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This is in a 2001.5 dodge 2500 auto with 300k miles, all stock.

 

4h worked in the morning, then after work jumped in truck and no 4wd.  When sitting still I can shift into 4h and 4L and no grinding and it is definitely getting onto 2l, but front won't engage.  When moving at about 20 mph, coasting, going straight, it grinds when I try to go into 4h.  The vacuum lines going to the axle look good, is there anything else I should look at?  Any common problems before I really get into it?

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Make sure you actually have vacuum at the ends of the hose, 

Next when engaging 4WD you should see the servos move .

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Permanent fix is vacuum eliminate by six states distributors it works like pto shift cable. Check vacuum lines on crossmember they can have leaks there. 

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i have seen the shift fork break inside the housing also. Do you get a 4WD indication on the dash? if you do, pull the actuator out of the housing and make sure the forks is in good shape. If you don't, mostly likely you have a vacuum issue.

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i have seen the shift fork break inside the housing also. Do you get a 4WD indication on the dash? if you do, pull the actuator out of the housing and make sure the forks is in good shape. If you don't, mostly likely you have a vacuum issue.
I have seen where the actuator binds up and wont move also.

Sent from my SM-G900R4 using Tapatalk

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Two problems I've had which I believe are common.  There's vacuum switch on top of the transfer case that reverses the lines to the vacuum operated CAD (Center axle Disconnect) on the front axle.  This plunger style switch is mechanically activated by the transfer case lever, sometimes it can get sticky and either not move or take a while to move.

The CAD on the front axle can get a hole in the diaphragm and not be able to build enough vacuum to overcome the spring pressure and pull the collar over.  I've had these issues either fail to engage the CAD 4wd or partially engage it which doesn't sound nice.  The switch for indicator light is on the CAD or if it's not coming on then the CAD isn't moving.

 

I've found this to be the most reliable solution.  It gets rid of the problematic vacuum system and makes a handy 2wd low range option which is nice for the manual tranny guys.

 

http://4x4posi-lok.com/app_dodge.html

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Does the heater move air from defrost to floor?

 

 

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The permanent fix is to jack up the radiator cap and slide pre-computer Freightliner  / international under it with Marmon Harrington running gear.!

Ok, all that said for these troll of trucks I use mine for snow plowing -

270K latter and mostly overhauled everything at least once.

Every problem known to man but the vacuum crap has not ever let me dwn.

I had the hose come off the valve assembly once

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The 4wd idiot light doesn't come on and the heater moves the air from defrost to floor and back no problem...

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I was hoping a rough road was going to jostle it loose on the way home but no luck.  Guessing it's the collar in the axle disconnect is stuck...

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Pull the vacuum line connector off at the CAD. Should be vacuum on one line nothing on the other, shift it into 4x4, now the other hose should have vacuum and the one that did before won’t have. If the vacuum is changing from one line to the other when you shift it then the CAD solenoid is suspect. If the vacuum isn’t switching between lines then it’s likely the switch on the transfer case. 

 

Typically if you have a significant vacuum leak (low vacuum) the heater vent will stay on defrost, sounds like you’ve ruled that out. 

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my sons truck is on the lift with the front axle tore apart, once it is out this weekend (hopefully) I'll see check the vacuum at the end of the lines

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22 hours ago, yogi_160 said:

Permanent fix is vacuum eliminate by six states distributors it works like pto shift cable. Check vacuum lines on crossmember they can have leaks there. 

I like this solution.

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In my 97 I had the cable, it was nice for backing in 2lo with the 5spd.  You also knew you were definately engaged

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Pulled it in the shop tonight and the vacuum lines on the x-member had both been blown off by slush i'm guessing - and the vacuum for the heater\defrost still worked fine as far as I could tell

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The only thing almost good about the doogde truck is the cumins engine .

Who could imagine a real trk with vacuum crap and wipers that cant be stood up whilst is parked?

Dodge could, what a joke.

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16 minutes ago, thumper 549 said:

The only thing almost good about the doogde truck is the cumins engine .

Who could imagine a real trk with vacuum crap and wipers that cant be stood up whilst is parked?

Dodge could, what a joke.

 

I couldn’t agree more, plus not build a normal crew cabling box for decades. I’ve always preferred the body and interior of a Ford, especially a crew cabling bed dually but the Cummins overruled those needs. 

Edited by Woodenhead

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I have firmly made up my mind- my next truck will be a mid-size international/ Freightliner with no computers.

 

They can be had for nothing and will go forever 

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13 hours ago, ish said:

Pulled it in the shop tonight and the vacuum lines on the x-member had both been blown off by slush i'm guessing - and the vacuum for the heater\defrost still worked fine as far as I could tell

 

 

The reason the heater controls still worked is that the vacuum goes through the valve in the transfer case before going to the axle. That and the encounter of flow restriction through all the little nes getting down that far, still allow the vacuum pump to create enough vacuum to still operate the heater controls. Just not as fast most likely.

 

a good friend of mine had the same exact thing happen to him, that just happened to you.

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1 hour ago, Torque Monkey said:

 

 

The reason the heater controls still worked is that the vacuum goes through the valve in the transfer case before going to the axle. That and the encounter of flow restriction through all the little nes getting down that far, still allow the vacuum pump to create enough vacuum to still operate the heater controls. Just not as fast most likely.

 

a good friend of mine had the same exact thing happen to him, that just happened to you.

Good to know!

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