schigara

Hello,

I have a 98 Suburban. In what situations does the heater control valve receive vacuum to close the heater valve to stop water from circulating in the heater core? I just replaced the compressor, condenser, orifice, TXV in the rear and flushed the a/c system, evacuated the system, split 11 oz of PAG 150 oil among the compressor, drier, condenser and low pressure hose going to the rear and then charged with 64oz of 134a refrigerant per specifications. The low pressure pipes in front and rear and the drier is ice cold but the center vent temp would not drop below 61 degrees when it was just 85 outside with 70% humidity so I found out the temperature blend door actuator had a cracked gear and was mixing heat with a/c.

I replace the actuator and started the engine(cold engine) and with close to the same outside temps @ 87 deg and 80% humidity but this time I was seeing 41 degrees from the center vent and felt cold just like it should until the engine warmed up and the vent temp rose to 55degrees. I know the blend door is closed to the heater core now but maybe the sealing lip of the blend door is cracked and still allowing some heat to mix. Next I checked the heater control valve, which is fairly new, and it had no vacuum on the line going to it so I put my vacuum pump on the valve and pumped it down to 15inches to close the valve and the vent temp went back down to 41-42 degrees with the valve blocking coolant flow through the heater core.

I know the AC system is operating perfectly with outlet temps of 41deg. At 85 degrees ambient, I am seeing low side pressure at about 44psi and high side at about 225psi @ 1500rpm. So, at what settings on the hvac controls is the heater control valve supposed to get vacuum to close the valve and do I need to pull the dash apart to check the blend door seal for shrinkage or cracks?

schigara

I found the problem. For future reference, if anyone has a problem of no power to the grey/black wire at the heater control valve solenoid. On the PCB of the climate control unit there is a circuit with a 3.8ohm resistor that sends power to the heater control valve vacuum solenoid which is mounted to the firewall next to the receiver/drier. After some probing with a DVOM, I found that resistor was blown. I replaced the resistor and the heater control valve is working the way it's supposed to now. Anytime the temp control **** on the climate control is turned counter clock to max or 1-2 notches from max cool, 12v power is sent from the climate control unit through the grey wire with black stripe to the heater control valve solenoid. The resistor is 1/4 watt 3.8ohms and the color bands are orange..gray...gold...gold.

I have had to replace the blower motor switch 2 times already and I think those and the resistor got burnt due to the alternator. The battery was getting drained due to a bad diode or 2 in the rectifier. The alternator was still charging but it was letting some ac power get past the rectifier. I just rebuilt the alternator last week so this should no longer be a problem.


Many thanks to Gawain Tomlinson over at SilveradoSierra for the write up and the schematics he created by reverse engineering the unit. SilveradoSierra.com ? How To Repair GMC CK Series Trucks HVAC Control Unit : How-To Articles

73shark

Welcome to the forum. Thanks for reporting back the solution to the problem as that's what really makes the forum work.

Geoffrey O'Brien

wow thanks....I replaced that resistor you mentioned and now getting voltage to the control valve solenoid. Saved me from buying another unit. Thank you