Another question about my '98 Jimmy: along with the HVAC issues I've recently solved I have been working on a clunking in the front end. Part of the diagnostic process was determining whether the front axle was erroneously engaged during the turns when the clunking was happening. Also, as part of the HVAC problems I solved today, I found the 4WD actuator under the battery was full of ATF. I replaced it with a new one.

To verify that everything was working after replacing everything I started up the truck and put it in 4HI. With both front tires on the ground I got under the truck and tried to turn the front drive shaft. It didn't turn, which would indicated that the front axle is locked. With it still in 4HI I jacked up the right front tire and tried to turn it. The tire turned. If what I know about the front axle is correct, when it's in 4HI the right side of the axle should be locked to the differential and the right front tire shouldn't turn if it's off the ground. Is that correct?

If that is correct then why would the drive shaft not turn when it's on the ground but the right front tire turn when it's off the ground? That doesn't make sense to me. I'd really like to get this issue settled so I can move on to fixing some other problems on this truck.

 

probably cause the driveshaft is locked in 4wd at the transfercase not at the axle. there is a vacuum actuator that engages the front axle, make sure your truck is running when you check the front end too. that actuator under the battery tray is what locks the front end in

 

It was definitely running when I checked it.

I thought that the NV233 transfer case was always in 4WD and the axle is what locked the wheel to the differential. Is that incorrect? I checked the front drive shaft both in and out of 4HI with the engine running and both wheels on the ground. In 2HI it turned freely. In 4HI it did not.

 

Engine at idle, transmission in park or neutral, 4WD selected, only the right front wheel off the ground: Right front wheel should not rotate.

Edit: The transfer case output shaft & gears are engaged, but are not powered in 2WD, thats why the shaft rotates by hand. In 2WD the front differential "powers" the drive shaft when the vehicle is in motion, this is what keeps everything rotating the same speed enabling "shift on the fly". When you select 4WD, the transfer case output shaft and gears become powered, the right front axle locks in, and the front differential becomes "live".

 

Thanks for the information; that makes much more sense.

So what does it mean that the drive shaft doesn't turn when the tire is on the ground but the wheel turns when it's up in the air? Is the front axle not engaged?

 

If the transfer case is in 4wd, the front drive shaft should not be able to be turned by hand unless you had the transmission in neutral and one wheel from both the front & rear axle off the ground (both rear wheels if g80 differential). If you can still spin one of the front wheels with just that wheel off the ground and the transfer case in 4wd, then the front axle is not locking in.

 

Thanks for the reply. Your info explains the situation that I found. Next I'll check the actuator and make sure it's actually locking the front axle as it should be.

Prior to the beginning of December the 4WD worked perfectly even though the HVAC controls did not. Now it appears that the Jimmy isn't really in 4HI even when the switch says it is. It also won't go into 4LO.

Perhaps this truck wasn't meant to have functional four-wheel drive and heater controls at the same time.

 

did you blow out your lines or replace them? if not you may have ATF in your new actuator. did you replace the faulty switch at the transfer case and check transfer case fluid level?

 

I blew out every single line, even the vent line from the transfer case. I replaced the vacuum switch on the transfer case. I refilled the transfer case to the bottom of the fill plug. I would be very surprised if there were any ATF in the new actuator.

 

Did you check the vacuum switch on the transfer case itself? I have learned from Captain, that switch tends to go bad and leaks vacuum into the T-case, sucking tranny fluid into the lines. If the switch is going bad, it may not route vacuum to the actuator to engage the right front axle.