I have a 2005 C55 that when driven like grandma it will refuse to down shift, or when it does it'll only give you a fraction of the power available. Its almost like a limp mode. I've reset the tranny via key in ignition, floor the throttle and turn key off release throttle. This works every other time, but then it goes back to no power mode. Checked and reset the ECU, No fault codes. Also did a tranny service and confirmed fluid levels. Its very strange. Any ideas? Thanks in advance.

 

Conductor plate going south?

 

Seems like it, or just some speed solenoids going defective.
Best bet is to just let the shop run some test and diagnostics on it.

 

I had problem downshifting gears at certain times . Solved my problem after I have done a transmission flush. I bought a transmission kit with filter online and had the mechanic replaced it . Now all gears shifts smoothly .

 

Well, only some codes will show up in the engine module, like those which the engine needs. However, the codes you want to check are in the transmission module, so you'll need a better reader to read them out. It could be as said above, some sensor or solenoid which are glitchy/bad. I bet most of the sensors are in the electronic plate, but once you know the error code it can be looked up.

You might save alot of money by reading out the codes before you try to replace the tranny fluid, because you'll need to do the same work 2 times if it turns out to be something messy with the electronic plate. It's hidden under the transmission cover, under the hydraulic body.

 

I just put one in today and tossed in a Sonnax sleeve kit as well. ATF134 is expensive stuff and anyone who believes these should be "sealed for life" has clearly never seen what one looks like with a few miles on.

The only complicated issue for me was the 13 pin connector.

 

Well, I can't believe that I haven't followed up on this post until now, some 2 years ish later. Anyways I thought I'd post a follow up just in case some information will be helpful to other peeps on this forum that may have experienced a similar issue. NOT a transmission issue... 1st attempted fix, drop the transmission conductor plate, check and clean all the sensors, reinstall with new filter and double triple check tranny fluid. Nope... Then decide to install a Eurocharged ECU tune and TCM tune. The problem actually went away for some time and the car was acting as it should. No bizarre limp mode activity when it was driven in stop and go traffic. Then the issues came back, which was when after driving in traffic or driving like grandma for several miles, if you would floor the gas pedal it would down shift but only give you maybe 1/4 power. I would just not go like it should. Back to the shop, knock sensor error code, replaced those and the car was fine for a few hundred miles until the same symptoms came back. A couple things that I should mention... I don't drive this car much(less than 5k a year) and when I drove it manual mode, I could kinda force it to give full power if I downshifted a couple gears manually. This just didn't make any sense to me, I also have a 2004 CL600 which has a twin turbo v12 pushing 500hp but basically has the same transmission. Never any issues with the CL, it has a completely different power band but I've never experienced a similar problem that I've had with the C55.
Second opinion at another shop, sometimes you just need another set of eyes. Replaced mass air flow sensor/intake manifold air sensor/breather hose kit, oh and again knock sensors. Cleared codes and all problems solved... I've only had an opportunity to drive the car for a few hundred miles since, but ya, no issues. I'm feeling extremely confident that this problem has been solved. Again, just wanted to share this info in case any other owners have experienced a similar problem.

 

Thanks for the contribution. I'm sure you know, but it's so helpful to be able to search these forums for a fix for almost any problem.