Newer HVAC panel blinks

[Jerrys1990]

Morning, over the winter I upgraded my '90 (broken) HVAC control panel to a good one from a 93-94 with the "MAX" button. The issue is, the actuators work, but the positions are not correct relative to the mode or temp selected, and the control face blinks. None of the actuators have been replaced, and I can hear the motors whirring. The symptoms are described here in this thread https://www.gmt400.com/threads/hvac-control-module-blinking-selector.66015/ . I don't have a service manual. Is there a calibration or reset procedure to follow? Or, could the actuators or feedback circuits be bad?

 

[AK49BWL]

Because the older version controllers and newer versions are not compatible, the wiring of the system is slightly different. Older trucks don't even have a connection to the recirc door from the control head.

 

[Jerrys1990]

Because the older version controllers and newer versions are not compatible, the wiring of the system is slightly different. Older trucks don't even have a connection to the recirc door from the control head.

Well that sucks, because another member here mentioned the 93-94 head was a bolt in deal. Also, the recirc, or MAX button, actually works the recirc door properly.

 

[AK49BWL]

Ah yes... The 90 head actually receives a feedback potentiometer signal from the actuator - 93-94 doesn't. This doesn't affect operation so that works normal. Looking between the two wire diagrams, I'm having a hard time figuring out why the actuators aren't moving to the right place, unless they used different resistance values for the feedback pots in the actuators, indicating a need to switch the actuators out too... But I guess I was thinking of something else in my first reply because the wiring is similar enough between 90-94 it should be working.

Complete pin-outs can be seen in these two PDFs.

1990: https://www.ak49bwl.com/other/wires/1990_Silverado_HVAC.pdf

1994: https://www.ak49bwl.com/other/wires/1994K1500/HVAC.pdf


Actually I'm a little confused now because I was under the impression the recirc door was actuated by a second high pressure switch on the compressor.... I just realized I have no idea how recirc works on the early models without the dedicated button lol.

 

[DerekTheGreat]

You can swap a later head unit into an older truck. It might work for a little while, but there's a difference between the voltages sent to the stepper motors. Once the head unit figures that out, it'll start flashing at you.

While it's nice to have the manual control over the recirc door, the earlier ones can be manipulated such that they stay in recirc when desired: Full cold + medium or high fan speeds with A/C will trigger recirc mode.

 

[Jerrys1990]

You can swap a later head unit into an older truck. It might work for a little while, but there's a difference between the voltages sent to the stepper motors. Once the head unit figures that out, it'll start flashing at you.

While it's nice to have the manual control over the recirc door, the earlier ones can be manipulated such that they stay in recirc when desired: Full cold + medium or high fan speeds with A/C will trigger recirc mode.

Different voltages to stepper motors......is that why the doors are not landing in the correct position? Will putting actuator motors in from the same year as the donor truck fix the issue or do I need to return to a non-broken early unit? I had replaced it due to the PC board solder traces being broken. Reflowing the joints didn't solve anything. Later units have ribbon cables, and also the recirc button.

 

[AK49BWL]

I thought maybe there was something to that different voltage thing, but at the very least according to the wiring diagrams, they get 5v from the head regardless of year. So I would have to guess the actuators themselves are different at this point.

 

[Jerrys1990]

I thought maybe there was something to that different voltage thing, but at the very least according to the wiring diagrams, they get 5v from the head regardless of year. So I would have to guess the actuators themselves are different at this point.

Tempted to try the actuators first....they're significantly cheaper than a used (and potentially bad) panel from the Fleabay.

 

[DerekTheGreat]

The manual I've got for '88 shows the actuators running on 8V. The later trucks ('92+) ran on 5V, IIRC:

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So I don't think changing your actuators will make a difference, as the head unit will be checking the reference voltage signal.

 

[packer0440]

I had this problem recently on a 1990, where setting it to come out of dash vents vents would make it come out the floor vents, and setting it to floor would make it come out of defrost. Oddly, at random times air would come out of the dash vents as desired but then revert back

I replaced the actuator by the accelerator pedal and it fixed the problem