I found there was an update for the deflector door or the blend door on the heater box.
I attached the TSB for it and a picture.
Do you hear the door move when you command heat and AC and back again? If you do then it is working.
If you do not then good chance the heater core is clogged. I would remove the hoses and run a garden hose through the core back and forth several times until you get a good even clear flow.
1988 C-G-H-J- & P-Body Passenger Cars and N-Body Truck
Warm Air From Demister Outlets
December 12, 1988
24-02-88 REVISION A
Technical Service Bulletin Date 10/17/88 is being revised to include Figure 1. Figure 1 was inadvertently left out during the original printing and distribution of Technical Service Bulletin 24-02-88.
Warm air coming from the side window demisters when the climate control system is operating in the A/C panel mode.
Before attempting further repairs, it is important that the following action be taken to insure this condition is not caused by a misadjusted temperature door.
1. Verify that the temperature door is hard against its stops when the temperature setting on the A/C control panel is in the coldest (not recirculate) position. On standard A/C, this can be done by rapidly moving the lever to the minimum heat position. On Auto Temp systems, it will be necessary to set the control to 66 degrees. When correctly positioned, the door will emit a "thump" as it hits its stops. If necessary, adjust the cable auto-adjusting clip to obtain the desired results.
2. With the engine idling in park or neutral, operate the climate control system in the A/C panel mode at high blower speed and at minimum heat (not recirculate) until the A/C discharge air and engine coolant temperatures have stabilized.
3. Measure the temperatures of the left center A/C outlet and the demister outlet. If the demister air is more than 20 degrees warmer than the A/C outlet air, the bi-level air door and vacuum actuator should be replaced.
4462526 Bi-level Door Assembly
4462918 Vacuum Actuator
This repair procedure involves replacement of the bi-level air door and vacuum actuator which will reduce the temperature and the volume of the demister air anytime the system is in the A/C panel mode.
1. Consult the appropriate 1988 service manual for the proper procedure and then remove the A/C evaporator housing from the vehicle.
2. Using the procedure as outlined in Group 24 of the service manual, remove the bi-level air door and vacuum actuator from the evaporator housing.
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3. Replace the existing bi-level air door with service replacement door, PN 4462526, as shown in Figure 1. Replace the vacuum actuator with service replacement actuator, PN 4462918.
4. Using the instructions in the service manual, reinstall the evaporator housing unit back into the vehicle. Complete all mechanical, vacuum, and electrical connections. Refill the engine with coolant. Evacuate and recharge the A/C system.
5. Be sure that the temperature door is adjusted properly and that all mode doors are functioning correctly.
6. Verify that the climate control system is operating satisfactorily by means of an overall A/C performance test as outlined in Group 24 of the service manual. If the climate control system does not pass the A/C performance test, the cause must be identified and corrected before delivering the vehicle to the customer.
POLICY: Reimbursable within the provisions of the warranty
TIME ALLOWANCE: Labor Operation No. 24-65-22-90 (C-G-H-J- & P-Bodies).... 2.9 Hrs.
24-65-17-90 (N-Body Truck)....... 2.1 Hrs.
FAILURE CODE: 59 - Internal Leak
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Sunday, November 24th, 2019 AT 4:41 PM