Digital Multimeters & Contactor Coils

Sooooo many issues, my arthritic finger may give out.

A "heating coil" is not the same as a contactor coil. A heating coil is a coil of resistive wire made for the PURPOSE of creating heat, as in a water heater. A contactor coil is an electromagnet intent on moving the contacts of the contactor to act as a switch. Not directly related to each other.

A contactor coil is just a long long piece of insulated wire, called "magnet wire", wrapped around a bobbin. That bobbin is then placed over a steel frame that creates the electromagnet. If you check the resistance on the bench, the only resistance you read is that of the wire itself because until

it interacts with the steel frame, it's just a short circuit. If your meter scale is too high, it appears to be "shorted" meaning virtually no resistance. If you read infinite resistance, that usually means the magnet wire is broken somewhere. But if your meter scale is too low, that might be reading the magnet wire resistance and interpreting that as open. Without knowing what the actual setting and reading was, it's hard to judge what you saw that led you to believe it was open. The apparent fact that it worked(?) would indicate you were wrong.

The little blue thing is likely a surge suppressor across the coil terminals, typically an MOV (metal oxide varistor) used on the coil to protect other sensitive electronics in the equipment from the effects of turning the coil on and off, called "inductive kickback" where the rapidly changing magnetic field of the coil creates a voltage spike on the source. Typically if the blue ones are still blue, they are OL because of they fail, they turn brown, or vaporize altogether.

Whew, made it...

The blue one on the right is still good, the one on the left has sacrificed itself in the line of duty. I'm showing this to allow you to see how different it would look if no good.