What Is Limp Home?
Well, that means, your car has lost a wheel so it is driving on three wheels to home.
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
)
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 on 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.