I have done the MB transmission service a few times in the past (drop pan, replace filter/sleeve/gasket, refill per procedure). It has always been difficult to get the transmission temperature up to 80 C; generally takes longer than I expect. When I have checked the level on my XJR just as an occasional precaution it's usually right at 80 C after I've driven it awhile. I've never seen it higher.

Today, on a whim, I decided to check the transmission fluid level after driving to my nieces graduation and back. It was a fair amount of stop & go driving with a couple of long lights. The engine temp never went above 205 F and settled at 203 F when parked in the garage (it's between 194 F and 198 F generally while driving). When I checked the transmission fluid level it seemed a bit too high, even though it was perfect the last couple of times I checked it.

I decided to check the transmission fluid temperature and it was about 85 C.

My question: Is 85 C too high? What is considered running "hot" for the MB transmission fluid?

It shifted perfect and was wonderful to drive today, so certainly no performance problems.

Thanks,

 

How are you checking the gearbox temp????????


Here is a TSB for checking the temp through the CAN bus (via the OBDII connector).


bob

 

Thanks for the link, Bob.

I use a method I learned reading a number of threads on the 722.6 fluid change procedure. I use an infrared thermometer. I've tested it in various ways on "known" temperatures and found it to be very accurate. I do expect that the average temperature of various points on the surface of the pan (the method I use) is probably still a few degrees off from the fluid temperature but accurate enough to get the fluid level pretty darn close.

Since I had no "trans temp" warning lights and everything worked perfectly it really just raised the question "what temperature is considered too high?".

I googled for a while thinking it would be a simple specification to find; not so much.

At what temperature range should I be concerned that the unit is running too hot? If my baseline (admittedly using the infrared thermometer) is 80 C then what increase from that should concern me?

 

I think the gearbox has a thermal limit that will trigger and let you know what is TOO HIGH!!!!!!


bob

 

Well, I hope that 16 year old sensor is reliable then. Haha

Thanks for the input.

I guess if I notice an upward trend in temperature I can just check the trans cooling system to make sure it’s not failing in some way. Otherwise I won’t give it much more thought.