Citroen AX Bosch MA3.0 Mono-Motronic ECU Repair
I have a 1994 Citroen AX, 954cc fitted with single-point petrol
injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control
Unit/Module (ECU or ECM). Once the ECU becomes faulty it will generate
fault codes and use default values causing the engine to run badly,
even though the components and connections may be sound. I have two
Bosch MA3.0 ECUs, both of which were faulty. I was quoted £360 for a
replacement unit from a Bosch agent, but was able to repair the unit
by replacing a single electrolytic capacitor.
First remove the plastic cover, the multi-plug and the four retaining
8mm bolts. Once indoors, remove the five T15 torx screws. The top
plate may need levering off as it is sealed with silicone sealant.
Remove four more T15 torx screws from inside and lift the plastic
cross member to free the circuit board. Avoid touching the circuit.
There is a single electrolytic capacitor on the board, a 100uF, 40V
radial - the line on the top indicates the negative terminal. Replace
this with a new capacitor of the same value and voltage rating
(available on eBay). Electrolytic capacitors dry out over time and
develop an excessive equivalent series resistance which reduces
voltage - they are by far the most unreliable electronic components.
The other problem on the Citroen AX is the throttle potentiometer
which wears out, and is only available with the throttle body (Citroen
Part No. 1920X4) at around £300! Bosch do an exchange unit for around
£170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets
(Bosch Part No. 3 431 015 900). But they wonít sell you the
potentiometer as a separate part!
From Haynes Automotive Engine Management and Fuel Injection Systems
Page 19.16 Fault code table (Citroen and Peugeot)
11 End of diagnosis
12 Initiation of diagnosis
22 Stepper motor
31x Lambda control
42 Injector or fuel pump control
52 Lambda control
53x Battery voltage
x Faults that typically will cause the ECM (Electronic Control
Module) to enter LOS (limited operating strategy) and use a default
value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code
Reader (FCR) multi-plug to earth for 4 seconds with the ignition on
while an assistant watches the fault lamp on the dashboard (or use two
long lengths of cable). To clear the fault codes short for 10 seconds,
or remove the ECM multi-plug. The FCR is situated by the o/s headlamp.
If you're buying second hand check the fault codes. Take the vehicle
for a run first in case the owner has cleared the fault codes from the
memory of the ECU!