My 98 SLT has been acting up since the HEATER weather started. Sometimes when I turn the fan blower on for the front heater, the blower sounds to slow and there is hardly any air blowing out. After tapping the plenum box under the glove box it runs at speed. Lately it's happening more and tapping the plenum is not helping. I would guess a blower bearing or connector under the dash.

Anyone have similar probs, or know how to troubleshoot this?

 

It could be several problems. The fan motor resistor is one, switch/wiring and worn brushes in the motor itself. The resistor is the most likely one but anything is possible.

 

I have an 01 suburban that had a similar problem. Only two of the five fan settings were working, but after tapping the plenum one of the settings returned. I finally replaced the fan motor resistor (two screws and harness disconnect) and all five settings are working normally. I think the part was about $25.

 

edpdx: welcome to the forum.

I would check all of the connections first before I replaced anything.