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GMC Jimmy 4WD Problems



  • No way - buy a 4wd. The transmission rear is diff, need a tranfer case. front diff, and finally the whole front suspension is diff so its nearly impossible and would be a headache and $$$$.
  • 4-wheel drive not working. Replaced switch,but will not engage into either 4-hi or 4-low. Anyone have any input as to what the problem could be?
  • I have vacuum to the actuator under the battery box. Today I replaced the vacuum actuator and nothing replaced the switch on front diff, switch on t-case, encoder motor traced the vac lines checked fuses again. Is the selonid on the firewall down behind the distributor? What about the modual?
    Thanks Jay :sick:
  • The solenoid is mounted on the firewall right under the hood seal - trace the vac line it goes thru it. If you push 4Hi with the engine running you should hear the vac actuator engage the axle coupler on front diff. If that occurs and the the new sensor switch you installed should tell the TCCM (computer behind the pass side in cab kick panel) that all is right engage 4HI and set the dash light to 4Hi. If you do not hear the actuator then it or the solenoid are bad. If they work then pull the battery cable for 30 min and reboot the TCCM and all may work fine - they get confused and hang sometimes, really just a cheap processor.
  • I can't seem to here it engaging at front diff... I can see the cable move from the actuator when I tested the vaccum actuator... What must I do now?
    Thanks Jay :confuse:
  • Buy a plastic in line connector and bypass the firewall solenoid (connect vac lines together) while running and see if it then engages. Solenoid is GM only at $45 and they do fail and leak internally - just changed my 99 but may be the sensor on the front diff also not seeing the coupler move - its a little ball that gets pushed up as the axle couples and sends the info to the TCCM then the dash light goes solid on for 4Hi. That sensor just unscrews off the axle so you can check it and the electrical connector.
  • I have a 97 Jimmy that I put into 4wd (hi) and when i try to switch back to 2wd all I hear is a couple small knocks on the transfer case. The 2wd button flashes but it will not switch back to 2wd. I have tried moving forward ,backward,park neutral,accelerating,coasting. Nothing seems to work.
    I need to get this problem resolved as I drive a long distance to work every day. Any suggestions?? TIA
  • Check the vac lines under hood and then the one down to the transfer case (its 5/32 and is T off at the drivers side under hood and goes back behind the engine and down. 98 and earilier have a NP233 transfer case with a 3 port vac switch mounted on it that engages the front axle coupler and the front driveshaft. Look up under the Blazer on the drivers side at the transfer case just behind where the front driveshaft comes out. You can get a replacement Motormite #49315 for $15 at Advance and its on the Help part isle. Use a 7/8 box end wrench to remove after simply pulling the formed rubber top connector off. The formed rubber fitting is keyed to the tab on the switch top so hard to get wrong. The switches leak around where the pastic top meets the metal bottom usually. If the plastic top can rotate it leaks for sure.
  • I checked all the vac lines and they all seem to be in good shape. I noticed than the actuator, under the battery case, does have vacuum to it. When I pull the line off the actuator, the cable running down to the front diff moves and the front axles are no longer engaged. Although the 4wd light does not go out. I also noticed that some front diff fluid leaking out of the diff. Further inspection found a loose front plug left that way from the oil change place. I still have plenty of fluid so I'm hoping nothing is damaged or seized. I assume that the switch on the axle housing is the one that turns the 4wd light on and off. Is it common for then to stick leaving the light on?? I'm not really sure how this system works as far as what happens when it goes from 2wd to 4wd and back. Could I do damage to the transfer case driving with the front actuator disconnected until I have more time to get more info as to where to look next?? Thanks Rob
  • After a double check on all hoses I was still unable to find the problem with my issue (not coming out of 4wd). I'm very curious as to what I am hearing in the transfer case below the vacuum switch. It is 2 sets of small knocks. Only when I push the 2wd button. I hear nothing when I push the 4wd button so I'm assuming that the transfer case may still be in 4wd. The 2wd light still flashes a couple times while the knocks are heard. I also hear relays or something under the dash. Is there somewhere I can find out what happens when one of the buttons are pressed?? (Relays,vacuum solenoids if any.)I use to be an ASE cert. Mechanic many years ago, before I started having problems with my wrists hands and elbows do to many years of air tool use.
