How we diagnosed the fault — process of elimination
Diagnostic Investigation
Mercedes-Benz SLK 200 Kompressor (R171) Diagnostic Investigation
Professional vehicle inspection evidence & findings
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Mercedes-Benz SLK 200 Kompressor (R171) – Won't Start / No-Crank Case Study
Reported symptom
Vehicle would not crank or turn over using the ignition key.
SP88 investigated a reported vehicle fault: Vehicle would not crank or turn over using the ignition key.
How the investigation progressed
Evidence-ledVerify the fault
The no-crank condition was confirmed. The vehicle appeared to have the required key/start authorisation but the starter was not being operated normally from the ignition key.
Initial scan & system overview
A full diagnostic scan was carried out. Numerous stored faults were present, including several Rear SAM communication faults, CAN communication faults and previous undervoltage faults. Relevant fuses were checked due to the Rear SAM-related faults and no blown fuses were found.
Testing performed
The starting system was investigated and the starter motor relay circuit was checked/tested. When the starter was commanded through the relay for diagnostic purposes, the engine cranked and started successfully. Once running, Reverse drove the vehicle backwards, Neutral removed drive, and selecting Drive still caused the vehicle to attempt to drive backwards. The gearbox selector mechanism and selector rod/linkage were then physically inspected.
Evidence-based conclusion
The transmission gear-selector rod/linkage was found disconnected at the gearbox. This is the principal physical fault identified during the visit and explains the mismatch between the driver's selected range and the gearbox's actual mechanical position. The stored Rear SAM/CAN faults are not being treated as proof that the Rear SAM requires replacement.
Additional scan / diagnostic notes
Diagnostic step
Full scan showed numerous stored faults including Rear SAM communication, CAN communication and previous undervoltage faults. Relevant fuses were checked and no blown fuses were found. Start authorisation appeared present. Starter relay testing successfully cranked and started the engine.
Key findings
After the engine was started, the vehicle drove backwards in Reverse and also attempted to drive backwards when Drive was selected. Physical inspection found the gearbox gear-selector rod/linkage disconnected.
NEXT REPAIR REQUIRED: Replace/renew the gearbox gear-selector linkage retaining clip/bush as required and securely reconnect the selector linkage. Simply pushing the linkage back onto the selector lever is not considered a completed repair. Inspect the linkage end, selector lever/stud and cable mounting for wear or damage and replace any defective retaining components. After repair, verify mechanical P-R-N-D matches the selected/displayed range; confirm Drive gives forward movement and Reverse gives reverse movement; verify normal starting in Park and Neutral and starter inhibition in unsafe ranges; then clear historic faults and carry out a final full-system diagnostic scan.
Outcome & recommendation
After the engine was started, the vehicle drove backwards in Reverse and also attempted to drive backwards when Drive was selected. Physical inspection found the gearbox gear-selector rod/linkage disconnected.
Next step: NEXT REPAIR REQUIRED: Replace/renew the gearbox gear-selector linkage retaining clip/bush as required and securely reconnect the selector linkage. Simply pushing the linkage back onto the selector lever is not considered a completed repair. Inspect the linkage end, selector lever/stud and cable mounting for wear or damage and replace any defective retaining components. After repair, verify mechanical P-R-N-D matches the selected/displayed range; confirm Drive gives forward movement and Reverse gives reverse movement; verify normal starting in Park and Neutral and starter inhibition in unsafe ranges; then clear historic faults and carry out a final full-system diagnostic scan.
What owners can learn from this case
A confirmed fault should be repaired and the vehicle retested before historic or secondary fault codes are treated as separate component failures.
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- Detailed recommendations and follow-up information