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1996 SL500
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Shift Clunk from P to R, R to D

What Is Limp Home?

Well, that means, your car has lost a wheel so it is driving on three wheels to home. :D

Seriously, when the Electronic Transmission Control module (ETC) detects intermittent or active problems, it prohibits gear shifting, in order to avoid serious transmission damage.

There are two different types of Limp Home modes, depending how serious the problems are. The names of these two different Limp Home modes may sound counter-intuitive but that is what Mercedes used.

Mechanical-hydraulic emergency running mode
(This is the light duty Limp Home)

In this Limp Home mode, the transmission locks onto the 3rd gear or the last known “good” gear.

This particular Limp Home mode can be reset by turning the ignition off and then on.

This mode could set generic OBD II code and also intermittent code in the ETC.

Electric emergency running mode (This is the bad boy :mad:)

In this Limp Home mode, the gear engaged at that time is retained and the assigned fault code is stored. After a shutdown, wait for more than 10 seconds, and restart, the transmission is locked in the 2nd gear and reverse gear only. When the shifter is moved from “P” to “R” or “N” to “D”, there is a huge clunk.

The transmission will be in this mode until the codes are cleared from the ETC by Mercedes HHT (Hand Held Tester) or SDS (Star Diagnostic System) or some third party special scanners or even the reversely engineered ones. Generic OBD II scanners cannot clear these codes from the ETC (although they can clear the corresponding generic OBD II codes).

When the transmission is in this “electric” Limp Home mode, it feels like it is completely shot. Many times (actually I will go out the limb and say most of the time) you do not need a rebuilt transmission or a new ETC.

What to do if the transmission is in limp home mode?

The most useful thing to do is to have the ETC trouble codes read. This requires a Mercedes HHT (Hand Held Tester) or SDS (Star Diagnostic System) or some third party special scanners or even the reversely engineered ones. These codes are pretty accurate at pointing to the fault that caused the limp mode. For example, it may have codes for speed sensors or the shift module.

If no Mercedes scanners are available, at least try to read the codes using a generic OBD II scanner. The generic OBD II codes are not as precise as the proprietary Mercedes codes but they might shed enough light to have a good enough diagnosis. For example, if a code P0715 is read, then it is fairly certain that it is caused by the speed sensors.

After fixing the underlying cause of the (electric) Limp Home, the transmission would still not shift. The ETC has to be reset (codes cleared) before it will act normal again.

Without an ETC reset, people often (unnecessarily) replace the ETC. Of course, a new or working ETC will make the car shift again but all is needed to be done is to clear the codes in the ("bad") ETC. When the ETC goes into (electric) Limp Home, it shuts off all the power to the solenoids so the ETC will test "bad" because of this.

There are different versions of the ETC and certain ETCs can only work with certain valve bodies so exercise caution when replace either the ETC or valve body. It is a good idea to use the existing part numbers to find replacements.
I reached to this clear and precise write-up, referred by Martin and Dean and others.
My car, 1996 SL500 R129, was and presumably still is in this Limp Mode. There is reason for saying this now.
I did replace the linkage bushing, cleaned the shift module, cleaned ECT, etc. etc. to fix the Limp Mode as I explained in other thread of 1996 SL500 transmission problem.
I just drove it out for test and the car runs good, or normal, which means the gear shifts automatically to reach 60 miles.
However, I felt the clunk when I tried move the shift lever from P to R, R to D, and back to R. This is unusual to me. So I figure I must find out how to fix this before I use this car again for daily use. Last time after I replace the bushing, I drove it out and it was OK for a little while and then came back to Limp Mode. So I redid everything again but I am still not confident.

So what should I look into in order to take care of this problem?
 

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1998 E320 base sedan @ 160kmiles
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You need to get someone to scan your TCU codes. I assume you have no check engine light, but if the car goes into limp mode, you definitely have 2 and 3 digit TCU fault codes.

The clunk means that the transmission is mechanically shifted from P to D to R etc. I still think that the TRRS module is bad in your shifter (the module that tells the TCU electronically) what gear the shifter is). If you had the TCU scanned you would get a 114 code for the TRRS fault.
Does your gear shift from 1st to 2nd to 3rd to 4th when you shift it manually with the gear shifter ?
 

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1996 SL500
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I will see if I can get it scanned by local Indy.
Yes, to your question, I drove with D, moved to down to lowest and shifted back to D, one by one.. and I find they are working fine.
You say the clunk is mechanical shift. What does it mean? Of course I shifted with lever from P to D, or vise versa.
 

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1996 SL500
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I called a local Indy for TCU scan, and he would charge $180 for the scan.
This kind of shift fault does not get detected by generic OBDII scanner? No CEL nor nothing codes.
 

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1996 SL500
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TCM cleaning

If they do not go away after you clear them (you did, didn't you ?) you then have persistent problems with the can_bus connection at the TCM. Hopefully it is a dirty connector (the one that has a pair of separated pins from the other which are for the can_bus. Have you touched the TCM (EGS52) to see if it was contaminated with ATF ? I guess you did. You call it "ECU". Just make sure the connectors to the TCM are clean and the plugs are installed properly. I would douse the connectors with electronic contact cleaner spray and let it dry prior to installation.
Yes, I did clean all those with electronic contact cleaner.
 

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1998 E320 base sedan @ 160kmiles
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6,707 Posts
Yes, I did clean all those with electronic contact cleaner.
The comment I made was for some other poster. Your car does not have EGS52 (facelift) transmission control unit.

