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Encoder Motor Problem for 97 GMC Jimmy

Everytime I drive and get on the throttle until I get up to around 40 or 50 miles per hour and then back off the throttle, I begin to hear a whiring noise from the front end, sounds kinda like a drill scoring metal. Anyway, I've changed the control switch on the dash, and had the encoder motor replaced and it still does this when I get on the throttle.

Does anyone have any ideas what exactly the problem might be? My mechanic says that it sounds like the 4-wheel drive is trying to engage at a high speed, however, I'm not sure anymore since I had those things repaired.


  • Encoder motor is on the transfer case and puts the low gear in for 4Lo only. You have a coupler on the pass side of the front diff that locks when you select 4Hi and 4Lo so the axles both spin then. There is a vac line that runs down to an actuator under the battery tray that may have a vac leak in the line or the diaphram leak on the actuator so the coupler is only partially engaging. Your system also has vac line connector on top of the transfer case (not on 98up) and a sensor on that axle and transfer case to tell the 4wd computer the state of the coupler and transfer case. The front diff and transfer case fluids could also be low and causing a whine.
    A final problem/fix these have is the computer sometimes hangs up so disconncet the battery for 30 minutes and then reconnect and see it the condition clears - it sometimes does.
  • My next question is, how costly is this to repair if it's indeed the vac line leaking or the diaphram leaking?

    I mean, I've put in a lot of money to fix this truck. First it was the fuel pump dying on me, and now this. I'm hoping this is the last repair I have to do to this truck.
  • Vac lines are simple and 5/32 or 7/32" line you can do. Actuator part isn't much its the labor. As for repairs assume this one has over 100k miles so there are some things that could go wrong still, common/normal for these are: AC, water pump and flush system, alternator, hubs, ball joints, ignition switch module (97 only), and then on all vehicles brakes. Have to do maintenence on all cases with new fluids and no one does so they fail. Electric pumps are all in the tank now and if drivers run low often they heat up and fail sooner. Decision on a vehicle is always a personal one.
  • Like the title says. Anyway, I'm going to just get it fixed, whatever it costs is whatever it costs. I just hate the labor aspect.
  • I do most of my own - have 4 of them so I know - seen many of the common problems and I'm in FL so sweat alot in the summer fixing them! Relax and enjoy life!
  • I had all the things you said checked, and nobody can find anything wrong anywhere. Fluid levels were good, vac hoses were good. So I don't know what else there might be. Any ideas?

    Also, I'm going to take my truck off of its battery for the half hour you suggested when I return from an errand, but otherwise everything else checked out.
  • 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
  • Read posts above and try 4Hi first - no encoder motor for that only the front axle coupler. You also have a solenoid on the firewall so see if that works - follow the vac lines.
  • I'm having the same type problem with my 97 Blazer but I can't get into 4 wd high, I can get into low 4wd, transmission garage looked at and they figured computer(control module) in dash($390 gm part without labor)- I then went to the local dealership and they diagnosised as the transfer case encoder motor (the gm motor was $580 without labor) I found the motors new at local parts distributor for 1/3 that, but with what I have read here it has cast doubts on the encoder the cost of these parts I'm trying to eliminate any question...
  • You lost me on that - encoder motor is only for 4Lo so if you got it into 4Lo then the motor worked and the front axle coupler worked so 4Hi should work - remove the battery cable for 30 minutes and see if that reboots the TCCM or control module and try again. Someone is wrong. Also there are sensors on both the front axle and transfer to send status to the TCCM (the ones that look like a round sender unit with an electrical connector on them. Check all vac lines before you go on!
  • Yes, I had it at the dealership 2 weeks ago and they diagnosised as the encoder motor, I had read your postings at this site and argued that it should not be the encoder , I had 4 wd low but not high, they stood by there diag...I was reluctant and refused the work, I posted my problem and your suggestion worked, I disconnected the battery and it did reboot, thanks for your help and sharing your knowledge
  • Hey repairdog I have a 2002 Jimmy with the 4 HI not working. Took it to the dealer and they said it was the Encoder motor, and that it would cost me $1500 for a new one with the install. Tried removing the battery cable for 30 minutes like you posted and it worked. Thank you very much for your knowledge.

    PS: You should be called repairgod.
  • Glad to help. I have 4 Blazers so have had alot of the problems and with three 4wd ones - got 2 with AutoTrac or 236 cases and a 98 with InstaTrac 233 case so keep learning new things all the time. Just as a note you can get many parts online at gmpartsdirect and rockauto and that encoder is about $200 and even the TCCM or 4wd computer is under $300.
  • repairdog, That sounded like a good call on that encoder motor. I have the same problem with my 2002 Blazer and can't wait to try disconnecting the battery to see if it reset itself. On the chance that does not work, how difficult is it to actually change the encoder motor? I have research some Motors Manuals and they don't actually give the details on R+R the encoder motor.
  • Depends on if 3 or 4 button case, 3 button (NP233)has a 3 port vac switch on the transfer that is most commonly bad and is $15 Motormite/Dorman #49315. If 4 button (NP236)I would look at that underhood firewall mounted vac solenoid as suspect also. GM#15165891 for $45 cause if they leak front axle coupler ops is slow and if they fail and pass vac no axle coupler - so bypass that with a simple inline vac connector and see if 4Hi works. Encoder motor bolts on and about $200 if you shop around but I would do that last. The TCCM is usually locked up and in some climates I've read that condensation gets in the connector and they blow it off and all works. There also is a "4WD" fuse in the dash panel by door and another called "ATC" in the underhood panel that should be checked and can be pulled instead of the battery.
  • Kudos to you, and many thanks Repairdog.
    Disconnecting the battery and reconnecting cleared whatever error condition existed. The 4WD works now, thanks for the advice.
  • well we got out 2001 LT blazer a little over a year ago. I love the truck its awesome, but recently we have had some problems. we were driving up to rockford hours from home, and our transmission made some nasty clunking sounds, and then wouldn't shift into reverse. so we took it to a local dealership, who were very nice and the warranty covered basically rebuilding the transmission,we didn't have the truck for a week. well just the other day before a big, ice/sleet/snow storm for the 1st time since we got it back I hit the auto 4wd button and nothing happened, I also got stuck in the snow. it won't shift into any of the 4wd gears at all, the panel just blinks and it goes back to 2hi. yesterday I took it back to the dealership I bought it from , and an hour later they told me the decoder motor went bad. they would have to order a new one. well we bought an extended warranty and that doesn't even cover it. they are saying $830 to fix it. I won't have it fixed, I can't afford that. I tried the disconnect the battery trick, and it just does the same thing. I still think them doing something with the transmission and this are connected because it worked when I gave it to them. any one have any solutions here ?
  • 99 Blazer...stuck in four wheel high. Will not drop out of four wheel until I pull the ATC fuse under the hood. Replaced the ATC control module..cleaned the contacts...still no go.
    Replaced the four button dash switch, no change. Still blinks and will not drop into 2 wheel, (nor four low) the auto four does stay steady lit. Vacuum switch on the firewall is good. You can here the encoder motor working when I push the buttons, can't say it's engaging...the puzzler is why does it drop out of four high when I pull the fuse?
  • Points to vac applied to the front actuator so with the vehicle running pull that vac hose off the solenoid on the firewall and see if you hear the actuator release - if so put a golf T in the source side and test it out - can leave till buy a new one but no 4wd. Those solenoids leak - internally sometimes applying vac or externally for slow engagement.
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