Mercedes-Benz W201 overvoltage protection relay (YJ relay) faults and soldering
Mercedes-Benz W201 YJ relay: where is it and what does a failed relay cause?
The aluminium-cased overvoltage protection relay (YJ relay) is found behind the battery, behind a plastic cover. If it does not work, cold-start enrichment and the electric idle control do not work. Symptoms can include an erratic idle, poor starting and the ABS light coming on.
How is the Mercedes-Benz W201 YJ relay repaired by resoldering?
The most common fault is bad solder joints inside the relay. The case is opened carefully from the bottom, all the joints are resoldered with fresh solder, and on reassembly you check that the insulating plate does not press the pins against the shell. Adjacent pins can touch each other, so check from the board which ones are on the same circuit.
Why does the ABS light come on or the idle run high after replacing the YJ relay?
The relay may not seat fully, leaving the contact open. In one aftermarket relay the smallest pin was about 0.3 mm thicker than the original, and the pin had to be ground to the correct size. Also check that the relay's base has not sunk down.