The module I was talking about is as attached. Does your shifter have one inside ?


https://www.fcpeuro.com/products/mercedes-neutral-safety-switch-mer-2105451332

This is the TRRS module and it basically has a wiper switch inside and as you shift gears, it will send voltage coded signals over 4 links to the TCU. Sometimes the wiper switch or the tracks the wiper travels on get bad and the TCU gets the wrong signal for a given gear. If WD40 got into the assembly, it can cause problems with conductivity.

If you cannot have the car scanned, the only thing I suggest is to measure the voltage levels at the TCU end (from the designated pins that come from the shifter, as you shift the gear.
 

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1996 SL500
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Clunking shift

The comment I made was for some other poster. Your car does not have EGS52 (facelift) transmission control unit.

The module I was talking about is as attached. Does your shifter have one inside ?


https://www.fcpeuro.com/products/mercedes-neutral-safety-switch-mer-2105451332

This is the TRRS module and it basically has a wiper switch inside and as you shift gears, it will send voltage coded signals over 4 links to the TCU. Sometimes the wiper switch or the tracks the wiper travels on get bad and the TCU gets the wrong signal for a given gear. If WD40 got into the assembly, it can cause problems with conductivity.

If you cannot have the car scanned, the only thing I suggest is to measure the voltage levels at the TCU end (from the designated pins that come from the shifter, as you shift the gear.
Yes, I have the thing in your attachment inside of the module, but I couldn't figure out how to take it out. But I cleaned with compressed air blow hard.

I just came back with test drive again, and found this mysterious phenomena;
I drove it like old lady steady and slow, no problem.
Coming back, I accelerated hard to pick up the speed fast(the way I usually drive this car to enjoy), then I went back to Limp Mode -- low gear running with high RPM, making funny hissing sound.
Sooo I stopped the motor and went thru "Tricky TCU adjustment" (Ignition on, push the pedal all the way down for 15 sec, Ignition off, key out, wait for 3 minutes, restart again..) Then it got out of Limp Mode so I circled around my neighborhood three times with hard acceleration. So far, other than feeling the 1st gear stayed a little bit longer than necessary, it shifts to higher gear according to speed OK.
That Tricky TCU adjustment did something I could not quite understand the logic.
I will wait a while and take it out again for longer distance with higher speed.
 

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1996 SL500
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I might have to check the voltage as you guys suggested after another test drive with another Tricky TCU adjustment. Last time when I cleaned the TCU at the passenger foot area, I did not see any contamination at all.
 

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1999 E430 Sport
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57 Posts
Want to read a 722.6 transmission horror story? Too bad. Here it is.

Over a 10-week period, my then-three-year-old 12,000-mile 1999 E430 was towed to Northern Virginia M-B dealers three times (and driven in three more) and, under warranty, had one valve body, one shifter, one torque converter, two transmissions, and four -- yes, four -- EGS control computers replaced.

As you might expect (or not, but bear with me), my shifting problems continued after the factory warranty expired, so two years later another EGS control computer, another torque converter, and another transmission were installed, this time under an extended warranty that covered only part of the costs.

So, after 40,000 miles and nine -- yes, nine -- service department visits, my car was already on its second shifter, its second valve body, its third torque converter, its fourth transmission, and its sixth -- yes, sixth -- EGS control computer.

Fast-forward to the present. I’ve learned to cope with an apparent software initialization defect in the EGS control computer. My car has just passed the 200,000-mile mark, and after 17 years I would probably keep it for another 17 were it not for the rust working its destructive magic on all major body panels and, most notably, the driver's side front spring perch.

But that, regrettably, is another story.
 

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1999 E430 Sport
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Three problems, relationships unknown.

First, on one occasion, after backing out of my garage, the car went into first gear and stayed there -- no upshift regardless of RPM. Driving around the neighborhood at 15 mph until the engine fully warmed up (this happened in February in Virginia) had no effect. The M-B service advisor initially claimed this was normal behavior that helped the engine warm up faster, urged me to be more patient, and refused to tow it in.

Second, on two occasions, after backing out of my garage, the car would not go into any forward gear. Associated symptoms included giggling neighbors driving by in their BMWs and my being late for work. The sixth service (see pdf chart) cured that problem. At the time the cause was widely believed (by M-B service people) to be spilled coffee interfering with the shifter mechanism. No such luck. During one of these visits the service advisor sat me down and, possibly in an effort to solicit a little sympathy, said of his mechanics, "my people are technicians, not computer programmers." I just shook my head.

Third, the car would occasionally start off in second gear (with W/S switch in S position, BTW), accelerate briefly, pause, then accelerate (harder) by itself (no foot on gas pedal) for about a second, and finally slam hard into second gear while emitting an audible hiss. That still happens on occasion but I don't know what the particular conditions are that induce it (temperature may be one). The last time I checked, the consensus of opinion (aka most popular speculative theory) is that the transmission/computer system requires a short time delay between the initiation of the power-on sequence (i.e., turning the key) and selection of a gear other than P. This behavior doesn't occurr as often as it did before the 7th visit, so I suspect a software update was installed during the 7th, 8th, or 9th that reduced, but did not eliminate, the size of the required software initialization time delay.

You can see from the dates on the chart that I learned to live with that one remaining quirk and that I'm now in my 18th year of doing that. That's longer than most guys keep their wives!
 

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MB 2001 320E
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When My 2001 w210 went into limp mode, my Indi immediately said it was the Transmission Conductor Plate before touching the car or any checking.
I bought a Transmission Conductor Plate on Ebay and he installed it.
Fixed the problem.
The speed sensor on the plate had become damaged and that caused the problem.
Just a thought.
 
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