X5 4.6is done 104k. First time I drove it home from buying it I found it had the harsh 2-1 downshift issue and also came up with a transmission code 'too much slip'.
Car went in to have the torque convertor replaced under warranty and have the transmission fluid and filter changed.
Got it back....and unfortunately the harsh downshift was still there. Checked the transmission fluid which was red (incorrect) and the sump had never been removed so filter had obviously never been done.
After complaining the car went back in to have the transmission fluid changed and filter done properly this time and I insisted they use the proper fluid which they ended up getting direct from BMW (more fool them!).
Got the car back....harsh downshifts were still there although they did seem to not be as bad - might have just been my imagination!
Anyway....I decided to investigate further myself and found that as soon as the transmission temperature rose to above 95degrees the harshdown shifts started.
First port of call was to replace the transmission thermostat which I did but unfortunately it made absolutely no difference.
I then decided that if I was going to make any signficant reduction in the transmission temperature then I was going to have to fit an external oil cooler. So this is what I did.
Parts used...
2 x -8AN Push on fittings (1x straight, 1x 90 deg)
4 x -8AN Fittings (2 x 90deg, 2 x 45deg)
4m -8AN Braided Hose
1x 19 Row Oil Cooler (-8AN Fittings)
1x Mocal 80deg -8AN Oil Thermostat
2x Rubber Bobbin Mounts
I basically cut into the return line of the standard piping system and used the push on fittings to attach to the thermostat then fed the cooler off the other side of the thermostat.
I'll let the pics do the talking...
...and the real test.
This was before the cooler was fitted - a typical 45 minute, 20 mile journey into work where the last 5 miles is stop/start urban type driving.
That's the transmission fluid sat at 107 degrees.
....and now after the cooler has been fitted..
That's the transmission fluid at 80 degrees - a differences of 27 degrees!
It usually sits around 76/77 degrees whilst cruising and only rises slightly in traffic - basically sat more or less on the opening temperature of the thermostat.
Has it made a difference to the shifts? Yes, yes it definitely has. Harsh 2-1 downshift has completely been eliminated. All gearchanges are totally smooth and seemless throughout. Massive improvement all round.
Would I recommend this mod to others? Yes I most certainly would. Anything to extends the life of these gearboxes. Even if it means I only get an extra 10k out of it it's definitely money well spent. It's even a good mod to do on a car with no transmission issues - cooler transmission fluid definitely makes for a happier gearbox all round - which is probably why nearly every automatic vehicle since the E53 X5s runs a separate transmission cooler.
