A Mercedes came in to our workshop. The check engine light was on. "All the previous attempts
to scan the vehicle failed" said the technician who brought the car to us. The check engine light
comes on when the engine control system detects a malfunction.
The first step was to interrogate the system. Without knowing what the fault codes were there
was no point in carrying on as you end up guessing which part to replace. We established
communication using Mercedes specialized equipment(i.e. a Star Diagnosis) and read out the
fault codes. Two codes were System overvoltage and ECU internal checksum failure.
Overvoltage means a fault with the charging system.
We scoped the alternator output voltage and current and got the result shown in picture two.
This picture suggests that one of the alternator diodes is faulty. The alternator needed to be
fixed first. We overhauled the alternator and found the faulty diode that had just started to
crack. The alternator was fixed and the bench test gave good results. So back in the car it
went. It was time then to deal with the second code.
to scan the vehicle failed" said the technician who brought the car to us. The check engine light
comes on when the engine control system detects a malfunction.
The first step was to interrogate the system. Without knowing what the fault codes were there
was no point in carrying on as you end up guessing which part to replace. We established
communication using Mercedes specialized equipment(i.e. a Star Diagnosis) and read out the
fault codes. Two codes were System overvoltage and ECU internal checksum failure.
Overvoltage means a fault with the charging system.
We scoped the alternator output voltage and current and got the result shown in picture two.
This picture suggests that one of the alternator diodes is faulty. The alternator needed to be
fixed first. We overhauled the alternator and found the faulty diode that had just started to
crack. The alternator was fixed and the bench test gave good results. So back in the car it
went. It was time then to deal with the second code.
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