1998 Blazer 4WD

General Description: After putting truck in 4WD HI while in park, and shifting in into gear, truck won't move under its own power, it just revs as if its in neutral. In fact, I can push the truck forward and backward, as if its in neutral. When putting it back into Park, there is a slight clicking noise of gears not quite meshing.

Specifics: When I have the truck in 2WD HI, the front driveshaft spins freely,as it should, and the rear driveshaft is fully engaged, truck behaves fine. After pressing 4WD HI, the front driveshaft engages and can no longer be spun by hand, same with the rear driveshaft. However, the truck will not move under its own power. In fact, it can be moved by pushing it, as if its in neutral. As my knowledge of the 4WD system is far from comprehensive, I'm not sure what would cause this.

I have gotten to go into 4HI a couple times, and when I come to a stop, something disengages and the truck behaves as if in neutral again. The only way to get it back into 2HI at that point is to put it in 4LO and then back to 2HI. I have read alot of the forums about 4HI not engaging because of the vacuum actuator not operating properly, etc, but since its also affecting the ability for the car to be "in gear" at all, seems more like a transfer case issue.

I have had some issues with the vacuum actuator under the battery getting "stuck" in the past, but by manually working that cable a bit, it seemed to free up fine.

I can put the truck in 4 LO, and can feel the front wheels engage. When I start moving in reverse in 4LO, I can tell the front wheels are fully engaged. When I begin driving in forward, I can tell all 4 wheels are engaged, the truck "jumps" as it ought to when in 4wd. If I go over about 3 miles per hour though, or give it any real torque, the rpms shoot over 2500 and I can't go over 3-4 mph. I know 4LO isn't designed for high speed, but something just isn't right.

Any help is greatly appreciated. Any pointers on what to check, etc, I sure would appreciate!!! I will post whatever results I come up with.

 

It sounds like your encoder motor is not reading the position of the transfer case shift sector shaft properly and when it is reporting 4HI, the transfer case is actually being shifted into neutral. Neutral is between 4HI and 4LO and will result in both driveshafts being tied together.

If you wind it out in 4LO, it should eventually shift into 2nd gear. The PCM should get a 4LO signal and alter the shift timing to occur at lower speeds.

 

Well, you are absolutely right about winding it out in 4LO and it eventually shifting into 2nd.

I had read somewhere when switching from 2HI to 4HI, nothing in the transfer case should be doing anything at all? Why do you think its moving from 2Hi to Neutral in the first place?

I should also point out I had no/low fluid in the transfer case recently, so I topped it off with fluid.

Thanks again, for your help!

 

I'm not sure where you read that nothing should occur in the transfer case... Since you have the NP233 transfer case, when you shift from 2HI to 4HI, the encoder motor rotates the shift sector shaft in the transfer case 20 degrees. For 4HI to 4LO, the sector shaft rotates 40 degrees (past neutral). Meanwhile, when shifted into 4HI or 4LO, a cam inside the transfer case activates the vacuum switch which sends vacuum up front to engage the front axle.

A low fluid condition could cause the shift sector shaft to some what lock up. You could check this by removing the encoder motor and trying to manually shift the transfer case.

 

This is where I read the bit nothing happening in the transfer case. http://www.automotivehelper.com/topic572914.htm

I was wondering, since when I put the truck into 4Lo, and it engages the front wheels, would that indicate I have sufficient vacuum, so I can rule out vacuum actuator issues?

Chris

 

I don't think your problem is with the front axle at all. That will not cause the entire vehicle to not move as the rear output from the transfer case is directly connected to the transmission in every mode except neutral (in the transfer case). Your transfer case is somehow shifting into neutral. This points to an encoder issue.

 

Out of curiosity , when I shift into 4hi by pressing the 3-button switch, are both 2hi and 4hi supposed to be lit? When I am in 4hi, both 2hi and 4hi are lit. When I am in 4 lo, both 4hi and 4lo are lit. Is this normal? I have read in another post where someone had a bad switch in the dash. I have a spare TCCM Im going to hook up too (Monday, dont have it handy). Just wondering if something before the encoder motor could cause it.

Chris

 

No, that is not normal. When you are in 2HI, only the 2HI light should be lit. Same for 4HI and 4LO.

 

when I am in 2hi, its the only one lit. When attempting to go into either other gear, they are both lit simultaneously. (2hi and 4hi when in 4 hi/ 4hi and 4lo when in 4 lo). I read somewhere that if you press both buttons simultaneously it will put the transfer case in neutral. Can you confirm that behavior? If so, do both lights stay lit in that scenario? maybe its the TCCM indicating its in neutral?

Ill try disassembling the switch and cleaning per the post below and see what happens.

https://blazerforum.com/forum/2nd-generation-s-series-1995-2005-tech-41/4hi-wont-work-4lo-will-50701/

thanks again.....

 

Yes, I do believe that you press both 4HI & 4LO buttons to get it into neutral. I do not know if it lights both though, but if both are lit when you press 4HI and only when you press 4HI, it would stand to reason. This would also mean that your switch is contacting both the 4HI and 4LO wires when you press the 4HI button.