First off, this problem is with our gas powered 97 1/2 4WD. The 4WD is not locking in and the light on the dash will not come on. I checked the hose's to the switch on the transfer case and there is vacuum to it. i then shifted the case to 4wd and and i had vacuum to the diaphragm on the front diff as well. I pulled the sensor out of the diaphragm assembly on the diff and when pushed in the light on the dash will come on. So, i decided to take the hose off and supply a little shop air to the assembly and it clicked like it was engaging. I know nothing about 4WD trucks so im not sure if its supposed to have vacuum or pressure to lock the diff. The truck has had an engine swap so it is possible that something is way wrong.

Long and short.....I have no idea what is wrong with it and i need it as the snow and ice is making it impossible to get home.
BTW....my 2WD Cummins is parked until it all melts off.

Anyone????

 

Well they are designed to operate on vacuum, between 20-25 inHG would be a good reading. The vacuum motor in the front axle operates the engagement or dis-engagement of the CAD. Sounds as if the vac motor on your Dodge is gone, if not before,it is now after the shot of shop air. You pretty much went thru all the steps of a good diagnosis, and eliminated items one at a time. I would think a trip to the dealer,and you will be on your way, with the benefits of a functioning four wheel drive system. Good luck.

 

So it should have vacuum on both lines all the time?

 

At the front vac motor: Black=4x4 Red=4x2 As mentioned above you should have a good vac reading at that point also. Things I see on this system occasionally: Vac line from engine heat damaged and restricted at rear of engine. Bad/sticky switch on transfer case. Leak in vac line. I haven't seen too many probs with the vac motor itself and it sounds from your diag by applying vac to each side that it is working ok. Hope this helps.

 

I had a 97 1500 4x4, and after I changed my sparkplug wires I some how moved the vacuum lines and they were hitting the back of the engine witch melted them. I cut out the bad section of line and replaced it with rubber line. Worked great drove at least 80k with it like that. I would find the lines coming from the engine and make sure they are in good shape. Since you have checked every thing else it might be this. Good luck

 

simple questions........
1. i have two new lines running to the transfer case...both have vacuum on them at the switch.
2. Both lines at the diaphragm have vacuum on them when the unit is in 4wd...is that the way its supposed to be?

 

It is my understanding that you should have vacuum on one line to engage and vacuum on the other line to disengage.

You need to sort out your lines and see where the screw up is.

If you have vacuum on both lines at the same time they cancel each other out and nothing with happen.


Sounds like the shift motor is fine, just the vacuum lines are not being fed from the right place.


Jason

 

You should have 4 lines to the top of the Tcase, 2 lines to the axle cad.
On the Tcase one is supply from vacuum pump one is a breather two to the cad.
On the cad the outer(towards right wheel) should have vacuum when in 2x4,
and on the diff side when in 4x4.
If you have vacuum at both on the cad someone has the lines tee'ed together or your switch on the Tcase is bad.