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XJ12 Heater Repair

XJ12 Heater Repair

Rob Reilly

I had to remove the heater unit from my 1974 XJ12L Series 2 (carbs and AC) to have the core rodded out. Here are a few tips and hints for anyone else who may be thinking of doing this to theirs.

  1. How it works: outside air is drawn in through the grille in the center of the cowl between the bonnet and the windshield. Leaves and dirt also come in here and either clog up the plenum drain hoses under the hood on either side of the engine or continue on where they get into the fans and ruin the fan motors or clog up the heater unit drains. The rubber bulbs on my plenum drains were full of debris. Keep those plenum drains clear. Vacuum operated flaps on the fan motors close off this outside air and draw air instead from inside the car under certain conditions known as the recirculating mode. From the fan motors the air goes through rubber branch hoses to the front chamber of the heater unit. Here ALL the air passes through the air conditioning evaporator and gets cold (if the compressor is on). Then there are four flaps controlled by mechanical linkages which can be seen on the right hand side of the unit. These control what percentage of the air passes through the heater core and becomes warm (if the water valve is open and the water is warm) on its way out to the vents. The upper front flap #1 opens or shuts off cold air to the dashboard side vents, center vent, and windshield defrost vents. This air has NOT passed through the heater core. Clockwise is open, counterclockwise is closed, viewed from the right hand side. The center vent and windshield vents also have vacuum operated flaps. The upper rear flap #2 opens or shuts off warm air, which HAS passed through the heater core after passing through the cold evaporator, to the same upper vents as the first flap. Clockwise is closed. The lower rear flap #3 controls warm air to the lower vents on the sides of the transmission hump and to the duct hoses going to the rear vents under the center console armrest. Clockwise is open. The lower front flap #4 controls cold air to the same vents as the third flap. Clockwise is closed. The bottom bypass link closes #3 as it opens #4. On the left side is a vacuum actuator and linkage which when the selector is on DEFROST will be up (vacuum off) and prevents the flap #3 from opening. The left hand control is a two-way variable resistor which should have a resistance of zero ohms at the 85 degree mark and 10,000 ohms at the 65 degree setting. There are three solder pins on this thing, one of which should not be used. You have to solder your wires to the right pins. Mine had a broken off pin and I naively soldered the wire to the unused pin, which made the resistance zero at 65 and high at 85, which was backwards. The shaft was binding on the bushing it fits through, making it stiff to turn. I loosened and retightened the tiny mounting screws and now I can turn it with one finger. The right hand control works some microswitches which do the functions as labeled: low, high, defrost, etc. I didn't fool with it. There is a servo motor on the lower right which operates some more micro-switches, vacuum line valves, and mechanical linkages, and is in turn controlled by the temperature setting and several temperature sensors through the amplifier and relay on the lower left. The small device mounted on the lower tube of the heater core is a temperature controlled on/off switch which disables the blowers until the water warms up.

  2. How to get the unit out: First, allow yourself about five days to get it out, the core repaired, and back in. Drain the radiator and discharge the AC. Disconnect the battery because you will have the doors open the whole time. The first thing to take off is the underdash panels and side panels, followed by the console window switch panel, console, the center switch panel, radio, dash-board cover, and the glovebox. Take notes on the wiring of the window and door lock switches. You do not have to pry out the defrost vent plastic covers. The black fiberoptic wires can be unplugged at the light source end. Remove the four bolts holding up the steering column because it has to be lowered to get the instrument panel out. Then remove the instrument panel and the rail across the top. All this isn't as hard as you may think, because there are multi-pin connectors in the wiring so you don't have to mark them. The vacuum hoses should be marked, and also make notes of the routing of the hoses, wires, and speedometer cable. Also disconnect the wire to the heater. Now remove the big intake branch hoses and their short metal tubes, which you twist 1/2" then pull. Next remove the hoses for AC and water under the hood, then the two mounting nuts under the hood. At this point the unit should be free, but mine was stuck to the firewall and I needed a prybar to get it loose. The factory was sloppy with the glue. The unit is awkward and fairly heavy, so slide the seat back, protect it with cardboard, and take the thing out the passenger's side.

  3. Getting the cores out: The AC evaporator comes out after removing the main cover panels. Clean the dirt off and straighten the fins. Mine were pretty badly mangled by the factory assembler. The heater core comes out the left side. Mark and remove the linkages and fasteners on the left side. There are no hidden screws. If the core won't slide out it is stuck with glue. Get the core rodded out by an experienced radiator shop. This kind of work takes practice to do well and is not expensive. My shop charged me $30. You might also ask your radiator shop not to paint the core because when you first start it up it will smell like burned paint for an hour or so.

  4. Putting it back: The first thing to do when you put the unit back is to connect the two little drain tubes, then the wiring, top vent hoses, then the mounting bolts. Then do the vacuum hoses, intake tubes, intake branch hoses, and top rail. Watch how you route the speedometer cable or you will get binding indicated by noise and a jumpy needle. Now put in the instrument panel. Connect the four hoses under the hood. When putting back the steering column use a little rubber cement to hold all the washers in place. Leave the bolts loose and pull the column back about 1/4" before you tighten them. If you don't you will get binding in the lower column universal joint and bumpy steering. The rest of the reassembly is pretty straightforward; center switch panel, console, radio and panel, window switch panel, dashboard cover, glovebox, underpanels and side panels. It seemed like a lot of work at the time, but I'm glad I did it and I have good heat now. The pleasure of owning the car still outweighs the pain of fixing it.
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