ac/heater

I have a 1998 Ford F150 8 cylinder automactic.The ac and heater both work , but only on the defrost setting,does not change to floor or other settings. Where to look for problem and how hard to get to? thanks for any information hlb

Reply to
HL B123
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On Ford trucks (and most other vehicles), the air direction doors in the HVAC system are controlled by vacuum. When there is no vacuum, the doors are moved by springs to their default settings, which is recirculated air out the defrost vents. This tells you you've lost the vacuum supply to your controls.

The first place to start looking is probably at the vacuum lines in the engine compartment. Not all vehicles are the same, but it is likely if you find the large line coming off of the brake vacuum booster and follow it, you'll find a junction block with a large line going to the intake manifold (that's your vacuum source), and one or more small lines for vacuum for other functions. One of those lines will go through the firewall into the cab to the heater controls.

Check all those lines to make sure they are attached and not cracked, degraded or leaking. If everything looks good, then you're going to have to find the supply line under the dash and check it for a good vacuum supply, make sure nothing has come detached from the heater control panel, etc.

Reply to
David Chamberlain

The AC works on every setting EXCEPT floor or panel, but when both floor and panel are selected for a mixed delivery option, then the AC does work. Any Mixed Setting and Defrost will invoke the AC. You cannot select Floor or Panel to get AC.

If you are saying that you cannot get air to come out anyplace but Defrost, then that is a problem with the vacuum system. On my truck, there is a check valve in the tiny space between the battery and the firewall. The hose on one of the connections --the valve forms a T -- had a hole burned into it allowing the vacuum to leak, and the heater control defaults to Defrost when that happens.

Reply to
Jeff Strickland

Thank you to both David and Jeff for your prompt and right on responses to my ac problems.I found and traced the vacuum line to where it should have entered the firewall right behind the battery .The hose was bad on the end,fixed it and I now have air at all the positions . thanks again hlb

Reply to
HL B123

Glad to help. I had the same problem, and asked here how to fix it. I pretty much got the answer I gave you.

Reply to
Jeff Strickland

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