I still call BS. A "resistive" ground path would still be better conductor (flow more current) than coolant.

Ground loops are a different "phenomenon".

And, what if my voltmeter had a very high input impedance and I used a zinc probe???

What was your initial voltage before the voltage started to drop?
As Graham mentined:

I can see a method something like this being of some use. But it needs to be properly researched. Type of electrode makes a difference. So does type of coolant. G05 and Dexcool are more acidic (actually less alkaline) that old style glycol green coolants.

We need a purpose designed meter. And a chart that says what voltage should be with different coolants. Would take quite a bit of research with coolants of all types and of different ages.

Seems to be the current on-line myth/story Hagerty and I think one of the vendors had similar articles recently.

My feeling - If you haven't changed your coolant for a while - get her done!

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5

I think it was around 0.3 v. I do not put too much trust into these readings since there are so many variables like coolant types and probe material. As Graham stated, better to just change your coolant on a schedule.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

It sounds reasonable that the voltage could droped,as you said,due to passivation of the electrode and due to the corrosion inhibitors.As it,maybe, covered the electrode with layer that decrease the potential voltage between the voltmeter probe and the negative metal.Is it right?
So,it looks like the coolant is good and it does its job,doesn't it?

I really don't know. That test needs to be better developed to be of any value.

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5

PH strips from any chemical supply house if still cheap should meet peoples needs. Actually in all probability once the ph is dropping towards acidity with time you can probably add an additive package to the antifreeze/water mix and restore it.

Only if the test strips indicate the need though. Variables with the voltmeter test probably just require comparison to a fresh anti freeze/water sample as a standard of calibration.

I have never given much thought of why it was always recommended to use only distilled water with anti freeze either. I may look into this a little as the need if I suspect what the reason was may be more needed today than ever. It is just as possible the need is no longer there as well perhaps. So I guess I better go have a look.

Thanks for all the responses.
Unfortunately,i didn't get an satisfied answer to:what is the reason that the voltage doesn't stay constant/why does it decreases the following next seconds and what does it mean(good,not good)?
Graham mentioned number of reasons for the decreasing:
Quote:
Originally Posted by Graham View Post
....There could be a number of reasons for the voltage to drop. Gas forming on electrode, corrosion inhibitors doing their job, passivation of the electrode..

But they are only possibilities.what is the true reason?
What is it mean?
I would like to understand the issue,it is not just to know if the coolant is good or not.
I can check the coolant,also,with PH strip as other mention.
But this is not my main reason formy question.

Thanks for all the responses.
Unfortunately,i didn't get a satisfied answer to:what is the reason that the voltage doesn't stay constant/why does it decreases the following next seconds and what does it mean(good,not good)?
Graham mentioned number of reasons for the decreasing:
But they are only possibilities.what is the true reason?
What is it mean?
I would like to understand the issue,it is not just to know if the coolant is good or not .
I can check the coolant,also,with PH strip as other mentioned.
But this is not my main reason for my question.

You should call up antifreeze manufacturers and ask them what they think about the voltmeter test.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Do you believe in experimenting? I tried a stainless probe (AC vent thermometer) in the coolant and the voltage started at 0.09 volts and did not drop. I also tried a copper probe (piece of roof flashing) and the voltage started at 0.3 V and dropped steadily, just like the HF meter probe and alligator clip probe I tried, which I believe are tin plated. Maybe the probe material, depending on material, is reactive with the coolant (or not) via electrolysis? Just a guess. If you really need to know the truth, I am sure you will find the answer if you put enough time, effort and money into it.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked