86 a/c gone crazy
Moderator
Joined: Nov 2007
Posts: 22,415
Likes: 92
From: Denver Metro Area, CO
Defrost is the default position to which the HVAC system will return during periods of low (or no) vacuum... it sounds like perhaps you don't have the vacuum reservoir properly hooked up? It's a grapefruit-sized plastic ball on the passenger side wheel cover and is attached to the vacuum system via a check valve.
Moderator
Joined: Jan 2001
Posts: 57,027
Likes: 2,763
From: Virginia
That's it. Going up a hill you are opening up the carb and the vacuum goes very low. Going down the hill and cruising the carb is almost shut and the vacuum is high.
To keep this high/low vacuum from affecting the HVAC controls, they put a check valve in the line going over to the pass side from the engine, and than after that they put a Tee and hooked a ball or can in the line to store vacuum, and then another line ran through the firewall near the pass side hinge to feed the dash system vacuum.
If you can't figure it out, you might need to find a friends truck and look how it's hooked up, or go to a junkyard and see how it's hooked up from the factory.
To keep this high/low vacuum from affecting the HVAC controls, they put a check valve in the line going over to the pass side from the engine, and than after that they put a Tee and hooked a ball or can in the line to store vacuum, and then another line ran through the firewall near the pass side hinge to feed the dash system vacuum.
If you can't figure it out, you might need to find a friends truck and look how it's hooked up, or go to a junkyard and see how it's hooked up from the factory.
Lead Driver
Joined: Jul 2009
Posts: 6,477
Likes: 19
From: Sheffield, AL
A vacuum leak anywhere in the system can cause the symptoms. My son's 1984 F150 had just had a stereo replacement and during the install, a small vacuum hose connected to the servo motor which mixes the defrost and heater vent position was pulled loose. As soon as it was reattached, the condition was corrected.
Listen carefully for a hissing sound when the engine is idling. Check all vacuum lines and vacuum ports for openings. Don't forget that the PCV system and power brake booster can be sources of vacuum leaks.
I just found this, using the "search" feature. Open the link: https://www.ford-trucks.com/forums/1...c-control.html
By the way, welcome to the forum!
Listen carefully for a hissing sound when the engine is idling. Check all vacuum lines and vacuum ports for openings. Don't forget that the PCV system and power brake booster can be sources of vacuum leaks.
I just found this, using the "search" feature. Open the link: https://www.ford-trucks.com/forums/1...c-control.html
By the way, welcome to the forum!
Last edited by 1986F150six; Jun 10, 2014 at 12:37 PM. Reason: Added link
Thread
Thread Starter
Forum
Replies
Last Post
pecanguy
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
5
Aug 18, 2013 08:36 PM
LARIAT 85
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
6
May 24, 2007 07:59 PM
FordandSkeeter
Cooling, Heating, Ventilation & A/C
10
Jan 28, 2005 09:03 AM
wildjon300ci
Cooling, Heating, Ventilation & A/C
7
Oct 9, 2002 03:14 PM
