still having issues with 4X4 stuck in.

metallica21156 · #1 12-07-2005, 08:13 PM

i still can't get the front diff to come out of 4X4. if i pull the vac line to the actuator then it kicks it out of 4X4 but i can put my finger over where the line hooks up and and its pulling a vac. and the trucks in 2wd. the transfer case has no problem switching from 2-4. its just it won't stop sending a vac to the front. what could be wroung here? i'm lost at it. does anybody know what the little thing behind the batt that hooks up to that vac line also that has a plug on it do? i can't figure out that or what could be causeing this. its been like this for prob about 2 years and last week i found out it was stuck. any ideas where to start? and the fluids are fine and only 1 year old.

slreno1217 · #2 12-08-2005, 05:11 AM

Quote:

Originally Posted by metallica21156

i still can't get the front diff to come out of 4X4. if i pull the vac line to the actuator then it kicks it out of 4X4 but i can put my finger over where the line hooks up and and its pulling a vac. and the trucks in 2wd. the transfer case has no problem switching from 2-4. its just it won't stop sending a vac to the front. what could be wroung here? i'm lost at it. does anybody know what the little thing behind the batt that hooks up to that vac line also that has a plug on it do? i can't figure out that or what could be causeing this. its been like this for prob about 2 years and last week i found out it was stuck. any ideas where to start? and the fluids are fine and only 1 year old.

check the vacume line that hooks to the canister that is mounted to the hood on the drivers side on the back of the hood. and trace it back to make sure of no leakes

metallica21156 · #3 12-08-2005, 06:09 AM

that would give me no vac. but the line is good. i had the truck off and used a vac tester. took it to 20 with no probs. with the engine runniung it would peak high then that but at idle it stayed very close to 20.

slreno1217 · #4 12-08-2005, 10:32 AM

Quote:

Originally Posted by metallica21156

that would give me no vac. but the line is good. i had the truck off and used a vac tester. took it to 20 with no probs. with the engine runniung it would peak high then that but at idle it stayed very close to 20.

not entirly true.. mine was loose at the ball and i still had vacume at the actuator.. but it wasnt enough to work it correctly.. but if you say you had it tested then i guess its good.

loots-42 · #5 12-08-2005, 10:49 AM

Quote:

Originally Posted by slreno1217

not entirly true.. mine was loose at the ball and i still had vacume at the actuator.. but it wasnt enough to work it correctly.. but if you say you had it tested then i guess its good.

I think what he is saying that his problem is that it won't come out of 4x4. If he disconnects the line (no vacuum), it comes out of 4x4. So the problem isn't a vacuum leak, because it's stuck in 4x4.

Cailen · #6 12-08-2005, 10:55 AM

Sorry, with the differing replies I'm not sure if this will address your problem but... have you tried resetting the encoder motor? Disconnect your neg battery terminal for 5 mins and see if that shifts 'er.

metallica21156 · #7 12-08-2005, 04:55 PM

i don't think the encoder motor has anything to do with it. the transfercase is coming out of 4x4 just itts sending a vac to the front. i did have the batt off at one point and it was fine. norm. vac when in 4X4 and not when in 2wd. then 2 days later i checked it and it was sending it again. yesterday i pulled the ecm fuse,4X4 fuse,prndl fuse and 2 other fuses trying to figure it out. any other ideas. for now i got duck tape over the vac plug and the tube hanging. if i need 4X4 i have to get out and plug it in. i already hooked it up tonight for the snow.

joeuser742 · #8 12-09-2005, 10:03 AM

I have the same problem and I'm still thinking of what to change first. It makes sense that the encoder motor can be the problem because that's where the vacuum comes from. I'm not 100% sure how the encoder motor works, but it sounds like it opens or closes some port to allow or disallow vacuum to the actuator. Another thing it could be is the VSS. If it's confused on the vehicle speed, then it may not release the vacuum. I think that I'm going to change the VSS first because I'm having this problem and also have the ABS light on. And I don't have an ABS scan tool.

