Heat / AC problem

[Ty1642]

I'm trying to troubleshoot a problem with the heat / AC in my truck. The blower motor runs on all speeds, but there is no air coming out of the vents. If I set the fan speed on high and disconnect then reconnect the purple wire to the blower motor while the engine is running, then air will come out of the vents. This doesn't work at any other fan speed. None of the controls (fan speed, temperature, mode setting) will start air blowing.

I'm guessing that there is a door somewhere in the duct that is sticking, and somehow starting the blower motor by unplugging / reconnecting the power hits it with enough pressure to blow it open. Does anyone have any idea where I should begin troubleshooting?

Thanks

 

[LOWBLAZERO1]

Re: Heat / AC problem

there is a flapper- like door behind the dash that is vaccuum operated, and may need new lines. or if the dash has previously been out, it may have gotten broken (very easy to do- ask me how i know). is it blowing out the vent under the dash?

 

[SY2932]

Re: Heat / AC problem

Sounds like you have a broken HVAC switch to me. Check out my post here.

 

[sly dvl]

Re: Heat / AC problem

Need more info.

Where is the air coming out when it's not coming out the vents?

Do the other settings work normal?

 

[Ty1642]

Re: Heat / AC problem

When the system isn't working, there is no air coming out anywhere. The motor is running and with the AC on I can just barely feel cold air coming out of the outlets on the front of the dash. (It's so slight it feels like it's air that's managing to leak past something.) When I 'jump start' the motor, air starts blowing out of the defrost vents on top of the dash and the vents by my feet. It keeps blowing out of those vents no matter what mode I set the right slider in.

The HVAC switch appears to be OK - I've disassembled the control panel enough to be able to see it working when I move the slider. Is there a trick to getting the control panel out? I've removed the two electrical connectors and the cable for the temperature, but the panel won't pull out far enough for me to get near the vacuum connections on the back. It looks like the hoses are stretched tight and I've afraid I'm going to break something.

 

[sly dvl]

Re: Heat / AC problem

The control cable is usually the hard one to remove (in my Sy anyway) since it's so short. There's a little more give to the vac. lines.

IIRC, there is no door that completely blocks off all air flow. So, even if the controls aren't working properly you should have air coming out somewhere.
With no vacuum applied to anything the air should be coming out the defrost vents. This is a default safety feature (in case of a vac failure) that is mandatory in all vehicles. Once you get the HVAC vac. lines unplugged from the control head see if there's airflow out the def. only.

 

[Ty1642]

Re: Heat / AC problem

When I 'jump start' the motor, air starts blowing out of the defrost vents on top of the dash and the vents by my feet. It keeps blowing out of those vents no matter what mode I set the right slider in.

Forget this part - I realized I was trying to switch modes with the engine not running and no vacuum ball. Air does come out of the vents in the front of the dash when I switch modes with the engine running.

I'll try to get the vacuum lines disconnected and see where the air comes out.

 

[Ty1642]

Re: Heat / AC problem

I figured out the problem with my blower fan. The contacts in the slider switches on the control panel were corroded. On low and medium fan speeds, battery voltage goes through those switches, the blower resistor, and then to the blower. The contacts were corroded enough that the fan wouldn't run on those settings at all. On the high fan speed setting, battery voltage goes through those switches but then it closes the blower relay on the firewall. When the relay closes the fan gets power directly from the 12V power source on the firewall. Somehow, on the high fan speed setting, the fan would run at a real low speed (maybe the relay was just barely closing? - it was a new relay) When I pulled the purple wire off the blower and reconnected it, it must have burned through the corrosion on the contacts enough to fully close the relay.

If anyone else is replacing their control panel, there are a couple of ways to do it:

- Replace the entire HVAC control panel assembly. The assembly is AC Delco #1571585 / GM #16074175. It costs about $110 through rockauto.com or gmpartsdirect.com.

- Replace one or both of the switches (the fan speed switch or the 'mode' switch. The fan speed switch is AC Delco #1571352 / GM #16032480 and costs about $7. The 'mode' switch is AC Delco #1571490 / GM #16047877 and costs about $13. Both those switches attach to the control panel with two screws. The replacement panel assembly includes both of those switches. Power runs through both of those switches before it gets to the blower, so it probably would be best to replace both if you are having problems.

To remove the control panel, you need to disconnect 3 electrical connectors (the two switches and power for the backlights), the vacuum harness, and a cable that connects to the temperature slider. The cable and the vacuum harness are held on by little circular clips that press over pins. The cable is held on by one clip and the vacuum harness is held on by two.

 

[gaugeprower]

Re: Heat / AC problem

You think this would fix myy problem with "weakness" ??? My blower is new, it has good connections... but the airflow coming out of the vents is so wussy... on the highest setting its blowing as hard as it should on the 2nd lowest setting...

 

[gkrcr882]

Re: Heat / AC problem

Bump for similar issue. On my S15, whenever I move the selector switch while the ignition is on, the blower fuse will blow. I can switch between "vent" and "heat" without issue, however once I select "defrost" or any A/C setting the fuse will blow. The kicker is, if I "pre-select" defrost, then turn the ignition on, the fuse will not blow. Blower motor works on all speeds without problem, no aftermarket radios or anything to mess up the wiring. I'm leaning toward replacing both the selector switch and blower speed switch, as well as the relay to be safe. I can't see much else electrical in the system besides these three. I've read about the blower shorting out on the ashtray, however even with it and the entire radio removed the problem remains. TIA.