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Autopapyrus · Citroën BX (1st generation) 1.6 (1987)
https://autopapyrus.com/guides/citroen/bx/diagnosing-no-injector-pulse/
Compiled automatically from service literature and owner experience. Specs are for the typical configuration — check them against your exact car before you start.

Citroën BX (1st generation) 1.6 (1987): Diagnosing No Injector Pulse

No injector pulse means the control unit is not sending a drive signal, or the signal is not reaching the injectors. The cause is usually one of three things: no power to the control unit and injectors, no engine speed or crankshaft position signal, or a break or corrosion in the grounds and connectors. The exact injection wiring diagram for this BX variant is not documented in open sources, so the general diagnostic approach is given below.

Difficulty: Medium Time: 1-3 hours (diagnosis) Steps: 9 Tools: multimeter, test light (LED or "noid light" for the injector connector), socket and wrench set, contact cleaner, oscilloscope if available

Part

  • 🔩 Don't buy anything in advance. Typical candidates if diagnosis confirms a failure: crankshaft position/speed sensor (or ignition coil/module, if the speed signal comes from them), fuel system relay, injection control unit, wiring harness and connectors. The cost depends on the fault found, from a basic relay to a more expensive sensor or control unit (check with your parts supplier)

Step by step

9 steps
  1. Make sure the battery is healthy (at least 12.4 V at rest, not below 9.5-10 V while cranking) and the terminals are clean. Voltage drop during starting is a common cause of lost control on older BX models.
  2. Check the fuses and the fuel system and injection relays in the fuse box. Wiggle and clean the contacts, and if possible swap the relay for a known good one. Turn the ignition on and listen for the fuel pump: if it is silent, look for the cause in the power supply circuit, the relay, or the circuit through which the control unit receives the engine rotation signal.
  3. Find the injector connector and unplug it. With the ignition on, use a multimeter to check for +12 V on the injector power wire. If there is no power, look for a break, a faulty relay or a blown fuse. If power is present but there is no pulse, the problem is on the control side.
  4. Connect an LED test light to the injector connector (between power and the control wire) and crank the engine with the starter. If the light flashes, the control unit is sending the pulse and the problem is in the injector itself or its connector. If it does not flash, move on to checking the engine speed signal.
  5. Check the engine speed and crankshaft position signal: inspect the sensor and its connector, check the coil resistance (compare with the specification for your sensor), the air gap to the reluctor wheel, and make sure there are no metal shavings on the sensor tip. Without a speed signal, the control unit will not fire the injectors.
  6. Check the grounds of the control unit and the engine: the battery negative terminal, the body grounds and the engine block ground. Clean the contacts and measure the voltage drop between the battery negative and the control unit ground contact (it should be well below 0.5 V).
  7. Inspect the harness and the control unit connector for corrosion, signs of moisture and chafed insulation, especially where the harness passes through the body and near the battery. Corroded contacts and dried-out wires are typical for a 1987 car. Check the injector control wire for continuity from the control unit to the connector.
  8. Check the resistance of the injectors themselves (typically a few ohms for low-impedance injectors and around 12-16 ohms for high-impedance ones; compare with the specification for your injector). An open or shorted coil can "kill" the control unit's driver stage.
  9. If power, ground, speed signal and harness are all fine but there is still no pulse, the control unit is likely faulty. Before replacing it, check the unit for bulging capacitors and signs of leakage, then try a known good unit of the same specification.

Common mistakes and warnings

  • Work on the fuel system away from open flames, and avoid sparks near the connectors if fuel has leaked. Do not short contacts "just to test" on the control unit itself, as this can burn out the output stages. If you have no experience with electrical and injection systems, it is best to go to a repair shop.