Missed with HVAC Parts Shotgun on '99 Suburban

[southsidesmoka]

To Whom It May Concern:

Recently, all of the illumination in my HVAC control head died. It was still functional, but I couldn't see it at night. I figured it was my fault - I probably damaged or broke something on the backside of the old control head when I was shoving it and a fire hazard's worth of audio wiring into my dash. So, I decided to replace the old and malfunctional $85 Amazon-bought unit with something "premium" from RockAuto - I prefer parts my Standard Brand (not their T-series) and don't mind paying for such quality. $200 later, I have a brand new, fully functional AND brightly lit HVAC control unit carefully installed in my dash.

Two days later, the unthinkable happens. The system stops blowing air. I thought the two-year-old blower motor died. So, I replaced it - to no avail. Followed by the high-speed relay and the blower motor resistor. Still, nothing. It was not until I changed the blower motor switch in the new HVAC control head that a hint of functionality returned - the new blower motor started blowing on High.

Neither the connector nor the wires for the blower motor switch show signs of excessive current. I cannot visually find any discolored wires or connectors anywhere. I broke out my DMM and did find that the Lo, Med-Lo and Med-High blower motor switch positions have 0 volts present at the positive connector for the blower motor while the only working position, High, has full system voltage.

I'm not sure what could be wrong and where said problem is located. As such, I would certainly appreciate some help and guidance in bringing my HVAC system back to full functionality. Thank you for your time.

- southidesmoka

 

[east302]

Disconnect the resistor and see if you get voltage on the blue, tan and yellow as the blower switch is adjusted. If you do, reconnect it and check the dark blue at the relay connector.

If no voltage at the resistor, then the issue is between the resistor and the panel - often a melted connector that coats the blower switch pins with plastic and results in a flaky connection. I note that you said you didn’t see any obvious damage, though.

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[southsidesmoka]

UPDATE:

Made a quick drive around the block and all blower settings are dead, again.

 

[Caman96]

What about putting the original functioning(with no lights) part back in. At least for the winter.

 

[southsidesmoka]

What about putting the original functioning(with no lights) part back in. At least for the winter.

I harvested the blower motor switch from that unit and put it in circuit with no results before actually switching out to a new blower motor switch that worked - for a few hours. This no blowing heat in my truck is a real *****. It's been getting so cold at night I can't even stand to be outside to work on the damned problem!

 

[Erik the Awful]

I broke out my DMM and did find that the Lo, Med-Lo and Med-High blower motor switch positions have 0 volts present at the positive connector for the blower motor while the only working position, High, has full system voltage.

On low, med-low, and med-high the current flows through the blower resistor. On high it bypasses the blower resistor entirely. This is almost always a sign of the blower resistor going bad.

...but you say you've already replaced that. Looking at the wiring diagram above, it is going to be the blower switch, resistor, relay, or a wiring issue.

I'd try pulling the blower resistor and jumpering wires to see if you can get it blowing with the switch turned to another setting. Don't run it long - if you notice, each of those run speeds has at least one resistor, and you'll be bypassing the resistors. If that gets it working, your blower resistor is bad.

If that doesn't make a difference, try jumpering from a hot source directly to the output pins from the blower switch to the resistor. If that gets it working, the blower switch is bad.

Finally, pull the relay out and test it, or just swap it with a known good one.

If none of these fix it, start chasing wires.

 

[east302]

before actually switching out to a new blower motor switch that worked - for a few hours.

Just to confirm, the connector to the blower switch is good and not melted at all?

 

[southsidesmoka]

Just to confirm, the connector to the blower switch is good and not melted at all?

Correct, good sir. The blower switch connector looks flawless - no melted plastic or burned wires.

 

[southsidesmoka]

UPDATE:

Blower set on high is working, again. The positive lead wire wasn't on the motor, either due to my forgetfulness, vibration or terminal looseness.

Going to investigate this new resistor as that seems to be where the problem may exist, by process of elimination.

 

[southsidesmoka]

12 V is present on the yellow, light blue and tan wires at the blower motor resistor connector with different blower motor switch settings. It's always such a pain in the ass to unscrew and remove this damned resistor matrix... but, what else could it be?