Thanks Mike, this give me some ideas of where to look.
On Thu, 07 Oct 2004 09:07:33 -0700, Mike Ellison
> wrote:

wrote:
>> Does any one know exactly how the low coolant level sensor works on a
>> 1987 C4, i am having a intermitant warning light that comes on, i have
>> replaced the probe that goes into the radator, (it is a single wire
>> type) and the module. I can not been able to find any short to ground
>> on the wiring. does the sensor work on a differential from temp levels
>> between oil and coolant? does it work on resistance to ground?
>> Diode, you got any info on this? any body else? my wiring diagraims do
>> not show the circuts inside the module, so i do not know what
>> condiotions it is looking for to turn the light on. Greg
>
>The same module & sensor part numbers are used on the '89. '89 shop
>manual shows that the module has a relay that provides a ground to the
>indicator light. So, I'd guess the lamp wiring is probably picking up
>an intermittent ground.
>
>Shop manual shows the module providing an a.c. signal at 100Hz to the
>sensor. Sensor resistance pulls down that signal, opening the relay
>and turning the warning light off. Shop manual shows the sensor
>providing a variable resistance (>45Kohm low coolant, 10Kohm or less
>with 'OK' level). Shop manual doesn't specify whether that resistance
>is measurable with a d.c. VOM or if it requires some special gizmo.
>(I've not a clue as to the resistive or capacitive properties of coolant
>or antifreeze so don't know what they are doing here.)
>
>Hope this helps........ Mike