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IF I am understanding this correctly... The trans oil cooler has a thermostat, sitting under the bottom of the engine coolant reservoir? For testing purposes, could I not simply remove the thermostat, and ensure trans oil temps are then significantly reduced? If so, then simply replace the thermostat with a new one?
↑ THIS ↑

It could also be a bad wiring harness on the valvebody. The temp sensor (not replaceable) is part of the harness...
 

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2001 325i Sedan
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Discussion Starter · #42 ·
↑ THIS ↑

It could also be a bad wiring harness on the valvebody. The temp sensor (not replaceable) is part of the harness...
But the sensor appears to be working fine. The readings I got were entirely reasonable, except when the tranny is stressed, it gets too hot.

Right now, I'm leaning towards replacing the trans thermostat (didn't know there even WAS one until today!), then doing a full trans oil & filter change.
 

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Discussion Starter · #45 ·
80C is the nominal opening temperature for the thermostat. Unlikely the trans would actually run at that temp under load. From all I've read, I'd expect actual operating temp to run up to perhaps 110-120C worst case (high ambient, high load). Under milder conditions (lower ambient, low load) I'd guess probably 90-100C. Mine seemed to be stable around 85-90C before I started really flogging it.
 

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If removing transmission thermostat to test, just replace it. Saves you some work, not expensive, and they frequently break on removal.
 

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Discussion Starter · #50 ·
The ATF is typically close to engine operating temperature, in the high 90’s C.
Logically, and scientifically, it would be impossible for it to be anything LESS than that!

The trans fluid cooler is using the engine coolant to cool the trans fluid. The engine coolant, under the best of conditions, runs somewhere around 80-85C. So, there is no way the trans fluid will be below even 90C, as the cooler is a FAR from perfect heat exchanger. The oil temp will certainly be well above the coolant temp. And if the oil was only 90-100C there would really be no need for an oil cooler, as trans fluid can easily handle that. So, it stands to reason the NORMAL operating temp of the fluid is likely above 100C, and well above that under extreme conditions. Based on my cars behavior, I think it safe to say 140-150C is TOO hot, as that is the temperature at which the transmission starts to misbehave. So, I'm guessing the real-world, worst-case "normal" temp is likely something around 120C, and certainly no more than130C.

I now know exactly what temp my transmission runs, under very specific circumstances, which I can easily duplicate. When I get the new thermostat, I will install it, and then immediately duplicate the tests I did yesterday, and see what temperatures I get. I will declare the problem solved if I see nothing over 120C, and that is what I fully expect. I will dance with joy if I see nothing over 110C.
 

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To confirm what the ATF thermostat does, it's actually two thermostats in one when you pull one apart. A single thermostat would either have the cooling path open or closed. This operates differently so it opens one path and closes another to allow non radiator cooled coolant to warm the ATF when cold, and changes this path entirely to cool the ATF when the coolant gets to above 80degC.

First thing I'd check is if the ATF temp sensor is correct. Do you have an IR thermometer. If so, get it up to temp and then measure the temp of the pan. It should be pretty close to the reading in INPA.

If this is ok, like you say I'd be changing the ATF thermostat. 150 is far too high. If it is getting to this temp, it explains completely the slippage. Higher temp = less viscous = less pressure at the same pump output flow. Clamping of clutches is all about pressure.

With regard to the valve body and it this possibly being the issue, besides EDS1, which controls the pressure throughout the entire transmission, there is no electrical input that could change the flow to the cooler. It comes from the TC path, and has a separate pressure control mechanism. To test, remove the ATF lines from the cooler, stick in a bucket and then crank the engine for a few moments. ATF should immediately start coming out. If so, this cooling path is working fine. Obviously fill the transmission afterwards.

For interest, I've put an image I made years ago when I was installing a new radiator that shows the fluid path. It might help.

929502
 

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Discussion Starter · #52 ·
More excellent information! Thanks much!

My plan is to swap the thermostat, and re-run my test, see what tempts I see, and if it misbehaves. If all is well, then I will do a full fluid and filter change.

Since it ran perfectly until the temp got above ~145C, and this is a relatively low-miles trans, I'm feeling reasonably optimistic the thermostat is the problem. We'll know by Friday!
 

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FWIW, I needed to go out so I connected INPA to check temps and took the car for a bit of a thrash. With little load it will settle a little higher than coolant temp. WOT throttle in 2nd only (so the TC clutch doesn't engage) over and over (brake, WOT, brake, WOT, etc, etc) it got as high as 110. Settles back down to normal temps slowly after resumption of normal driving. I'm interested to find out if the thermostat fixes the problem. I'd be surprised if it doesn't.
 

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Discussion Starter · #55 ·
Just removed the trans thermostat, which was, at least, completely intact, and came out quite easily. The new one will be here in a few hours.
 

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My transmission didn’t have any limp mode/clunk or slippage.But curious to see how the your temps go after the thermostat change.
I had this same issue you have with high oil temps. I took thermostat out completely and still got into the 120deg+ zone (but took longer to do so). Purchased a new thermostat and I still would see 120deg+ oil temps.I’ve chosen to ignore the temps these days and just drive the car. Even took the car to a track day for 90+ minutes of track time and still no issue 🤷🏼‍♂️. I still think 120deg is too high for the trans oil though.
Just thought I’d add my 2cents.


Sent from my iPhone using E46Fanatics
 

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Discussion Starter · #57 ·
New thermostat is in. It's a really trivial job - perhaps an hour all in.

I'll beat on it in the AM, but I expect it'll be fine. I tested the two elements in the old one, and the lower element seemed sluggish, and did not always move full stroke.

I'm hoping to see, ideally, something under 100C tomorrow, since ambient temps are pretty mild here at the moment.
 

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Discussion Starter · #58 ·
Sadly, did not get the result I was hoping for. It is definitely better, but I think still not good enough. While driving to my "test site", the temperature came up gradually, and settled at about 95C, After two hard runs up the steep grade, temperature was up to 115C. At that same point before the thermostat replacement, it was at 150C. So it is better. However, after driving, normally, several more miles, it eventually made it's way up to a fairly stable 135C, which seems too hot to me. I suspect driving it hard at that point would have likely triggered a fault, but did not push it to find out.

So, where does that leave me? Is a valve body rebuild, and lots of crossed fingers, the next logical step?
 

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Sadly, did not get the result I was hoping for. It is definitely better, but I think still not good enough. While driving to my "test site", the temperature came up gradually, and settled at about 95C, After two hard runs up the steep grade, temperature was up to 115C.
Slipping clutch plates generated a lot of heat.
 
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