AC/Heat Not Blowing Air when starting up or idle

[TrueBurner]

Hey Y'all

Having issues with my AC/Heater. Good thing is that they both blow hot/cold air when the truck is moving. But when you stop or are idling or when you initially fire up the truck no air comes out even though it's still making sounds. Haven't done any troubleshooting yet, thought I'd come here first to potentially save some time on diagnostic steps.

 

[Rdnck84_03]

I bet it is blowing out the defroster until the rpm comes up to build vacuum. They defult to defrost with no vacuum.

I would guess you probably either have a vacuum leak or the vacuum pump is very weak.

James

 

[IDIBRONCO]

I would guess you probably either have a vacuum leak or the vacuum pump is very weak.

That's my first guess too.

 

[TrueBurner]

Thank you! Any tips on finding a potential leak? Does a weak pump mean I just need a new one?

 

[IDIBRONCO]

Does a weak pump mean I just need a new one?

Yes it does.

Any tips on finding a potential leak?

The first thing that I'd do is to test the vacuum right at or as close to the pump as you can get. That way you eliminate all of the rest of the vacuum system. Since all you need vacuum, for is the HVAC system, you could plug off all of the other vacuum lines and just leave the ones to it. Since I noticed that you have a hydroboost brake set up, I'm really guessing that you have a weak vacuum pump. You may be able to get away with using an electric one like came out in the Super Duty line of trucks with the diesel engine. If your cruise still works, you may not be able to. One thing that I do recommend is installing a vacuum gauge somewhere in your vacuum system and have the gauge mounted inside your cab.

 

[Nero]

Before we get too far ahead of ourselves, which climate control head unit do you have? Do you have the button for ac, or is it a sliding lever to turn on your ac?
The button version does not use vacuum for blend doors, whereas the slider does.

 

[TrueBurner]

Turn Knob

 

[Nero]

That should be vacuum actuated doors. Definitely go down the rabbit hole of Checking vacuum.

 

[franklin2]

Check the large hose leaving the vacuum pump. It goes from the pump to the pass side and lays on the inner fender. Over the years of the engine shaking, it can rub through on the underside of the hose where you can't see it.

 

[bumblebeer]

Definitely check the vacuum pump. With hydroboost brakes, you may not have noticed a poorly performing vacuum pump. It should be able to pull 21 inches plus at idle. A lot of times a failing pump will pull little to no vacuum at idle, but will otherwise seem to work fine once at speed.

If you want a quick hack to get your air conditioner blowing at idle until you can fix the issue, look for the vacuum actuator for the dash vents. It's just above the passenger floor board area. You should be able to look up and see it from the floorboard without having to remove anything. Push the lever to the fully closed position and zip tie that booger. Since the blend door is cable controlled, and the air has to go by the dash selector door before it can get to the floor/defrost selector, this will keep cold air blowing out of the dash vents regardless of vacuum. You can also zip tie the vacuum selector closed on top of the air box in the engine compartment. This will keep the AC on cab re-circulation instead of pulling outside air.