Sorry to bring this post back up again but I've got identical issues and I'm contemplating fitting an external transmission cooler. What I'm wondering though is has anyone changed the standard one before doing this?

My story goes like this...

I bought my X5 4.6is around 3 months ago on 99k. I got it back from picking it up and immediately clocked onto the harsh 2-1 downshift.

I scanned the gearbox for codes and it came up with the ‘too much slip’ code. Car went back under warranty to have the torque convertor replaced and the gearbox oil and filter changed.

I got it back but was still getting the harsh downshifts. I scanned it again but nothing came up.

I got under the car and had a look and could see the gearbox sump hadn’t been disturbed at all and so the filter hadn’t been changed. I took a sample of gearbox oil too and found that it was red and so they had used standard ATF fluid rather than the proper Mobile/Esso fluid. I complained and they agreed to take it back and do the filter and oil.

Unfortunately due to circumstances and time they didn’t get the car back for another couple of months or so and I’d done 3k in the car in the meantime.

I regularly tow with the X5 and the week before last I was towing a car trailor and the transmission got ‘stuck’ in 3th gear for around 10 miles. Nothing I did worked to try and get it to change – stopped the car, switched it off, holding it at certain revs, tried to go through the gears manually nothing worked. However after around 10miles it just changed into 4th then 5th by itself and then continued to work fine.

I scanned the gearbox again and got codes ‘3 Pressure Regulator EDS 3’ and ’34 Transmission Oil Temperature Sensor’. I hoped that the filter and oil change would rectify this the following week.

The car went back in to have the fluid changed for the proper spec and the filter got changed.

This weekend I towed again and again after around 2 hours of towing it again got stuck in gear – this time 4th gear only this time it continued until I got to my destination a further hour or so later.

After a bit of reading I found out about the transmission cooler thermostat. I got one ordered and fitted before I had to tow home again. Turned out it was the original one. I towed home and this time it was perfect with no hanging in gear although there is still a slight harshness with 2-1 gear after about 40mins normal driving. This to me still indicates a possible temperature issue especially as it’s perfect before this.

So I’m now contemplating replacing the standard cooler incase it’s slightly blocked and/or possibly adding an additional one too just to be safe especially considering the towing I do.

Any thoughts anyone?