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GMC Jimmy 4WD Problems



  • The encoder motor is ONLY for 4LO and there is a sensor to tell the TCCM that its engaged or not and both 4LO and 4HI need the front axle coupler engaged and the sensor on the axle to read it is or light will flash. Work on 4HI first and get that to work then worry about 4LO and encoder motor.
  • Where is the sensor located at on the front axle? Or is it the encoder motor?
    Because if I get under the jimmy I can put it in 4 wheel drive by hand, it's just when you try to put in 4 wheel drive with the push button it will not go.
  • To do searches on past posts enter the query in the "search" field when you are at the main page so it looks in all Blazer/Jimmy sections.
    Front sensor is on the pass side so look under the vehicle and see the electrical connector plugged into the soild outboard part of the front diff. Encoder motor is in the transfer case for 4Lo only and a sensor on top of the transfer for that too (hard to see). The TCCM or 4wd computer in the pass side compartment kick panel controls all operations. I must assume you are only engaging the encoder motor for 4LO by hand and if the front axle is not engaged and sensor tells TCCM then the light blinks and no go. #1 check all vac lines from the engine down to actuator first.
  • My 2000 Jimmy wouldn't shift out of 2x... Tracked the vacuum line from tranny to somewhere under battery. It came out through plastic harness to the firewall. About dead center just below the hood gasket theres a black vacuum actuated switch. It is either the switch (i doubt it) or the vacuum line is dry and crcked. Look right where the line slips on the nipple. It gets VERY hot in the engine compartment and alot of problems are from cracking vacuum lines...
  • guestguest Posts: 774
    Vac line runs from the intake by the PCV hose, over to the drivers side fender/firewall where it Ts to the vac reservior can in the fender and to the 4wd and heater(hard plastic one). Then to that switch on the firewall so check all lines and replace 5/32 (and 7/32 for larger line) vac line. See if when in 2wd there is vac to the inlet side and none out the oulet. Then select 4Hi and see if vac on outlet - if not its probably the switch. The vac then goes down to the actuator under the battery tray that has a diaphram and it pulls a cable to the axle coupler. If that doesn't engage no 4wd. A sensor on the axle to tell the TCCM or computer that its enagaed or not. $lo needs that first also then the encoder motor puts in or engages the Lo gear in the transfer case. Once all is fixed and it still won't work pull teh 4wd fuse or battery cable for 30 min to do a TCCM reset (sometimes the logic gets confused).
  • lanae2lanae2 Posts: 1
    I have a 1995 gmc jimmy 4wd ,,,Does any-one know how to R&R my front passanger hub..... :cry: :cry:
  • repairdogrepairdog Posts: 948
    Buy the bearings and seals and a 36mm hub socket and go for it.
  • I have a 99' Chev Blazer and went to use the 4x4, but to no avail. I can press the button on the fly and in park and in neutral and nothing. All it does is flash the 4hi button and goes back to 2hi. I can hear it trying to engage at the transfer case but it sound like the encoder motor is seized or dying. It does this as well when I try 4lo. I'm at the end of my rope this ride as it only has 120,000 km. I've replaced so much it's rediculous. I need to fix to sell....Thanks Jay
  • I bought a 1996 Jimmy knowing it was a two wheel drive truck just because I was desparate for a car. But now I was gonna start working on it and fixing it up. I would love for it to be a 4x4 but I have no clue as to how hard this would be or what I would have to do or what I would need to do this. If anyone has ANY help it would be greatly appreciated. Thanks! :confuse:
  • Push the 4Hi button and see/listen if the front axle coupler engages - thats all that should happen. If not follow the vac lines under hood and to the firewall center where a solenoid is mounted - is vac to it and when you push the button does it open and allow vac thru?
    Transfer encoder motor is only for 4Lo.
  • 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.
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