HVAC Problems

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HVAC Control Information - How to tell if you have the upgraded HVAC

1992-3 Control Head1992 - 1993 are completely different systems. They use R-12 refrigerant and have mechanical cable operated controls just like the military Hmmwv's.

1994-5 Control Head This is a 1994 and early 1995 Control head. Later 1995 models came with the later head used in 1996. These vehicles came with a electric servo-motor flap control system and a button for the A/C. The heater valve on these systems is electrically controlled.

1996 ControlsThe picture on the left is from my 96 which had the old obsolete hvac. If you have buttons on your controls you have the old system. Some 1995 and 1996 thru the first half of 1997 vehicles came with a electric servo-motor flap control system with a recirculation button for the fresh air flap and a button for the A/C. The heater valve on these systems is electrically controlled.

These systems had so many problems that AMG stopped using them in the middle of 1997 and replaced the old ones that were still under warranty.

Problems with these systems include poor performance; especially the A/C, control heads that burn out and the primary problem, leaky heater cores that are all but impossible to repair.

The control heads on these trucks are the same as Chevy-GMC trucks and C/K pickups. Nacho Corella gave me permission to put his article that has directions to fix the control heads on these trucks on my website.

98 Controls1997.5 and later, as well as trucks with the factory upgrade A/C system have a vacuum servo controlled flap system with only a MAX A/C position on the control dial; no buttons. The max position is the outside air closed/ recirculate position. On all of the trucks the temperature blend door is controlled by an electric servo motor and they all use R134a refrigerant.

On the left is the control panel from my 98 which uses the new hvac system.

Vacuum Valve There are 2 kinds of heater control valves depending on the year of truck you have. 1997.5 and newer have a vacuum controlled valve pictured on the left.

The 1994 to 1997 trucks (old style HVAC) have an electrically controlled valve pictured below. The photo on the left shows the valve with the actuator removed. The picture on the right is the electrical actuator. There were a lot of problems with this setup. The electric actuator would twist away from the valve lever and the valve wouldn't open or close. If it got stuck open the A/C wouldn't get cold because all the hot water was heating the heater core. If it was stuck closed you wouldn't get heat.

Heater Valve Electric heater valve actuator


The Front Passenger Noticed Water at her feet. There Wasn't Any Smell so it Wasn't Antifreeze

Drain tubes Your condensate drain is plugged up. That's the water at the passengers foot. Drain lines are located under the passenger kick panel. These have been known to pull open. These lead to a black tube that goes under the truck from the A/C evaporator. It ends in a duck bill (flat end). It gets plugged up under the truck and backs up the condensate from the A/C and spills over in the passenger compartment. I've also seen the tube end broken off the black plastic trough.

According to AM General TSB, the Kazoo attachment on the end of the condensate drain should be removed on civillian H1s. This is a short tube with a flat duckbill end that the condensate water from the airconditioner drains through. It has the flat outlet to keep water from coming up into the hvac unit when you are fording in deep water. They have a tendency to block up causing the water to backup into the passenger compartment.

Blower Problems Page

I'm not getting any A/C and the compressor clutch isn't engaging

My A/C of course stopped working on the first hot day of the summer. When I turned it on I didn't hear the 'click' of the compressor clutch engaging.

The first thing I did was check the fuse for the compressor. It was fine. Then I measured voltage at the clutch connector on the compressor and there was none. When the hvac control head calls for A/C it outputs 12v through a high pressure cutoff switch, low pressure cutoff switch, ambient pressure switch and a thermostatic cycling switch which are all in series. If all of these switches are closed it energizes a pin on the ECU telling it that it is ok to start the compressor.

The ECU then outputs a 12v signal to energize the compressor relay which in turn energizes the compressor clutch. All of these switches were fine and there was 12v on the wire going back to the ECU.

The next thing on my list was to check the compressor relay. The problem was that I couldn't find it. After making a few calls it was suggested that I look under the passenger side crash pad and kick panel. I pulled the kick panel off and there were 2 relays just hanging there. I grabbed the relay block to get a closer look and found that the relay on the left wasn't pushed all the way into it's socket. I pushed it in and checked out the A/C and low and behold it now works. What luck!

I'm not getting any heat in the winter and the A/C is blowing warm in the summer.

heater control valve

First check the outside air temperature. The Hummer has a cutout switch in series with the air conditioning compressor clutch that will prevent it from actuating if the outside temperature is colder then approximately 45 degrees. It is located on the right side of the cooling stack tied to the side of the radiator. It is plugged in to the harness near the A/C's filter drier.

The rear auxiliary heater/ cooler is on the same system as the front. When the defrost is selected it calls for the A/C to come on. This also sends refrigerant to the auxiliary cooler. When it's cold outside it would cause cold air to blow out of the rear auxiliary unit freezing out the passengers. The cutout switch prevents the A/C from engaging when it's cold out. If the switch is defective it will keep the A/C clutch from activating.

All the trucks have a heater control valve. This valve lets hot coolant flow into the heater core when the control is set for heat and turns off the hot coolant when the A/C is on. If this valve gets stuck it will cause the above problems.

