Tranny temp sensor causing Failsafe in 5HP-24?

[-PM-]

Here's a bit of a mind-boggler:

A couple days ago, I was driving along for about 20-30 min and decided to downshift to pass a slow moving car. The revs kicked up and then a thud, and "trans. failsafe prog" came up.
It got stuck in 4th and I limped home.

Let it sit for 45 min and took it back out. Shifts perfectly. After a few minutes, it starts slipping, and then kicks back into to trans failsafe.

So I let it sit for 6 hours, did a quick drain & fill (4 quarts), and took it out for a drive. I got about 10 min of problem-free driving until it started slipping and mis-shifting again. Then goes back into trans failsafe.

So it seems that when it's cold, it's perfect. As soon as it warms up, it will not drive correctly.
When warmed up, it will not move when shifting from P to D (no issues at all when cold). Rev it up and then thud, trans failsafe.
Reverse works fine when cold or hot.

Checked tranny fluid temp with INPA while driving. 120-130C. Seems very high.
But here's the kicker: I let it sit overnight (16 hours to be exact) in a cold 35 degree F garage.
Checked temps in the morning (via INPA):
Engine is 6 degrees C but tranny fluctuates between 20-21 degrees C. Doesn't seem right.

So, bad temp sensor?? Can it cause the improper shifting/slipping?

I'm really hoping I don't need a rebuild.

[jsoto]

Yes, it can. I changed out the harness in mine...

[-PM-]

Originally Posted by jsoto

Yes, it can. I changed out the harness in mine...

Thanks. I ordered the harness. I'll probably take out the pulse generators and clean them up as well. Hoping it works out.

[Junkycosmos]

Might also want to check the transmission thermostat which is on back of radiator. Surely would it not cause the wrong readings you found but might contribute to bad temp management of trans. The sensor or harness fix I bet is more what you need here. Yes the internal temp and fluid level are key to proper shifts.


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[Junkycosmos]

It's part number 20 in this diagram.
http://realoem.com/bmw/showparts.do?...91&hg=17&fg=05

In my case mine was stuck open and had issues taking longer than normal to heat up during extremely cold days. I believe this thermostat controls how much engine coolant flows through the transmission oil intercooler.

If it is the same part used on your X then it is easy to change since the swap only involves coolant pipe disconnects.

I doubt that this could cause your trans to overheat from normal driving and surely would not cause a bad internal sensor reading but might be worth considering next.

Also suggest you ensure the trans refill you did is at the specified temp so the fluid level is not too low. Lots of posts about shift issues after trans fluid refills being done at too high of a temp (so the fluid is too low when it cools off). Probably something you are well aware of given your 1st post.

Gluck and curious to see your fix



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[RRPhil]

By far the most common problem I come across when repairing 5HP24s is the failure of the axial bearing between the B-clutch hub and the C-clutch drum



which allows axial movement in the input shaft assembly leading to the O-ring seal on the A-clutch to split



When the fluid is cold, the oil pump is able to keep up with the leakage through the seal so the transmission operates normally. After around 20 minutes of running though, once the fluid is up to temperature, the reduced viscosity of the fluid means that the leakage is so high that the pump can no longer maintain pressure on the A-clutch and it slips.

The A-clutch is used in 1st, 2nd, 3rd & 4th gears so the transmission controller has no option other than to engage 5th (usually with a bang, because it doesn’t have an accumulator to damp the engagement) and flag the transmission failsafe message. The A-clutch isn’t used in reverse.

When the transmission cools down it functions perfectly again and then the whole cycle repeats. Do you think that this might be the same problem that you are experiencing? Might it be 5th gear that your transmission is engaging when it flags the failsafe message rather than 4th?

You could easily test the PTC sensor, which is built into the electrical wiring loom, just using a multimeter rather than risk spending money on parts that you don’t need which could be put towards a rebuild





The temperature signal is transmitted via pins 13 and 14 on the electrical connector at the back of the main casing, and the resistance between the pins should follow this characteristic :



Phil

[g300d]

^Wow. Fascinating info, thanks for sharing!

[-PM-]

Whoa. Lots of useful info! Thanks!

I ended up pulling the harness and the output speed sensor crumbled. I have ordered a new one and should be getting it in today. I have a new harness as well.

I'm hoping that it's not the axial bearing. I really don't feel like pulling the tranny in the middle of winter.

[-PM-]

RRPhil - My issue is similar to what you described, but it happens after 7-8 minutes of driving (after warmed up from completely cold). And it goes into 4th for sure. I noticed my rpm's are higher than normally in 5th at 60mph.

[-PM-]

Well....bad news. No luck.

Replaced the harness. Fresh fluid. Still same thing.
Felt great for about 15 min, and then started slipping gradually until I couldnt really move from standstill. It will start moving a little and then, with a light bang, it'll pop into 4th (or maybe it is 5th) and trans failsafe pops up.

RRPhil, do you think its that bearing?