For models W140 and R129, 1991-1996, the instruments' electrical harness can be checked. Using a digital multimeter (only), a few important signals can be probed on the harness connectors. NOTE: Do NOT use a mechanic's trouble light, as this feeds back power, and may cause damage! IF you do not have electrical experience or are uncomfortable checking, skip this step or seek the help of an experienced electrician! Be very careful not to short any signals to ground! The following pins are seen from the perspective of the harness ends :
CONNECTOR 1
CONNECTOR 2
- Set multimeter and test leads for DC Voltage check (Max. 20V).
Use grounding harness bolt (pictured above) for reference ground.
Check for constant good positive (12V) on pin 2.12.
- If pin 2.12 is correct, use it as a constant good reference positive (12V).
Check for good ground (0V) on pin 1.10.
- Put the keys in the ignition and turn them slowly until the fuel pump turns on (do not start the car).
Use grounding harness bolt (pictured above) for reference ground.
Check for good constant positive (12V) on pin 1.1.
Check for good constant positive (12V) on pin 1.5.
Activate turn signals and remeasure, ensuring voltage is stable.
- If pin 1.1 is correct, use it as a constant good reference positive (12V).
Check for good ground (0V) on pin 2.11.
WARNING: Do NOT attempt to connect wires to the harness or introduce voltages to them! Irreparable harm may result to your vehicle! Only licensed Mercedes dealers can work on the electronics of your vehicle! Do not attempt to connect voltages to the instruments, as they are very sensitive and susceptible to total failure with incorrect or mis-timed voltages! A burnt wiring harness or power surge to any ECU will result in permanent vehicle and electronics damage!
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