05 6.0L Fx4. When it put my truck into four-wheel drive the A/C leaves my vents and will only come out of the defrost vents. I had it into a mechanic and he pressure tested the lines and says the hubs hold pressure. He pulled the little black box on the passenger side of the truck and tested it for pressure and it held pressure. He also messed around with the pump and some valve connections. We then started the truck and the A/C worked again in four wheel drive. However, about 5 miles away from the shop I tried four wheel drive and it was back to the defrost vents again. Anyone have any idea why this is happening?

Also, I don't know if this is connected or not but the back of the dust seals on each axle broke off and are hanging on my axles. He said that they didn't play a part and that they didn't even really do much as long as the seal itself was still in place.

Thanks,

Mike

 

maybe its a misspeak, but he should have tested it to see if it holds vacuum.the hoses could be starting to crack on the ends, and perhaps they temporarily sealed when he put it back together

 

he didn't test the lines at the hubs. he pulled the lines apart in the engine compartment and hooked a handheld pressure pump up to the hose going down to the hubs. The pump held pressure after he pumped air into the lines.

 

i was messing around with some things trying to figure this out and came upon this discovery. if i sit long enough with it in 4x4 the ac will again come out of the dash vents. i assume this happens after the pump stops running. but i don't get the rough turning typical of 4x4 when on concrete. i do get that if i have the hubs in lock but not in auto. how do the auto hubs work on these trucks? does the 4x4 only come on when needed when in the auto position?

 

When you switch to 4x4, after the transfer case shifts, the Pulse Vacuum Hublock (PVH) solenoid applies vacuum to the hubs for a period of time. I believe it's around 30 to 45 seconds. Then it shuts off vacuum to the hubs and they remain locked.

What is likely happening is that there is a leak in the lines to the hubs, or the hubs themselves are leaking. This results in a loss of vacuum. Without vacuum the control door in your heater defaults to defrost. When the PVH finally closes, vacuum is rebuilt by the pump and your heater returns to normal operation.

The problem is most likely a vacuum leak in the line going from the PVH to the hubs, or the hubs themselves are leaking.

 

thanks chamberlain.

where is the pvh located?

 

On the passenger side firewall. It has two vacuum lines, one coming from the pump/reservoir, the other going down to the hubs. And an electrical connector with three wires.

 

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Here's what the mechanic says:

He says he tested the lines going to the hubs after the PVH and they held pressure, so he doesn't think thats the problem. He thinks that the pump is going bad and is supplying a weak vacuum. He is going to replace the pump first and then tear into the axles if that doesn't work.

If he is just replacing the pump I'm going to sit there while he performs the work because it won't take long. How much faith do you guys have in it being the pump? Do you think I should schedule another car to drive for the day?

He's also going to replace my egr valve thats throwing a p0405 code?

Thanks,

Mike

 

Based on what you're describing, it's not the pump.

Edit: The lines should be holding vacuum, not pressure. If he's not testing them with a vacuum pump and gauge, he's not properly testing them.

 

He used a handheld gun with a gauge on it. If you pressed your finger to the hole of the gun and pressed the trigger the gauge would build up pressure/vacuum until you released your finger. When you release your finger the air then came rushing out. He connected this gun to the tube after the pvh and it held pressure/vacuum.

 

Was it pressure or vacuum? They are pretty different.

 

i just called him. it was a vacuum gun.

 

If the lines from the solenoid to the hubs are holding a vacuum. And a vacuum is being held when the solenoid is not activated (as evidence by your HVAC working right), then I think I'd be looking at a leak in the hub side of the solenoid.

 

i can only find one leak and i don't even know if its related. there is a line that runs from the radiator back towards the cab, under one of the ac lines and into a black plastic box near the top of the firewall. where this line connects to the box there is quite a bit of air blowing out. the other 6.0 sitting right next to me does the same thing just not nearly as much as is blowing out of his. btw - his 4x4 works fine.