Ignition

Spark and glow plugs

Citroën originally fitted Champion spark plugs but specifies Eyquem and sometimes Bosch equivalents as well. The best choice would be the first one, however, there is a major problem: Champion and Bosch plugs are manufactured at many plants all around the world. Although the technology is supposed to be the same everywhere, there are significant differences between the quality of those plugs. Those manufactured in the Belgian and British factories of Champion and in German plants of Bosch are excellent but those from other countries are average or often quite bad. You can, unfortunately, find no indication on the package disclosing the actual production plant. So, unless you can be absolutely sure where your plugs come from, you should use Eyquem instead which is not worse than Champion by any means and is manufactured in a consistent quality.

The following engines use diesel fuel and have only glow plugs.

Idle speed and timing advance

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Some engines accept both unleaded and leaded fuel, others only one of them.

The need for adjusting the advance timing arises from the fact that leaded fuel explodes at a slower pace. As a consequence, you have to set the ignition to an earlier time so that it will not be late for the piston passing over the top dead center position.

Ignition systems

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Stroboscopic timing light

Traditionally, stroboscopes were built with xenon flashlights such as those used in photographic flashguns. However, these flashlights require a 400-600 V voltage and even more for igniting them. Needless to say, electric shocks of such voltages are often lethal. Unless you have experience in building such high voltage circuits (including proper insulation) and also have measuring equipment to help with the process, you shouldn't even think of building one.

Fortunately, with the advances of contemporary semiconductor technology, there is no need for those flashlights. There are very high intensity LED diodes available which are perfectly suited for this task. These mostly AlInGaP (Aluminum-Indium-Gallium Phosphate) LEDs come with an intensity level in the range of 10,000 to 25,000 microcandelas, which is an awful lot compared to the 5-50 mcd value of common LEDs. If you even put it into a socket with a reflector and a focusing lens, its light will be so bright that—although it's not a laser diode—you should never look directly into the light rays. Such LEDs are about ten times more expensive than regular ones but in absolute figures that's not so terrible at all: around 3-5 EUR each.

The stroboscope circuit is also much simpler than the traditional one where a DC-DC converter was needed to generate the high voltage from the 12 V DC available. This converter used high voltage capacitors, diodes and a transformer. We can spare all these components as our circuit will be perfectly happy to use the 12 V it receives from the car's battery.

The functioning of the circuit is quite simple. The +12 V and ground wires should be clipped to the battery terminals (the D1 diode protects the circuit from accidental reverse polarity). The trigger input must not be connected directly to the ignition system in any way, use an alligator clip clipped around the insulation of the first cylinder (closest to the distributor) spark plug HT lead. Using a shielded cable for the trigger input line is essential to avoid false input from the other cylinders.

As soon as you switch the circuit on, the C2 capacitor starts to charge with the current flowing through the R2 resistor but nothing else happens as the Th1 thyristor is initially closed. When the engine is running, the trigger input collects this signal via a capacitive coupling. The first pulse opens the thyristor and the C2 capacitor discharges through the R4 resistor. The T1 transistor senses the voltage dropped on this resistor and opens, switching the LED to the +12 V supply (through a customary current limiting R5 resistor; you might need to alter its value according to the current of the LED you actually use). As soon as the C2 capacitor is fully discharged, the T1 transistor closes again and the LED goes out. The Th1 thyristor doesn't remain open because the resistance in its anode circuit is quite high, meaning that the current flowing through the thyristor is way below its holding current. Finally, everything starts again from the beginning.

The components R1, D2, C1 protect the thyristor.

Where to aim with it?

Start from the left hand side of the engine head and go down at the side of the engine, past distributor, thermostat and whatever there is, down and slightly to the front, until you reach down to the crankcase. You'll be stopped there by the clutch which is bolted on to this side of the engine. Just where the clutch connects to the engine, somewhere between the two top bolts holding the clutch in place, you'll find a longitudinal aperture that lets you look into the crankcase, to see the outer top edge of the flywheel. The aperture had a small cover plate originally, you might need to remove this. The rotating marks are on the flywheel, the stationery ones on the edge of the aperture. Clean the area and, if the marks are not visible, reinforce them with a little bit of white paint (the best paint for this is the correcting fluid used when typewriters were still topical, but a small amount of white car paint stamped there using a baby-care swab or a very fine brush is also excellent).