NOVA71 · #9 12-09-2005, 02:51 PM

I had an 85 that had the same prob. I used to have to unplug the vac line from the actuator under the hood to get it out of 4wd. The prob was with the vac switch on the side of the transfer case. It has a check valve inside that would stick. You may want to check this, it's pretty east to get at. It's on the drivers side of the transfer case and has the vac lines going into it. It's tricky to get a wrench on though (I cut an old wrench very short to make it easier to get in there). I got another vac switch from a buddy of mine who works at the dealer. He told me that the check ball will stick and you can soak the switch in WD40 and work the little check ball free. I had 2 of these switches; one in the truck and one I kept in a jar filled with WD40. When the one in the truck would start sticking, I'd just take that one out and put that one in the jar to soak and put the other one in the trunk. Hope this helps.

Cailen · #10 12-09-2005, 03:09 PM

The encoder motor is a backwards forwards motor that operates the actuator in either direction. If it's screwed up it can "think" that the truck is in one position when really it's in another. Removing the negative battery terminal restores it to its proper state... I think that's basically the same as removing the line you're talking about, isn't it Nova?

BlazerLT · #11 12-09-2005, 03:48 PM

has the vacuum actuator actually been changed?

NOVA71 · #12 12-09-2005, 03:51 PM

I'm not totally sure how the encoder motor works, but my buddy at the dealer said that it's not uncommon to have trouble with them and that they run around $900+ (that's Canadian) for a new one. I've had 3 GMC Jimmys, an 85, 94, and an 00. The 85 always had a sticking vac switch and would not come out of 4wd, which I heard is very common. My 94 and 00 have only had trouble shifting into 4wd. In both cases it was a rotten vac line behind the distributor. Actually, I just replaced that line on my 2000 today (we're supposed to be gettin about 12 inches of snow tonight). I did in the past unhook the battery on both trucks figuring it may reset the encoder or clear any codes, but to no avail. Luckily both times it was just the vac lines and not the encoder. When new vehicles work good they are great, but man when they don't they can be a real beeeotch!

loots-42 · #13 12-10-2005, 05:08 PM

Metallica,

Seems like we've got the same problem so I hope you don't mind me joining in on this post. Mine is a 98 Jimmy.

I just replaced that vacuum switch. Still doesn't work, but at least it was only a $30 part.

Dealer did diagnostics on my 98 4x4 problem and said that the encoder motor on the transfer case shorted and took out the 4x4 module (by the passenger's feet) and I need a new switch. $1200 for the shift motor, the 4x4 module and the switch and labor. Does that seem right? If it had shorted, would the TC still shift between 4HI and LO, and sometimes into 2HI. Doesn't seem right to me. Any other thoughts?

Is there a position sensor on the front axle that maybe isn't working? I've check and tested the vacuum actuator under the battery and that seems to work ok. Man this is a tough one.

Thanks

metallica21156 · #14 12-11-2005, 09:16 AM

mine has no problems shifting in the case. it goes into 4lo 4hi and 2wd. i'm going to check the vac switch on the transfer case today and see what i find. i have heard they can also wear down and not work so i'm looking there first since its the chepest. as for the encoder motor i think mines fine because after the recent pulling of the fuses it made no diff to it but if the switch checks out then i might pull the batt and see what it does. i praying its ok. i think it is. worse case i just spend the 130 and get that posilock cable and not worry about the vac since everything else is ok. if the transfercase is shifting like it should then the encoder really can't be it. i also tried changing out the switch in the front axle because it was telling the vcm that it was in 4x4 and changing the trans shift patteren as well as the way the abs would kick in. with the vac disconected you can tell a deffinent difference in the way it drives and shifts. i'll post back on here later what i find. i'm gonna have fun hunting for the switch since theres so much crap under there. o and with the recent snow it had no problem. never spun once in 4x4. i was backing out of a driveway and you could see where it drug the front wheels from being locked in.

loots-42 · #15 12-11-2005, 09:36 AM

The vacuum switch on the TC was pretty easy to change. A 7/8 combination wrench (use the box side) fit just fine. Its on the drivers side, upper part of the TC.

Is there a switch/position sensor in the front axle. Is it hard to change? My vacuum actuator seems to work ok though.

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