# 02 F350 heater/defrost problem



## Tony350 (Feb 6, 2010)

I was talking to my dad today and he was telling me today that the heat will only come out of the defrost. It doesn't matter where he puts the dial it will only come out of the defrost. I know that is the default if there are problems, many old fords at some places I have worked had this problem. What I am wondering is what may be broken to cause this. A simple vacuum leak, plenum broken or the arm going to the plenum? Any ideas please let me know. Thanks again.

For what is worth it is an 02 7.3 diesel 4wd ext cab long box.


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## 07PSDCREW (Sep 4, 2011)

Passenger side, above the floor area is a buncha vacuum lines... Theres a group that may have colored lines with a clear connector. This plug came apart and caused the same issues for me In my old truck. Stuff stacked on the floor or people's feet can dislodge it. If it is still connected, try unpluging it and plug it back in. Check for vacuum supply in the hvac system... It runs on vacuum to move all controls/doors. Diesels dont really produce vacuum, so you should have a vacuum pump under the hood. Some were theres a separate electric pump with a tank or some had engine driven vacuum pumps. It could end up just needing a new pump. Good luck.


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## Tony350 (Feb 6, 2010)

Thanks a lot. Do you know what else uses vacuum pressure to operate maybe help determine if it is the whole vacuum system or something with the hvac?


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## 07PSDCREW (Sep 4, 2011)

Just looked real quick. 02 7.3 should have an engine driven pump. Check to see at the passenger side fender well there's a black vacuum storage tank. Check for vacuum there. If its there, check above the passenger floor for a leak. Keep tracing till you find the source of the leak.


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## 07PSDCREW (Sep 4, 2011)

Tony350;1655322 said:


> Thanks a lot. Do you know what else uses vacuum pressure to operate maybe help determine if it is the whole vacuum system or something with the hvac?


If you have auto 4x4 hubs...those use vacuum..


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## dieselss (Jan 3, 2008)

Pass side inner fender towards the firewall is the vac pump. What you can do is turn the key to run and listen for a "pump running" sound. If it doesn't stop within a couple of minutes, you have a vac leak. As was said sometimes pass feet kick the line off


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## 07PSDCREW (Sep 4, 2011)

dieselss;1655328 said:


> Pass side inner fender towards the firewall is the vac pump. What you can do is turn the key to run and listen for a "pump running" sound. If it doesn't stop within a couple of minutes, you have a vac leak. As was said sometimes pass feet kick the line off


02 7.3 has engine driven pump above power steering pump. That trick won't work...

Edit... It SHOULD have an engine pump but may have electric... Sorry not trying to be an ass...just not sure of the cutoff..


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## dieselss (Jan 3, 2008)

Ummm. I think the cut off was obs 1998 for engine driven vac pumps 
1999.5-2003 7.3 are just about all the same. No engine vac pumps


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## clark lawn (Oct 7, 2005)

Its an electric vacuum pump. On the right inner fender.


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## dieselss (Jan 3, 2008)

Ok. That's what I knew. When you turn key, do you hear the pump running, or do you hear the pump running constantly?


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## weareweird69 (Dec 10, 2010)

What they all said. Black is your source line for vacuum. The vacuum pump is on the passenger fender behind the battery.


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## Tony350 (Feb 6, 2010)

Thanks for the help I will see what I get figured out this weekend if he has time to bring it out to the shop


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## MickiRig1 (Dec 5, 2003)

If the vacuum pump is not working you would know it. You would have to _stand on the brakes_ to stop the truck. I do believe the heater control is vacuum operated. Check that no line is off the control dial. Make sure you have vacuum to it. Then check the lines leading to the heater box. Make sure they are not disconnected somewhere. I have had the problem with some of my trucks and every manual is real vague on it. I go to the library and _every book is vague_! I think the mode selector is buried deep inside the heater box. It has to be a rare problem to be so non-addressed as it is.


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## weareweird69 (Dec 10, 2010)

The HVAC mode selector is the dial itself. Pull the HVAC panel out and all your lines are right there. The hot/cold door is electric but selection is vacuum.


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## dieselss (Jan 3, 2008)

Having an issue with the vac pump will not affect the brake system on this truck . We have already determined that its the small electric pump NOT a belt driven pump


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## weareweird69 (Dec 10, 2010)

Yeah. That's established already. But if you have no vacuum. You have no control of where the air comes out. a loss of vacuum to the HVAC panel creates a defrost ONLY condition


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## dieselss (Jan 3, 2008)

I did an edit. I had my thoughts and words wrong.


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## weareweird69 (Dec 10, 2010)

Gotcha. And yes your right. The vacuum has nothing to do with brakes on a Super Duty. They are hydro boost and use the power steering pump for assistance


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## Jguck25 (Oct 30, 2009)

I had the exact same problem with my 99 f350 deisel and used it like that for three storms, I couldnt take the ice cold feet anymore and looked into it, it ended up being the pump in the engine compartment on the passenger side, pretty cheap part, cant remember exactly but less than 100. Took about 20 minutes to change and it worked fine after replacing it.


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## Tony350 (Feb 6, 2010)

Thanks for all the replies. I haven't had time to look at it. However my dad was out cutting wood and setting up some deer stands and bottomed out kinda hard and now it works. He isn't worried about it so I guess I don't need to be. Thanks again.


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