Four wheel drive; GMC Suburban

Hello there! I have a 1996 GMC Suburban that has started acting funny when put into four wheel drive. When I push the button on the dash to pop her into four wheel, I can hear a thunk, thunk, thunk noise and I can also feel it in the floor. It almost reminds me of a cogwheel that is too loose to fully catch and causes a slipping and thunking noise; I really don’t have much of an idea of how four wheel drive works. This had only been happening while the four wheel drive was engaging, and I didn’t think anything of it. But, today, it continued after the four wheel drive was fully engaged and also happened after I disengaged the four wheel drive. I can never remember this happening before, is it normal? Any ideas of what the problem may be? Thanks in advance!

Does the thunk-thunk noise start just as soon as you push the button, and does vehicle speed affect it?

Sounds like one of the actuators is malfunctioning or not completly engaging or front cv joint etc. Take your pick. Take it to a shop, have it put on a lift and have the 4 wd system evaluated including the other drive line components. It’s the only way you can tell for sure. Continuing to drive around this way can cause problems with other components other then one that may need replacement or adjustment.

4WD doesn’t always disengage immediately. There is a transfer case bolted to the back of the transmission. The T-case has the mechanism to transfer drive to the front axle and a front driveshaft attachment. The front axle has a mechanical clutch that hooks the wheels to the center of the axle so the front will drive. This stuff is disconnected when you turn off 4WD to save gas. When you turn 4WD off, the T-case unhooks the front driveshaft and the axle clutches release so the internals don’t turn.

I agree with @dagosa, Sounds like one of those actuators at the front axle have failed OR something in the front axle has failed. Either way, don’t use 4WD and get it to a service place for a look.

Ok, this all started after I took it to a shop to have them do some work. They had to take the back axle all apart because of a bad internal seal. Could I have been caused by something they did, or do you think that it would be completely unrelated? Thanks!

There is no internal seal in the rear axle, only external ones. I don’t know what you mean by “all apart” but maybe they had to remove the rear cover to pull an axle to replace an axle seal. I call that $300 apart. “All apart”, to me is $1500 apart. Unlikely this is related.

I’m not quite sure. It was like a $1000 deal. There was some pitting that they needed to replace too.

It sounds to me like you have a bad CV joint or U-joint whichever your truck has. You should be able to crawl under and inspect them. They will be located on the two front axle shafts near the backside of the Hub.

As far as the rear end, I doubt it is related to your problem now.
I too think the just replaced the axle seals, but the pitting…It’s always best to replace the old pitting with fresh pitting.


Service the transfer case and related parts ASAP,this stuff isnt as robust as one might think,a transfer case that has been neglected oft times needs replacing