PIWIS hell - Mileage increasing but no speedometer!

[geordiepingu]

Hi all

On this episode of f*** around and find out...

 

The story goes that a particular coding attempt failed (errored out) and I've been fighting the good fight since, when trying to re-code some features for PCCM+ in development mode. I've since deleted all codes, reprogrammed the gateway for good measure, re-coded from scratch by comparing with an old vehicle analysis log in PIWIS and now have the following REALLY bizarre issue.

 

If I turn the steering very fast or roll the car backwards or forwards, my odometer begins to clock up extremely rapidly. The speedometer - both analogue and digital stay at 0mph or 0kmh. PSM detects movement in the wheel speed sensors and reports what seems to be reasonably accurate.

 

Once the odometer starts, it will continue clocking up while the ignition is on. If I turn the key off then on, it continues clocking. To stop, I have to reset the unit by pulling the fuse (F1 Row B if memory serves - or maybe C, it's a 15 ampere) or disconnect the battery.

 

I've spent the past day reducing codes down to a known error with my door handle and still scratching my head where I am going wrong with this. For a while I thought the steering control module was knackered until I got rid of the codes - perhaps it still is - all sorts of CAN codes for being unable to read drive and comfort of the module. Seems to have disappeared however and all functions on the wheel and stalks are working correctly.

 

The tachometer, fuel gauge, temps and pressures work fine, as do the lights and display when tested.

 

Has anybody got some advice on where to start?

 

Many thanks

 

Video of odometer: https://i.imgur.com/mKz4jEf.mp4

Edited July 23 by geordiepingu

[geordiepingu]

I have some new findings, in that it looks like there's speed being picked up by the PSM CU (control unit), but not being distributed over CANBUS. When the car rolls, not even the convertible top or the climate control gets a speed signal. Presumably this lack of signal when the front CU and PSM CU detect movement is causing chaos to a few other CUs.

I've isolated out the front CU, DME, rear CU, park assistant, steering control modules to find I still get the mileage issue. Naturally taking the fuse out for the instrument cluster and then subsequently replacing it makes it stop, until I roll the car again (get any wheel speed in any of the inputs). I did also try a gateway from another vehicle without coding or programming it to see if the issue can be replicated - and unfortunately it can. I did code the unit with a new VCI - not trusting my old one given what's gone on - and unfortunately same issue.

I saw a diagram somewhere that described how the PSM deduces wheel speed and sends it on to the bus - it may have been from a chap diagnosing his tiptronic unit on rennlist or planet 9, but hell if I can find it again. It would be useful to double check with an oscilloscope and my CAN bus dumping tools to make sure the PSM CU is not sending out a signal before I just try and replace it. I did pull the PSM CU and of course it meant I had no issues with the cluster, but then of course, nothing was getting vehicle speed!

I should also add that I've had my scope on and CANBUS dumper to see if there's any dodgy signals or shorts to ground in the CAN Drive circuit (albeit with the PSM module unplugged...) and without further specification or workshop documentation, that doesn't show any obvious signs of anything wrong - no packet loss/malformed packets or shorts to ground pulling the signal down.

TL;DR: No more fault codes, my educated guess is I've had bad timing and done something in on the PSM CU. Would appreciate any documents (rather than just PIWIS wiring diagrams) that could help me with my diagnosis. Many thanks