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I have an odd one. I'm doing a mirror upgrade for our plow trucks that use a tow plow. Swapping the right stationary mirror for a motorized one. Both left and right mirrors heated of course. The heated mirrors won't operate. I unplugged both to see if the switch would light and it did....1 time. I pulled up the wiring diagram and everything I see is checking out. I have power to the switch at #2. When I press the switch #4 terminal lights like it should. I swapped the relay with another one with exact part number. The HTD mirror fuse at f15 is good. I replaced the switch just because I have one. Any help would be appreciated.

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The mirror motor works as it should. Checked powers and ground. not always understanding the circuit. The circuit goes thru the instrument cluster module. Wondering if that's the issue??

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Not sure if that's a new enough Mack to have CAN bus wiring, but with CAN bus if there's more current draw than the computer expects it may shut down the circuit or do weird stuff. Probably should check the Mack body builder manual which I think is a free download if you don't have one.

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Its a fairly simple circuit, that's what's driving me insane. I swapped dash, relay and switch. All those components work good in another vehicle (13 GU)

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Are these new mirrors?  Does the heater circuit have a resistance spec you can measure to verify it?

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right mirror is new. I've checked for continuity and its good. Should the switch light with both them unplugged ? 1 Unplugged?

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Looking at Mack Body drawing 22525262 the mirror heater requires key in ign position to work, moto mirror runs off batt position. 

Unless your circuit is different the “switch mirror defrost” switch should light when on regardless of mirror plugged in 

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According to the wiring diagram, when the switch is turned on there should be voltage at pin87 on the heated mirror relay. The instrument cluster controls the relay ground circuit (pin 86 on relay). The instrument cluster may also have a heated mirror icon which may be why the switch power (pin 4) goes to the cluster plus it acts as a signal to turn on the relay ground. It possibly could be a feature that needs to be enabled through the dealer or using PTT2 if you have it. 

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turckster, I concluded that the instrument panel controls the ground circuit. But you cannot just ground that circuit, you'll pop the fuse. ZDB I felt I should have a lit switch with out the mirrors plugged in. Here is what I have.

At the switch I have power at terminal 2 with the ign on.

When you push the switch I have power at term 4 on the switch, and that power goes to term 27a on the instrument panel.

With ign off I have power at pins 85 and 30 at the mirror heater relay. With the relay plugged in I have power at pin 86 at the relay which sends power to pin 22b at the instrument panel.

I have continuity from pin 12 on the switch to the positive at the mirror. The Ground at the mirror is good.

ALSO.... I took the dash, the relay and the switch and installed them all separately in another 2013 gu and every time the heated mirrors worked.

I'm so lost on this

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Also when I jumper pin 4 and 12 on the switch, press and hold button, I get 12 volts at the mirrors

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When you turn the heated mirror switch on you should have voltage at pin 87 at the heated mirror relay. When there is voltage at pin 87 there should also be voltage at pin 12 on the H/M switch (for the lighted part of the switch). If you don't have voltage at pin 87 (relay) then there is a wiring issue somewhere. You have verified the components(switch, inst. cluster, mirrors and relay) are ok. Measure resistance from pin 4 (switch) to pin 27A (instr. cluster), should be 0-5 ohms. Measure resistance from pin 86 (relay) to pin 22B (intsr. cluster). Wiggle the wiring to see if the resistance jumps around. If those circuit look ok jump pins (relay) 85 to 87 and 30 to 87 , if the mirrors heat up then you know those circuit are ok. 

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ok, so I measured resistance , pin 4 to pin 27a... .5 ohm. Then pin 86 to 22b... 5.0 ohm. I wiggled the harness no changes. So then with everything connected , ignition on I pressed and held the switch and wiggled the connection at the dash panel and the light came on. So I have it positioned where it'll come on every time (so far ) but what would you recommend. I had my suspicions early on that the dash panel itself was bad. But I don't know what to do now. Should I order a dash unit? If I order a dash panel will it need to be programmed at the dealer?

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When you say dash unit do you mean the instrument cluster? If yes I would think the issue is the harness connector terminal. Either the 27a or 22b terminal is loose on the pin. If you are careful you can remove the terminal from the connector and carefully pry the contacts. The reason I say that is you tried this cluster in another chassis and it worked, correct?

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Actually I did pull the connectors apart and pried the connector yesterday. Put it back together and turned on the mirror heat. Then I wiggled the harness at the back of the panel and I could not get it to go off. Thanks for the help turckster!!

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