D-Jetronic was developed by Bosch as early as the sixties. Citroën had already been using it for the DS IE, and launched the SM IE with it in 1972 (1973 Model Year).
The 'D' in D-Jetronic stands for "Drück" and it's a Speed-density system, meaning that no air flow meter is required. Instead a MAP (Manifold Absolute Pressure) sensor is used to measure load. Speed-Density systems are quite common even today.
The ECU is competely analog and is a pretty sturdy unit. Injectors are low impedance, requiring ballast resistors in the ECU.
The D-Jetronic system is good for what it is. The primary issue with the system is that it does not use a lambda sensor, and cannot compensate for wear on the engine and sensors.
D-Jetronic uses seven sensors. Faults in either of these sensors will yield different running problems:
- Coolant temperature sensor
- Air temperature sensor
- Ignition trigger (located in the bottom of the distributor)
- Manifold absolute pressure sensor (behind the hydraulic reservoir)
- Full load sensor (on the firewall)
- A throttle opening rate sensor, controlling acceleration enrichment. A second sensor on the other bank is not connected to the injection system and solely controls the secondary butterfly opening and closing.
The computer itself generally does not fail.
- NEVER start or run the car with the original Bosch 0 280 150 003 injectors. Replace immediately with 0 280 150 043. YOU HAVE BEEN WARNED. The original injectors leak internally, and can lead to catastrophic fire.
- NEVER run the car with the original external fabric sheath fuel pipe, however good it looks. Replace every piece if present with modern fuel injection piping. YOU HAVE BEEN WARNED.
- The ECU may go lean when the engine is very hot. Check the thermostat cover bleed pipe is clear.
- A larger/modern alternator (see Alternator) makes the car run much better, even though the fuel computer can operate as low as 5V, as the car will be charging even at idle.
- Check that the cooling fans have not dropped a magnet internally (e.g. both take the same current). This can be characterised by a sharp crack when they turn on. A failed magnet causes much more current draw, lowering the voltage avalable at idle.
- http://members.rennlist.com/pbanders/ -- lots of pages about the D-Jetronic