1995 Dodge Ram 1500 Overdrive Vacuum Loss
Kind of hard to visualize the items you are talking about. the vacuum line that goes in side the car is to drive your air doors for your defrost, and vents. This vacuum line should go from the intake to a vacuum tank. In that line should be a check valve. Then from the tank to thie inside of the vehicle. The check valve is to keep a vacuum leak inside the truck from causing driving problems. When you described before the vents changing when you went up a hill, makes me think that maybe that check valve has gone bad, and maybe you have a vacuum leak.
The cables that go to your throttle body should only be two. A crusie conrtol servo cable, and a throttle cable. The down shifting is all done by computers.
I would make sure that vacuum system going to your inside is in good shape, and I might even disconnect it a plug the hose to eliminate that as a problem. Then see if your truck runs good. If it does, then find the problme in that vacuum system. If it doesnt make a difference, then place a vacuum gauge on that line and see how you vacuum is. It should be around 18-19 inches and steady at idle. If you have a large leak, then it should run rough.
This is the Cruise Control Servo Unit. If your cruise control doesnt work it need to be replaced. It could have a vaccum leak internally on in the line that feeds it.
Suggested Repair
Check the intake manifold gasket and the cabin vacuum circuit, including the vacuum tank and check valve. Plug the interior vacuum hose to see if the drivability issue changes, and confirm about 18-19 inches of steady vacuum at idle. Also inspect the cruise-control servo under the battery and its vacuum feed for leaks.
Conversation Summary
A 1995 Dodge Ram 1500 with a 318 lost transmission strength and would not downshift from overdrive on inclines; the vents also switched to defrost uphill, suggesting low vacuum. After a transmission rebuild, lockup and takeoff improved but the overdrive symptom remained. Replies pointed to an intake manifold gasket leak, a failed vacuum check valve or leak in the HVAC vacuum system, and a possible leak in the cruise-control servo under the battery. Downshifts were described as computer-controlled, and isolating and measuring the vacuum system was recommended.
