Bosch Motronic common failure symptoms
(35 & 55 pin ECUs)
Bosch Motronic engine management ECUs with integral ignition power stage are increasingly suffering failure as ECUs age. These ECUs are commonly fitted to Alfa Romeo, BMW and Vauxhall(GM) cars from 1987 up to the introduction of distributorLESS systems. Click here to see photo of ECU.
To identify Motronics with integral ignition power stage look at the underneath of the ECU. There are 2 small screws (Philips or Torx head) which locate the connector body to the ECU base plate and either 2 or 4 larger screws (Philips or Torx head). ECUs with 4 larger screws have integral ignition power stage – if 2 of the larger screw holes are blanked off with plastic plugs then there is a separate ignition power module(s) and THIS PAGE DOES NOT APPLY – THERE IS A SEPARATE IGNITION AMPLIFIER LOCATED IN ENGINE COMPARTMENT. Click here to see photo of ECU. NOTE:Bosch Motronic ECUs with 35 pins fitted to Peugeot or Citroen cars (up to 1992) do NOT have integral ignition amplifier – there is an ignition amplifier module located below inlet manifold and this is the most likely cause of loss of spark on 2 cylinders on these 2 particular makes.
1) Car breaks down (sometimes without warning) – apparent loss of spark and NO fault codes.
2) There may be a history of intermittent starting or misfire problems but note below for Vauxhall cars.
ALL VAUXHALL CARS with this system – REPLACE large fuel injection relay for vehicle non start or running problems – before condemning anything else!
1) Check that injectors and fuel pump are working normally.
2) If yes, check for very low ignition pulse – very weak spark sometimes visible.
3) If yes, check all LT and HT connections for defects and exchange coil if available.
4) If above OK then ECU confirmed failed.
If in doubt about whether ECU is cause of problem on your car, try our low-cost ECU inspection service. This is fast, low-cost and many Motronic ECUs with this common problem can be fixed during inspection – avoiding costs of rebuild.
DO NOT REMOVE ECU COVER – THESE ECUs FEATURE HIGHLY SENSITIVE MICROELECTRONIC INTEGRATED CIRCUITS THAT CAN BE DAMAGED BY NORMAL ENVIRONMENTAL ELECTROSTATIC CHARGES.