it is a hard line in the main wiring harness.

 

OK,

I located a hard white plastic vacuum line in the wiring harness above the blower motor that enters the cab. I believe this line feeds the actuator motor for the blender door located under the hood adjacent to the blower.

When I put a vacuum on this line nothing changes - the vents and the AC do not work. I am not confident this line runs vacuum from the pump to the vent/AC controls. I think it might control vacuum from the cab to the blender motor. Is there another hard line in a different wiring loom?

 

Once More With Feeling

I am out of ideas and hoping someone will throw me a bone. I have checked vacuum on the following:

Under the hood:

the vacuum pump
the brake booster (including the check valve)
the plenum chamber
the low vacuum switch
the hard white plastic line on the passenger side in the loom above the blower motor


In the cab:

All five coloured vacuum lines at the function switch:

White
Black (it is capped and terminates on the passenger fenderwell near the battery. I assume it was a superduty option for those trucks with the additional electric vacuum pump)
Red
Yellow
Blue

I can function all of the vent acutuators if I bypass the interior switch and draw a vacuum on each line. I can't find any leaks, yet the truck remains in default defrost mode with no AC.

Last time - does anyone know the source of vacuum for the function control switch in the cab? How does the vacuum pump physically connect to the function switch in the cab?

 

The Saga Ends

The black vacuum line on the passenger fender well is not meant to be capped. It is the source vacuum for the function switch in the cab. It must be connected directly to the vacuum plenum located next to the evaporator under the hood. The check valve on this black line is then connected to the vacuum line from the plastic manifold. That took too long.

 

DJW,

Oddly enough, I have this very same issue. I've done all the basic stuff you've mentioned but still now vacuum to the controls under the dash. Your last post mentioned the black line connected to the check valve which is connected to the plastic manifold. My question is where is this plastic manifold? I'm thinking my vacuum line from it must be broken.

Thanks, Don

 

Don,

The plastic vacuum manifold is located on the driver side wheelwell under the hood. Mine is just below the tymar air intake and has four lines attached to it:

1. a vacuum line from the vacuum pump (source of all vacuum for the vehicle)

2. a vacuum line to the brake booster

3. a vacuum line to the low vacuum sensor, and

4. a vacuum line that crosses the engine bay above the turbo and connects to the check valve in-line with the vacuum plenum (source of vacuum for the in-cab requirements)

PM if this is unclear. I can provide more detail.