Vacuum Valve

There are 2 kinds of valves depending on the year of truck you have. The 1994 to 1997 trucks (old style HVAC) have an electrically controlled valve and the 1997.5 and newer have a vacuum controlled valve pictured on the left. The electrical valves had all kinds of problems. The actuator would twist itself out of the valve body leaving the valve stuck in a random position. The vacuum controlled valves are dependent on 18 to 22 in of vacuum from the vacuum pump. They also can get stuck if their mounting clamp is too tight.

Removing the heater valve can be a real pain. Getting to the clamps is the hard part. Sears Craftsman (00947390000-dlv) and Snap on make a special tool for removing the spring clamps. The Craftsman tool is 40.00 and the Snap on tool is 120.00 (in 2005).

Hose clamp toolCheck the air doors for proper operation. Sometimes you have to reset the linkage at the actuator. You loosen the nut that holds the linkage and set the temp on the control head to full hot. Once the actuator rotates all the way to full hot, adjust your linkage so that the blend door is full open and tighten the nut back up on the actuator. Go to full cold and watch the actuator close the door completely. Your heater / water valve under the hood inward of the surge tank will have a vacuum line going to it and it is made of black plastic. Make sure that the plunger is all the way down or open when the heater is on.

How do I know if I need a new heater core?

If your heater core is shot you will get hot sweet antifreeze smelling vapor fogging up your windows. This means your heater core has probably developed a leak. Read this for information on heater cores. Do not pressure test a heater core over 15 psi. You will distort the tubes and stress the structure.

If I turn on just the fan, it’s hot air that blows out. Not quite like having the heater on but close.

If you have just the fan on you won't get any really cool air. If the air is warmer then the outside you could have a leaky heater valve. This valve routes hot water to the heater core when you call for heat. It should be off when you call for cool. If it's leaking hot water into the heater core it could negate some of your A/C. Other problems could be problems with the control vents or the control head. Very early trucks use mechanical cables to control the vents. In the 95 era the controls were electrical. There were lots of problems with the electrical actuator on the heater valves. All the newer valves are vacuum controlled. You should check the vacuum pump for 18 - 22" hg.

The air conditioner does not blow real cold. We charged it up and it just blows kind of cold.

When I re-charged my A/C, the local shop had a new product called ICE 32. I was skeptical at first, but they added ICE 32 to my A/C system and it made for a SIGNIFICANT difference in A/C performance. The A/C blows freezing cold; colder in the front than before and freezing for the rear HVAC. Other owners report that alternate refrigerants don't really help.

Make sure all the air control doors are operating properly. You might want to seal up all the duct work with silicone and duct tape.

For pre 97.5 trucks: Unfortunately the A/C in your truck is marginal at best. It will probably never really blow cold. Make sure that you have the proper amount of R134 in the system. Too much or too little will cause degradation in the cooling. AMG had a heavy duty A/C upgrade (I had this on my 96) which consisted of a new condenser, electric fans for the top of the cooling stack and a new drier. This somewhat increased the capacity of the cooling system. If I remember this option was around 600. I don't know if they even offer it anymore because AMG has completely scrapped the old style systems. The only real fix is to get the retrofit new HVAC system for your truck.

One experienced Hummer tech said that if you install manual shutoff valves on the heater core your A/C will work much better. He claimed that hot water will get into the heater core even though the heater valve is closed.

The blower is running but air isn't coming out of the vents or when I change the control the air won't shift between the upper and lower vents.

This is almost always the vacuum pump. (on trucks with the newer style HVAC system 97.5 and up) A diesel engine doesn't produce vacuum like a gas engine does so it has a belt driven vacuum pump. This pump drives all the vent doors, the heater valve and the turbo waste gate.

We pulled apart the control head and checked the vent doors. Nothing was moving. We had 5" of vacuum which seemed like it was enough. We went over and checked another hummer and found it had more then 20". Then I looked in the book and it said we should have 18 - 22" of vacuum.

You can find a hose to hook a vacuum gauge to near the fire wall on the passenger side. The vacuum pump line runs from the pump which is below the alternator on the passenger side of the truck to the top of the engine where it meets up with the waste gate. A vacuum line runs from there to the firewall where is goes to all the HVAC door motors. There is a bundle of thin control lines that continues on across the doghouse to the control unit. Look for pulled out or kinked lines. In some cases the control head or a door actuator is defective.

We installed a new pump and the HVAC worked perfectly. I noticed that my engine sounded different. I have a boost gauge and could see that the engine was getting much more boost then it ever had. I used to pull away from a light and get 4 to 5 psi. Now it shot right up to 8 to 11.

The Napa part number for 6.5TD engines is 641018. Otherwise get it from AMG.

Replace Vacuum Pump

Why did my A/C compressor blow out?

The fan clutch is what pulls air through the condenser (radiator for the ac is in front of the real radiator). If it isn't working properly then you will have high head pressure on your A/C system which can over stress and ruin your compressor. It is probably the number one cause of compressor failure.

