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GMC Jimmy 4WD Problems

rcfrcf Posts: 1
I have a 2000 GMC Jimmy SLT that I purchased knowing the 4WD didn't work. Afetr the sale I put it in a small,private shop and they said (if I understood correctly) that an ABS sensor was out of alignment and something about a bearing was not allowing it to go into 4WD. They fixed it and after I left I tried it out. It now goes into four wheel drive but the following occurs.
1.If you press 4 Hi from 2 Hi.....the 2 Hi light goes out and 4 Hi light blinks but does not stop blinking although 4WD is engaged(I can feel it while turning).
2.When selecting the Auto 4WD button the Auto light starts blinking and instead of being in the 4WD "on demand" mode it goes directly into 4WD.
3.No code or check 4WD lights appear on the dash.
The repair facility had closed for the day when I discovered all of this and I plan on going back but I just wanted some ideas as to the problem before I do. Thanks for any help.


  • repairdogrepairdog Posts: 948
    Heres the story:
    The front hubs each have an ABS sensor in the 1 piece hub for the ABS and Autotrac (only 4wd mode effected) to work. The rear sensor is in the transfer case.
    The passenger side front axle has a coupler that engages the axle to the front differential when any 4wd mode is activated and a sensor tells the TCCM computer in the cab located in the pass side kick panel its engaged. That axle has a vac actuator under the battery that is vac activated and it pulls a cable from it to the axle. 99up have an additional electric switch for the vac mounted on the firewall. CHECK all vac hoses from the intake to reservior (in drivers front fender) to the firewall switch, to the actuator under the battery (5/32 small and 7/32 larger). Note that both front axles CVs always turn as well as front drive shaft. Thats covers Autotrac and 4Hi. For 4Lo the transfer case has an encoder motor that is on the transfer case that engages a lower gear when you stop, put trans in neutral and push 4 lo and a clunk is heard. Again a sensor on the transfer case to tell TCCM it is engaged. Hope that wasn't too confusing. Blinking dash light means the sensor did not work/actuator did not work/ or the TCCM computer is confused - yes it can happen. After you check all operations try and disconnect the battery for 30 min and then retry and that should have cleared any error. Finally, check the front diff fluid and make sure its full and the transfer case requires blue autotrak II GM fluid - this has a GM bulletin on it so do not use Dexron red in it due to the clutch packs in yours.
  • my 98 jimmys lights flash when you try to put it in 4wd but doesent engage. the only way I can get it to engage is to put it in neutral hit the low lock button and while its flashing put it back in gear. but it will only go into low any suggestions?
  • repairdogrepairdog Posts: 948
    Many posts on this if you search the main area.
    Assume 3 button 4wd (no Autotrac):
    1. Check the vac hoses from the untake over to the reservior and T off to the actuator under the battery tray.
    2. Chack the actuator (crawl under the pass side front) by looking /listening to see it the cable pulls the axle coupler.
    3. Check the front axle coupler sensor(wires to it) by unscrewing and pushing the ball in) which tells the TCCM or 4wd computer that the axle is coupled for all 4wd modes.
    4. Check the similar sensor on the transfer case for the encoder motor that engages the low gear for 4LO.
    5. Check all connectors are tight and wires not cut all the way to the transfer case.
    6. Disconnect the battery for 30 min and see if the TCCM reboots (must be a windows system - thats a joke) cause often this does fix it as it hangs up.
  • I bought a 1986 blazer and having vibrations and leaks fluid.I changed transmission fluid I am thinking it may be transfer case. I have enabled 4xhigh and have been driving around for a short period time. Is this a good idea to loosen up gears,etc.? This is my first 4x vehicle.
  • repairdogrepairdog Posts: 948
    No, 4hi is only to be used off road where the wheels can slip and not bind up the drive train. Snow is OK too. Its 20 years old so will leak at the seals. Change the transfer case fluid too. The drive shafts exit from the transfer case and there are seals there that can be replaced easily. Check/change the front and rear differential gear oil and the pinion seals can leak there too. Vibration must be traced to type - tire or drive train. If the U-joints are original and have no zerks on them they may be loose. Check the front wheel bearing and tire balance.
  • I can't get my Jimmy to go into 4X4, did anyone else have any problems with this? Please help me out.
  • repairdogrepairdog Posts: 948
    Do a search in the forum on this and many answers already posted on the components to check. I'll give you more after you do that.
