73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Interior & Equipment => Heating, Ventilation & Air Conditioning (HVAC) => Topic started by: doneebaker on February 14, 2026, 04:52:36 pm
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Hello folks , new user here. I have a 74 c10 with a/c that I cant get the heater motor to do anything. It will run on slow mode when I turn on the key but nothing else. While poking around in there I see an orange wire hanging from the hvac vacuum switch on the right side of the controller just hanging there not connected to anything and I can't see any other wires hanging down it should be connected to. I assume that it needs power but we all know about assuming things. Every thing else is intact. So if anyone would know where that wire goes to or what it is connected to that would be a huge help. I tried to find diagrams online but no luck. Thanks in advance for any help..
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This is exciting! It's been quiet around here lately. Welcome to the Forum! (Que the music Zieg!)*
Using a test light, does the "dangling" orange wire probe battery voltage (B+)?
Assuming factory A/C, is the 4-wire harness with brown, yellow, light green, and light blue wires plugged into the blower switch (see attached diagram)?
Does the brown wire connected to the blower switch probe 12 volts with the ignition switch on?
* Inside joke...
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Thanks for the reply. So I probed the brown wire and it does have power with switch on. The orange wire does not. The orange wire in question is the one on the bottom right of the diagram. Connects to 52 , 2977253, in the picture. The diagram does not show where that goes to, and that is my question. And yes it is all original factory stuff.
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As a WAG that dangling orange wire is for a different model application and should NOT be connected to anything on your truck.
Unplug the blower switch and jump the brown wire to the light green OR light blue wire in the blower switch connector. With the ignition switched ON, does the blower motor spin at Medium or Medium High speed, respectively? Now, jump the brown wire to the orange wire in the blower switch connector. With the ignition ON, does the blower spin up to High Speed?
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I checked them tonight and the motor does spin up on the blue and green wire, but does not on the orange wire.
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At the very least, it sounds like you have a bad blower switch. But, there is more to the story and problem. Refer to the posted wiring diagram. The 18-gauge orange control wire from the blower switch is for high blower speed; it energizes the high blower relay to supply B+ power directly to the blower motor via a 15-amp inline fuse and 10-gauge orange wire that are physically restricted to the engine compartment, bypassing all lighter-gauge wiring inside the cab. Odds are that the plastic inline fuse holder connected to the firewall B+ junction block (located somewhere above the passenger-side engine valve cover) is melted, severing the high-blower fuse connection to B+ power.
Does the high blower relay "click" when you jump brown to orange at the blower switch with the ignition ON? If "YES", replace the factory inline fuse holder and AGC 15 glass fuse. I recommend upgrading to a 20-amp mini-ANL fuse and holder to help alleviate a repeat failure. If "NO", replace the high blower relay, then recheck the high blower function (an underlying problem still could exist with the inline fuse holder). Before replacing the high blower relay, read the Technical Article, "Blower Motor Relay info (Keeps blowing fuses) (http://forum.73-87chevytrucks.com/smforum/index.php?topic=24383.msg203763#msg203763)."
Edit: The foregoing assumes the relay control wiring is intact and properly connected. Before replacing the high blower relay because it doesn't 'click', use your test light to validate control power to the relay coil via the 18-gauge orange wire from the blower switch. You should also confirm that the relay coil is effectively grounded via the connected 18-gauge black wire.
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Check this fuse and replace if blown.
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Well I did some probing so to speak. I replaced the inline fuse as suggested and verify power to the relay, all working . Next i installed a new control panel to spruce it up inside with a new switch. Plugged in the switch and it would work intermittently. I discovered one of the terminals inside the plug was broken and not making good contact. Repaired the broken terminal and it works as should.. Thanks for all the help I really appreciate it.
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Thanks for the update. Glad you got it sorted out. We're always happy to help a brother in need.
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the power wire to the heater is brown per the wiring diagram. The orange wire may be for your cigarette lighter. At least it's close on the diagram. In that your fan turns slowly you have power going to it. The old trucks had a constant low fan output whenever the ignition is on. Kind of a fresh air thing. Anyway, the fan wire ties to the resistor pack which is inside the AC box and easily removable with just two screws if memory serves. The speed switch switches in more and more circuits from those resistors, which look like little heater wires, but they don't get hot. Just low resistance resistors.