Hello,

My 2002 XKR is experiencing the common "Gear Box Fault / High Tempurature" issue, but only under specific circumstances. If the gear selector is left in the "D" position there are no issues whatsoever. However, if gears are manually selected (2,3,4, etc) then the fault mentioned above quickly appears and the car goes into "Limp" mode.

If I clear the fault with a scanner, disconnect the battery and wait a few minutes before reconnecting, I am able to restore the car to full capability. Then, provided that I only use, D, R, P or N, the car operates perfectly. The moment 2,3 4, etc is selected, the fault returns. The valve body has already been replaced and no MIL codes persist. I'm convinced that it's an a electrical/elecronic problem but don't know where else to look.

Anyone able to shed any light on this?

 

I have had that problem in the past as well but only on long trips up to Scotland. It crops up in the most random places and I can't think why it would do it. Turning the car on and off and clearing the code fixes it, I assume that it really is just overheating the transmission but I know these transmissions are good for much more power then what our cars has. Mine hasn't shown up in a few years but I haven't taken it up to Scotland in about that time. Could be the temp sensor in the transmission? Last time it happened it went into limp home mode and wouldn't drop down gear and would be locked into a really high gear all the time, I had to use the cruise control to get up to speed. It wasn't a very good drive home as a result. It doesn't help that my code reader can't read the transmission codes. There are a lot of up hill and downhill roads in Scotland so that might not help but I've gone up and down hills before with no problems.

It doesn't really make much sense though because the last time it happened I had already done lots of up and down hill around the Yorkshire Dales area with no problems at all and then drove up to Scotland where it went into limp home mode well after doing all that more taxing driving. I don't know if I was in the "manual" mode on the J-Gate though? I would like to say it's just a sensor problem and not actually a real problem with the transmission because the problem goes away like you said with a rest and battery disconnect. It's a yellow limp home mode as well not a red one so the fault likely isn't that bad and it's just protecting itself from damage because it thinks something is wrong because a sensor is acting up perhaps?

I just take a socket wrrench with me and my code reader on long trips so I can remove the codes, I also had a problem with the car going in restricted perforamnce mode but I just used a screwdriver to take the intake off and clean the throttle body a bit and that cleared the codes. I've cleaned it out really nice now and that one hasn't shown up since. Cleaned the MAF and intake temp sensor with MAF cleaner and used carb cleaner ont he intake. Best to treat these cars as classics and not a brand new car, they are 20+ years old at this point so they are getting on in years.

 

You don't say, but I assume your 2002 is the 4.0 car with the Mercedes gearbox, rather than the 4.2 with the ZF?

Have you checked what the computer thinks the temperature is with a scanner (one of the more advanced ones that can read the modules)? You can also read the transmission temp using an IR gun on the oil pan and see if they agree.

The workshop manual says to put the transmission in D or R to read the trans temp (trans temp is only displayed in D or R) - maybe that is a clue as to why you only get this fault on one side of the j-gate, maybe the computer is confused as to where the gear selector is.

 

Another possible complication, when not in D or R the TCM substitutes engine coolant temperature. Is it possible this is where the "heat" is or maybe conductor plate and/or plug guide sleeve has gone bad and need renewing.

 

It just shows how short of memory/computer power or connector pins they were back in the 1990s that they couldn't send the trans temp all the time.

The OP says he has replaced the valve body, which I assume means the conductor plate - if not its certainly worth changing, along with the plug sleeve, as well as cleaning all the connections. I think the j-gate is worth looking at, maybe a microswitch not working as it should, confusing the computers in the process.

 

If the original conductor plate was replaced, what was the reason for doing so ?

I’ve found out the hard way not all conductor plates for the Mercedes 722.6 transmission are created equally. And having the temp sending unit on the plate makes it very important to have the real Mercedes part in place, not a Chinese knock-off or a counterfeit part.

I had the plate replaced during a routine transmission service, and didn’t keep the original one. Oops !

the replacement turned out to be a counterfeit M-B part, and now I have occasional and infrequent random “High Transmission Temperature” warnings regardless of the actual transmission temperature. It’s even happened on a cold engine ! The warnings don’t last but for a few seconds, but they hey still give a heart attack.

I can check the transmission temp (in Drive or reverse of course) with the code reader, and it is always normal or less.

Some day I’ll get a Genuine M-B plate installed and try to get back to the future.


Z