I just lost a heater core in my 02 F150. I'm not replacing it until I make sure I don't have an electrolysis problem.

How am I supposed to check the voltage in the coolant when the only way to access the coolant is through an insulated plastic bottle? I'm getting different readings from the coolant in the overflow bottle (.25v) and the aluminum radiator (-.33v, yes it's negative). Which one should I believe?

 

I finally figured out that the overflow bottle is connected to the lower radiator hose. I thought it was just connected by the overflow tube.

I'm going to flush the heck out of the motor this week (drain, fill with distilled water, drive, repeat in 2 days) and check for electrolysis again next weekend. Hopefully getting all the old and nasty coolant out will improve things.