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Heater only blows defrost...

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Old 02-11-2010, 01:39 PM
beefey beefey is offline
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Heater only blows defrost...

Hey all,

My truck's blower is stuck on defrost, it does not blow to any other location however I feel the switch is fine because it still works for on/off operation. I've never taken the dash off, how does the system regulate where the air is sent? Is there some kinda flapper controlled with a servo or similar?

One other hunch I have is the Blower Motor Relay... does this sound like a possibilty?

Any help would be greatly appreciated since I don't have much idea on where to start diagnosing this.

I have attached what Chiltons considers a wiring diagram lol...

http://www.frontiernet.net/~burlroof/forumpics/Wiring%20Diag.jpg

Thanks,
Beefey


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  #2  
Old 02-11-2010, 01:47 PM
beefey beefey is offline
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EDIT: blend door actuator has nothing to do with this problem....
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Old 02-11-2010, 01:49 PM
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dsquared dsquared is offline
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No info on your truck in signature, so a SWAG here:

Vacuum leak to you front 4x4 hubs (ESOF).

let us know....
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Old 02-11-2010, 01:59 PM
beefey beefey is offline
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Truck: 2002 7.3 4x4

A vac. leak to the front could cause the switching action to fail? The system is dependant on vac pressure to switch blow locations? How would one go about testing this theory?
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Old 02-11-2010, 02:05 PM
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The blend door actuator is vac operated if you have a vac leak the door defaults to the defrost position. Pinch off the lines going to your ESOF system one at a time when the door moves that is the line that needs replacing. This is a very common thing that happens to these trucks. Which is why some of us chose to go with manual lockers
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Old 02-11-2010, 02:11 PM
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If you have ESOF see this Thread for some details.
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Old 02-11-2010, 04:13 PM
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If it's a vacuum leak then it is most likely BEFORE the ESOF system. The ESOF solenoid only sends a brief (30-40 second) vacuum pulse to the hubs to lock or un-lock the hubs. Once the solenoid shuts the blend door will re-align once vacuum builds back up when the pump actuates to restore the vacuum level.

If it is the ESOF system, then he has at least two problems. One is a failed open solenoid valve and the other is a leak in the system that would allow the vacuum to bleed off and never build up to actuate the blend door to another position (the blend door defaults to defrost only for 0" of vacuum).

Just my .02
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Old 02-11-2010, 04:26 PM
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It could also be the Vac pump itself not sure if they are all in the same location but mine is on the passenger side inner fender. You can here it run when you first turn the key on if you are under the hood and have a helper turn the key on.
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Old 02-11-2010, 04:36 PM
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Quote:
Originally Posted by serviceguy View Post
It could also be the Vac pump itself not sure if they are all in the same location but mine is on the passenger side inner fender. You can here it run when you first turn the key on if you are under the hood and have a helper turn the key on.
I don't know what engine beefey has (not listed in his profile), but I think only the diesels have a vacuum pump. The gassers get vacuum from the engine IIRC.
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Old 02-11-2010, 05:05 PM
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Quote:
Originally Posted by beefey View Post
Truck: 2002 7.3 4x4

A vac. leak to the front could cause the switching action to fail? The system is dependant on vac pressure to switch blow locations? How would one go about testing this theory?
Quote:
Originally Posted by brian42 View Post
I don't know what engine beefey has (not listed in his profile), but I think only the diesels have a vacuum pump. The gassers get vacuum from the engine IIRC.
He listed a 7.3 so he's got the vacuum pump. It's the classic ESOF vacuum leak issue.
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Old 02-11-2010, 06:52 PM
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If it's constantly at defrost, the vacuum leak is UPSTREAM of the solenoid (unless he has a bad solenoid AND a leak in the ESOF, or the vacuum pump itself and its associated lines).

A quick test would be to disconnect your vacuum line from the ESOF solenoid (which is on the fender wall just above the right front wheel well) and plug that line (could be a friend that holds their thumb over the opening). Turn on the truck and see if you can get something besides defrost. If you have normal ventilation control, you have multiple ESOF issues. If you still have your defrost only issue, you have a vacuum leak in your HVAC system (bad pump, bad lines to/from the reservoir, etc.).

Hope this helps.
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Old 02-15-2010, 12:04 PM
beefey beefey is offline
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Ok guys this is all good info! I know I have a vac. pump however I don't have auto-locking hubs. They are manual but there is still a selector dial in the cab. This means that obviously it cannot be the ESOF because I dont have one, correct?

If my vac. pump was out wouldn't I have full brake failure? They are completely operable.

Thanks,
Beefey
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Old 02-15-2010, 12:10 PM
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Quote:
Originally Posted by beefey View Post
Ok guys this is all good info! I know I have a vac. pump however I don't have auto-locking hubs. They are manual but there is still a selector dial in the cab. This means that obviously it cannot be the ESOF because I dont have one, correct?

If my vac. pump was out wouldn't I have full brake failure? They are completely operable.

Thanks,
Beefey
Two different systems. That's why you still have breaks. I would try to find someone that will let you swap solenoids and see if that's the issue. Usually the solenoid sticks open and burns up your vacuum pump. So you may have to replace both.
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Old 02-15-2010, 12:12 PM
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On diesels the power boost for the brakes is from the power steering pump not vacuum. The vac pump is only for heater controls, ESOF and maybe a few other things but not brakes. If you have a dial on the dash your truck originally had ESOF so it is possible that something in the system is still intact and now leaking depending on how much they removed when switching to manual hubs.
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Old 02-15-2010, 12:28 PM
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Quote:
Originally Posted by jkidd_39 View Post
Usually the solenoid sticks open and burns up your vacuum pump. So you may have to replace both.
This is what happened to me. Solenoid stayed open, pump ran continuously, and pump finally gave up. Check the lines to the hub while you're at it. Look for cracks or splits in the lines, and make sure all the connections are secure.
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