Date: Sat, 28 Jul 2001 08:14:20 -0500
Reply-To: Larry Alofs <lalofs@ENTERACT.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Larry Alofs <lalofs@ENTERACT.COM>
Subject: Re: Coolant resistance thru coolant level probe
Content-Type: text/plain; charset=us-ascii
Leon Korkin wrote:
> Larry,
> That's scientific approach! Valuable results too.
> My question is, if trigger point is at 42K than using ordnary tap water will
> not cause that blinking light to blink, would it?
Right. Although I hope that some other people will try to check the trigger
point because my van has been flakey in the past and it is very possible that
this value is not typical.
> Why then some people had
> it blinking with tanks full of coolant?
That is the main question. In the past mine has blinked for weeks at a time
with the tank completely full of a reasonable mixture, and then healed itself
again. David's statement about a marginal capacitor on the circuit in the dash
being affected by humidity seems to make sense. I guess I should go into that
or learn to ignore the flashing or replace level sensor part with a different
circuit.
>
> Perhaps the sensor is capacitive? Being more sensitive than resistance? Just
> guessing.
I don't think it could be capacitive. The capacitive between probes that small
and that far apart even with liquid between them would be so small that it would
be comparable to the capacitance between the two wires leading to the probe or
between the wires and any nearby metal. The change in resistance would be much
easier to measure.
>
> Leon
> 85 Subwagen Westy
>
> Larry Alofs wrote:
>
> > Volks,
> > I finally tracked down a coolant leak that had been bothering me for a
> > month. Had a small crack in the pressure tank between the pressure cap
> > and the coolant level sensor. Since I had to disassemble things anyway,
> > I decided to take the coolant level sensor and attempt the definitive
> > study of the resistance of various coolant mixtures and then see how
> > these values compare to the resistance needed to trigger that &%#*
> > blinking light.
> > The coolant level sensor was placed in each liquid sample about 1/2 in
> > deep in a clean plastic cup. Initially as removed from the van the
> > probes looked mostly clean and shiny with a few particles of dirt
> > covering less than 20% of the surface. After the first measurement, the
> > probes were rinsed between measurements.
> > Note that the resistance of liquids generally cannot be measured
> > accurately with a typical ohmmeter using a DC test current because
> > chemical reactions occur which quickly coat the electrodes and change
> > the readings drastically. These measurements were made by using 14 V AC
> > applied to a series circuit consisting of the coolant probe and an
> > adjustable resistance as a voltage divider. When the voltage at the
> > junction was half of the total voltage, I assumed that the adjustable
> > resistor had a value matching the resistance of the liquid sample
> > between probes of that size and shape. For each sample, I then
> > disconnected the circuit and measured the resistance of the adjustable
> > resistor using a DVM.
> > The coolant from the van only tested to be good to about -10 F with a
> > hydrometer,. so it was not the recommended 50/50 mixture.
> >
> > 1. coolant from van, probes "dirty"------------ 333 ohms
> > 2. coolant from van, probes wiped off--------- 356 ohms
> > 3. same coolant, probes sanded lightly, rinsed, wiped--- 365 ohms
> >
> > 4. 100% Prestone extended life (silicate & phosphate free) --- 1667 ohms
> >
> > 5. 50-50 mixture Prestone & distilled water from grocery store--- 387
> > ohms
> > 6. Tap water (Chicago)------------------------------------------3603
> > ohms
> > 7. 50-50 tap water & Prestone ---------------------------------- 384
> > ohms
> > 8. distilled water, two different brands, same reading
> > ------------47000 ohms
> >
> > Since I wanted to investigate the statements I had read about the wrong
> > mixture triggering the coolant level warning, I wanted to see how high
> > the resistance had to be to start the light flashing. Got the digiwife
> > to sit up front to report on the flashing and reset periodically by
> > turning off the ignition. This test was done with the engine not
> > running. I adjusted various potentiometers which I had attached to the
> > connector for the level sensor probe until I found that the circuit
> > triggered at a resistance of about 42000 ohms.
> > This circuitry in my van has been acting correctly for the past month
> > or so, although it has had bouts of random unexplained flashing over the
> > past year (hence the motivation to gather more info).
> >
> > My conclusion is that most any mixture of water and antifreeze should be
> > sufficient to keep the light from flashing, assuming the rest of the
> > bizarre circuitry is working.
> >
> > Whew. Sorry this is so long.
> >
> > Larry A.
> > 91 GL (no coolant leaks and no flashing...today)
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