Connect the negative lead to ground, and dangle the positive lead in the coolant via the radiator fill neck. See what voltage you get. Less is better, and it better be a small fraction of one volt.
There's two causes of "radiator voltage".
1. Improper grounds. The vehicle electrical system is grounding through the coolant.
2. Worn-out coolant. The dissimilar metals in the cooling system are using the coolant as electrolyte, the metals touched by the coolant are effectively a big battery.
I've mentioned before on this forum i think, but the best flush product i've found for iron blocks is Motocraft VC9.Your correct. I run pink CAT ELC coolant in everything. Mostly because it’s readily available and it fights electrolysis.
As for system flush, I’ve used a lot of crazy stuff. Cascade for dishwashers, mineral spirits, degreaser etc. I would use at your own discretion but my point is not to put much thought into it. Drain, fill, drive repeat a few times and your good.
The only way to get a voltage that way is if your leads are made of dissimilar metals.Finally, dangle BOTH leads into the same pool of coolant (e.g., at the radiator filler neck). THAT reading should be quite close to 0V... and if non-zero then is likely due to the probe's metal tips.
The only way to get a voltage that way is if your leads are made of dissimilar metals.