You shouldn't need battery connected to measure coolant voltage. Regardless, changing coolant will take care of the problem of old coolant causing electrolysis if there is one.

There are arguments all over the place regarding whether an additive is required for a flush. I personally use water from a hose. Just make sure that the engine is cold when you begin. It probably doesn't matter but I'd hate for cold water on a hot engine part to crack the part.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.