5HP24 Fault Code 34 Sump temperature

[wayneawh]

Long story short... My 2000 540ia has ZF 5HP24 transmission and it had failed a little bit more than 2 years ago.
At that time, it failed due to no reverse and losing forward drive. This was expected as purely mileage (~270k km)and age related.
Sent to a reputable transmission shop and rebuilt the whole transmission.
Transmission was working fine for approximately 8 months and started to experience the following symptoms:

1. Sudden downshift from 5th to 4th while cruising
2. Will stuck at 3rd and rev to high rpm at cruising speed/low load
3. Won't get out from the above 2 issues unless car is restarted or allowed to cool down

I took my OBD scanner and got transmission fault code 34 ATF sump temperature.
However, due to the recent maintenance, I highly doubt there was really a problem with the ATF temp.
With a IR temp gun and checked against scanner reading and confirmed there is a +20 deg C offset.
IMG_4703.jpgIMG_4702.jpg
Called the transmission shop, they did the same diagnostic with came with the same conclusion that there is a problem with the temp sensor
They replaced the harness under warranty.

Car was working perfectly until a few days ago.
The same symptoms came back and this time is same... with a +20 deg offset.
This time I am out of warranty already and started to think that maybe the transmission did not put in a new harness last time.
So this time I decided to DIY and get this right. Ordered harness PN 24341423719 from FCP euro and ATF.

Crack open fill plug
Unscrew drain plug
Remove pan bolts
Remove the 2x T27 bolts holding the clips the front and rear speed sensors
Unplug the speed sensors
Unclip connectors to solenoids
Flip the main connector clip down and unscrew it
Push the main connector inside the case to remove harness
Reverse the above procedure to install new harness
Follow fill procedure and make sure fluid level is good (better just slightly overfill)

The trick to insert the main connect back through the case is to align the single tab to the notch inside the case.
The notch is at a slight angle and you will see the notch when you shine light inside the case.
Remember to lube the double o-rings on the main connector with ATF.

The culprit of the all fault code 34 ATF sump temp..
The yellow connectors confirmed my theory that the transmission did not put a new harness.
New harness connectors are white in colour.
The sensor is filled with gunk and probably affected the resistance. Hence giving +ve offset readings.
IMG_4728.jpgIMG_4727.jpgIMG_4729.jpg

It's been 2 days after installing the new harness and there is no more offset in ATF temp reading.
And the ATF temp is very consistent at ~90 deg C.

If your 5HP24 is holding at 3rd or 4th gear with no trans safe prog/CEL and got fault code 34 ATF sump temp, this is very likely the cause and only a new harness can fix it.

[Jaaap]

Very informative. Thanks!