Originally Posted by Diseasel300

MTC is a known-bad part. There are hundreds of reports of the MTC cartridges not working from the day installed here and on various other forums. Your first recourse should be to replace with an OEM cartridge and see how things fare. They've gone up in price since the $35 days, but it seems to be about the only reliable way to get a working cartridge. You can also swap the diaphragm and rubber parts from the MTC cartridge to an OEM core assembly for a working solution, but that requires having an OEM core to swap the parts onto.

Electromagnets will last forever if they're not overdriven or abused. The electromagnet coils in gas dryers fail because they are marginal to begin with and are in close proximity to a heat source. They suffer insulation failure or simply burn out. DC coils last considerably longer than AC coils do as well, so you shouldn't be worrying about that part of the system.

As wwii pointed out, the ground side of the monovalve circuit is switched. If the key is on and the ACC is powered, you will have 12V present at one of the leads all the time. Since it is the ground side of the circuit that is switched, the only way to test it is to leave the coil in place and test each wire to the chassis with a multimeter. One will read 12V, one should read near 0V. If both wires read 12V referenced to the chassis, you know the ACC is not switching.