Blower Motor/Climate Control Issues?

2008 GMC SIERRA
230,000 MILES • 5.3L • V8 • 4WD • AUTOMATIC
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LOGANC573
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Hello, I just picked up a very used truck listed above, and I was initially told that the blower motor wasn't working on it. I did a little bit of research and decided to throw a new resistor in it to start with. That didn't fix the issue, so I went ahead and bought a new blower motor as well. Also, no good. So, I've been scratching my head at this point trying to figure out why I can't get the blower motor to work. I've tested the power from the head unit to the resistor and I'm showing just under 12v. Coming out from the resistor I'm only showing roughly 7-8v. But the blower motor still has no life to it. I tested the signal wire and ground from head unit to resistor and I'm not showing much of a signal at all. Roughly .06V. As I change fan speed, it will move, down to .05, .04, etc. I jumped from the power and ground of the head unit to the blower motor rather than running it through the resistor and it fired up immediately. So, I'm losing power somewhere between the head unit and the fan. Any ideas on what to look at next?
Nov 9, 2022 at 10:52 PM
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STRAILER
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I don't show a speed resistor like in older cars it has a speed control module, is that what you replaced? Here is the location in the diagrams below and I have the blower motor wiring diagrams to help you see how the system works. Please check the grounds again. Check out the images (below). Please let us know what you find. We are interested to see what it is.

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Nov 10, 2022 at 9:26 AM
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LOGANC573
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Yes, that’s correct! I replaced both the control module and the blower motor.
And I apologize, I’m not super familiar with reading these diagrams. Where would be the main ground locations that I need to double check and ensure they have a solid connection?
I also work odd hours, so I apologize if my replies aren’t exactly fast.
Nov 10, 2022 at 11:29 AM
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LOGANC573
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I found what I believe to be G200 under the dash trip near the A pillar on the passenger side. It looked fine, but I took it apart and sanded the connector a bit. Put it back together and still no luck.
Nov 10, 2022 at 9:54 PM
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STRAILER
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No problem, can you please turn the system on and see if you are getting power to the red wire of the speed controller?

Here is a guide to help:

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

Please go over this guide and report back.

Nov 12, 2022 at 9:12 AM
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LOGANC573
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So, I had temporarily jumped power around the control module as we had a bit of a freeze. We had noticed a high amount of resistance on the connector between the control module and the blower motor itself. After replacing that, we now have 10.5 V running through the connector at the lowest speed, but voltage is dropping as you turn up the speed. Currently 5.5 V at the highest speed. This should be working the other way around, correct? I would assume the higher speeds should be using more power. Even with power making it through the connector, the blower motor isn’t working at all, though it worked when sending power straight to the motor rather than through the control module.

But it all in all it does seem we have power making it through the system now. I’m beginning to wonder if the new control module is a dud.
Nov 20, 2022 at 9:05 AM
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LOGANC573
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I had my brother prod around a little bit and he’s saying it appears there isn’t much amperage being delivered into the resistor/speed control module. He said he was only reading roughly 45 milliamps into the speed control module and basically nothing into the motor itself.
Nov 20, 2022 at 10:18 AM
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STRAILER
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So, are there 12 volts at the red wire for the speed controller? Please check out the HVAC wiring diagrams above to see what I am talking about.
Nov 20, 2022 at 12:51 PM