Finding the cause of overheating
Check the coolant fluid level on a cold engine. If it is low, especially if you have a coolant leak, you might have found the problem right away. Otherwise, start with checking the fuses #1 and #4 on Series II models or #5 on the older ones. If they are OK, put your finger on the leftmost relay behind the fuses; when you turn the ignition on (don’t start the engine), it should click. If it doesn’t, replace it. If you have electric windows fitted, either the second relay from the left or the second from the right would be of the same type as the one serving the cooling fan; you can use them as a quick roadside replacement.
14 engines: The fan control switch is screwed into the right bottom of the radiator. It has two blue wires, the feed coming from the relay has a mauve sleeve, the other going to the fan has a yellow one. Jumper these two connections. If the fan does not start, check the wire leaving the fan for a common ground behind the battery.
16 and 19 engines: The fan control switch is screwed into the right bottom of the radiator. It is a double temperature switch with three blue wires: the ground has a brown sleeve, the low speed contact has a yellow one and the high speed contact has no sleeve. Jumper the brown to the yellow and the fan should rotate in low speed. Jumper the brown to the sleeveless and the fan should rotate in high speed. If it does not work in either speed, check its ground: touch the wires mentioned above to some exposed metal part instead of the brown sleeve wire. If only the low speed fails, the low speed resistor (an aluminum cylinder with a 3-pin white connector at one end) located right behind the left headlamp is suspect.
With air-conditioning: These models have two fans. In low speed, they are connected in series, so both should run (and because of the circuit, if one fails, the second fan stops running, too). In high speed one fan is always running, the second is switched on together with the A/C. The fan control switch is the same as with other 16 and 19 engines, the green wire serves the high speed and one of the blue wires the low speed (the other blue wire is earth). To check the high speed, touch the green wire to some exposed metal part: with the A/C on, both fans should run in high speed, without the A/C only one of them. To check the low speed, touch one of the blue wires to earth (if nothing happens, try the other blue wire): both fans should run in low speed. As the low speed is obtained by using both fans, there is no low speed resistor on these models.
Although no circuit diagrams I have describes this, there are a few 16 or diesel models out there with their control switch connected the BX 14 way, between the positive feed from the relay and the fan instead of between the fan and the ground. Check for this situation first before you jumper anything to the ground.
If this resistor fails (or is missing by any chance), replace it as soon as you can. It’s very easy to test: feel it with your fingers after a trip in town. If it feels warm, it was working (caution! it might be hot!). At first sight this resistor doesn’t seem to be very important (if the fan doesn’t start at 88 °C, it will at 92 °C, the engine will not overheat, so why bother?) but it is an essential part of the system. Starting the fan in a high speed from stillstand requires significantly more current than the amount consumed either in low speed start, low-to-high speed transition or when running at any steady speed. Thus, in order to reduce the strain on the temperature switch, it starts in low speed and switches to a higher one when required. Consequently, if the resistor circuit is malfunctioning for any reason, the wear of the temperature switch is accelerated seriously. Wearing of this switch means that its switching temperature will rise continuously, reaching a point when the engine overheats without the fan starting even once.
So, if the fan is working as it should, check that temperature switch. The most precise way would be to remove the switch and test, however, you can make a simple but sufficient test in the car. In a cold engine, make sure that the coolant fluid level is OK, start the engine and run it at high idle (it is much easier if you ask somebody to help you). As the engine warms up, the radiator stays cold for a while (feel both the rubber hose between the thermostat and the radiator and the radiator itself at the left side with your hand; as the thermostat is still closed, no coolant—no heat, that is—should come that way). As soon as the temperature reaches around 80 °C, it should open, sending the coolant through the radiator. If the hose and the radiator stays cold above 80 °C although the engine temperature rises steadily, the thermostat does not open and should be replaced.
If the engine temperature rises faster than it normally used to be, start the heater inside the passenger compartment and set the temperature and the blower speed to the maximum. If you find the air entering much cooler than the engine temperature should dictate, the coolant might not circulate because of the malfunction of either the coolant pump drivebelt or the pump itself.
If the thermostat does open, watch carefully. As the temperature goes up, the fan should cut in. In 14 engines, that would happen above 90 °C; in 16/19 engines it cuts in below 90 °C at low speed. In the later case, disconnect the resistor behind the headlamp quickly. The fan stops, the temperature rises. Above 90 °C, the fan should cut in again, this time in high speed. If it does or you have a 14 engine and the fan did start above 90 °C, stop the engine: everything seems to be fine. Don’t forget to reconnect the resistor.
If the fan remains still, the control switch is probably at fault. The warning lamp (even the flashing or yellow pre-warning light with models without a temperature gauge) comes to life way above the fan cut-in temperature. If the fan doesn’t start even when the warning light goes on, the temperature switch is dead without any doubt. Tell your assistant to shut off the engine as soon as the warning comes on; there is no need to overheat the engine once more.
If everything else is perfect and the engine still overheats, you might suspect the radiator. When the engine is hot, touch various parts of the radiator looking for cooler spots. If you found some, drain the coolant and remove the radiator. You can try to flush out the blockage directing a flow of water into it. Reversing the flow (entering into the bottom hose) and using some cleaning agent as Holts Radflush or Speedflush might remove bad contaminations as well. If this doesn’t help, you’ll have to find a radiator repair specialist or to replace it with a new or reconditioned unit.
If your BX only has warning lamps instead of a temperature gauge (most of them have no gauge), you should proceed the same way: check the thermostat first, then wait for the fan to cut in. Constantly monitor the warning lamps. If the fan does not cut in before the warning starts to flash or light, the control switch has to be replaced.