  • jenfen2jenfen2 Posts: 3
    I have tried a new motor , light just flashes and won't go into 4 high or low. I also had a brain box from a junk yard and 2 wheel drive just flashes and won't go into 4 high or low. I can take motor off and put in 4 wheel drive by hand, so I know the acuator works.
  • Thanks, I will do a search now. I am new to this so I really don't know where to go for my answers.
  • I have a 2000 Jimmy SLT and 91,000 miles when I started hearing a thumping noise underneath when I would start to drive or slow down and stop. My mechanic says it is the transmission, that a gear is getting worn down and will eventually break and I will lose Reverse, 2nd and 4th gear. Is there any truth to this and is there a quick easy fix? I thought maybe it was just the U joints, but he says no that I will need a transmission overhaul and that is a common occurance with certain transmissions.
  • repairdogrepairdog Posts: 948
    Sorry I don't always see the Jimmy stuff.
    That rev/2/4 thing is caused by an internal broken Sunshell and the neck breaks off all at once usually from drivetrain shock as hard 4wd or towing. Noise could be many things as u-joints, hubs, brakes, tires etc so need to look under it and start checking. Also look at the transfer case fluid and change every 50k as well as trans fluid/filter.
  • On my 2000 Jimmy, when I press the 4WD button, I hear the clicking and I feel it shift. REALLY feel it shift! Feels like I've got the brakes on. Vehicle starts to slow down slightly and there is a low whine. When I press the 2WD button. There is a loud Bang :surprise: or thump and it goes back to 2WD. My daughter recently had a ball joint replaced by a shade tree mechanic. I switched vehicles with my daughter so not sure how it has been cared for. She did take it to the dealer last year due to a loud...REALLY loud siren like whine from the 4WD. That cost $800+. Winter is coming so would like to get the 4WD working correctly. Thanks all for your help.
  • Assume you are pressing 4Hi and you get the bang.
    Front drive shaft always turns and keeps both diffs at same speed, so when you depress the 4Hi vac makes the actuator under the battery tray pull a cable that couples the pass side CV axle to the diff (drives side always connected). If thats the bang then see if the actuator vac line is good and the actuator pulling the cable. The 4Lo will only work at a stop in neutral or a 3mph or less roll in neutral, then the lower gear in the transfer case engages. That really loud noise FIX needs to be looked at to see what they really did - if the 4Lo gear is being engaged when 4Hi is pressed you have a sensor problem - probably best to go back to the last work shop and ask or pay the dealer to determine - this gets very expense very fast. If you have 4 button Autotrac the transfer cases sometimes have clutch problems and use special blue syn fluid to smooth out.
  • The Bang comes going back to 2WD. When going into 4WD, you hear the clicking of the relays and feel it shift and hear a low whine/rumble. 4LO seems to be working as advertised. I brought the car to a stop and engaged 4LO and then 4Hi and there is an obvious difference so it is going into 4WD.
  • I am somewhat confused - so you have regular 3 button 4wd and are engaging it on the hard street and driving it, then say it bangs going out of 4wd - it should cause you are probably binding up the drivetrain so do it on a soft surface like dirt and report back. These 3 button systems are NOT designed for street use at all - snowing or dirt allows slippage between front and rear diffs. 4Lo engages the transfer case low gear and you hear a clunk as it engages and disengages.
  • Yes, it is a 3 button 4WD. Today I took it out in a dirt parking lot and along a dirt road and it still does it. Goes into 4wd with a low whine and goes out with a bang. On the hard surface, was at low speed and going straight.
  • If you mean 4Lo then the encoder or shift motor on the transfer case is binding up when its disengaging the low gear so first see if any fluid in the transfer case - make sure its full and I'd change it.
    If its 4Hi then have to look at the front actuator under battrey going to the pass side axle.
  • What should I be looking for? This is my first 4WD vehicle.
  • They all clunk loudly when the 4Lo is disengaged and engaged so maybe try doing it at a 2-3 mph very slow roll - thats the best way so the gears can unmesh easier - I do that and it aviods much of the noise. This is for 4LO only of course as the 4Hi can be done up to like 55 mph (stated in your owners manual). Maybe that will help or when you see another Blazer ask them to engage and disengage to compare - many will be glad to help you as they have had similar things hard to explian without comparing. I have 3 4wds so often see if the others do the same thing - ops, guess thats really not a problem but a Blazer design quirk.
  • I have tried a new motor , light just flashes and won't go into 4 high or low. I also had a brain box from a junk yard and 2 wheel drive just flashes and won't go into 4 high or low. I can take motor off and put in 4 wheel drive by hand, so I know the acuator works.
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