I got a 1990 W-250 pickup.The 4 wheel drive never worked on it when I bought it 2 years ago.I,m getting around to it now,tinkering with it. I suspected the front axle vacuum modulator on it.I took the old one off,put my hand vacuum pump on it and yes it did not work.I went and got a new one and installed it on the truck.Still no front wheels turning.The front shaft on the transfer case spins, but nothing turns on the axle.The truck is on jack stands in my shop. Does the truck have to be on the ground and rolling forward or back in order for the axle to engage once the transfer case lever is thrown in gear???? Or do I have little vacuum at the modulator? I put my finger on the end of the hose,there is some vacuum, but is it enough??? My next step is to pull the front axle cover off and see if the ring gear is stripped of teeth..If so,I have to pull the gits out of the axle.It is a Dana 44 FBJ. I,m havinh a lil hard time tryiong to find the special tool needed to remove the inner axle shaft. Chrysler number D-354-4 the tool and the adapter D-354-3.Any body know where I can get that at a reasonable price?????

When you bought the new vacuum unit did you test it before you installed it? Can you use the hand vacuum pump to make the vacuum unit work on the front axle? Can you connect the hand vacuum unit to the line on the truck to read the vacuum available on the truck when it is running and in 4x4? Just some of my first ideas before tearing into the axle.
Bryce

The only thing the vacuum modulator does is lock the axle on that side to itself so it will turn in the differential. without it locked in, the wheel end of the axle will freewheel. With both front wheels off the ground, I'm not sure you will be able to tell if the gear is locked in or not.

The vacuum only pulls the sliding gear in or out, depending on which vacuum port has vacuum. Once it is slid in the direction the port dictates, there is no need for the vacuum to hold it in place, but I believe the vacuum remains.

One of the biggest issues with this setup in the vacuum lines. They are thin steel lines with rubber hose ends, and the hoses go bad and the steel lines rot out.

You can manually lock the gear in by removing the vacuum unit, sliding the mechanism into the lock position, and shifting the vacumme lever to the side making it hold the slider in gear, then reinstalling the vacuum mechanism. To disengage the 4x4, you have to remove the vacuum mechanism disengage the slider, center that vacuum mechanism and reinstall it.

Pulling the cover to inspect a differential is never a bad idea, but unless its really broke, your probably not going to gain anything concerning the 4x4 function. Gene

if you don't mind a bit of wrenching, this may fix the problem: http://4x4posi-lok.com/app_dodge.html

I tested the new unit with a hand vacuum pump.It slid the fork over as it should when I pumped the handle.I did connect it to the unit when I installed it on the axle of the truck.I pumped the handle, but the front wheels did not engage and turn.I felt vacuum on the one hose with the tip of my finger.I don,t know if it is enough.I will try and connect a gauge on the end of the hose and start the truck to see how much is there.My next step is to pull the front cover on the axle and look inside.It will only cost me a new cover gasket,or if I,m careful,just a lil bead of silicone if cover is removed carefully. Hope I don,t find a ruined differential inside the housing....

use your hand pump to engage the front axle, spin 1 front tire, if the other tire moves in either direction (likely opposite the one your turning) then the front end is "in gear".

I pulled the cover on the front axle tonight.Only thing I saw was 3 spider gears with a small amount of tooth missing at the heel of the tooth,not enough to affect performance of the other gears. I put the vacuum gauge on the vacuum hose to the modulator,it read a steady 19 Hg,s. Only conclusion I can draw at this point is the shift fork is not moving the sliding collar in place{ I looked carefully to make sure it was aligned to the collar when I installed the modulator mechanism or something is broke with that shift collar internally,whicj will involve pulling the left side axle assembly out{I will need that special tool now} I had the truck running and in gear and watched everything turn in the front differential. Has to be area of the shift mechanism.I,m still wndering about the vacuum at the modulator.Is it enough or is there supposed to be a "Blow" effect with the other small hose that is "Paired" with the vacuum hose that both go to the modulator???

It's pretty straight forward of a setup. The connecting collar just slides over and engages both splines when locked. You probably already know that though. If I recall, the collar stays on the outer stub of axle when in 2 wheel drive, and then when 4x4 activated, it slides towards right side of truck when activated. The vacuum signal remains for either the 2 wheel position or 4 wheel position. So if you had a live vacuum hose handy, you could slip it on one or the other of the 2 actuator nipples, and the fork will go one way or the other depending on which nip was chosen. I must be the only one who has not had issues with this CAD system, although many here have had rusted lines, cracked hoses, or the switch on transfer case problems.

What do you mean by "Switch on the transfer case" ???Is there something on top that supplies extra vacuum to move the actuator on the axle??I will pull the actuator off the axle and look inside more closely at the sliding collar...

Yes there is a vacuum switch, that the internal transfer case linkage activates, to apply vacuum to either side of that vacuum actuator on the axle. Initial vacuum source comes from engine through a hose.

I got it....I removed the actuator assembly on the axle slowly,with an inspection mirror and discovered I missed engaging the sliding gear collar with the actuator fork.I had a suspicion that might have happened.There is a lot of space before the fork slides over the collar.Put it back together and NOTHING..Now What....I swapped the lil vacuum hoses going to the actuator and PRESTO........The front axle now turns when transfer case lever is engaged and transmission is in gear....... The indicator light does not light on the dash telling it is in 4x4 mode.I will chase that down with a meter at the end of the actuator tomorrow.I got to run to O-Reilly,s and get some silicone sealer for the front cover,I discovered I was out of it...Hope all this helps someone else who is working on their truck.....