1998 Jimmy SLT heater problem

Sounds like you have a bad heater door actuator.

If the engine is reaching normal temp, and both heater hoses are hot, you may have a problem with one of the blend doors under the dash.

Because the Main Gear in the Blend Motor has cracked or teeth have chipped and is now not keeping the heat/cold door to the Hot position. 3 fixes: Buy a new Blend motor, takes about 5mins to replace. Buy a new Blend Motor Gear, will take about 30mins to fix. Repair current gear with Epoxy, takes about 30mins and 12hrs for the glue to set up strong enough to use it.

Suggestion: see if the blend door opens or closes completely to allow all air through the heater core. To see the blend door moving, remove the panel that says "Cut here for service" under the hood along the firewall. I have a 99 Bravada where the blend door keeps moving back and forth constantly. Changed the blend door actuator but still keeps moving. The air inside is only warm since the door keeps allowing bypassed air through.

Most likely the problem is a broken blend door inside the heater box or a broken adjuster cable going to the blend door, the blend door inside the heater box is responsible for heater output depending on your temp adjustment on your heater control panel, most likely the blend door is stuck in the cold position allowing cold air to come through the vents verses allowing the blower motor to blow warm air past the hot heater core to produce heat, check the temperature adjuster cable behind your heater controls and make sure its connected and not broken, lastly make sure the blend door isnt broken and operating freely.

may be the blend door actuator

sounds like you need a new blend door motor. this motor is in the heater box and controls the blend door that lets eather cold or hot air inn or a combonation of both

I have the Heater/blend door to replace the old one in my 98 gmc jimmy. I cant get the part to slide on the shaft. Any ideas?

The heater blend door breaks on the 1997 to 1998 Explorers, you will need to replace the heater plenum case.

I fixed the problem. It was the blend door actuator, but the wheels continued to turn because the reset operation requires resistance to know when the blend door is at its limit for the hot and cold side. At least that is what I assume. If the actuator is tested off of the blend door, it will simply turn until it can't turn anymore. After installation, it worked fine.