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HVAC electrical issues

Tiny
CHUCK WALKER
  • MEMBER
  • 1995 CHEVROLET SUBURBAN
  • 6.5L
  • V8
  • TURBO
  • 4WD
  • AUTOMATIC
  • 421,000 MILES
None of the actuators seem to function. Replaced each one. Each one had broken gears inside so I felt that was the problem. But they still do not work.

So I replaced the control module. That did not solve my problem either. I checked all the fuses. So now I have no heat because the blender door is in the "cold" position and I only have air blowing out the floor vents because that is what position it is stuck on.

Everything inside the air box is free and moves easily. It is like the actuators just are not getting any commands to move.

There are three wires going to the actuator that controls the hot/cold blender door. With the key on my volt meter says there are twelve volts on all three wires. As this is a plastic part mounted to a plastic box, one of those must be a ground. I can see one wire moving the door in one direction and the other wire moving it the other direction. So having voltage on all three has to be a problem. I put the old control panel back in just in case but it made no difference.
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Monday, December 11th, 2017 AT 4:10 PM

10 Replies

Tiny
STEVE W.
  • MECHANIC
  • 13,577 POSTS
Your description screams, You lost the ground for the actuators. One side of them gets twelve volts, the other ground, the center wire feeds back to the control head with a voltage between those to tell it to stop at a particular spot. If you are seeing twelve volts the ground for that part of the system is open.
It commonly grounds on the back of the cylinder head and is shared by the blower motor. As the blower motor is working I would check the area of the splice, or simply run a test ground from battery negative to the black ground wire on any of the actuators.
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Monday, December 11th, 2017 AT 8:56 PM
Tiny
CHUCK WALKER
  • MEMBER
  • 7 POSTS
Actually, I had the same problem with the blower motor over the summer, but everything else was working at the time so I cut the factory ground wire and just ran a new ground wire to the blower. Did you mean to say cylinder head or control head?
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Tuesday, December 12th, 2017 AT 6:18 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,577 POSTS
The "normal" ground is spliced to the ground that goes into the engine bay and grounds on the block. Sounds like it has failed. As you already have a new ground for the blower the high current draw is taken care of, you just need a new ground for the rest of the system.
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Tuesday, December 12th, 2017 AT 5:33 PM
Tiny
GRGROVE
  • MEMBER
  • 4 POSTS
2008 Suburban with 5 wire connectors to the actuators. I'm getting 10+ volts on all 5. I'm thinking I've got the same issue. How should I test the ground coming into the HVAC Control Unit? Just a continuity check? If it's shorted, how do I fix it? New wire to ground/frame? Thanks!
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Sunday, July 26th, 2020 AT 11:38 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,577 POSTS
Easy way to test it would be to run a test ground jumper from a known ground to the ground feeds on the HVAC. Those are the black wire on pin 10 of connector X1 and the black wire with white stripe on pin 3 of the same connector.
Wiring wise for all the actuators the light blue with black wire is the 5 volt feed. The yellow wire is the low reference -ground. The brown wire is the 12 volt feed. If putting a good ground on the module doesn't do the trick you can try one attached to the yellow ground wire on one of the actuators.
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Sunday, July 26th, 2020 AT 1:06 PM
Tiny
GRGROVE
  • MEMBER
  • 4 POSTS
Thank you sir. I appreciate the quick response. I pulled the head unit and there are 4 connectors: J1-J4. J2 is the only connector with a black and black/white wires. But that blk/wht might be black/yellow. What do your think? How best should I jumper in a ground wire? I don’t think my voltmeter probe will fit down to check it.
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Sunday, July 26th, 2020 AT 2:45 PM
Tiny
GRGROVE
  • MEMBER
  • 4 POSTS
On a side note, I disconnected my battery and waited 30 minutes. I checked the parasitic drain and was seeing 4.6 amps. I went through my fuse box and pulled fuses until I got to the horn fuse and the drain dropped to.1 amps. I just left that fuse out, but still no joy on HVAC. I'm wondering if I have a ground short somewhere.
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Sunday, July 26th, 2020 AT 2:49 PM
Tiny
STEVE W.
  • MECHANIC
  • 13,577 POSTS
Well that is unique. The horn fuse only feeds the two horns, it's after the horn relay even. I would think you have an issue in the fuse block itself. Do the horns work?
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Sunday, July 26th, 2020 AT 5:04 PM
Tiny
GRGROVE
  • MEMBER
  • 4 POSTS
Thanks. I'm thinking the same thing now. History: I took this to the dealership to help troubleshoot a similar HVAC problem I was having, long story short, they found a Pos+ wire under the fuse block that had shorted until almost severed. They repaired this wire and and it fixed the issue. Wondering if this a reoccurance. I pulled the fuse block out as far as I could and couldn't see any glaring issues. Perhaps it's time to take it back to the dealership again and pay out the nose.
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Sunday, July 26th, 2020 AT 6:21 PM
Tiny
STEVE W.
  • MECHANIC
  • 13,577 POSTS
It could be collateral damage that happened at that time and is just showing up now. If the wire got hot in that spot it may have also been warm enough to melt in another spot enough that the insulation became thin and it finally failed. Or it may be internal to the fuse box itself but caused in a similar manner. Hard to say without taking things apart and tracing the wiring.
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Sunday, July 26th, 2020 AT 7:36 PM

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