whats the vacuum for?

[K5dreamer]

ok, so i was trying to fix a fueling problem with the truck when i noticed some rather dry rotted and crumbling vacuum lines on the truck. Needless to say there arent alot of them so i didnt know what they were for, i tracked them going from the vacuum pump, to the injection pump, and then to a hard line that dissapeared over the back of the engine.

my question is. whats the purpose of the vacuum line to the IP??? some sort of vacuum advance?

 

[kennyw]

That is for the vacuum modulator on the TH400. It controls when your transmission shifts. There should have been a hardline from the front of your motor down to the modulator though. Someone problably wrecked it and bypassed in the military.

 

[K5dreamer]

AH HA!!!! brilliant! that explains why the truck hasnt been shifting into third until 50-55mph instead of 40-45 like it used to. But the hard line is still intact. Ill try to post a picture of what it is im talking about on thursday, but the vacuum line has a breif stop at the injection pump before heading back to the transmission.

the vacuum line goes from the vacuum pump, forward to a dual port connection on the passenger side of the injection pump, some black plastic thing mounted (if memory serves) opposite to the throttle linkage on the driver side, and then the second port of that vacuum thing is connected to the hard line via a roughly 6in piece of rubber tube. at the front passenger corner of the intake.

 

[kennyw]

Yes, get that fixed and your truck will drive a lot smoother.

 

[Wrench]

The thingy on your IP that the vac lines go to/from is the throttle position sensor. Get your vacuum system closed up so you have vacuum again and your tranny will shift properly . . . .