99 Tahoe front drive shaft working in 2HI

[chris peter]

My Tahoe seems to have the transfer case ingaged when the push button is in 2HI. I have the auto 4wd system the switch indicator does not stay lit all the time and when in 4HI the front differential does not lock in. However the front drive shaft spins all the time it does not matter which buttons is pushed. Any ideas as to whats wrong I did change the transfer case fluid and it was blue and normal anf the front differential fluid was good when i changed that also.

[j cAT]

Originally Posted by chris peter

My Tahoe seems to have the transfer case ingaged when the push button is in 2HI. I have the auto 4wd system the switch indicator does not stay lit all the time and when in 4HI the front differential does not lock in. However the front drive shaft spins all the time it does not matter which buttons is pushed. Any ideas as to whats wrong I did change the transfer case fluid and it was blue and normal anf the front differential fluid was good when i changed that also.

with the 4wd tahoe NOT AWD , the front diff actuator locks the front axles together in any 4wd position ...this is all it does....the transfercase encoder motor places the vehicle into 4wd when 4wd lo/hi is selected or when in auto 4wd the pcm/computer senses a slip....

the vehicle is not to be operated in any 4wd position on solid dry pavement ,,,,this is only part time 4wd....

the newer vehicles do have full time 4wd but I an guessing this is not a vehicle with this type of drive train ...


place the vehicle in 4wd and lift front end one wheel , and see if it is locked ...then restart engine and when in 2wd hi see if wheel turns if so its good....

if the front axle does not lock it is the front diff locking actuator ..////or the circuit ///wiring ///defective switch ,and or soldering joints on switch ...

you should have a service 4wd light come on if there is a defect...if so get the code pulled ..

[chris peter]

Thanks for the reply j cat, My tahoe has the NP8 system I just checked all the 4wd system fuses and found them all good. I did notice when I pulled the #2 fuse on the side of the dash waited 5 min then pluged it back in I heard a click under the dash so that must be the 4wd module I'm not sure if that is good or not. The first owner of the truck for the first 10 years only used the 4wd a total of 4 times. He did get a the fluids changed as required. I'm going to change the switch first since that is the cheapest part to start and the lights on the switch go on and off like it has a bad connection. I would think if the encoder motor was bad it would blow fuses in the system. I would like to know if the front drive shaft should be turning all the time or not. On my suburban when its in 2wd it does not turn but that does not have the auto 4wd feature it just has the 2wd 4HI 4Lo push button.

[j cAT]

Originally Posted by chris peter

Thanks for the reply j cat, My tahoe has the NP8 system I just checked all the 4wd system fuses and found them all good. I did notice when I pulled the #2 fuse on the side of the dash waited 5 min then pluged it back in I heard a click under the dash so that must be the 4wd module I'm not sure if that is good or not. The first owner of the truck for the first 10 years only used the 4wd a total of 4 times. He did get a the fluids changed as required. I'm going to change the switch first since that is the cheapest part to start and the lights on the switch go on and off like it has a bad connection. I would think if the encoder motor was bad it would blow fuses in the system. I would like to know if the front drive shaft should be turning all the time or not. On my suburban when its in 2wd it does not turn but that does not have the auto 4wd feature it just has the 2wd 4HI 4Lo push button.

these motors do open and the wiring has had issues with some , then the rodents chewing on the wires ..not all failures wil blow the fuse ..

the switch is a low cost part...normally you will get the service 4wd LT if you have any 4WD defects...behind the switch is the 4wd contoller, this is where the 8volts dc is generated to operate the system and those dash switch lights...it is possible that this is defective but you would be the first,, I have never seen this go bad ...


by following the previous posting instructions you will find out if the 4wd / 2wd is working correctly..

[chris peter]

j cat your test was correct thanks for your help. The 4wd was not working. I then install a new switch and when I started the truck all the indicators lit up on the switch then I pushed the 4wd and tested again and it works. The switch was bad I got lucky $65 part.

[cah3311]

OK - just to be clear. On Auto-Trac 4wd's, the front drive shaft turns ALL the time. Thats why lift kit manufactuers require the use of a dual cardan driveshaft. the factory drive shaft creates high vibes at high speeds when lifted.