Hi all and Merry Christmas!

I'm new to the forums here as I joined yesterday.

I saw this thread,as I was looking for help on troubleshooting my trucks HVAC system.


I own a 2001 F250 7.3 PSD crew cab Lariat 4x4 with 118k. I love this truck as I have all my Ford trucks,and plan on keeping it for years to come.

My problem is that all the HVAC systems air,comes from the windsheild's defrost vent,no matter where I have the system switch? No matter what position the system selector switch is put in ie A/C, heat , defrost,it blows out of the defrost vents? The A/C works fine ,but blows out that vent.

I suspect that this may be a vacuum issue? Last August (2010) it gave the first signs of this issue.If I selected the A/C ,the A?C would start as normal,but would initially blow cold air from the defrost vents,then the normal dash vents.This would take minutes,and would sometimes not work at all? Now it's at the point where it will only blow from the defrost vents.

I've only just begun to T shoot this ,and thought I'd ask some of you ,to see if you can tell me what's up?

Not sure if this would be related,but when I select 4x4 high,the unit initially engages the 4wd but then slips out ? This has been an issue for about 2years. I had it in for service and the Ford dealer said it was vacuum operated,and that the seals at the hubs were not holding the vacuum,and that the seals would have to be replaced. Was costly so I elected to wait,and just manually switcth the hubs. Could the two issues be related,if both driven from the vacuum source?



Rick
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One easy check of some of the plumbing is behind the glovebox.

You can force inwards, the two verticle sides of the glovebox and that will release the catches and allow you to remove it entirely. Behind there, you'll see the lines that feed the various components of your dash system of valving.

Beyond that, I'm lost.

 

Hi Rick and welcome to FTE.

The first place to look would be the vacuum lines that go to the hubs. They tend to rot off at the connection at the knuckle. Then, as you mentioned the yellow o rings on the unit bearings. Replacing these o rings isn't a bad job as long as you have a few basic tools.

And yes, the 2 issues are most likely related. Both systems are vacuum operated.

 

Most likely cause for HVAC system to default to defrost is a leak in one of the hubs.

Diagram of ESOF vac lines.





Remove one and plug it. Will HVAC come off defrost? It can take a few seconds. If yes that is the leaking hub. If not try the other side. With both vac lines plugeed and it is still defaulted to defrost check vac system under hood. Make sure all vac lines are tightly connected.



Another way to isolate leak to hubs is follow the hub vac line to a 90 degree elbow fitting. Pull the vac line going back to the vac reservoir and plug it. If the HVAC comes off defrost one of the vac lines or hubs has a leak.

Also, if the pump doesn't run there will be no vacuum for the HVAC system and it will default to defrost.
1999-2001 Under Dash fuse #28 is for the vac pump among other things.

Edit: A leak in the vac reservoir tank will also cause default to defrost.

 

looks like they have it covered, just replaced line to hubs, cured mine

 

Welcome! I am a new member also!!! I have the exact same problem with my 2000 f250 PS. I have replaced the PVHS and it worked for a little while and then it quit. It quit the second time after my Dad put the truck in 4wd to come pick me up and my deer. It is winter here and time for me to catch up on all my maintance. I will watch this post closely. Good luck!!!

 

Great Pics Robin.

You can start at the hubs, since it is the furthest point in the vac system, and one at a time undo each connection and plug it. Whichever one you plug up and makes it go from defrost to the other vents, there is a problem between where you just plugged it and the previous spot you plugged it. Its just like tracing a wire that is broken inside the insulation pretty much.

The way I do it is take the main vac line off at the pump going to the tank, and rig it up to hook to my air compressor, I regulate the pressure down to about 5-10psi, you would most likely be able to hear the leak, if not, you can start spraying a soapy water mix in a spray bottle on the lines and connections to find a leak.
That is the quickest way to find a leak, and i've used it numerous times, but remember the only place that will not find a leak is at the hubs.

9/10 times you will not have a leak on the small 1/8'' lines, it will usually be on the hub itself, the soft line going down to the hub, or the soft rubber connectors or 90* connecting the lines together.

Welcome to FTE!! and goodluck

 

Thanks Cory and great tip on using compressed air to find a hub vac line leak. Reps sent.

 

You have some as well bro, thanks!

 

Wow Robin and company. I don't know what more I can say but Thank you!!!

What a great amount of information.

I'm visiting with family at the moment,and did not have a chance to see this until now.