  • Under the hood on the drivers side find the vac lines and at the last T you will see 1 line that turns to plastic for the heater and the other runs to the transfer case 3 port switch (line runs up over the PB assembly and down the rear of the engine pass side then crosses over to the transfer) - pull that one off and cap the T off and that will disengage 4wd and allow you to drive till you fix it. Most likely is the 3 port switch for $15 but hard to reach so if bad wrists go to a trans shop and a quick change and you will even stay clean!
  • I removed the line at the actuator. I just wanted to be sure there was nothing in the transfer case that also had to move (electronically}like another gear or fork to disengage anything in the transfer case. I will be replacing that switch today. I'll let you know how it turns out. Still not sure what turns the 4WD lights on and off. Thanks, Rob
  • I installed a new vacuum switch this morning and found it was stuck in the pushed in position. I checked the hole (with mirror and light) that it screws into and found that the lever that pushes the button up was covering the hole. I'm assuming that something has moved in the transfer case to cause this to happen. I checked the transfer case fluid and found it was about an inch or so low. I am hoping that nothing has seized in the transfer case. I have been told that I should have a synthetic fluid in the case but it looks and smells like burned ATF. Anyone know what is really suppose to be in there? obviously NOT burned anything
  • guestguest Posts: 770
    1.1 qts of Mercon-Dexron II red fluid. There is an electric shift motor that moves that lever controlled by the dash switch and the lights are set by the TCCM located behind the middle of your dash (pretty sure on that year - they moved them several times).
  • After hours of changing fluids and checking switches I was finally able to coax it back into 2wd. After talking to a very good GM "Heavy Repair" mechanic I was not able to get in touch with for the past few days, He told me that sometimes the TCCM that is located in the passenger side kick panel sometimes gets moisture in the connector. He said Disconnect it and Spray the connector ONLY with WD40 and blow it dry with compessed air should take care of this problem. He has seen a lot of this happen. He has also replaced quite a few Encoder motors on the transfer case. I sprayed the connector on both the TCCM and encoder motor and blew them dry as instructed. I also gave the encoder motor a couple wacks with a small PLASTIC mallet hoping I would shake something loose or back where it belongs. I pulled the fuse before I started the other things and after I was done,I replaced the fuse and pressed the 2wd button and was very happy to see that something worked. I cant be sure what actually worked I'm just glad its out of 4wd for now. (Till the snow starts flyin'.) I will have to truly diagnose the problem when I have more time. Thanks for the replies it did help narrow the problem.
  • I have been messing with my car to get it into 4wd. it will not engage, in the past the 4hi light has blinked and i would have to unplug the battery for a minute and plug back in and then the 4wd would work for about 3-5 switches from 2hi to 4hi. but lately, the past 24 hours of trying to switch into 4hi and even 4lo the lights will engage but the 4wd itself will not be engaged. i have tried pulling the 4wd fuse out, as well as the trans fuse. neither have done anything. right now i have my battery unplugged and will leave it for a half hour to see if that will help reset the transfer case. when i try to engage 4lo, it will engage but the 4wd wont, i can hear the transfer case parts moving and the transmission goes into the lo range gear but no 4wd just 2wd. from what i have been reading it sounds like the vacuum switch on the transfer case, its 10 degrees here and snowy so i haven't felt like getting under my car to check the lines yet. another possibility i have been reading about is the selnoid on the firewall. anybody have ideas as to what is causing this and a fix for me?
  • You should have a 233 transfer case, 3 button so this is based on that. There is a vac line that runs down to the drivers side of the transfer case to a vac switch that controls the front axle actuator. Its a $15 part that I would say is the most common problem with all 92up 233 cases. Its a 3 port vac switch Motormite/Dorman #49315 and I just did a 98 same thing. Need a 7/8" box end wrench to replace and the formed rubber connector is keyed so pull it off. Hope that does it.
  • It "sounds" like i have an electrical problem!
    The internal warning speaker (not sure of it's technical name somewhere inside the dash) is ALWAYS on as long as the key is in the ignition. It started last week, coming on periodically and now it progressed to non stop being all the time. i checked all fuses and they are ok, and i have no error codes displayed on my dash. Any ideas or advice? Were exactly is the speaker so i can remove it?

