HVAC problems

[flomulgator]

Hello all, again :D

Skippable prelude:
So for what seems like the 100th time we have decided to resurrect the beast. It just keeps being cheaper than a new truck, and everyone loves Rojo Grande so it stays. The last trip through the Rockies in the dead of winter did some grim work on a 20 year old, 270k frame though, and the recent turbo install highlighted some existing problems. So I got quite the laundry list to restore this truck. Discovering LMC Truck was like finding a Starbucks in Mogadishu, however, and I now feel equipped to tackle the challenge.

Problem:
HVAC blower will randomly "fall out" of venting anything useful. The speed selector still works and you can hear it blowing furiously, but like a trumpet with a mute. Air kinda leaks out all the vents at the same time, ineffectively. This is a real problem when it's pounding rain or icing up and I need the defrost, and when it's -10 it's hard to get the truck warm this way. It tends to do this after driving for quite a while.

I've looked at the wiring manual to no avail. I got the dash all torn up, and am looking at a pretty worn looking door to the heater core. Haven't the foggiest on how to take it apart though, the core has pressed studs and no one sells the parts that I've found anyways (?). Could it be the selector wheel? The door/air vent valve actuators? This is a pretty serious problem for someone living on the wet side of the mountains and so I need the fix this, no matter what the cost/difficulty.

[87max]

what year are we talking about? Are they cable operated blend doors or vacuum? If it randomly comes out everywhere it is blend door (flapper) related.

[flomulgator]

1991 Suburban

The temperature appears to be controlled by a cable. The blower doors are all adjusted via a vacuum selector wheel on the back of the dash unit. Where the vacuum cables go I can't see.

When it goes haywire I can't tell if it's coming out of all vents simultaneously or blowing through some defect into a random area. The sound becomes muted and unusual compared to any normal setting.

Sidenote: So far I hate vacuum. Is there any reason to like it or is it a good riddance from a bygone era?

[DmaxMaverick]

If the vacuum directional valve (the wheel) isn't leaking (you should hear it), the most likely suspect is a failing vacuum pump. Vacuum leaks are usually easy to hear. Get your head under the dash and listen. Other than that, the directional gate can break at the bellcrank, which leaves it only to "suggestions" as to where you want it to go. When you switch from DEF to Floor, you should hear it "thump" into place. It it doesn't, weak vacuum or broken bellcrank.

[flomulgator]

When it is working it will make a hissing noise while changing from one mode to another. Does this indicate a leaking wheel?

I had a shop check the vacuum for me recently while they were doing a compression check. They found 24psi vacuum at the pump which they said was good.

I looked at it again today and the big trap door that heads upwards to the passenger vents definitely needs new rubber on the door and the rods that actuate it look less than ideal to. Are these parts replaceable and how hard? (I saw nothing in LMC Truck! :eek:)

Lastly, what is a bellcrank?

[DmaxMaverick]

A bellcrank is a "handle", that changes the direction of motion. The lever (where the cable attaches) that moves the mixing gate is a bellcrank.

24" HG (not PSI) is good for the pump. The selector valve should hiss while doors/gates are in motion. If it hisses after, it's leaking. The valve can be separated and cleaned/greased (use dielectric grease). They aren't too expensive to replace, if it comes to that.

Poor door gaskets cause blower air leakage, but it's usually not bad enough cause problems, or even notice, in most cases. The ductwork joints are sealed with foam/tape, and will be long gone by now. What's left will probably crumble in your hand. When they get this way, the ducts can fall out of position, and cause the blower air to cool/heat/ventilate everything under the dash, and little else.

[flomulgator]

Thanks for the letting me know about the bellcrank. I had previously replaced all of the accessible seals (they were crumbling). When I pulled it apart it seemed like it was all well connected, but then again I couldn't really access the defrost before now. THat's my primary problem anyways, losing defrost. It didn't seem terribly well attached, but I just don't know if that is the entire problem.

