2nd Gen Non-Engine/Transmission Cab heat/Vacume system question

[Dennis Garrett]

Since new, my 97 has had pretty good cab heat once it finally warmed up. This winter is different. The only way I can stay comfortable is to use High on the temp knob, and Recirculate. If I turn it back to floor/vent, or floor, the temp drops to luke warm. But lately driving around town, I've noticed if I put the truck in nuetral and kick the RPM up to about 2000, it blows hot air again within just a few seconds. But when it idles down, just as quick, it goes luke warm again. Then out on the road crusing at say 1700 RPM, it goes luke warm again until I go back ro Recirculate, then it blows hot again. Wondering about a vacume failure someplace. Can't hear a hiss with the engine running, what about the pump or vacume motor under the dash? All the lines under the hood look OK, but kinda don't know where to look behind the dash. I did take the blower fan out and every thing up in the cavity looked OK, and the door seemed to seal up.

 

[RBratby]

check your coolant level. my wife was complaining of the something in her car. i checked the coolant level and it was very low.

 

[McLaughlin]

Is your fan running at full speed? I've read some reports the resistor that controls fan speed on the 2nd gens has a tendency to fail.

 

[ubcummins]

the dash controler may be at fault if it is failing to shut off the air from the outside,are your vents all switching properly?

what is your engine temp?if t-stat is bad are stuck open the water won't get hot enough to heat the cab.

also check the control valve on the heater hose,have someone turn the heat on and off while you watch to see if it is working. :-laf :-laf :-laf

 

[Dennis Garrett]

I don't think it's any of that, I changed out the T-stat a while back, but it had stuck open in the fall. The eng temps seem normal, and the lines in/out of the core are hot & hot as hell, so I know hot water is going to the core and goving up some heat before returning. The water level is where it's supposed to be, so that's not it. I think the problem is RPM related which makes me suspect either low vacume at the pump, or a leak someplace, causing the mixer door or something to not work properly. When the RPM jumps up to 2k, things get normal pretty fast. When it slows down, things quickly cool down. I do have all the positions on my selector switch so that's not it. I haven't taken it out to look at cable slack when closed, might need to shim it or adjust it somehow if it's holding the door open a little. (not sure how it all works under there yet)

 

[MattSt]

Dennis,





Did you ever figure this out? I have been having the same issue with my 01 this winter. I have a 10 mile commute to work and even though the temp guage is 180 I only get barely warm heat. I haven't tried the recirculate trick, in fact I don't think my truck even has that option unless in cooling mode.

 

[nps]

On a 97 the brake booster is vacuum, so I'd expect that the brakes would be out before the heat. I'd suspect something in the area of the blend door controller or the vacuum source leading to it.

 

[Dennis Garrett]

Never got to the bottom of it. On the nice days I forget about it, and when it's cold, I don't want to be out there tearing it apart. I do think it's something behind the dash, because at idle it takes maybe 20-30 seconds for the controls to change the flow of air in the vents. At 2,000 RPM, it's faster, but still takes too long. And, then sometimes I have hot air, sometimes I don't. I can't hear it "hiss" when I change anything, so I think I have a weak vacume motor or pump. Maybe when it warms up... .....

 

[MattSt]

I hear ya! It's been so warm here that I had forgotten about it. But last night it got down to 15 and the ride into work wasn't pleasant at all. :{ I have been considering getting a 12v cab heater and just not worry about it. The real problem is if it starts snowing and I need to plow I can't keep the windshield warm enough to melt the snow.

 

[MattSt]

Dennis ,



How are you switching to recirculate? Mine does not have that option. I just got back from lunch and tried your defrost/floor option trick and it blows heat after about 20 blocks! I can't decide if I'm more ****** that it doesn't work when set to floor or that I have been freezing my *ss off for months and all I needed to do was switch it to that position. If/when I get a chance to actually fix it I will post. At least I will be warm for now. Thanks for the tip.