[Owen81]

Hello,

I have a 2009 335d, fully deleted with the EGR cooler removed and "bypassed" with a tube as was common with the early delete packages. Ever since I deleted a few years ago and started monitoring temps, I noticed my transmission temperature never seems to get up to operating temps (88°C) during my commute. I chalked this up to having a 20-30 minute commute in Canada where the ambient temps are usually fairly low. However, I thought the coolant/oil heat exchanger would be heating the ATF until they equalized, then it would "cool" it down to engine temp as they called this a "heat exchanger".

I was monitoring it extra close this past week as I had completed a full trans service including sleeves and solenoids and had reset my trans adaptations. I was going through the long process of the adaptation procedure and it took my transmission about 45 mins or so to get up to ~80°C with ~1°C ambient temps.

After I finished two iterations of the second part of the procedure I figured it would be enough as it was shifting beautifully so I did some full power pulls to see how the trans reacted. I started logging and noticed my trans had dipped to ~65°C and wasn't climbing anymore, even after a few full power pulls which was perplexing as it should have at least held at the coolant temp?

So I did some digging into the coolant system and found the following:

Opening the thermostat in the gearbox oil cooler
The automatic gearbox oil is cooled in the cooling circuit. A thermostat is installed in the gearbox oil cooler. The thermostat is opened or closed on a temperature-dependent basis.

>
E81, E87, E90, E91, E92, E93

The thermostat regulates coolant volume that flows through the gearbox oil cooler depending on the gearbox oil temperature.
The thermostat opens above a gearbox oil temperature of 92 °C

So this states that the coolant flow through the coolant/ATF heat exchanger is regulated by the gearbox oil temperature, and only allows coolant flow when the ATF hits 92°C. So it isn't really a heat exchanger in that sense and more of a cooler.

This brought me to a few questions:
If the coolant is 88°C how could it cool the ATF to ~65°C and keep it there?

What is the point of the thermostat in this equation at all? Wouldn't it be better to just have it heat the ATF up to engine temperature?

Is the T-stat in my heat exchanger malfunctioning? If it was stuck open, this would heat the ATF faster? Perhaps it is stuck closed but this wouldn't really explain the temp drop I experienced.

Is it better to block the EGR cooler coolant flow ports, and is this affecting the system? It looks like it bypasses the T-stat but according to the functional diagram the coolant goes to the same point in the system regardless (cold side of engine T-stat). See below:



Here is a screenshot of the logs that day. NOTE: I edited the spreadsheet to have all of the pulls together, so it looks like they were back to back but it was actually over a time frame of about 10 mins.



Hopefully someone has some logs to compare to or other insights.

Thanks for reading!

[ranthum]

For a transmission lower temps are better. Also, there no thermostat that I'm aware of on the transmission heat exchanger. Also, the coolant goes through the lower portion of the rad prior to entering the heat exchanger.

My theory is that the roughly 90c coolant travels through the lower portion of the rad where it cools substantially prior to entering the tranny heat exchanger. I think you're trying to solve a problem that doesn't exist.

[Mistercamp]

There definitely is a thermostat for the transmission, at least for the e70.
https://www.realoem.com/bmw/showpart...81&hg=17&fg=30

[Owen81]

There is a thermostat internally on the 335d exchanger as well, #4 on the diagram above.

Any non-deleted cars happen to have a transmission temp log with ambient around freezing?

[robnitro]

No log, but on normal 20 min local commute with some stop and go, I hit usually 50-60 C in freezing temps.
The transmission doesn't create much heat unless you're driving with unlocked torque converter to which is only for up to around 15mph.

[czrny]

Hi, did you manage to figure this out? Having exactly the same issue. After 30 minutes of driving (outside temp - 5 degrees C) engine oil temp is stuck at 82 degrees, gearbox oil temp at 60. It took some pulls and 45 minutes of driving to get gearbox temp to almost 80 degrees. I'm suspecting gearbox heat exchanger thermostate.