Heater and 4x4 issue - AC Auto Repair Help Forum


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Heater and 4x4 issue

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malbrecht10
New User

Dec 20, 2008, 9:21 AM

Post #1 of 3 (1509 views)

Heater and 4x4 issue	Sign In

I own a 2003 Ford F250 6.0L diesel with 71,000 miles. The heater is default to the defrost setting even when the switch is on panel or floor. Also, the shift-on-the-fly four wheel drive is not engaging. I was told this was a vacuum pump problem, but the vacuum pump is running. I replaced the pump, and it didn't fix the problem.

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Dec 20, 2008, 10:13 AM

Post #2 of 3 (1506 views)

Re: Heater and 4x4 issue	Sign In

I'm not going to be a ton of help but I'll try with what I understand so far. MVAC controls almost always default to "defroster" ducts when something fails - that's normal by itself. More and more are electrically actuated - you may just look for a vacuumm line entering thru firewall and see if in fact there's vacuum available.

Vac pumped or by natural intake manifold vacuum is still vacuum and probably stored in a container "ball" or "can" of some sort and also have a "check valve" to allow one way only flow and lock it there for some use with engine off.

The two failures together are probably the same issue. See if you can follow vacuum and measure it. Handy to have a hand held vacuum pump and gauge with an assortment of "T" fittings so you can watch what the vacuum is and the pump will apply vaccum to items one by one as a test too and show leakdowns if they exist.

A big enough leak will disable most all vacuum controlled items with vacuum assisted brake booster usually last to give out. Try to pinch off if hoses are in good enough shape to tolerate that and see if you can isolate the area of what I think you'll find is a large leak or total break somewhere. Good luck,

T

malbrecht10
New User

Jan 6, 2009, 1:16 PM

Post #3 of 3 (1468 views)

Re: Heater and 4x4 issue	Sign In

I used a vacuum pump/gauge and found that my hub lock solenoid had a huge leak in it. I replaced the part and can now control the four wheel drive and heater from the cab!

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