So just as winter hits my 4WD stops working. My assumption is a vaccuum line somewhere. But I've checked and replaced quite a few of them. But not all. I have not given up on a vacuum leak. But here's an interesting twist:

Whenever I plug in an OBD2 scan tool, in this case a Snap-On Ethos, it starts working. Although it seems to swap between 4Hi and 4Lo by itself every now and then.

The TCCM does spit out 3 values on the Scanner: Front Diff lock, Front Diff Request lock, and TC lock. When the scanner is first hooked up, TC lock, and Front Diff Request all give a "yes".

But the Front Diff doesn't lock. Scanner says so and you can feel it in the drive. The TC switches, it'll even go into low gear while it attempts to lock the Diff. Again, you can feel it in the drive.

However, keeping the scanner hooked up, turning the key to "Off", then back to "On", allows everything to work just fine for the duration of that run. Even if the scanner is disconnected.

I've confirmed it to do this atleast 3 times.

So, what do you all think? Anything useful is appreciated.

 

Hi, I have a 98 and had some problems also! First there is no vacum involved at all! There is a transfer case module under the right kick panel which just pops off! Try unplugging and pluging back in, helps to use elec. sil. grease on connections , and make sure it is tight,,grounded!This is a place that gets a lot of water! Try with key on and then key off! usally works! This signals the BCM (body control module) which is right next to gas pedal, over trans. hump! Sometimes you may need to replace this as a last step! These are what control your transfer motor along with speed sensors and position sensors!! wow thats what i said when i first looked at the diagram. good luck!! P.S. There is a vacum that is located under the batt. that pulls via a cable,, the front diff. to lock poss. and off!! their is a signal switch there!! sorry until you said diff. I didn't think of that vacum!!

 

ljd, I'm not sure where you are getting your information from, but the BCM's role in the 4wd system is really very limited on the s-series trucks and EVERY s-series truck has a vacuum actuated front axle engagement system from the factory.

bando, It sounds like you have the NV236 (4-button) transfer case.

I woulds start by checking for an electric switch attached to the front axle. It should be in the same housing as the front axle engagement cable (just to the passenger side of the diff). This switch should show open when in 2HI and closed when in Auto4wd (if NV236), 4HI, & 4LO.

 

Sorry for any misunderstanding! new to forum's, and didn't read the entire posting first,, but did put a correction in one minute! In the title he had a problem in the tranfer case shifting,, the TCM is a always haveing conn. via moisture. its an easy check!! The BCM is indirectly involved, but I have switched many and solved many shift lock problems! after reading further I corrected or added to the fix. also that cable for the front diff goes bad! sorry if you didn't like my correct info,, but it was free, and just trying to help! seen tons of wrong info, or partial! Try hard to be nice!!!

 

So update. It suddenly started working.

Took it for a trip to Sears and it shifted perfectly.

I'll still take a look at the switch on the diff, as well as check out that cable tomorrow.

 

Your edit did not occur before my post was made so I never saw your correction, but I stand by my statement. In both my work involving these trucks for friends/family/co-workers and on this forum for the past 6+ years and I have never seen a BCM cause a 4wd problem. EVER. Just sayin'. Check out the wiring diagrams and let me know how many wires the BCM & the TCCM (either NV233 & NV236) have in common.

Glad it's working. It is possible that the cable is getting sticky and not fully pulling the shift fork into the locked position, therefore intermittently not contacting the switch properly. The fact that it is working now means that the switch is likely ok, but still check out the cable.