Back on 27 July 2001 I posted what I felt were definitive test results
on this topic, at least for a van with the same circuit as my '91.

I concluded that any plausible coolant mixture was conductive enough to
satisfy the level sensor.  Pure distilled water in a clean container
would be about the only thing that would not trigger the circuit.
  It's interesting how some urban/vanagon legends live on in the face of
objective evidence.

Larry A.

Dennis Haynes wrote:

>No way is that circuit senstive enough to detect coolant concentration. The quality of the water supply will have more of an effect on resistivity than the antifreeze concentration. There is a huge difference btween hard tap water and de-ionized water. When the ignition is first turned on the warning should blink for a few seconds as the bulb check. The circutry for that function is part of the foil circuit board. Sometimes the timing capactior goes and the light will blink for a long time. Sometimes the coolant level control relay will go bad making the light blink forever or never turning it on.
>
>Dennis
>
>----- Original Message -----
>From: John Rodgers <inua@CHARTER.NET>
>Date: Wednesday, February 8, 2006 3:37 pm
>Subject: Re: How to - or not - bleed the cooling system
>
>
>
>>Robert Smythe wrote:
>>
>>
>>
>>>My coolant light flashes at startup, then goes out, temp guage
>>>
>>>
>>never goes
>>
>>
>>>past half. Am I fooling myself in my speculation that its just
>>>
>>>
>>testing> the
>>
>>
>>>function of the circuit with this behavior?
>>>
>>>Robert
>>>90 MV
>>>
>>>
>>>
>>>
>>You are correct.
>>
>>The specs call for a coolant mixture of 50/50 coolant to water. That
>>mixture gives a specific resistance between the two electrodes on the
>>coolant level sensor. Evidently on startup the circuit assumes an open
>>circuit or infinity resistance until the correct voltage for the
>>circuitis read,  at which time the light goes out. If the mixture
>>is incorrect,
>>or the tank is low on coolant and the electrodes exposed, the
>>resistanceis incorrect, the voltage reading is wrong and the light
>>will come on.
>>Important to keep that coolant mixture at 50/50, as well as
>>keeping the
>>tank full.
>>
>>Regards,
>>
>>John Rodgers
>>88 GL Driver
>>
>>
>>
>
>
>