98 C1500 HAVC blower intermittent

[HotWheelsBurban]

The '95-up 400s are notorious for eating the HVAC switch in the dashboard. We had to replace the one in our first '99 Burb 3 times to get a good one, all Delco. 15 years ago when Delco was still good stuff.
Recently on my crew cab, the blower was slower, then quit altogether. I took the motor out and checked it, directly to the battery, and it worked. So I put everything back together, checked the HVAC fuse (25A) in the dashboard fuse box, replaced that and everything works again. Fuse did not look bad or blown, but I've noticed that my 400s like brand new fuses better than old ones.

 

[kolgeirr]

I have replaced the blower motor and resistor but still no fan. Ideas?

It's a pretty simple circuit outside of the HVAC controller. Assuming your new motor and resistor are actually good, and the connectors for both are OK, the only things really left to check are the fuse mentioned above and the HVAC controller itself.

 

[Sean Sills]

I saw what looked like a relay next to the resistor. Is that HVAC?

 

[kolgeirr]

I saw what looked like a relay next to the resistor. Is that HVAC?

It's been awhile since I've been in mine, but assuming I'm remembering correctly, the resistor handles the Low/2/3 fan speeds, and the relay is for the High/Max fan speed. When set to High at the knob, the controller clicks that relay on and the fan is directly powered through those wires, bypassing the resistor.

 

[Orpedcrow]

the resistor handles the Low/2/3 fan speeds, and the relay is for the High/Max fan speed

Yea that sounds right.

Here’s some pictures of the harness if it will help.

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Some things to check would be the terminal connectors on the black and purple fan wires, make sure they aren’t melted and crusty. The ground terminal (brass L shaped in picture) on the new fan needs to have a clean connection to the metal case of the fan motor (you can scratch the paint off with a razor blade). Check the 2 wire black plug for melting and corrosion. Check the fan speed switch in the hvac control module also for melting and corrosion.

 

[Sean Sills]

Right now I have zero fan speeds.

 

[kolgeirr]

Right now I have zero fan speeds.

Check voltage between the two plugs that go on the blower motor with the knob set to max fan and the key in the Run position; should see 12V. If not, you have a controller/relay/wiring issue. If 12V on High but 0V on any other speed, probably the resistor. If you have voltage at all settings between those connectors you probably have a DOA blower motor and need another new one.

Can test the blower motor while out of the truck by applying 12V across the two terminals; positive is the insulated one closer to the center that gets the purple wire. Lower one in the above pic.