Hi, so I bought this 1998 ram 2500 v8 (gas) 2 weeks ago and im trying to get the hvac to work properly. The air in the cab only comes through the defrosters which i have read is a common vacuum line issue. The vacuum line leading to the cab is old and brittle (even broke when i moved it around a little), I repaired the brake with some heat shrink and spare tubing. I have checked all other vacuum lines and repaired them with heat shrink as neccessary. Now all the lines look to be sealed, but air is still only coming through defrosters. So i know i have another leak somewhere but i am unable to find it. If you guys have any wisdom to shed on this that would be awesome, thanks.

Also, cruise control doesnt work and the light for it wont even turn on. 4x4 works fine, so i know i am at least getting vacuum somewhere.

 

I think the heater and Ccontrol get vacuum from the same port. Plug the line to the heater where they split and see if the ccontrol works. I think it was common for the line to the heater to break where it went through the firewall.

 

You should have two check valves, One right out of the manifold and the other by where the vac line goes threw the firewall. The check valves go bad also so you may want to check them. Make a note how they are installed before you remove them. Then see if you can blow into it just one way. If so it's good, Both ways it'd bad. You can get them ant napa https://www.napaonline.com/en/p/NDP7301349

 

I would be more tempted to put the check valve between the T, and the source, that leaves the reservoir in the circuit, for those times when you have low manifold vacuum. (like, a long hill climb while towing.......)

 

You might also want to have a look at the "PANEL-DEFROST DOOR ACTUATOR" itself. I had the same issue when I got my truck and by sheer luck the whole thing fell down when I was going along a rough dirt road. So I knew where to look Probably not the same issue but see if everything is moving freely and you could also check if vacuum is present.

I don't believe the cruise control not turning on is related to any vacuum issues.

 

One way to find a vacuum leak would be to spray WD-40 on a running engine. When the spray finds a vacuum leak the idle will increase. I have had issues with both Moparites and DerTrucks suggestions.

 

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