Hummers produced between 1994 and 1997.5 have poorly designed heating and air-conditioning systems that were supplied to AMG by Acme. The reason AMG specified this unit is because it was physically small. The first Hummers that came out for civilian use were targeted for commercial / industrial / and the mining market. They wanted to have as much space inside the cabin as possible. Creature comfort was not a priority. As it turned out AMG misread the market and the biggest user by far turned out to be us civilians. The Acme unit became a major headache.
Aside from it's poor A/C performance it is plagued with numerous problems. The largest problem by far is leaking heater cores. The cores develop pin hole leaks which can be immediately recognized by a strong sweet smell of antifreeze and the major fogging up of the front windshield. Along with the heater core there have been reports of air conditioner evaporator leaks, blown controllers, and sticking heater valves. The Acme wasn't built to be repaired. The heater core and the evaporator are one assembly and almost impossible to disassemble.
AMG decided that this system was so bad that they started replacing the whole thing (except the A/C compressor) complete with core, fan, wiring, and all the controls in trucks that were still under warranty with a completely new system. The newer system comes standard on all trucks that are 1997.5's and newer. At the same time they discontinued all repair parts for the older Acme system.
If your truck is older then 1997.5 and your heating and/or air conditioning system develops a problem you have 3 options:
1. Your best bet is to Upgrade to the "new and improved" system which costs about $2800.00 in parts. The 2003 version HVAC uses an aluminum heater core and a rubber hose connects it to the manifold at the firewall. There are also other improvements. The fully installed price for an upgraded heating / air conditioning system with labor is about $3,800. This modification is major and requires new holes to be cut in the truck. If you do this your HVAC will work the way it was always supposed to. This modification will also add resale value to your truck
2. You might be able to find a rebuilt unit and either install it yourself or have it installed. The labor for the install is anywhere from 600.00 to 1,000.00. This will still leave you with a under performing system.
3. The third option is to repair/ rebuild the unit yourself. Most people think that it is impossible to fix the existing unit, because it cannot be taken apart and re-assembled properly. That's why AMG always replaced them. The Acme units are assembled with rivets and black mastic glue. Dealers estimated that it would be an 8 to 9 hour job to disassemble and repair an Acme unit. At 75 to 95 an hour (not including parts) it can get expensive. Spend some time yourself and save the big bucks.
New Heater Cores from the new style HVAC system leak also. At least they can be repaired and replaced easier. Do not pressure test a heater core over 15 psi. You will distort the tubes and stress the structure.