95 dodge , 5 inch lift , 360 motor. I have replaced the vacuum motor on the front rear, the solid vacuum lines under the truck going to the vacuum motor, and the vacuum switch that is mounted on the transfer case. My problem is , is that when I engage the 4x4 the truck feels like you are driving on a rumble strip on the highway. The light for the 4x4 comes on on the dash. Once you hit about 25 mph the light goes out and so does the 4x4. You hear a big bang and then the light comes back on and so does the 4x4. I don't know if the transfer case is bad or is the front rear bad? What would cause this to happen anyway.The switch on the front rear I think operates the light on the dash to tell you that you are in 4 wheel drive when the vacuum motor moves the axle colar and hits the switch. I would think it is losing vacuum somewhere, but why the "rumble strip" feeling when driving. Any help will be great !

 

The rumble strip part of it could be that it is not fully engaged. You need to check vacuum at the lines connected to the CAD. Should be around 15lbs. One of them will have 15 in 4wd and the other will have 15 lbs in 2wd.

 

I'll have run out and grab me a tester for the vacuum. I kind of thought it might be something like that, and that's why I replaced the solid lines. I will report back after I check it out. Thanks

 

Keep in mind that if you do have a vacuum leak, it can be totally unrelated to the 4wd system. Like the cruise control if you have it, or heat/ac controls, and the check valves or hoses on the passenger side of the intake manifold for instance.

 

Well, sitting at idle in the driveway I tested it in park in 2 wheel drive. 17 lbs. , then I put it 4 wheel drive, also read 17 lbs. needle did not move at all , stayed smoothly at 17. I tested both vacuum lines. One read 17 the other read 0, then I flipped the vacuum lines on the tester and it read the opposite way like it should. I wonder if the colar that connects the 2 axle halfs is bad. Doesn't explain why it would go in and out of 4 wheel drive when driven. Seems that if I go over 25 mph it just doesn't work. That leads me to think that maybe when the vacuum pressure gets higher as the engine speed increases it may be colapsing one of the vacuum lines?!?! Any other ideas will help. I should replace some of the vacuum lines on the engine. Hasn't been looked at in the 8 years I've owned the truck.

 

Any chance your heater vents move from dash to defrost when you are on the throttle like going up a hill? And i don't think the collar is bad, but the fork has two plastic shoes on it that wear , and there is a bushing on the end of the right side axle where it mates with the intermediate shaft inside the cad. But, most of the time that will just make it hard to get it in 4wd. usually if it pops out of 4wd it's a vacuum problem or the shift fork inside the TC sorry to say.

 

no. i think you have some kind of mechanical problem, and the stress is overcoming the vacuum and kicking the collar off.

the fact that your 4wd light comes on, is a good indicator that your vacuum is working, and the cad is working, at least at first, and the internal cad shaft is sliding over and touching off the indicator switch for the light.

the rumbling and banging are not good. i'd remove the cad and see what it looks like. check for anything out of alignment, or broken, or missing e-clips, or something out of whack. consider putting all 4 wheels up on jack stands and run it.

 

hmm, good question on that. if the t-case jumps out of 4wd, does the vac switch to the CAD change its flow, and uncouple the front axle.

mysterious BS like this is why i like a posi-lok.

 

I've got a posilok also, and it certainly eliminates most troubleshooting probs.

 

I checked the vacuum motor and the fork and plastic peices and they are all good . No missing e clips or anything. I just ripped it apart to see. I slid the colar back and forth and it seems to be pretty solid on the 2 axle halves. I did find out that one of my u joints is on its way out, on the driver side though. What is a posi lok? Explain if you could.