The AC clutch would kick in and out repeatedly. It started after recharging the system with a can of refrigerant. We were charging the system due to a suspected leak. Seemed the low and high pressure switches (one located on the AC dryer and the other at rear of AC compressor) were OK because we swapped them from another truck and observed no change. Finally got it working OK or so I thought. It was cooling better than it ever had. A week later I noticed it putting out much warmer air and assumed the suspected leak was not fixed right. Then the AC compressor started making a big racket every time I would turn it on. Turned out it contained 4 pounds of refrigerant instead of the normal 2 pounds. We unknowingly overcharged the system and it cost me a new AC compressor.

My compressor burned out twice on my 2001 wagon. The problem was traced to an electrical problem that was causing the clutch to engage and then disengage rapidly. This must have heated it up which caused the exact symptoms you described. (including the long tow trips). The first time the dealership replaced the compressor. It failed again about 6 months later.

The same thing happened to my 99. It ended up being the low pressure cut off switch; the little rubber boot with the 2 wires sticking out screwed into the filter drier. It had came loose and kept engaging and shutting off causing it to finally seize up. This switch is wired in series with the low-temperature cutoff switch and the compressor pulley engagement coil.

Intermittent rear A/C unit blowing cold then after a few minutes blows hot

I've seen this on my Hummer. It only happens when the main A/C drain hose is blocked, which fills the A/C unit with water. When this happens, the rear A/C blows hot and cold as you describe. I think it is because the evaporator is blocked by water, and the 'cold freeze up switch then kicks in.
I don't think expansion valve. My guesses in order:

1. Plugged drain
2. freeze up switch
3. controller
4. Bad expansion valve


Another tip to cool your hardtop down in 100 degree + Weather

Reflectix insulation on the roofI glued Reflectix insulation to the metal roof of my 98 wagon. for temperature control, not acoustics. Reflectix is a bubble wrap with heavy foil on both sides. It's sold in home improvement stores for home insulation. A 2' wide roll should cost about $20. Just cut it with a scissors and use spray on glue.

Tropical Severe Climate Kit

This is only for the older style HVAC systems between 1994 - 7. The A/C on my 1995 really didn't put out enough cold air for my liking and I live in the Chicago area where it's hot in the summer but not nearly as hot as the southern states. When I got my 1996 I wanted something to jack up the cold. I found that AMG had a kit available for the A/C that would increase it's capacity. It consists of a new condenser, filter drier and an electric fan on the cooling stack. If you have a 1997.5 or newer with the new style A/C you probably don't need this.

The part number for the kits are:

05744670 for the 1992-1996 models 05744668 for the 1994 only models

Installation Instructions 05744669 1
Fan Control Wiring Harness 05744667 1
Fan Power Relay 06004055 1
Fan and Mounting Legs kit 05744657 1
Condenser Cooling Fan 05744656 1
Cooling Fan mounting legs 05744655 1
Condenser Installation kit 06003578 1*
Condenser stand-off spacer 05743412 4*
5/16-18x2.5" Socket Head Bolts 05744143 4*
5/16 Flat Washer G 02436162P2 4*
Receiver/Dryer 05743473 1
#6 A/C O-rings 06006173 3
#10 A/C O-rings 06006175 1
1/4-20 Nut w/ lock washer G00271172 4
1/4-20x3/4" Bolts MS 000907286P2 4
Tie Wrap MS 000033671 6
Tie Wrap MS 000033673 1
* Not needed /included in the 1994 model kit.

The kits, are approx. $700-$750 for the 1992-1996 kit and $225-$250 for the 1994 kit.

For 97.5 and higher with the new style HVAC system you can use the following parts which add just the electric cooling fan to the truck. This might help the A/C stay cool while the truck isn't moving.

Fan Control Wiring Harness 05744667
Fan Power Relay 06004055
Condenser Cooling Fan 05744656

92 and 93 Have Different Systems 7/03/2004

The HVAC system used by AMG in '92 & '93 HUMMERs was designed and built by a company called SGM out of Ohio. I have the dual R12 system in my 93 and can offer a few points. R12 is plentiful now (7/2005) that demand is down and the price is down also, although it still is twice as much as R134. George Goble who is or was a member of this board's company makes R406 which is a drop in replacement for the R12 and sells for just a little over R134. The system in the 93 is designed for extreme duty and will outperform the systems in any other Hummer. On calibrated professional thermometers the cold air coming from the front evaporator is 38-39 degrees and the output from the rear evaporator is 34 degrees. These temps were taken at 80 degree outside ambient using R406 refrigerant. If you do not turn down the system the inside of the 4dr or ambulance body will become like a refrigerator inside. Don't bother with trying to convert to R134 as the system will not perform as well, and if you do not remove ALL mineral oil it will end up a dead system. Hope this helps.

Is Your Older Truck Stock?

Over a decade has since past, but as far as I recall about every third 1992 and about every fourth 1993 HUMMER was originally built WITHOUT A-C. Of those 1992s that were built WITH A-C, none were originally built WITH an auxiliary A-C/Heat Unit. Of those 1993s that were built WITH A-C, approx. 20% were originally built WITH an Auxiliary A-C/Heat Unit.