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98 SL 500 TCU for transmission

2K views 9 replies 5 participants last post by  SLautoparts 
#1 ·
My tranny is slipping and it was diagnosed as the TCU [electronic controller] rather than the pump or the torque converter. i understand that this electronic controller determines the shifting sequences. If I turn off the car and restart it the problem often disappears. I have two questions; where can i buy this controller module and how hard is it to install?
 
#3 ·
It is most likely that the only problem is fluid incursion to the TCM.

Replace the pilot bushing and clean the TCM with non residue type electrical cleaner. Then have the codes cleared by a proper interface.

All of this is well within the abilityof a DIYer except for clearing codes.

Good luck.
 
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#6 ·
I'd recommend that you start by reading through the excellent FAQ thread on the 722.6xx transmission in the W210 DIY/Help stickies.
http://www.benzworld.org/forums/w210-e-class/1463460-mercedes-benz-722-6-transmission-faq.html

Here's a stripped-down, simplified summary.

The transmission in your car is a mechanical device, but it's electronically controlled. Think DBW (drive-by-wire). The TCM connects to the conductor plate (which sits atop the valve body) via a harness that slides through the pilot bushing (connector/harness adapter) in the front right of the transmission. The pilot bushing has been redesigned at least three times since your car was made because they have a tendency to leak fluid into the harness, where it runs up the wires and soaks the TCM, triggering fault codes. (The starting point, though, is to pull the TCM and see if it is wet with fluid, if not my recommendations would change a bit, so do that first.)

The scanners that read codes can clear them from the ECU (regular old OBD-II compliance), but that doesn't clear them from the TCM. So you do a bunch of work, heck, you could even replace the transmission, and the TCM will feed the codes back to the ECU and voila, you're back in limp mode. So you need a system that will talk directly to the TCU and that limits you to SDS, Snap-On, carsoft, etc. Any decent shop that works on German cars will have the proper interface.

There are a couple of DIYs on replacing the pilot bushing in the 210 forum DIY/Help stickies. But what you need to know as tips. First, you rotate a ring on the pilot bushing to release the harness connector, then you pull the harness connector STRAIGHT OUT. Do NOT try to rotate it (this applies double when you are reassembling, if you turn it you will bend one or more pins on the conductor plate, and your day just went downhill in a big hurry). Second: there is a single very small bolt in the center bottom of the pilot bushing. It threads into the conductor plate and serves only to secure the two pieces together. It is not strong enough to pull the pilot bushing into place, if you do that you will break it or strip it. You must fully seat the pilot bushing in the transmission housing, and then snug up (not much more than finger tight!!) the little bolt. I use a nut driver on it (IIRC it's a 7mm head).

Be sure to use spec fluid, MB 236.14. If you don't know when the trans was last serviced you *could* service it at this time, but I would not (since there is a possibility you'll have to replace the conductor plate and then you'd lose all the new fluid and it's not cheap).

The only special tool you will need is the transmission dipstick tool. You'll read about that in the FAQ thread.

BTW, the reason the shop said replace the TCM is twofold. One, they don't like to clean and clear, they like to replace. Less trouble and time for them, less chance of a comeback. Two, they aren't that familiar with these cars, since they didn't mention replacing the pilot bushing, which is the likely cause of the problems in the first place.

So, to sum up: check the TCM for signs of fluid incursion. If so clean it and replace the pilot bushing, set the fluid level, have the TCM cleared, and you should be good.

Do some reading, post back if you have questions.

Good luck.
 
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#5 ·
I repair these transmissions, and like check codes said. Your TCU just needs a cleaning, or the circular connector needs replacing..The TCU rarely fails, and the failure is failure to communicate to the testing computer.

More than likely your computer has oily connectors, and oil in it. All because the circular connector failed-The fluid wicks-up the wiring harness

If you are in limp mode(you only have 2nd & reverse gear), you will need a computer to reset it. No mater if you fix the problem. It will not reset by disconnecting the battery for a while.

I wrote this for a guy, but the process is the same...Follow(post 2) the links!!!

http://www.benzworld.org/forums/w140-s-class/2359593-1997-s500-trans-both-p0720-p0715.html

Check your fluid first, and you need a testing dipstick...

All the best to you,

Martin
 
#9 ·
Of course I'm in Sac???

I can answer questions, but I really work on rear-wheel drive vehicles because some (if not all) one has to drop the sub-frame. Hold the engine up on a swing-set-up, and raise the car over 3 to 4 feet above the ground. Too many magical tools needed to do front wheel drive cars..

Any rear-wheel drive vehicle is light-years easy to remove a transmission, and basic tools are only needed..

Front wheel drive vehicles were designed to lower cost of assembly, but for the most part front-wheel drive vehicles are designed to have a quicker economic-obsolescence, and sadly the lower to middle class cannot afford a premium rear-wheel drive vehicle for lower maintenance costs.

Consequently, the V engine front wheel drives are the expensive ones to own as basic maintenance can cost four to a ten fold than the average rear wheel drive car

Martin
 
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