Transmission heat exchanger regulator - stays in 3rd gear and hard downshifts.

**dville** · 07-20-2015, 08:51 PM

Anyone on here done this fix for overheating trans on e38?

http://www.bimmerforums.com/forum/sh...iT)&styleid=50

**dville** · 07-22-2015, 12:54 AM

Originally Posted by MLue1

My story and other info

My story and the reason that I put this write up togeather. About a year ago last spring my 540iT started down shifting quite hard during stop and go driving; ya I know what you’re thinking but this was like suddenly hitting a wall at low speed, scary as hell when it surprises you. A few times it went into "fail safe mode", and would only reset after sitting for an hour or so. Initially we thought that I need a new transmission and even one reputable shop determined as much. After some research, we concluded that the most likely causes were either the thermistor in the transmission harness acting up or the transmission was worn out and require rebuilding. We ruled out the transmission thermistor, when my buddy David used his INPA software and OBD cable to confirm that the tramsmission harness thermister was operating correctly and that the transmission was not being cooled at all. We monitored the operating temperature of the transmission using a laptop with the INPA software loaded and cable connected to the OBD port inside the car, we watch the transmission temperature climb steadily to 272 degrees F, while I drove around in stop and go traffic, with shift quality degrading as the temperature rose. FYI the operating temperature of the transmission is supposed to be about the same or lower than that of the engine.

When we removed the tramsmission cooler Regulator using the above procedure, we discovered that "cool side" passage on the Heat Exchanger was plugged by a clay like substance (see pic). Basically, the cool coolant from the rad could not flow because of this substance, hence no cooling. I used a flat head screw driver to remove as much of the substance as possible, back flushing initially did not work but back flushing with compressed air got things flowing. I then hooked up a garden hose with a reducer and back flush the rest of the substance, if I could not accomplish this I would have had to replace the Heat Exchanger (BMW Part# 17217505823). As mentioned in an earlier post, the Heat Exchanger/Regulator is located at the lowest point in the cooling system and the coolant passages are small, so any substance in the cooling system will likely wind up in this area.

The +95 degree heat wave the last few days has basically validated the success of the repair, the transmission has gone back to operating as it did when I first got the car.

Lesson learnt:
If you have this or a similar problem(s), try changing the coolant Regulator or service the automatic transmission cooler first. It's better to spent hundred(s)...instead of possibly thousands of $ replacing a transmission unnecessarily.

Symptom of a Transmission Running Hot

-The transmission would down shift un-expectedly with a jolt.
-Holds on to the lower gear when accelerating longer than expected or sometimes down/up shift by itself.
-When driving in stop and go traffic, the transmission will lock into 3rd gear and will shift neither up or down. This is a sign that the transmission is locked into “fail safe mode” in response to an abnormal condition.

post

**fingersmazda** · 07-22-2015, 02:40 PM

I've had the voltage regulator replaced; and the mech blew out/cleaned the heat exchanger. Cured my problems. May not cure yours - could also be a tranny temp sensor (excluding possible larger issues). There are some write ups on this and how to distinguish between the 2. Basically you need to take the tranny temp directly (INPA?): if it's hot, it's likely an exchanger/regulator issue. If it's not hot (but still gives you problems), likely the tranny temp sensor (thinking the tranny's hot when it's not).

I tried to do the fix myself based on the post you referenced. I'll say on the E38 it is more difficult to get to the heat exchanger and voltage regulator; I gave up and took it to my mechanic. Think my mech took off the front bumper to get to it?? Removing the fender liner really didn't give sufficient access. That being said, I'm not a great mechanic, so you may do better than me.

**Grantnz** · 07-24-2015, 07:10 PM

I had the same issue a few years ago and had tracked down that same post dville linked to. I cleaned out the heat exchanger and briefly contemplated removing the thermostat mechanism from the regulator, but that isnt possible because the body is plastic. The repair worked, but the problem returned in the summer with the higher temperatures. I finally got around to ordering the replacement regulator from Pelican parts last week and will fit it in the near future. The reason I have tolerated the issue for so long, is that the E38 is not my daily driver and on the shorter trips I usually do, the problem never arose.
The previous work was done through the left front wheel arch, there was no need to come in from any other direction, and no need for me to remove the front bumper.

**dville** · 05-28-2016, 02:00 AM

I actually have not had this problem return after flushing the transmission heat exchanger and replacing the transmission cooler thermostat also known as the "voltage regulator" in the parts database.

- - - Updated - - -

**Grantnz** · 06-01-2016, 03:58 AM

Yep, replacing the thermostat solved it for me. I washed out the coolant side of the trans cooler while I was in there. I never disconnected the oil lines, although I will be doing that when I do the trans oil and filter replacement in the near future.

**Andrei7** · 06-18-2016, 07:59 PM

HELLO can somebody help me with this , I keep getting Trans Failsafe Prog, and the car jerks when I put in reverse, I crawled underneath the car and i looked at the transmission and I found this broke wire, can somebody give me some detailes about it or where i can purchase the whole cable?
[IMG]file:///C:/Users/Andrei/Desktop/nnnnnnnnnnnnnnnnnnnnnnnnn.jpg[/IMG]

**haynes740** · 06-20-2016, 02:17 AM

Good post gonna check mine cause I've got the trans fail safe woes as of yesterday. Was driving hard on Mullholland Dr. So Cal yesterday.

**Grantnz** · 06-21-2016, 05:46 AM

Originally Posted by haynes740

Good post gonna check mine cause I've got the trans fail safe woes as of yesterday. Was driving hard on Mullholland Dr. So Cal yesterday.

Check your battery condition/charging first. These trans are very fussy about battery voltage.

**haynes740** · 06-21-2016, 07:57 PM

Originally Posted by Grantnz

Check your battery condition/charging first. These trans are very fussy about battery voltage.

Battery and charging system are good. Indi is saying transmission is on its way out. Said I should start looking for a replacement. Would like to convert to the step tronic with a 3.15 differential.