One of the house and woodworking magazines gave the following procedure to check if the coolant is too old and needs to be changed:

1. Open pressure reservoir cap.
2. Warm engine until thermostat opens and coolant circulates.
3. Use a high impedance digital multimeter to measure voltage
between battery negative terminal and coolant. That is stick
one lead into the coolant reservoir (that's why you opened the cap
before warming the engine) and one on the battery negative
terminal.

4. If voltage <0.4V coolant is still good.

That's the procedure. Now how accurate (or right) it is I don't
know. I guess the idea is to find out if the additives in the
coolant to prevent corrosion are still working. If they don't work
anymore one could expect galvanic currents and a small voltage
potential between ground and the coolant.

Martin

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