2nd Gen Non-Engine/Transmission Temperature Control

[Fireman Dave]

I'm having trouble regulating the heat and A/C temperature in my truck. The heat isn't getting very hot - even with the heat control set to high heat. If I run the fan at high or second from high, the heat is ok - but not as hot as it used to be. I think the problem is the actual temperature of the air coming in just isn't as hot as it should be. If I turn the heat control knob toward cold, the temperature doesn't seem to change at all until I get about 3/4 of the way to full cool, and then it pretty much goes to full cool - no gradual change. I noticed in the summer when running the A/C that it was full cold and at about that same point (3/4 cool) it would go right to heat - no gradual change.

I have fresh coolant in the truck, and the t-stat is working correctly. The heater core is not plugged. Vacuum seems to be working fine because all of the other controls are working. I'm thinking the blend door is ok since it does change temperatures - just not gradually. I'm wondering if it's the temperature control switch. The FSM hasn't led me to any conclusions, either. Has anyone else had this problem, and if so, what was the cause? Thanks in advance for any help.

 

[fest3er]

Freak!

Is there any difference in the speed of the hot-to-cold 'switch' and vice versa between fan on low and fan on high? Do you hear any difference? Or feel a 'percussive' effect?

Looking at the FSM (for my '98), that ¾ point makes sense if the door broke in a particular way: such that you can still push the door one way or the other, but when the air flow hits it right, the door gets shoved rapidly to one side or the other. Faster under high air flow, slower under low air flow. But it's unlikely breakage would still let you control the door in both directions. One, yes; both, highly unlikely.

There's a chance one end of the control cable sheath is loose. In fact, this might more readily explain what's happening.

 

[Fireman Dave]

Hey Freak Neal! Thanks for the reply. I haven't noticed any difference in relation to the fan speed, but I'll experiment with it a little more tomorrow. I agree that the door seems to be working - at least to some degree. I've fixed a few blend door actuator motors on Ford minivans at work, and when those break, the door is stuck wherever it was when it stopped working. I don't hear any unusual noises, either. I wonder if the servo motor has a problem, and I'm not sure about the cable you mentioned. I'll try to check into those things as much as I can without pulling the dash. I'm still hoping someone may have had the same condition and can lead me in the right direction.

Thanks,

Dave

 

[BTMaltby]

Hey Fireman Dave,
I have the same thing on my truck - either hot or cold with no in between. Also no defrost as well as a noxious mist from the heater - classic heater core shot symptoms. So this morning I will start by replacing the heater core and then go from there. More later.

 

[Fireman Dave]

Well, I've pretty much ruled out the heater core. Both heater hoses are the same temp - and hot. My best guess is my problem is in the blend door area. I may end up taking it somewhere eventually. I don't think I want to remove the dash myself - especially without being confident I even know how to fix the issue. The FSM is a little lacking with this particular problem. It mentions a temperature control cable as a possibility, then it never mentions or shows that part again. I think that part was discontinued on our trucks at some point, and I'm not convinced my truck even has one (perhaps this was a leftover mention in the FSM that should have come out for the 2000 model year).

BT - your defrost issue could be vacuum related. Although, I think if there's a vacuum problem, it defaults to defrost, so maybe not. I had that once and had to replace a vacuum hose to fix it. I recommend that if you're in there for the heater core, also replace the evaporator as long as your there. I'm on my third one, and I think that one has a small leak now, too.

Thanks,
Dave

 

[klcammie]

One thing to check pull heater fan motor out and look in to see if condenser is plugged. If so it could be blocking the air, maybe that's why you need fan on high. my 2 cents.