so I have seemed to misplace my haynes manual for my truck and have developed a weak vacuum. Ive checked all the vaccum lines and boots and have found the suction to be pretty weak coming out of the small box bolted to the driver side fender. can anyone tell me what that is or has a diagram of the vacuum system. It appears to be some sort of diaphrame maybe. Ive lost my cruise control, heater vents wont change and had to replace my 4x4 with a posi lock kit due to the weak vacuum.

 

Ok, I also have this mystery box on the drivers inner fender. There is a vacuum line coming off of mine that is capped off and some wires that go who knows where? What is this? Just got this truck and already chasing vacuum gremlins.

 

the mystery box is on the fender so it does not get wet or exsposed to direct sunlight, just remember not to feed it after midnight....

sorry I couldn't resist

Cummins vacuum system is pretty simple, start at the pump and verify pressure.. or lack there of since its vacuum that is being measured, then move forward joint by joint until you fun a significant difference

the box with wires your describing sounds like the low vacuum warning switch, it basically will turn on your red brake lamp if vacuum sucks, or doesn't suck enough in this case... sry I had a few beers , long day at work and I'm doing my best to be funny

anyways, your vacuum pump should create roughly 8-25 inches of vacuum , if this truck was in my stall I would plug everything that enters the firewall and smoke the system, then do the same with the inside vacuum lines i.e. HVAC controls

this should get you started in a good direction, let me know how it turns out

scott

 

Have you lost cruise control? That "mystery box should be your cruise control box.

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That would make sense with my truck, the brake light stays on even when the brake is released, and i know i got weak vacuum cause my Pacbrake doesnt always kick on