Citroen AX Bosch MA3.0 Mono-Motronic ECU Repair

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 Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot)
Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM
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!
Happy motoring!
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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 Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot)
Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM
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!
Happy motoring!
Add pictures here
‚úĖ
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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