Hey guys I’m having some heater/blower issues I’m hoping someone can help me with. A few weeks back I was having issues with the AC not blowing cold enough. I refilled the AC and that seemed to fix the problem except for that I noticed the blower will only actually blow at full power for 15 minutes or so before the power drops significantly to the point where I have to open the window to cool off. To be clear, the AC always blows cold. It’s the fan itself that seems to lose power. This morning I turned the heater on but am not getting any heat at all. Heater worked fine up until this morning. Any ideas? Could the no heat/losing power from blower issues be related?

 

In 1994 the heater core is always ON, it is covered by a cable operated "Blend Door", when you move the COLD-HOT Slider you are allowing air, from the blower, to pass thru the heater core(HOT) or be diverted around the heater core(COLD) by this Blend Door
It you open the glove box and fold it down, you can see the cable above the heater air box, move the slider and you will see how it works.

The 3 Air vent selections, panel, floor, and defrost, are Vacuum operated, on the top of the Heater/blower housing in the engine bay is a small Vacuum Reservoir, this is what "powers" the Air Flow selection.
If there is low or no Vacuum, air will only come out of the Defrost Vents, defrost is the default/no vacuum setting for the 2 vent doors.
So if after "15 min" the air flow from blower switches from panel vent to Defrost vent then you have a leak in the vacuum system, usually in engine bay

The Blower motor gets 12volts at all times, its speed is controlled by its GROUND wire.
There is a resistor block in the heater housing in the engine bay, it passes the blower Ground thru 2 or 3 resistors to lower its speed, for HIGH speed, these resistors are by-passed and blower gets full Ground.

This is a cab ground, so make sure the Ground strap from the back of driver's side head to the Firewall is there and tight, no corrosion


The heater core on this model is easy to change, and it does need to be changed as it gets plugged up over the years, if you notice the temp gauge going up and down randomly then its because heater core is clogged up, doesn't overheat just randomly goes above 1/2 then back down to below 1/2

 

your saying if there is a leak, the airflow from the panels will be redirected to the defrost? Without manually switching to the defrost option? As far as I can tell no air flow is being redirected to defrost. Even when I manually switch it to defrost I have very little air flow.

As for the heat, it seems to be working again at the moment. If the heater core is pluggged would I have intermittent heat or would it stop working entirely? I do definitely notice my temperature gauge rising and falling like you said.

 

your saying if there is a leak, the airflow from the panels will be redirected to the defrost? Without manually switching to the defrost option? As far as I can tell no air flow is being redirected to defrost. Even when I manually switch it to defrost I have very little air flow.

As for the heat, it seems to be working again at the moment. If the heater core is pluggged would I have intermittent heat or would it stop working entirely? I do definitely notice my temperature gauge rising and falling like you said.

 

If air flow is not being diverted to defrost then vents are working as designed

I would pull out the blower motor and have a look inside the housing and also check that the blower blade/cage is firmly attach to blower motor shaft, the set screw can loosen up so you get weak or no air flow even with motor on high

Yes, flow thru the heater core can be intermittent
It only takes about 20 minutes to pull out the heater core to see if its plugged up
Remove the two hose clamps in the engine bay
Then 4 screws, 8mm, in the cab, to remove the core

There is a plastic cover under the heater core in the cab, there is one 8mm screw at each corner, one is higher up behind glove box, if you lower glove box down all the way you can see it

Once cover is off put a towel down under the core, because you will need to tip it after you pull it back out from the firewall and slide it down and out
Dump coolant out and heater core should be light as a feather, shake it and if you still here coolant inside its plugged up

Heater cores for these are cheap, and super light for shipping, lol

With the heater core out you can also check the movement of the Blend door, put your hand up inside the core area and move the slider back and forth