  • Do you mean the key in ignition buzzer noise - if so only the 97 (your lucky day) has a known ignition problem with the ignition module (wires and connector in the steering column) but called the ign switch. Change it out and good to go.
  • Greetings,

    I am about to replace all the 4x4 vacuum hoses just to make sure they are right. I have read that they are 5/32" and 7/32". Some questions...

    1. Tis is ID, correct?

    2. Silicone hoses are OK?

    3. Most hoses I find are metric, that is 4mm, 6mm... research is not clear but I believe I can use 4mm and 6mm, correct?

    4. Approximately how much, in feet and meters, of each do I need to replace all the 4x4 related hoses?


  • repairdogrepairdog Posts: 948
    4 and 6 or what my conversion came up with also but never had to find metric ones - alittle slow down here getting on world standards. Never used silicon. If a 3 button with the vac switch on the transfer lots needed but normally only about 2 feet/meters of smaller under hood are bad. Make sure you get the lines right if you run all the ones to the transfer case on 3 button.
  • RD, thanks for your reply.

    Things are getting worse and worse! I have some questions but I’ll fill in details first.

    1998 GMC Jimmy
    3 button 2 Hi - 4 hi - 4 lo
    NP01 Transfer case (233?)

    Original problem, TCCM needs to be rebooted on a regular basis.

    Done so far in this order:
    1.Replaced actuator under battery – no change
    2.Replaced TC sensor – no change
    3.Replaced TCCM with a used one from ebay - no change
    4.Replaced at same time –
    a.TC fluid
    b.Replaced Front Diff oil
    c.Replaced TC encoder motor
    d.Replaced vacuum lines to TC

    Now no 4wd at all, 4 hi and 4 lo flash then back to 2-hi

    I am stationed in the Persian Gulf and am unable to do the work myself, physical injury, but, I have a reasonably knowledgeable local mechanic that does what I tell him, that is I supervise!


    1.Is this indeed a 233 TC?
    2.I have seen the phrase "firewall solenoid" in the newsgroup, is this the actuator under the battery or is it a different part?
    3.When my guy ran the new vacuum lines from the TC sensor, he told me one was a breather. Is this so? If not where do all three 3 hoses go? What did he miss?
    4.I have seen repeated comments regarding the encoder motor, 4 hi and 4 low. Before installing the new motor we connected it to the plug and pushed the 4-hi button and it did move. So last and for all, does the TC encoder motor do anything when shifting from 2wd to 4 hi?

  • More Information,

    New parts installed:
    Vacuum actuator
    TC Vacuum switch
    TC encoder motor
    vacuum hoses from TC vacuum switch
    Front Differential oil
    TC fluid (Red dexron II)

    Checked vacuum at actuator under battery, none!

    Hooked up direct vacuum to actuator and it does pull the cable!

    So this means no vacuum from 3 port switch on TC to actuator! Further research on vacuum:

    I have identified the hoses on the switch on the TC:

    1 constant vacuum from intake set up
    2 to vacuum actuator under battery
    3 is release vent for sifting back to 2wd.

