STOP light on BXs

A frequent question which appears is why does the STOP light come on when
cornering? The most usual posted reply is low LHM level, this is not
necessarily the case.

A examination of the electrical circuits for the BX shows that the oil
pressure switch is used via blocking diodes to test the other warning lights
(low lhm level/low LHM pressure, and low water level) on the panel when the
ignition is turned on and before the engine is started. It's a great pity
that the Citroen drivers handbook does not explain this. Strangely on my
1990 turbo diesel the over temperature lights are not tested but do operate
the STOP where appropriate.

>From the Citroen workshop manual early BXs appear to have a light test
switch as well as the blocking diodes.

If the oil pressure drops at any time to a level where the low oil pressure
switch operates then the STOP light AND other warning lights will
illuminate.

The reason for low oil pressure is usually low oil level and the car right
cornered fast. Oil is thrown to the gearbox end of the engine away from the
pump, result, low oil pressure and STOP and other lights.

The STOP light will also come on when cornering right if the LHM level is
low for exactly the same reason, the LHM level sensor is on the right side
of the LHM tank and on cornering the LHM fluid will be thrown away from the
sensor. Similarly for the water level sensor.

What this all means in simple terms is

1. If the STOP light, low oil pressure, low lhm pressure/level AND water
level ALL come on then the problem is low oil pressure probably caused by
low oil level.

2 If the STOP light and the LHM light come on the problem is likely to
be low LHM level but could be low LHM pressure, the latter should be obvious
on cars with power steering.

3 If the STOP light and low water level come on then the problem is low
water level. See note A

4 If the STOP and one or more of the over temperature lights comes the
problem is overheating. There are many different temperature warning light
arrangements on BXs, on some only one light causes STOP on others both
lights do. See note A

If the car is properly maintained and the daily checks as recommended are
done then none of the low fluid level problems will exist!! Unless of course
a fault develops during the course of the day.

If these tests fail to fix the problem there is every possibility that there
is a fault in the wiring somewhere, faulty insulation or broken wires are
likely.

Note A

Be aware that BXs are prone to intermittent contact on plugs and sockets.
Depending on the model a high resistance or open circuit can cause no
indication at all or as is the case on my 1990 turbo diesel an open cicuit
on the water level sensor will indicate low water, a high resistance on the
water temperature sensor will indicate overheating. It would be worthwhile
checking what your BX does before you have a problem.

Bob Smith