Back to website first page

Mercedes Common ECU Failures

    ALL MERCEDES MODELS - Intermittent poor/non start or poor idle/running. Has vehicle got overvoltage protection relay (also called OVP relay or surge protection relay)? If yes REPLACE! any problems here can cause fuel injection system mis-operation.


    Late type SLK (from 1999) and new C class (from 2002) suffer from 'check engine' light illuminating with PO30/32 codes (misfire). This is caused by defective soldered spur connection in wiring harness (+12V feed to coils). Resolder to rectify.


    Late type distributorLESS engine management systems on all models from 1992 (manufactured by Bosch, Siemens and VDO) are now failing in large numbers with loss of spark on two cylinders.

    Identifying which engine management system is on your car is important to establishing whether or not we can rebuild ECU. Basically we can rebuild HM type ECUs but not (currently) PMS type ECUs. HM = Hybrid Management System, PMS = Pressure Management System.

    PMS type ECUs manufactured by Bosch and Siemens can be identified by two rectangular connectors on top of ECU (opposite metal base plate) and vacuum hose attachment. Please click HERE for photo of offending ECU.

    HM ECUs manufactured by VDO and Bosch are cast alloy rectangular boxes with top and bottom covers held in place by small anodised torx screws. There are two rectangular connectors on the side of ECU. Please click HERE for photo of offending ECU.

    VDO ECUs are also fitted to SsangYong Musso and Korando vehicles from 1992. CAUTION: These ECUs are difficult to source for SsangYong as the UK dealer network has closed down.

    Failure symptoms for all ECU types are loss of spark on 2 cylinders. On Bosch and Siemens PMS type ECUs, this failure is caused by ECU design - ignition power stage transistors are glued to ECU base plate. This glue joint fails and transistor junctions melt once thermal conductivity is lost. We are now able to rebuild these ECUs. Please follow link in next paragraph.

    On VDO and Bosch HM ECUs, the cause of failure is more difficult to ascertain, but the system design and components appear to be underspecified for automotive applications. We now offer a low cost rebuild service for VDO and Bosch HM ECUs. If you would like a rebuild quote please complete the quote request form you can find on this page.


    Bosch ignition ECUs types 0227 100 023 and 0227 100 042 fitted to many 1980's Mercedes models suffer common failure with loss of spark as main symptom.

    ECU is a rectangular cast alloy box approx. 10cm deep by 8cm across by 4.5cm high with 3 mounting screws. It has 2 circular connectors (1 pin and 4 pins). Cause of failure appears to be deposits of dissimilar metal corrosion building up on (alloy) base of ECU where it is mounted on steel. This reduces ability of ECU to dissipate heat and leads to premature failure of power transistor and associated components. If you own a classic, preventative maintenance should include cleaning deposits off base and remounting with thermal grease.

    Back to website first page.