From me lurking on a previous post earlier of (Robins) ,I was able to gleen a little schematic knowledge of the vacuum pump and hose routing.

While out sledding with the family,I went to the truck with my son to warm up a bit. I found that the vacuum pump is not running at all now,but till now,did not know where to check for the fuse. I didn't have a meter so checking for voltage is out for now.

I did simple test with my son. I turned on the heat so it would blow from the dash,and had my son sit inside.As usual,the heat blew out the defroster.Then I pulled the vacuum connection apart for the HVAC ,and sucked on it to create a vacuum.The heat was diverted from the defrost,to the proper dash duct. At least I know now that the inner workings are OK,and now it's time to concentrate on why the vac pump isn't working.

I will check the fuse (#28) as described by Robin. I may not get to it till tommorrow.I will keep you all up to speed as to my progress ,and will let you know what I find.

Thank you all again.

Rick

 

OK ,

Had the chance this morning to check the fuse (#28) underdash,as Robin had pointed out.

Fuse checks out OK and next,I checked power at the vacuum pumps connector. I have about 14 volts there,so power is being supplied to the vacuum pump.

I went to a local Ford dealer to check on price and availability of the vac pump. They did not have it in stock. The pump assembly (Ford part # 6C3Z*2A451*A ) was $235.00 .

I won't be in a good position till later this week,to remove the pump ***.. I'd like to inspect it to make sure it's bad before buying a new one.

Any leads on a better value would be greatly appreciated.

I will post what I find ,when I get to the bottom of this.

Thanks,

Rick

 

Where are you located Rick?

 

Ziggy,

I'm located in Elmhurst IL. 60126

Just west of Chicago.

Thanks,

Rick

 

Well.... It's been a few weeks ,but I have some good news to report.

I located the problem,of course after all the great information you all posted ,which helped me diagnose it.

Of course,things are not always as simple as they may seem.

The problem was that the vacuum pump,was not working,so the default for the HVAC system is that all system will blow out the defrost vent,as there is no vacuum to control the various HVAC diaphrams.

I removed the pump,and took the little cover off the motor. It smelled like it was burnt. OK time for a new pump.

I found this place online : Ford Superduty / Dodge 5.9L ELECTRIC Vacuum Pump
and thought that I would order one from them,as they state that it's an OE made in the USA Ford part for $106 plus shipping. Ford wanted $235 so this seemed a good alternative,to get the same part. I decided to check a local truck parts place first,before placing my order. I found that that place could get me the part for $153 plus tax,so I decided to order it locally,even though it was more money.I like the idea of having face to face recourse. I go to pick it up after it came in and the parts guy tell me it's $203 and that he screwed up. Not going back there again,but took the part.



I installed the new pump and hooked it back up. Pump ran upon key on position.And ran and ran and ran. Oh Oh! Must be a vacuum leak too! Decided to plug the port that goes to the FWD solenoid,and it pumped right up and then the pump shut down.Tried the HVAC control,and it worked like a charm!

I then hooked it back up the way ity's suppose to be,took the port that goes from the FWD solenoid down to the hubs,and plugged it. Ran the vacuum pump again,and it would not shutdown. Looked like the FWD solenoid switch was bad,so I removed it to inspect.Yep see pictures.It is cracked.



My guess is that condensation froze in there and cracked it.

Looks to me that once this was cracked,and the vacuum loss occured,the pump just kept running and running until it burnt out.

I did two more quick experiments. I hooked the outlet from the vacuum reservoir directly to the line that goes down to the hubs,but I put a vacuum gauge in line with it. It would not pull more than 6 inches of vacuum,and the pump would not shut down. That told me that there is some other vacuum leak somewhere between the reservoir and the hubs or at the hubs. (see picture)


After confirming that there was something leaking after the FWD switch between the reservoir and the hubs,I decided to see what the vacuum pumps output should be,and where it is set to shut down. I hooked the vacuum gauge up to the reservoirs outlet,that goes to the FWD solenoid switch,and it went up to 12 inches of vacuum,and then the pump shut down. (see pic.)



I hope that all of this makes sense,and can help someone else.

I'm going to keep the ports plugged that go to the FWD solenoid for now,so that I can use the HVAC controls. I will troubleshoot the issues with the FWD hub circuit,when it gets a little warmer outside (Chicago winter) colddddddzzzzzzz.

Thanks again for all the help !

Rick

 

Nice job Rick on the pics and write up. Reps to you.