TECH - Coolant Level Warning Circuit

[ranahan@charter.net (John Erickson)]

Flashing Low Coolant Light Circuit Description.

The trigger for the warning circuit is the sensor in the 
top of the coolant expansion tank.  Make sure you have the 
proper level of coolant and the electrical connection on 
top is clean.  Assuming that there is electrical 
continuity everywhere in the circuit (no broken wires or 
dirty electrical connections), there can only be two 
things that cause the light to flash when there is the 
proper amount of coolant in the expansion tank: a faulty 
sensor and/or relay.

If you look at the sensor, it consists of two small metal 
rods (probes) that extend into the coolant.  If current 
can't pass between the probes (low coolant condition), 
then the relay will activate the warning light in the 
temperature gauge.  I really don’t know how the sensor 
could fail, but perhaps if the cooling system was 
neglected for a very long time, corrosion might be an 
issue or perhaps there might be a build up of crud on the 
probes inhibiting current flow.  You can remove the sensor 
and clean off the probes.  You can also test it via a 
simple continuity check (probe-to-connector).  New sensors 
cost US$10.

The relay is in position #18: third row (top) far right 
hand side. There is a "43" on the old relays (new ones are 
blank).  Its only function is to pass current through the 
sensor and to trigger the light if it can't.  Its not a 
standard electro-mechanical relay, by the way.  You cannot 
take it apart and clean any contacts.  It has a small 
printed circuit board inside with some sort of silicone 
device (chip) on it, a few resistors and perhaps a diode 
or two.  I'm pretty sure its an electronic relay, but it 
may be slightly more complicated than that.  If you remove 
the relay, you will still get the flashing coolant level 
light when you start the car.  That's a bulb test function 
not associated with the relay or the sensor.  New relays 
cost US$30.

Continuity issues:  Before you start replacing parts, 
clean the relay contacts and the connector on the sensor. 
  The connector on the sensor is terribly susceptible to 
corrosion due occasional baths in coolant.  The connection 
also tends to get loose from frequent use.  If you spend 
much time in that corner of the engine compartment, the 
expansion tank gets moved a lot.  As a result, the 
connector gets yanked a lot.

There is very little information about the low coolant 
warning light in the Bentley manual.  The circuit is not 
covered in the electrical schematics.  I found the relay 
in the manual, but, according to the book, my car doesn't 
have one.  It's called the "low coolant indicator/idle 
boost control relay."  Perhaps on some cars it's the low 
coolant indicator relay and on others it's the idle boost 
control relay.