How to test heated mirror ?

[Green Machine]

2001 GMC 1500. Last time we had ice, I think the passenger mirror was not heating, we did get more ice on that side, but the rear window and driver side seemed to clear off OK. Of course it warmed up before I had a chance to look at it.

Is there an easy way to test if the heater in the mirror is working OK without taking the door apart ? Ohmmeter ? Is this a problem area on these trucks ? Would like to fix this summer while it is nice out, don't want to wait until it is cold again to test and repair it.

Thanks.

[DirtyLarry]

The service manual is less than sterling with testing the heated mirrors. It mentions checking the heating element supply voltage circuit (which would be infinite resistance on a DVOM) for a short to ground but that is about it.

It does say to verify if the rear defrost is working (system check) first then check the HTD MIR fuse, then check for poor connections if no fault is found with the previous (if the power movement function is working then the connection issue would be unlikely).

Outside of that, you may have to wait until old man winter returns again to test it. I have not heard of these failing often but did notice they change colors when the defrost is on. Does your mirror turn a greenish color when you turn on the rear defroster?

[Green Machine]

Well it seems to me that yes, the driver side has a greenish tint and the passenger side did not, I'll have to double check. Will the heated mirror function kick on when the temp is above freezing ? I assume the HTD MIR fuse is independent of the rear defrost, or same fuse ? Thanks.

[DirtyLarry]

Originally Posted by Green Machine

Well it seems to me that yes, the driver side has a greenish tint and the passenger side did not, I'll have to double check. Will the heated mirror function kick on when the temp is above freezing ? I assume the HTD MIR fuse is independent of the rear defrost, or same fuse ? Thanks.

You’re right, if the glass is very hot it may not come on at all so you will want to do this out of direct sunlight.

I am not sure if the rear defrost fuse is the same as the MIR fuse or not. It is quicker to look in your owner’s manual than I can dig up the diagram.