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Heater Core Removal and Replacement:
For Pre-1997.5 Hummers
by Chuck Kopelson

How to Remove and Replace an Older Style Acme Heating/AC System

Removing the doghouse and passenger side dash trim piece. Removing the doghouse gives you more flexibility in moving wire looms and allows more light into your workspace.

Remove the passenger front seat via the 4 bolts above the slide mechanism.

Remove the two trim panels in the kick area of the foot well.

Drain coolant from your radiator.

Have A/C system evacuated.

Remove the coolant overflow container

Remove Air intake:

Inside the Air intake, you will find 3 bolts. All three must be removed. The bottom one supports the bulk of the unit. I would advise just loosening it until ready for final removal.

 
Below the coolant overflow tank are the main connections for the coolant and Freon. These need to be disconnected. Make sure that you keep the freon lines clean. I recommend plugging them so you don't contaminate the A/C. Around the connections there are 3 nuts that hold the unit in. All three need to be removed completely.  
 

Remove the relays on the blower unit.
Remove the blower unit and the defrost diverter. Be careful to note which screws go where.

Remove the drain line and the vent line at the floor.

Remove the last screw in the Air intake.

With some creative movement, the unit will come out. Remember that the unit has foam insulation and lots of RTV on the engine side, so some force is necessary.

 
 

Replacement units from Lefty's include all temperature sensors, motors and harness connectors.

 
A New Reconditioned Heater / AC unit from Lefty's.  

Replacement is the reverse of assembley. Make sure that you clean up the area where the old unit came out so all surfaces seal correctly.

The entire process should take about 7-8 hours. With some experience, it could probably be cut to 6-7 hours.