Is there an internal gate that moves air up to the defrost? I feel like I've taken apart everything that can come off but there is still a lot of stuff in there riveted on or hidden from view. I do not know how the air is routed. I have a valve and will check vacuum where I can lacking any better solution.

Oh, and thanks for all the advice!! I feel like I'm already a lot closer to the solution.

[DmaxMaverick]

It's been many years, so bear with me. I replaced the HVAC unit in my 85, and a couple others, but all more than 10 years ago.

There is a "main" gate that directs air up to the defrost, and the bellcrank is on top of the unit. The ones I replaced all had the bellcrank break just inside the housing. It's a metal rod that runs vertical, supporting the large gate. They broke where the rod is molded/inserted into the bellcrank. This doesn't prevent some motion, but prevents the gate from fully opening or closing. If you have some air at all vents, regardless of the selector position, then it's probably broken there. This is the "suggestion" selection I referred to. The mechanical connection between the bellcrank and gate is sloppy, and prevents it from opening/closing fully. More sloppy is less effect to the selected position.

The last unit I replaced was about $150, but fixed all the problems. I may still have an old (disassembled/destroyed) unit in my barn, somewhere. If I find it, I'll post some pics. Replacement isn't too difficult, but there are some tricks to getting to the nuts in the engine compartment, and maneuvering it out of the dash and ductwork. It's not too bad a job.

[flomulgator]

Okay that's feeling more and more like the problem. Nuts in the engine bay, eh? Sounds like fun. I have the Shop Service Manual and Wiring Diagram but this whole system is not diagrammed anywhere; I'll be flying blind on this (again). Is there any publication that would describe the removal or at least have a diagram?

What all would I need to pull to inspect/replace?

[DmaxMaverick]

There is no "inspect" with these, in my book. By the time you get it out, just replace it. The entire unit is replaced. All the duct outlets, vacuum actuators, gates, doors, the whole enchilada. It's all in the one housing, and none of the individual parts are replaceable. I tried "gluing" it the first time. Lasted about a month. The only things added to it is the blower motor, heater core, and resistor module (or was that on the evaporator....). It's been a long time.

There are several nuts on the firewall (5 or seven, I don't remember, but it seemed like an odd number), holding the unit in place. The unit has studs that pass through the firewall. They are under the insulation cover (covers the evaporator and blower motor). Don't try to do this without removing the cover, it'll just continue to foul your efforts. Once that's out of the way, the nuts can be accessed. It takes a bit of manipulation to get them, but aren't too bad with a wobble socket extension. Disconnect all the electrical, ducting, remove the blower motor and heater hoses, and it will come out onto the floorboard. It's easier if you remove everything in the way, such as ash tray, radio, and glove box. One of them I did just fell out. The others required some force. The gasket is double-sided tape, and can be really stuck if it's never been done before. The new units I got had the gasket attached to the unit, with a peal-off strip to reseal it to the firewall.

[Robyn]

If this 91 is a diesel, I would start by checking the vacuum pump.

These can and do fail. Yesss, you were told it was at 24" but is it that way now.

The HISSSSSSSSSSSSSSING sound is not normal and could be a leak in the HVAC control valve or a hose/tube

The vacuum runs the HVAC doors and if the truck has a TH400 it provides vacuum for the modulator on the tranny.

If its a 1500 series with a 700R4 then the vacuum is just for the HVAC system.

If the issue is off and on, check that vacuum pump.

It needs to be delivering about 20+ inches of HG at the pump.

If you have good vacuum at the pump then check to be sure you have it at the HVAC controls.

The top lever runs the vacuum doors. This is done via the little vacuum switch on the far RH side of the HVAC control panel.

There is a hose assembly that connects to the little valve assembly.

Unhook the hose assembly from the valve, start the engine and make sure you have vacuum at the supply hose.

The line could have broken where it enters the cowl area.

Another common area is the plastic "T" fitting at the vacuum pump. These will crack and or leak

Just check the vacuum pump, hoses and fitting under the hood and to the line that runs into the cowl near the HVAC box, then go inside and check.

Keep us posted

Missy