*4lo but no 4hi*

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rob_z71

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so guys ive been chasing this problem for a while now. truck is a 1998 gmc k1500 with the np1 transfer case. the truck goes into 4lo but will not go into 4hi. when i first bought the truck a few years back it would go into 4hi no problem. as time went on it would only sometimes go into 4hi and now it wont go in at all. it only goes from 2 to 4lo

things ive replaced (dont worry i returned everything because it didnt fix the problem):
front diff actuator, transfer case encoder motor, push button on dash, and transfer case control module.

im at my wits end here and NEED 4hi for the winter. the only things i think are left are the transfer case itself or some sort of wiring problem somehow. i would just swap to a manual floor shift but the money isnt there right now and its much more work. any input would be greatly appreciated, thanks guys
 

rob_z71

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Well, it seems like you've narrowed it down to a wiring issue or a transfer case problem.
any ideas within those sub categories??? what in the transfer case controls 4hi that doesnt affect 4lo, where do the wires differentiate between 4lo and 4hi, etc. etc. ?????
 

tinfoil_hat

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This might be a stupid question but when it goes into 4-lo are you 100% sure you actually have power to the front wheels? The t-case will still shift down to low even if the actuator never engaged. This might make you think that 4-lo works and 4-hi doesn't when in reality neither is working. You can test the actuator by pulling it out of the diff but leaving it plugged in, then hit the button and see if it extends. If not, you narrowed your wiring problem down considerably.
 

rob_z71

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I know it goes into 4lo because I've used it in the snow. Recently I've also changed the transfer case switch on top of the transfer case and I still only have 4lo. Another point to add is if I pull the transfer case fuse and/or the 4wd fuse and then put it back in. The truck will go in and out of 4hi for a few minutes until the light for 4hi doesn't even illuminate when pressed just like before but I'll still be able to go from 2 to 4lo
 

Btp55

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Any way to fire this thread back up!! I have the EXACT same problem as Rob Z71... I can't figure it out
 

Schurkey

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Both you and rob_z71 need to do a better job of explaining what's going on, what does and what does not work.

Best I can figure out:
You have 2wd high-range. If the transfer case can switch between 2hi and 4low, the high-range/low-range system seems to be OK. Therefore, high-range works in the transfer case. If there's no 4wd Hi, I'm assuming that it's the 4wd that doesn't work, not the high range failing to work. In other words, you can drive around in 4wd High, but the front wheels aren't powered. The rear wheels are.

First Guess: Problems with the transfer case switch or perhaps the dash switch, that causes the front axle actuator to not work properly in 4hi.
 

Btp55

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4 lo works every time without fail... I use the truck to plow snow I know for certain that the front end is locked... If I unhook battery for 15 minutes or so 4hi will work for a short time then it switches over to 2wd on its own then I put it in 4lo and like I said it locks in every time without fail... I've replaced actuator, tccm, encoder motor and also the selector switch in the dash...
 
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