Sharp R-1874 shuts off after 59 seconds

That would be a problem in the damper assembly, either a bad switch or motor, or the damper door is jammed or blocked.

Consult the attached diagram for general reference.

The damper switch is the same as the primary interlock (door) switch.

You can get Sharp parts from Tritronics at http://www.wehaveparts.com

I had the same problem. Turned out to be the micro switch and damper motor. I replaced both myself and fixed the problem for less than $40.00. The best advice I got was replace the switch, reconnect the motor and make sure the motor runs through a damper door cycle. It didn't run, even though it showed continuity and had power, so I replaced the motor which solved the problem. The micro switch which is activated by the cam on the damper motor has to be cycled within one minute, or the unit shuts itself off. The damper motor on my model R-1870 is found in the right rear section of the assembly when facing it. You have to remove the unit, and remove the outer casing to get to it. The damper is a simple sheet metal door, activated by the motor, and the switch mounts at the top of the motor and is activated by the same cam that activates the door.

My problem turned out to be the control switch which was a $15 bill plus another $90 in labor. I had to remove the microwave from the motor home and take it in for repair. The removal can also be handled through the motor home dealership and the cost will be higher.

In the next couple weeks mine will be going into a shop that only repairs microwaves.
I can't get it unmounted from my motorhome so that has to be solved first.
I will post back here when I get the final fix.