    To me, this means that when the ball is pushed or released by something in the TC to either release or send vacuum to the actuator to engage/disengage the front axle. What pushes the Ball? When? I presume something in the TC moves, probably moved by the encoder motor? I hear the TC encoder motor running when pushing the 4-Hi switch.

    Is it possible that the hoses were connected incorrectly to the TC vacuum switch? I know it is keyed, so this shouldn't be possible but hose 1 (constant vacuum) and 2 (vent) are the same size and could theoretically be switched and would explain why there is no vacuum to the actuator under the battery. For the sake of argument, which hose should be where, clockwise from the "key" on the switch?

    Also, is it possible to install the TC encoder motor in the wrong position, i.e. the motor thinks it's in 4wd but the truck is in 2wd? I hear it run when I push the 4-hi and 4-lo switches, so it's doing something!

    Last but not least, I am out here on the fringe of the world so getting parts is a battle! The last one I can think of is the switch on the front axle/diff; anyone have the part number?

    Again HELP!!!!!!!!!!!!!!!
  • repairdogrepairdog Posts: 948
    You got it. The 3 port vac switch on the TC is the most likely item to fail but its new. When in 2wd the ball on the switch bottom is down and the vac shut off while the vent line (also connected to the TC vent) is connected to the front actuator so its uncoupled the front axle. When 4Hi selected the encoder motor steps to move a fork in the TC that engages the front driveshaft and pushes the ball up on the 3 port vac switch that opens the port so the engine vac now is to the front actuator (and shuts off the vent port to actuator). How about the dash switch - I just posted 2 answers on how that works with 8 volts and TCCM watches return from switch each mode with a diff resistance value (Dorman about $35). That front coupler sensor is gm#15598481. The firewall solenoid is for autotrac 4 button cases only so you don't have. You can find most parts online at the dorman, smpcorp, or rockauto sites all with pics and many cross ref #s.
  • I have a 2002 Blazer 4x4 with 2wd 4hi 4lo and auto 4wd buttons the transfer case is always engaged (front drive shaft always turns) is this supposed to be that way? is there reason for concern? and if its not supposed to do that is there a common problem I should look for?
    thank you
  • repairdogrepairdog Posts: 948
    The 4 button Autotrac transfer cases NP236 always have the driveshaft connected by desigh so no worries. This is always spinning to accomodate the Auto4wd selection that senses wheel spin and engages power to the front differential when needed. (The 3 button cases NP233 does not and it engages when 4wd is selected)
  • I have the same Auto 4wd system on my 2001 Jimmy 4x4 (Diamond Edition). My 4wd will work for a while, then the Service 4wd light will come on. Then a week later they 4wd works fine again, then a week later... well, you get the point.
    Any clue to what might me causing this? I would like to get a direction before I start tearing into this thing. And with 6-10 inches of snow in the forecast for tomorrow, I really should go to town on it tonight! Also, is there an easy way to change the transfer case fluid? Thanks!
  • As for fixing look back up at all the answers I posted - covers most. As for fluid there are 2 plugs - bottom to drain and top to fill and use the blue syn fluid GMonly 2 qts (not red Dexron!). Cold can cause some binding and sensor sticking. If its the computer pull the AWD and 4wd fuses for 30 min or the battery cable and see if the reboot fixes.
  • big ice strom in the notrh east left me trying to engage my 4wd on my 98 jimmy 3 button panal on dash the 4wd low and 4wd hi just star blinking and the go back to 2wd after I shut the car down I here a major clicking sound behind the glove box
    I took the glove box out and the sound seems to be coming out from behind the back of the wheel well I dont want to disconect any electronics untill I have a better idea what im looking for any idea what im hering ?
  • TCCM or 4wd computer is in the pass side kick panel and that maybe it or the front axle has the axle coupler on the pass side that must engage - push switch, vac form engine down to transfer case 3 port switch activated by the encoder motor on the case. Check vac lines, switches and sensors - many posts on all that.
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