Vents stuck on defrost

I believe this works off of vacuum. Can anyone shed some light? Where do I start/look?

Thanks in advance!

I agree... most likely a vacuum leak.

Is your engine running ok? If you have a big enough leak that the engine is surging, then the leak is probably under the hood.

Otherwise it is probably a smaller leak under the dash. You might be able to hear it. Otherwise you'll have to locate the hose as it travels through the firewall to the control head and then out to the vacuum motors.

The engine seems to be running fine. It def. doesn't surge. If anything it idles a little low. I'll have to check the vacuum lines but I don't really know where to start. Where does the line come through the fire wall? That would be a big help.

Sorry... idk. Look for the vacuum canister (looks like a juice can) and see if there is a line that runs from it to the fire wall.

This is a thread a put together for my windstar, and although it isn't specific for a bronco, if you understand it, you will be able to transfer the knowledge gained to troubleshooting your truck.

http://www.fordforumsonline.com/forum/heating-air-conditioning-cooling-systems/153-windstar-rear-ac-problems.html

EDIT... I posted the wrong link yesterday.

That's perfect Broncojoe. Now I know where to start at least. Thanks for the help.

yo BESSIE,

JOE has a lot of 1st hand vauum leak diagnosis info!

I believe you have a 92?

If so or for earlier years & up to 96;

Vacuum Control Valve Location pic by deleplank at SuperMotors.net



The white PVC plastic material vacuum hose is far more susceptible to UV light than any other color & the gap along the edge of the hood allows it to degrade.

Vacuum Line (Hose), Mini-Tube Damage Repair in a 92 Source: by JohnMcD348

The climate ctrl air handling system is designed to provide defrost when no vacuum is applied to any of the three air door vacuum motors; Esp. the Vacuum Control Valve. This is done to prevent a situation where defrost cannot be obtained due to a system vacuum leak. Instead, a leak in the vacuum control circuit will send all airflow to the defroster outlets. This condition may occur during acceleration (slow leak), may exist at all times (large leak) and may happen only when certain specific functions are selected, indicating a leak in that portion of the circuit. from F-150, F-250, F-350, F-Super Duty and Bronco, F-Super Duty Motorhome Chassis Workshop Manual

http://www.thedieselstop.com/faq/9497faq/maint/stj/stjc0021.htm

Air Conditioning Control Diagrams & Overview in 92-96; "...In the OFF position, all doors are in the vacuum-applied position with the exception of the PANEL door. The blower & compressor clutch circuits are not energized. In the MAX A/C position, all vacuum operated doors are in the vacuum position. A/C damper inlet door (19C802) air is shut off and the passenger compartment air is recirculated to maximize cooling. Discharge is through panel register. A/C clutch & blower circuits are energized. In the NORM A/C position, outside air is allowed to enter the passenger compartment. The air passes through the A/C evaporator core (19860) and is cooled before reaching the passenger compartment. Discharge is through panel register. A/C clutch & blower circuits are energized. Ventilation is delivered through the instrument panel registers when the function selector **** in the A/C control (19980) is set in the VENT position. In the VENT position, no vacuum is applied to the OUTSIDE/RECIRC. vacuum control motor (18A318 ) and the door is open to the outside. The air coming in through the cowl is discharged through the panel registers. The blower circuit is energized, but the A/C clutch is not. In the FLOOR position, no vacuum is applied to the vacuum control motor and the OUTSIDE/RECIRC. door is open to the outside. Air is discharged through the heater outlet floor ducts (18C433) with a small amount going to the windshield defroster hose nozzles (18490). The blower circuit is energized, but the A/C clutch is not. In the MIX position, outside air is discharged through the windshield defroster hose nozzles and the heater outlet floor ducts. A/C clutch & blower circuits are energized. In the DEFROST position, outside air is discharged through the windshield defroster hose nozzles with a small amount going to the heater outlet floor ducts. All doors are in the no vacuum position. A/C clutch is engaged in ambient temperatures above approximately 10�C (50�F) & the blower circuit is energized. The PANEL/FLOOR, FLOOR/DEFROST and OUTSIDE/RECIRC doors are vacuum operated. For maximum cooling, the temperature **** should be set in its fully counterclockwise position; the function **** should be in the MAX A/C position; and the blower motor (18527) should be set for a desired rate of airflow. Even though the function **** is on MAX A/C, the temperature ****, being manually controlled, may be set to modify the temperature of the air and the path through which the air flows. Another characteristic of the MAX A/C setting is the increased noise level of the blower motor. Speed does not change when the OUTSIDE/RECIRC. door is moved to either of its two positions. The difference in noise level is that an open recirculation door exposes the passenger compartment directly to the noise. When insulated against the noise with the recirculation passageway closed, the speed appears to be less. The control **** operates an A/C switch that is attached to the backside of the A/C control by one ***** and retainer tabs. Five hoses (black, white, red, blue, yellow) extend from the A/C control just below the electrical connector for the mode selector switch to the vacuum control motors and vacuum supply. The solid black hose goes to the vacuum supply through a tee-shaped A/C vacuum check valve (19A563), which attaches the A/C vacuum reservoir tank and bracket (19A566) and engine source. The white hose actuates the OUTSIDE/RECIRC. air door two-position vacuum control motor. The blue hose actuates the FLOOR/PANEL air door two-position vacuum control motor. The red and yellow hoses actuate the FLOOR/DEFROST three-position air vacuum control motor. Each end of each hose slides onto the ****** of the vacuum port to which it attaches..."

The vacuum hoses used in the passenger compartment control circuit are constructed from PVC plastic material. The vacuum hoses used in the engine compartment are constructed of Hytrel. Because of the materials used, the vacuum hoses should never be pinched off during diagnosis to locate a leak. Use Rotunda Vacuum Tester 014-R1054 or equivalent to locate vacuum leaks. A wood golf tee can be used as a plug when it is necessary to plug one end of a vacuum hose for leak test purposes..."

Source: by Ford

More vacuum leak diagnosis by JOE @ http://broncozone.com/topic/14269-code-reader/page__pid__74587__mode__threaded

and I have some @ http://broncozone.com/topic/22770-low-idle/

Here is a picture of a vacuum motor, On the passenger side, under dash.

I looked at our '90, it appears that the vacuum gets through the firewall by going through the Air Conditioning blower box on the passenger side.

Here is a link to EVTM for the heater air conditioner vacuum system for a '94 should be the same for your '92, because it is the same for my '90

http://www.supermotors.net/registry/media/909465

You guys are the best. I traced the line from the motor and in from the firewall. There was a juncture under the dash where the line came off. I put it back on and ****! Working.

Bessie,

You were good to begin with... you recognized that it was likely a vacuum leak. Good for you! /emoticons/[email protected] 2x" width="20" height="20" />

Now you are even better :thumbsup: