how to test if the heater inside the air dryer is working?

What year/make of truck are you working on??? Either way it should be keyed power. But theres no standard from one make to the next until you start getting newer. Older Freightliners would grab power for the air drier heater off the reverse switch on top of the transmission. So if your reverse light works then it could be the wire is broken between there and the drier. But more often than not, the wires get switched around during a transmission install and makes the drier heater work only in reverse.

But your not driving a 2000 FLD, the. I can’t guarantee that any of that will actually help you...

 

But to test one they are pretty straight forward. Unplug it. If you have key power and ground then the truck side is working. To check the heater it’s self, ohm the twi pins in the drier. I don’t remember the temperature for sure, (maybe 40 deg) but it should show Open Circuit above and show a resistance below that ambient air temp.

 

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Your truck is in the same era as the 2000’s I worked on. Yes one of the wires should have a 12V keyed power. If it doesn’t, the heater isn’t working. My Next suggestion is to check your reverse light to make sure it gets power. If it does THEN I would figure out how to get to the top of the transmission. Back in the day I was small enough to fit my head and an arm up in-between the cross member and right rear corner of the transmission and test the poles on top of the switch. It’s a little button switch with two Phillips or flat screws sticking out of the top. The side with key power should have two wires going to it (key power & drier heater). The single wire should go to your reverse light. .... .... but that was 15 years and a rough 100lbs. I suppose if I can’t point and grunt at the Noob I would have to take the center floor out of the cab to get to it.

In all reality it would be easier for my fat ### to run a new power and ground to a keyed breaker in your fuse panel. But I still like to keep things factory so I’d probably just try to stick my arm on top the tranny (no head) and do it by feel.

 

I would definitely check ground before running just a keyed power. If I remember right, the ground is usually run to the to or the transmission as well.