Technically Speaking
Author: Mike Riley, Technical Editor
Subject Matter: Mercedes 722.6
Issue: No upshift condition
The frustrated owner of a 1996 Mercedes S500 called to complain that his vehicle had been acting up for some time and he was at his wits end. Apparently, he and others had taken a swipe at repairing the problem with little success so the vehicle was towed into the shop to get a look with a fresh pair of eyes, so to speak.
The Mercedes was equipped with a 5.4L engine and a 722.6 transmission and even though it had 187,000 miles on it was in quite good shape and ran well, in first gear at least. The transmission had good application when shifting to drive or reverse, however upshifting just was not in the cards. In addition to the no-shift condition were some indicator lights flashing on the dashboard.
As the story unfolded from the car owner it became obvious that fixing the problem had no simple solution since the transmission had already been replaced once, but to no avail. In addition, someone had decided to change the gear shifter in the console because of technical information indicating that it could also be the source of the problem.
To make matters worse, the initial problem and attempts to repair had been dragging on for some time, according to the car owner. He stated that the vehicle had not been usable for more than a year, and why it was taking so long is anyone’s guess, but his directive was get it fixed now. As with other situations of this nature, the question is what to focus on first.
Unlike newer vehicles that have enhanced diagnostic information more readily accessible to handheld scanners or computer programs, the 96 S500 was rather limited when it came to retrieving information. The option of merely attaching a scanner to the vehicle OBDII connector to verify that the computer was in fact sending a shift command to the transmission did not exist, so the technician had to go to plan B.
Even before jumping onto all the transmission-related stuff, it was noticed that one of the dashboard lights involved the ABS system. Since one area of a vehicle can impact another area, the technician needed to rule out anything brake related so that was addressed first. As it turned out, the right-rear wheel-speed sensor was not providing a consistent signal to the computer and needed to be changed. Hopes were high that replacing the sensor would take care of the shifting issue because on that vehicle a no-shift condition was certainly possible. Unfortunately, the sensor was replaced that took care of the dash light but did not affect the transmission upshift problem.
Aware of the work previously done to the vehicle, the technician started to rule out external items such as harnesses and connectors that may have been corroded or damaged. Everything appeared to be OK as far as the connections went and without good data-stream info the tech decided to check solenoid operation manually, so off went the pan.
Not knowing the extent of the first transmission replacement, it was questionable as to whether anything was transferred from the original transmission to the replacement, such as solenoids or other electrical components or even the valve body. To avoid overlooking any possible cause of the problem, the valve body was removed and the solenoids were tested individually. Everything seemed to fire as intended and the solenoid-connector-plate terminals were all in good shape. The valve body and solenoids were reinstalled along with the pan. What a shame that the transmission could not have easily been hotwired to see if an upshift would occur.
Confident that the transmission was probably not the issue, the technician turned his attention to what else could be responsible for a no-upshift condition such as non-transmission sensors, the computer or the gear shifter. While testing various inputs and outputs of engine-control items, it was determined that everything was functioning as needed, therefore the remaining item on the hit list was the gear shifter. The shifter had been somewhat ignored due to the fact that it also had been replaced and with a brand-new OE component.
In the repair business it is pretty much a given that a brand-new part purchased from an OE dealer is virtually foolproof, however there is always the exception to the rule, which as it turns out was the situation with the Mercedes. When the technician started to test the gear shifter inputs and outputs, he noticed that in drive range the signals were inconsistent as opposed to the other gear positions. The computer of course needs that input to make things happen downstream.
Unfortunately for the vehicle owner, this fiasco had been going on for so long that he was unable to return the shifter for replacement or credit.
Assuming that the problem had been identified and to avoid purchasing another new shifter costing more than $500, a used shifter was found in a boneyard for a fraction of the cost (Figure 1). When the used shifter arrived no one jumped for joy since the gear shift handle wasn’t exactly the same as the original, but with a little tinkering, the correct handle was installed and things were back on track. Once everything was reassembled the vehicle was taken for a road test and the transmission shifted great without having any other issues.
Transmission systems have come a long way since the mid-’90s but so have computers and gear shifters. It seems as though many later-model vehicles come equipped with the tap shift capability and are much more compact and streamlined (Figure 2A). In addition to the new cosmetics is the fact that newer shifters have much more integrated circuitry as well as diagnostic capabilities that make troubleshooting a bit easier (Figure 2B).
Why the owner of the Mercedes chose the actions that he did is hard to say, however it was pure bad luck that the brand-new OE shifter that was purchased malfunctioned and led everyone involved astray. At least the vehicle is back on the road and hopefully nothing else pops up that has to be dealt with.
