1999 F350, 7.3 Diesel, Lariet, 4x4, dually. I never hear it running by noise with the ignition key turned to power or when the engine is running. Also placed my hand on pump and never feel a pulse. Thought to pull a vacuum line to the reservoir but vacuum line will not budge when pulling or twisting. Afraid I will break the attached plastic inlet to reservoir.
Thought by removing this line it would cause reservoir to leak to ambient (zero vacuum) and send a signal to the pump to turn on. Is this the correct way to test pump?

 

Tried to remove electrical connector to pump motor but it will not disconnect.

 

You will get a better response posting this up in the 99 - 03 forum. The 94.5- 97 rigs use a mechanical vacuum pump and not electrical. But to answer your question it's pretty hard to hear with the engine running but with just key ON it is quite noisy, like a compressor running. It only turns on for a short period of time though, maybe 3 - 4 seconds or so on a cold start.. Not sure why your' asking though, if it was bad and not running the heater - ac controls would not work properly through the vents in the cab.

 

Cycle through your ventilation settings when the truck is running. If it changes appropriately then you have enough vacuum and your pump is maintaining that. Our diesels don't make vacuum which is why we have a pump.

If your ventilation goes to defrost (when you are not selected to defrost) then your vacuum has dropped to 0" and you have a leak that your pump can't keep up with or your pump is dead.

The pump is not designed to run continuously and will only come on to maintain vacuum above a certain level (don't know what level off hand). The tighter the system, the less it runs.

 

^^^^ this^^^^
if the heat /ac controls are working properly, the vacuum pump is working. i have never heard the pump in my 02 come one in the 11 years i have owned it.

 

I didn't realize this was something you would hear either. I have never heard this in the almost 12 years I've owned my truck.