Question about 1998 Ford F150 Regular Cab

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F150 98 4wd

Heater or a/c will switch up to defrost on its own.under moderate accell.or going up any incline with a load on eng.will switch back when throttle has backed off, or after leveling off from incline

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  • heavy105 Jun 30, 2008

    I have the same problem with my 1998 F150. It will switch to blow air up on the windshield and then go back to where it is spposed to be blowing. It doesn't matter is you are using AC, heater or just blowing fresh air out hte vents. I was guess that it was a vacuum problem but I haven't investigated any farther.

  • randy67 Jul 01, 2008

    The day I purchased my 1998 F150 new I had the same problem when I speed up my AC would stop blowing out the vents and blow on the windshield them when I let up on the gas it would then blow out the vents. I believe they replaced a vacuum check valve. Now my truck has started doing it again. I plan to repair it myself if I can find out how.

  • jlw1caz Jul 12, 2008

    My 1996 F 150 4X2 302 does the same thing. I was thinking that it was perhaps the transmission vacuum modulator. There is an obvious correlation between the vacuum opening and closing the duct and the transmission. The problem is, even though there is only 1 bolt holding that rascle onto the transmission, the band color varies from vehicle to vehicle. When I have checked on this part at Autozone, I was told that my truck, engine combination calls for both of them, even though there is only one!

  • mayelaine519 Aug 08, 2008

    what did u do to fix this problem

  • sp61584 Aug 24, 2008

    Mine does the exact same thing, does anyone know the location of the part needing to be replaced and what it is called, this is embrassing with customers in y truck.




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I repaired this problem on my 1998 F150 4.6L. It had a bad vaccum line between the vaccum check valve and the vaccum storage tank. The vaccum check valve is located between the battery and the firewall. The vaccum storage tank is attached to the rear of the battery case.

Posted on Sep 10, 2008


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2 Answers

My Winshield Defrost Quit Working. Heater, AC, and vents all working except the windshield defrost.

I'd have to check actuator movement for defrost. The actuator is electric according to my database. The actuator should be under dash at end of hvac housing.

Dec 10, 2015 | 1989 Chevrolet C1500

1 Answer

7.5 gas eng in 1995 F 350 ! When you accelerate real hard the AC blows out of the defroster ! Got me whupped !

Most Ford products have vacuum actuated control valves in the heating and a/c system. Ford designed a fail safe system into their cars and trucks that if all vacuum is lost that the system would default to defrost so at least your windsheild would stay clear. Typically there is a canister under the hood mounted to the fender well that looks like a metal tomato juice container from the 60's. That cannister is the vacuum reservoir that is supposed to hold the vacuum under high load (heavy acceleration) conditions. If that "tomato juice can" is either disconnected or if the check valve in the vacuum line is bad, then whether you are heating or running the a/c, under hard acceleration air will come out of the defroster vent. Check all your vacuum lines running from the intake manifold to that canister and back to your heater and a/c system for cracks or leaks, Also check that the check valves in the vacuum lines are in place the right way (they should only flow one way- toward the engine.) Listen under the hood when the engine is running for the hiss of a vacuum leak. Fix any leaks that you find. I hope that starts you off in the right direction.

Jun 29, 2013 | Cars & Trucks

1 Answer

When I turn on the ac or heater I notice that my engine starts making a funny noise. When I step on the accelator it sounds like a pulley is making a strange noise

you may have an a/c compressor clutch and or clutch bearing going bad. may be time for a new a/c compressor or compressor clutch assembly. the a/c comes on when you have the defrost or recirculation on in heater mode.

May 30, 2011 | 1996 Buick Century

1 Answer

I keep the heater in my 2001 S-10 on the floor or defroster setting but the heat begins to come from the vents, it switches back to floor and back to vents over and over.

This sounds like a vacuum control problem. Check on the top side of your engine (Manifold or throttle body area) for the vacuum feed for the heater controls. You will find a plastic vacuum line that goes thriugh the dash. Follow this line from the engineto the firewall. Somewhere along this line you will find a check valve. Replacing this check valve should fix your problem.

For verification that this COULD be the problem do this: drive the vehicle and make a point to notice whether this switching is taking place while the vehicle is under moderate to heavy acceleration. If this is the case, the check valve is definately the problem.

Jan 25, 2011 | 2001 Chevrolet S-10

1 Answer

Air only comes out thru defrost but when you let let go of the accelator it starts to come out thru the vents 2003 mercury grand marquis

It sounds like a vacumn problem. When the heater controls loose vacumn it defaults to the defrost position. When your car decels your engine vacumn is at its highest. Check your vacumn lines for damage. Check the control panel for a vacumn leak. If you take out the controls and hear a suction noise than the control switch is leaking.

Jan 11, 2011 | 2003 Mercury Grand Marquis

2 Answers

Ford F-150 2001 when you run the A\C and go to

this is very common problem, theres a vacuum leak.check the vacumm lines under the hood also check the one that goes inside to heater controls, whats happening is that it looses vacuum when you accel because the motor takes it,theres a one way valve that holds the vaccum when this happens but with a leak it cant hold vacuum,heres a vacuum diagram to help you check the vacuum lines,i dont know which engine you have in that 150 but this will be close. take good look at the circled areas in the diagrams,good luck, hope this helps you.d3fa7c0.giff57ca3c.gif

Mar 06, 2010 | 2001 Ford F150 Styleside SuperCrew

2 Answers

Heater problem, not very hot & not switching from defrost to floor

either the AC\Heater panel switch on your center dash or the heater box vent door is stuck or not operating. Most likely panel switch or disconnected \ broken vacumm lines at back of switch

Jan 13, 2010 | 1996 Chevrolet Blazer

2 Answers

Truck heating system changes from heat, defrost, and vent

thats normal, you are taking vacume from the heater system when you are under a load,but it returns when you let back up on pedal,if you wanted to alter this affect you might try installing a vacume ball to the heater vacume hose,

Jan 02, 2010 | 2001 GMC Sonoma Extended Cab

1 Answer

My van Heat/ac is stuck in defrost only, regardless if it is heater or A/C. Is this an easy fix?

yea ....
there is a vacuom hose should go to the evaporator box maybe have damige or disconect u can find it by listn carefully to the air vacuom leack under the dash or behind the eng

Mar 15, 2009 | Ford E-250 Cars & Trucks

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