I measured the usual 1 kHz and 800 Hz PWM signals. This is what it looks like in P:

In R:

In N:

And in D-1:

A "larger view" of the signals looks like this (back in P, if I remember correctly):

In tabular format the duty cycles look like this:

Gear level position	Sensor 1 DC (%)	Sensor 2 DC (%)	Sensor 3 DC (%)	Sensor 4 DC (%)
P	50,57	49,90	78,05	51,56
R	50,56	49,90	49,95	87,03
N	50,47	49,90	50,00	51,37
D	50,47	49,90	78,42	22,95

Interpretation

As stated above, I don't see any major issues with what I measured. A few observations:

1. The PWM signals are nicely centered around 50%, + 25%, -25%. The only exception is maybe the 87% of sensor 4 in R
2. The signals all seem to be quite repeatable. When the axle is in the same position, the resulting signal is very close to each other.
3. I don't like too much how the yellow signal looks (but also the others to a lesser extent): There is a clean edge for the most part, but then there is this capacitive-effect like slope which might or might not cause issues

For me this all means that the sensor at least with cold gearbox oil (engine ran idle for about 20 minutes), seems to work just fine.

I'm especially curious about what these signals look like in other cars. If there is anybody around who has performed these measurements as well, please share!

Next steps

I left the probes and cables attached to the car for now. I've seen that the scope I'm using has some features to analyse acquired signals in a more advanced, statistical way. Maybe I'll have a play with that this afternoon or tomorrow.

As I discussed at the start of this post, I now would like to use a PIWIS to validate how the TCU is calibrated for these signals. If all signals are all shifted by 3 mm, then only a small real signal shift can cause the firmware to think something is really off. And if I can calibrate that 3 mm out, potentially I have more margin before this actually happens.

Then I will start driving the car. If the issue is solved, I will be a very happy man indeed. If not, I will break down, curse, get the car to the small workshop and start working on replacing the position sensor from within the car...

And then, for sure, I'll curse again when having to pay the bill, but will be able to cruise around in my lovely 981 once again.

Hopefully ...

 

[glennvdb]

Hello all,

Almost 7 months since my last post here. I know it's not good practice to revive old threads, but I thought it would be better to keep all information concentrated in one thread.

After my investigations last winter, at the start of spring I carefully started driving the car again more regularly. At first a wee where I used the car as my daily, pootling around commuting and going to the shop. No issues at all. Hopes getting up.

And they kept going up as I did some more spirited driving in the Italian Apennines which are literally in my backyard. Afraid at first, but opening up the taps my confidence increased and a big smile appeared on my face again. At last.

Then a big morning with some motoring friends (an E93 M3 and an Alfa 4C complete our small club of driving enthousiasts) consisting of 3 hours of driving pleasure went by without any problem.

Also yesterday I crossed the Apennines in a 3 hour drive to reach the Tyrrhenian coast on the other side. The car behaved perfectly, also on the way back home over the autostrada at a more docile speed, even if I registered 40,5°C outer temperature and the aircon was struggling to keep the car cool - I now remember that a new condensor was part of the todo-list once the gearbox issues were solved.

All was good and plans for some trackdays and roadtrips later this year when the Italian heat has subsided, were forming.

Until this morning.

I wanted to leave for the gym and the car started up with the dreaded messages that started all this: "Reverse gear not available", "Start/Stop disabled" and on a second restart also "Motor management issue". And of course, no gear selection possible. Pushed the car out of the garage and took out the Skoda-barge (which I equally enjoy, albeit in a completely different way). A second workout later, the Porsche is now back in the ever so slightly uphill garage, still no gearbox.

So, back to where I was almost exactly one year ago. This time, however, not stuck in Switzerland, but in the relative comfort of my own garage at home. Also feeling lucky it happened on my driveway, instead of on the highway at 40°C ...

Now the hunt starts to get access to a computer to reset the errors, hopefully to re-enable the car to get going again, as happened last time. And the troubleshooting process starts over again.

The only thing I can think of other than replacing the position sensor inside the gearbox, would be to replace the (still original) battery, which might cause intermittent issues on long drives, due to the charge controller saving some fuel by disconnecting the alternator every now and then.

Opinions?

 

[glennvdb]

Hello all, I finally got access to a decent code reader and was able to reset the codes and drive the car.

The code I cancelled was P1732, the implausible signal from the shaft for gears 5 and 7.

Different from the errors I received initially, but still related to the distance sensor.

Time to get my hands on one of the sensors from @jjrichar , I would say ...