My vacuum pump I noticed is running constantly and also my vents are stuck on defrost.

I narrowed it down by taking off the lines going from the tank to the 4x4 solenoid and the tank to the HVAC line. If I plug both lines I get pressure to both but it still won't shut the pump off. Why won't the pump shut off? Leaky tank?

If I plug the red line back into the solenoid I cannot get pressure to come out of the HVAC line. (Switch in cab set in 2wd)

I can make the pump shut off by plugging the main inlet to the pump.

I took the tank out and can't find any cracks or leaks. Could something inside the tank be broke?

Also why can't I get pressure when the red line is plugged into the selinoid?

I can't figure it out. Almost seems like the 4x4 solenoid and the tank are both the problem. Any help would be greatly appreciated.

 

The 4X4 and the air vent diverter are vacuum powered. A leak in any of the vacuum lines will prevent the hubs from locking and the vents from moving away from the default defrost position (good place to have it stuck in the winter).

You need to find the vacuum leak, wherever it is. A Mity Vac with a gauge will allow you to test each component.

 

I don't think there is any leaks because I eliminated all the lines and plugged the 2 coming out of the tank. The pump still won't shut off. It builds pressure but keeps running.

Could the pump motor be weak and maybe not be building enough pressure to shut itself off?

 

I had the same thing happen to me a couple of years ago.

I know for a fact that it was not at all related to the 4x4 hubs, because I don't have ESOF... I have MSOF (Manual Shift On the Floor).

I also know for a fact that I didn't have a vacuum leak of anykind... because just as mysteriously as the problem appeared, the problem also went away a few days later.

What I surmised the issue to be was some type of ice formation/block, only because I was operating in the mountains of Wyoming in winter when the problem occurred.

Fortunately, the default was defrost, so I unplugged the vacuum pump connector to keep from burning up the pump motor. I finished my trip, which ended back in sunny warmer temps, and surrounded by the comfort of my own tools, reconnected the electrical plug to the vacuum motor to diagnose.

To my surprise (and admitted relief), it functioned normally, cycling off when sufficient vacuum was stored, as normal. I didn't fix anything. It fixed itself. The only thing I could think of was ice formation from temperature differentials... but even that is fairly far fetched, given the underhood temperatures from sustained operation on a long trip.

So all of this is to say that you may be right... you may not have a leak, and you may not have an issue with your 4x4 hubs, as so many others have had. You may just have the same mysterious fault that my system had, which resolved itself just as mysteriously as it appeared, and which to date I have not diagnosed... but haven't had a problem with before or since.

 

Thanks for you reply. I noticed my air controls kicking onto defrost off and on ever since I bought the truck but I could always just mess with the lines by the pump and get it to work again. But this time it's permanent.

I've been messing with it some more and plugged the tank off so there are no lines involved. It builds pressure but won't shut the pump off even after running for 4-5 minutes. I let it build up and I shut it off then waited 10 minutes and there was still pressure in the tank. So absolutely no leaks.

I can shut the pump off by plugging the intake hole with my thumb though.

So I guess I'm going to take the plunge and buy a new pump.

I noticed one line on one hub came off and I've been using 4wd quite a but lately so I know that pump has been running constantly for a while and I think this is the root of my problem.

 

By "pressure" I assume you meant vacuum.

We know your pump operates and builds vacuum, so a new pump would not be the first part I would throw at it, especially because it is the most expensive part.

When I had this problem, before it mysteriously resolved itself, I was going to check the service manual to find out how the truck knows that it is time to shut the pump off. Where/how is sufficient vacuum being sensed? If it is part of the pump, is that serviceable without replacing the entire pump?

 

If the pump shuts off when you plug that inlet then the pump shouldn't be the problem. I'd hook straight to the tank and test it and do the same with the heater control. My hub solenoid was after the tank on my truck when I unhooked it but thePO might have done that.

 

I plugged the tank off and let the pump run for 5 minutes. It holds lots of air even for a couple days. But it doesn't shut the pump off. So I decided to buy a new pump.

I also plug everything except the tank and the hub solenoid. It won't hold pressure in the tank when I do that. So the solenoid must be leaking.

My theory is that the solenoid started leaking causing the pump to run constantly for a few weeks which messed the pump up.

I hope it solves the problem. Already got 2k in this pickup in repairs since I bought it in March. It's about to go back on the market and I'll stick with my Duramax. Lol