Sooo.... I got tired of waiting for my carsoft v12 multiplexer to arrive from Belgum, so I decided to poke at the MB with sticks. Or at least that's what I feels like to me when you can't access the REAL data stream.
CARSOFT Mercedes V12 System
The problems that were presenting were:
No top operation.
Signal Bulb Indicator On
Transmission in limp mode (2nd gear only).
ABS/ASR lights on
Brake wear sensor lit.
Codes P1747 No CAN BUS Data to Transmission
P0500 Speed Sensor A Error
So I jacked up the car and pulled the wheels for the first time, and found that there was a new wheel speed sensor on the front left. MMMMMkay
I also found that the RF brake wear sensor wire was broken.
I fixed that with two waterproof butt splices. $0.20
I have spent a lot of time reading on SLK and MB forums about issues that are common on these cars.
This led me to pull the k40 relay box. This is a relay board that essentially does the job of 5 relays and fuses. Nothing complicated, but it is annoying to have 5x the chance of failure.... anyway.
When I turned the car "on" only one relay closed. Interesting.
However, on the dash, the brake wear indicator was now out.
I checked for 12v signals at the relay coils and 3 were actually being commanded on.
I knew the horn did not work. I actuated the relay for the horn. BEEP!
I checked and one of the non responsive relays was for the TCU circuit.
I closed it manually and immediately the engine ran better.
I held it shut with some cardboard and cycled the engine off and back on.
The brake warning light went out and via the generic OBDII scan tool I read that the two codes were now clear.
Since it was up on stands, I put it in drive. It shifted 5 times.
Next I tried the convertible hardtop, and whhhrrriiirr click click buzz whhrrrhhrr clunk, it disappeared into the trunk.
That's a lot of stuff off of the list.
For the final fix I will either repair, modify, or replace the k40 Relay Module.
To reconcile the PO's experience.... they were being told that there was a wiring problem "between the sensor and the computer". That was wrong, and it is what happens when you use generic OBDII codes on an older German car. ( I also have a z3). LOTS of stuff gets lost in the translation. The REAL tools use OEM proprietary connections under the hood to access ALL of the modules.
So a lot of time and $$ in parts I suspect had been tossed at the big issue (limp mode) until the owners just gave up. In the end it was just a bad relay that wasn't turning the TCU on. If the TCU wasn't on, it could not talk on the CAN bus (P1747) and report a speed (P0500).
On top of all of that, the first step in the roof module logic is to make sure the car is not moving, and with no speed reported, it would just shut down and not open the roof.
The ABS/ASR lights are still on but I think that there is a good chance that they may only need to be reset with the proper CARSOFT interface now that the transmission reports a speed.