Heater Controls
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Master of Disaster
Joined: Apr 2010
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Heater Controls
My heat is nice and hot, but its stuck on defrost.....is the heater controls ran off vaccum? I dont think its the blend door because i can make the heat hot or cool it off with the temp control ****....any ideas?
Fleet Owner
Joined: Nov 2003
Posts: 27,918
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There is a valve that can block off supply of hot water to the heater core if you lose vacuum. It is located near the fire wall - not far from the MAP sensor. I have a pic somewhere if you need it. The vacuum line is very prone to coming off of it. I cut the original end off of the vacuum line and used a piece of small vacuum hose as the new connector. Works a lot better.
Fleet Owner
Joined: Nov 2003
Posts: 27,918
Likes: 3,636
The system defaults to defrost with a loss of vacuum. Besides the heater control valve, there are a number of other vacuum system leak points.
Is your truck a 4x4? If so, the 4x4 solenoid and the hubs are common leak points.
There is another common leak point at the blend door area.
I can send you more details if you are interested.
Is your truck a 4x4? If so, the 4x4 solenoid and the hubs are common leak points.
There is another common leak point at the blend door area.
I can send you more details if you are interested.
Fleet Owner
Joined: Nov 2003
Posts: 27,918
Likes: 3,636
PM me an email address and I can send you more detail, but here is a start:
Start up at the pass side engine bay, you will see vacuum lines going to the vacuum pump. Trace all the lines you can see and look for any that are knocked off or cracked. Then look for a gray vacuum line that will either be connected to a stud on the HVAC box, sort of behind the oil fill up tube. Some trucks were equipped with a inline heater control valve the is on the heater hose going to the heater core. If you have one of these valves, the gray line will go to this valve. If you don't have one of these valves, then the gray vacuum line will go to that stud on the HVAC box.
You can also install a vacuum gauge on the line to the heater control valve. Disconnect vacuum pump lines one at a time and plug them (or block them off), and see if you get vacuum reading on the gauge.
Truck must be running to check vacuum this way.
Start up at the pass side engine bay, you will see vacuum lines going to the vacuum pump. Trace all the lines you can see and look for any that are knocked off or cracked. Then look for a gray vacuum line that will either be connected to a stud on the HVAC box, sort of behind the oil fill up tube. Some trucks were equipped with a inline heater control valve the is on the heater hose going to the heater core. If you have one of these valves, the gray line will go to this valve. If you don't have one of these valves, then the gray vacuum line will go to that stud on the HVAC box.
You can also install a vacuum gauge on the line to the heater control valve. Disconnect vacuum pump lines one at a time and plug them (or block them off), and see if you get vacuum reading on the gauge.
Truck must be running to check vacuum this way.
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