jj3500

2001 3500, dually, in sig. 6 spd. 4x4. quad cab & 8' bed.

i've had this problem once before when i first bought truck (new). brought to dealer, they said, "loose wire underneath". they didn't get specific.

it happened again. no 4x4. i pull lever in cab, no light on dash to show and it doesn't engage. I was stuck in my driveway.

is there a little harness that gets unplugged on the underside near the diff? or anything simple that I can remedy?

the 4x4 is not problematic. it either works or in this case, not.

it just stopped working from one use to the next and it was sitting in the driveway.

its gotta be something quick...i'm thinking.

thank again for all the helpful hints.

Tallguy67

When your transfer case is engaged by pulling the lever in the cab it opens a valve in the vacuum system that uses vacuum power to actuate the CAD (cener axle disconnect) on the passenger side of your front axle. When the vacuum actuator slides the CAD collar across and makes your axle solid, a small contact switch closes the circuit so the 4x4 light comes on, on your dash.
Let's test the simple thing first, find the CAD on the backside, passenger side of your front axle. Make sure both vacuum lines are connected. With engine running try cycling the 4x4, move it back and forth a bit if it won't engage just to make sure the CAD isn't hanging up on a spline on the axle. If problem isn't solved, put a jumper across the two connectors for the 4x4 light. Again, strart truck. All this will do is confirm that the indicator light works. Once you have completed both of these steps, check back in and give us a progress report.

Tallguy67

Our 4x4 system is actually fairly simple but the vacuum lines are tiny and rather problematic. We will get you through this just fine.

SIXSLUG

Also, check the vacuum "Tee" at the rear of the firewall just above the fuel filter. The rubber bit of hose connecting the tee is notorious for degrading and allowing vacuum leaks...

torquefan

Good advice above on checking the vacuum system, that's what causes the vast majority of 4wd engagement problems on these trucks. The vacuum supply line runs from the firewall, down along the top of the transmission, to the vacuum switch on top of the transfer case. From there, two plastic lines go down the t-case to the crossmember where they turn into steel lines and head to the disconnect on the front axle. Give them all a good visual check for damage before you go any further, and let us know how it goes.

Edit: I should also mention, if someone has just changed the fuel filter or done anything else in that area, it's very common to knock loose one of the vacuum lines in that area, from the firewall forward to the vacuum pump on the driver side of the engine.

jj3500

THANKS GENTS for this info.
its been really nasty here in NY. Frigid temps and now freezing rain.
when this clears enough, i'll crawl underneath.

thank you much again.

cupressus

I don't know how often, but CAD diaphragms do go bad as well.

The PO on my truck had opened the housing and drilled the CAD shaft, installing a pin so the front axle was always locked in.

I got in there and replaced it with a posiloc (cable actuation system for the CAD), eliminating the need for vacuum so you lock the axle together using a cable in the cab.

Or you can spend lots and lots of money to get a manual hub kit.

Just throwing the options out there (hope it's not just adding confusion)

C

Tallguy67

You will notice in my signature that I have the PosiLok but I didn't want to make that recommendation until we know what was wrong.

It is my humble opinion that PosiLok is the best thing to ever happen to the 4wd system of our trucks.