Hello everyone,
I’m encountering a problem with the convertible top on a 1994 Mercedes Benz SL500 (R129), and I’m hoping to get some insights from those with more experience on this system. This vehicle came into the shop with a few issues: the right front (RF) window was inoperable, the fuel level gauge was consistently reading empty, and the convertible top was not functioning at all.
Initially, the RF window was stuck closed. I could hear the window motor running, but the glass wouldn’t move. Interestingly, if I manually pushed the window down to the fully open position, the convertible top system would operate without any issues. This particular SL500 is an early production model (November 1993), which uses window limit switches, unlike later models that utilize hall effect sensors in the window motors to determine window position for convertible top operation logic. Upon inspection, I discovered that the plastic mounting on the original window motor was broken, preventing it from engaging with the window regulator gear. Additionally, the instrument cluster was only outputting 1 volt to the fuel level sender. We decided to send the instrument cluster out for professional rebuilding.
After a few weeks, we received the rebuilt instrument cluster and a replacement window motor. Upon reinstalling the cluster, the fuel level gauge began working correctly. The new window motor, which appears to be an updated design with metal threaded mounting (still a two-wire motor), also resolved the window issue.
However, after these repairs, the convertible top still refuses to operate when attempting to open it. When I activate the convertible top switch to open, the red light in the switch flashes continuously. Notably, the convertible top warning light on the instrument panel does not illuminate. Furthermore, neither my Autel MaxiSys Elite nor my Iscan SF diagnostic scanners can communicate with the RST (Roll Over Bar and Soft Top control module) via the 38-pin diagnostic connector.
When I push the convertible top switch in the opposite direction to close the roof (while it’s already closed), the rear fabric section of the top attempts to push down into the storage compartment cover. There are no flashing lights or any other indications, it simply tries to lower the rear bow. Typically, the next step in the convertible top sequence would be to raise the windows after the rear fabric bow is confirmed ‘locked’. The fact that it continues to try pushing the rear bow down leads me to suspect a fault with the lock-position microswitch, possibly indicating an ‘unlocked’ status when it should be ‘locked’.
To investigate this, I consulted the pinout charts from Alldata and checked all relevant microswitches. According to my measurements, all microswitches are reading ‘Closed’ or ‘Locked’, which corresponds to 0v or continuity to ground. I verified both voltage and continuity at the back of the RST module for all microswitches indicated in the wiring diagrams. It’s also worth mentioning that the roll bar system is functioning correctly, moving up and down as expected. Again, there are no warning lights on the instrument cluster related to the convertible top or roll bar system, with the exception of a persistent SRS warning light. The SRS system is reporting five stored codes: 72 (no description), 71 (no description), 65 (no description), 17 (RF seat belt buckle switch with AB and ETR or programming error), and 19 (LF side airbag sensor defective), these codes were present before the instrument cluster rebuild.
The only changes made to the vehicle recently are the instrument cluster rebuild and the replacement of the right window motor. The replaced window does move down completely and engage the microswitch (S21/8). Referring to the power window wiring diagram, this switch should read 0v when the window is fully open, triggering the switch, and 12v when off the switch. I’ve confirmed this voltage reading at the convertible top control unit (N52). The initial step in the convertible top opening sequence is to lower the windows until the window switches are activated. However, in this case, the red light on the switch just flashes in the open position, and the system doesn’t even attempt to initiate the opening sequence.
I’m at a bit of a loss on where to proceed with troubleshooting this Mercedes Benz Sl500 Convertible top issue. Any guidance or direction for further testing from someone experienced with this particular system would be greatly appreciated.
Thank you for your time and assistance.
-Aaron