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Just to be complete, here is the circuit diagram of a traditional flashlight stroboscope:

When buying the components, note that two capacitors are rated for AC voltage. The transformer in the DC-DC converter can be a simple 220 V to 9 V mains transformer for PCB-mounting, 1.5 VA is sufficient (from Schaffer or other manufacturer). The 9 V coil will be used as the primary coil. If you want to make it yourself, the primary coil has 100 turns, the secondary 2540 turns. The autotransformer driving the flashlight is a standard flashlight ignition transfomer, you can buy these at electronics stores.

We intentionally don't present a printed circuit board for this second stroboscope. First, the best solution would be to find an old flashgun and put the new circuit into its housing, retaining some components (flashlight, autotransformer, in-out switch). In this case the PCB should be designed to fit the available room. Second, we stress again that such high voltage equipment should be built only by those who have the necessary knowledge and experience; those people will find designing a PCB from scratch an easy task.

Trouble with tachometer (diesel)

You should start the troubleshooting at the sensor in the engine. Use a voltmeter in AC measurement position and observe the voltage at the two terminals of the sensor as you rev the engine up (have either an assistant or a long meter probe). If you measure absolutely nothing, the sensor is suspect (although you can check it with an oscilloscope if you can find one somewhere). If you obtained some readout, repeat the check at the instrument panel, at the blue connector mentioned (one pin is ground, one is +12 V, but the remaining two should exhibit the same behavior as the terminals of the sensor). If the readout is identical, the tachometer is suspect. If you obtained no readout, trace the wiring back through the bulkhead to the engine compartment and repeat the measurement at every connector it passes. If you found +12 V in any of the sensor wires, you have a shortcut somewhere.

Retrofitting a tachometer

You can always have an aftermarket tachometer sold in many car parts stores. However, these are additional units you have to place somewhere on the dashboard; not a nice thing to do if you want to keep the looks of your car as original as you can. But if you replace your instrument panel unit with one having a tachometer, you can have the same layout BXs with higher trim levels originally had. With a little work you can change the tachometer only but it would be even wiser to look for a complete GTi instrument panel and retrofit the coolant temperature gauge, too, which is also lacking from your dashboard. Study the description of this modification first.

You can connect the tachometer using the 4-pin connector at the bottom of the unit. Pin 1 (arrowed on the picture) is the feed switched by the ignition key (in the original layout this wire forks from the one coming from the mainboard V/5 (yellow 4-pin) connector and goes to the oil level gauge as well). Pin 3 connects to the low voltage connector of the ignition coil (the one connecting pin 1 of the ignition module to the coil). Pin 2 goes simply to the ground. Pin 4 is not used. You can cross the bulkhead wall with the necessary wiring beneath the battery.

Diagram ???

You will also have to think about the redline zone on the tachometer. If the meter you have just installed comes from a BX with a different engine, the redline zone will be different. It is highly recommended to modify this marking to reflect the behavior of your engine. You might not forget it but some nitwit at a garage might not pay attention and torture the engine into too high a revolution during some tests or repairs. You can make red marks with red touch-up body paint. First, cut out the negative image of the required shape from Scotch Magic Tape or similar and use this as a stencil pattern to give it a nice look. Some paints may attack the plastic of the front panel or might not leave a nice clean edge when you peel off the adhesive tape used for a pattern. You might want to make a first try on the invisible edge of the front panel, covered by the facia frame.

As a second option, you might also buy a red self adhevise label or print a shape onto a transparency film with an inkjet printer and glue it onto the front panel--this second solution is easier to rectify if it does not look nice for the first try.

In theory, you can do the same in a diesel BX, too, the only difference is that the tachometer has more connections and an additional sensor is also required as there is no ignition signal to feed a tachometer in a diesel engine. As soon as we locate the necessary wiring diagrams, we will describe the modification for diesels, too.

Ignition parts on the CX

The 8-digit part numbers in parentheses are the corresponding Citroën part numbers. NA means models not fitted with any anti-pollution device. Years denote Model Years.