TECH - Coolant Level Warning Circuit

[bencoates@yahoo.com (Matt Coates)]

great write up.  I would add that the connector to the temp sensor is not a one
way connector.  I chased a blinking coolant light for 3 weeks and found the
connector was plugged in backwards.  Turning it over fixed the problem.


--- John Erickson <ranahan@charter.net> wrote:
> 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.
> 
> 
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