Q: I’ve read about electrolysis causing cooling system problems. I put the negative probe of a digital voltmeter to engine ground and the positive probe in the coolant at the surge tank on my motorhome. It measured about +250mV (.25 volt) DC with it running; the engine was cool when I did this. Is this normal?
A: Electrolysis erodes the radiator, heater core and internal engine parts. The damage is usually caused by old acidic coolant, but can also be caused by bad electrical grounds on the engine or chassis. To properly check for electrolysis voltage the engine should be running at about 1,800-2000 rpm, at operating temperature with the thermostat open. To do this, remove the radiator cap only when the engine is cold, then start the engine and allow it to warm up. Place the negative probe of the voltmeter on the negative battery post and place the positive proe in the coolant. Keep your hands away from hot surfaces and coolant and ensure that the + probe is only touching coolant. If the voltmeter reads higher than 400mv (.4v) DC the coolant should be flushed. After a flush perform the check again. If excessive voltage is still present the electrical system grounds should be checked. You may also need additional ground straps.