I was guessing like that before as the new trans fluid is dark brown within 3,000 km. So I decided to replace a new transmission thermostat - Dayco brand (the one sitting between expansion tank and heat exchange). The new thermostat has 80C spec. I did run a test for the new and old thermostat by boiling water with infrared gauge. The old one (genuine BMW) and new one started to open at 80C (not full). At 90C the new one opened fully while the old one opened just half way.

Based on this I put the new one in the car. However this didn't fix the issue I have.

On my last trip (as I mentioned earlier) and with the TFP popped up, the trans. temp was around 96C and engine was 66C, I stopped and turned off the engine on 10 separate occasions. But it didn't fix the problem. Every time I turned on the car it started normal until the gear selector was switched to D (even without engine running). So then I decided to move the selector between P, R, N only for around 3 - 4 times, and then moved to D. It didn't trigger the error. So I don't think the thermostat is the issue here.

66C is way too low for the engine temp if you've driven more than a few kms- at that temp the DDE will still have the engine in warm up mode, while the transmission is pretty much up to temperature. The engine temp should be around 85-95C in general running.

Failure of the engine coolant thermostat is a common problem in the M57 diesel.

Wpoll, It looks like when I the outside temp is below 10C and I am on highway (above 80 Km), my temp will hover around a quarter mark only. Last January, I was taking the car to Tasmania where the temp was 10C, a bit drizzle, the speed was 110 km and I was driving non-stop for more than 5 hrs, the temp. was only a quarter mark.

So you reckon that due to my DDE is still in warm up mode, while the trans temp is up to the working temp that would trigger the error? My mechanic told me that once a diesel engine temp is on half, it would never go down - unless the engine stops.

Took our X5 on a trip around the South Island last summer and had no issues - temp gauge sat at 12 o'clock all the way. Then during the winter I noticed it was struggling to get to 12 o'clock, even pulling a trailer load of firewood. Checked the temp via the BC and noticed that most trips it was never getting past about 66 Deg.C.

Changed the thermostat and everything is back to normal. Temp gauge is back to 12 o'clock now. Car is using less fuel, blows much less soot and feels smoother.

Could it cause the trans code issue. Quite possiby but I never left it long enough - I was lucky to spot the failure pretty early. :dunno:

Not sure about the your mechanic said but if I watch the temp via the BC it does vary up and down a little - in the 85-95 Deg.C. range. Gauge never moves though. It's designed not to move once the temp is in the "normal" range. It's not a linear gauge like you would expect.

Looks like your next job is to change the thermostat - if you do this yourself, I wrote up the process on here somewhere - I'll post a link if you want. :thumbup:

Please send me the link. I'll fix the engine thermostat & change the trans fluid again as I'm not feel confident with the darker colour fluid although just been changed less than 3k km. I'll update you the result later on. Many thanks.

I haven't done a step-by-step write-up (yet) but I have outlined the process in a couple threads.

The basic process, with links...
http://www.xoutpost.com/1084162-post23.html

And some pictures of the job on my car...
http://www.xoutpost.com/1079662-post7235.html

HTH.

Wpoll, just an update as I have replaced the thermostat. The engine temp is quicker to get to half mark now. However after driving it around 20 minutes with the engine temp was half mark (or around 87C) & trans fluid was around 91C, I got the same TFP message again.

Don't know what to do now. The only error I have on my portable scanner is 'gear selector switch' - which I have replaced with a used unit.

Well done for sorting the engine temp. I was kinda hoping (like you) it might resolve the TFP - I guess not. But it needed doing anyway.

There was another forum member that had a similar TFP issue after (doing an amazing job of) rebuilding his V8 - not sure if that thread ever was resolved - I'll hunt it down.

If you clear the gear selector error, does it return? If so, perhaps that could stand further investigation.

Found the thread - it was williamx5 that was rebuilding his V8, along with his 6HP26X trans. Doesn't look there's a conclusion yet but it's been quiet for a couple of months...

http://www.xoutpost.com/bmw-sav-foru...roject-13.html

When the error appears, I turn off the engine and wait until the TFP message disappears and re-run the engine again - it will start normally. P light is on and the same with R/N light is on when I switch to R/N. Only when I switch it to D, then all the gear light disappear and TFP message pops-in.

If I switch it to N or P, and clear the gear selector switch error, it will back to normal again. So I can clear the error at this point. However, when I move the gear selector (whether with engine on or only the key on) to D, then the TFP pops-in and all the gear light is gone.