4wd Problem k1500 1992 Silverado

Hey everyone, I have a 5.7ltr 92 Chevy truck with 4wd that wants to try and engage the 4wd actuator if I hit bumps. You can hear the actuator followed by the gear grind sound. This is happening while the 4wd shifter is in 2wd. I replaced the transfer case switch(no change). I’m running out of ideas.

Winters coming!

I am not familiar with your truck. I know on fords if you have a vacuum leak in the 4wd system it will automatically try to engage 4wd and cause the grinding of the gears trying to mesh. not good to drive with this problem. you will cause more damage if it has not already.


I presume your truck is configured as free-running hubs, front hubs lock via vacuum control (part time 4WD), no need to stop & exit vehicle to switch from 2WD to 4WD. With that configuration when you switch to 4WD (purposely), both transfer case and hubs change their gearing configuration, so either could cause a grinding noise I suppose.

My guess, actuator switch is faulty or wiring insulation has degrading and shorting out going over bumps. Where is actuator physically located? Is it a pushbutton switch on dashboard? Does it have a corresponding dashboard mode light? If so, when this symptom happens does the mode light switch briefly to 4WD? You may have to determine if both hub control and transfer case control is switching mode via bump in road, or just one of them.

GM uses wax filled actuators with a heater inside to lock the front axles. When they fail, they fail open.

This sounds like maybe the transfer case is worn inside causing gear clash or maybe an electrical short causing the electrical actuator to try and enganbe 4wd.

Do you mean by “open” they fail in 2WD mode, hubs unlocked, free spinning front wheels? On my 4x4 Ford truck anyway, if front hub unlocked (free spinning), engaging xfer case to 4WD can cause gear clash problems. Mode not allowed. Ok for front hubs to be locked & xfer case in 2WD, but not other way 'round.

Heat expanding wax method to actuate hubs? Sort of like how a thermostat works. Not something I’d have thought of. Probably why nobody at GM asked my opinion … lol … But it seems like it should work pretty good, and not sensitive to vacuum leaks.

Yes. Unlocked. Whether you select 2 or 4wd, the front hubs are disengaged.

The expanding wax works well, like a t-stat, and the failure mode doesn’t really cause any issues.

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I should have mentioned this is converted to the electromechanical actuator from the old thermal type.

The physical actuator is located on the front “pumpkin” on the passenger side. Also its a floor shifter type. So normal 4wd engagement is pull the 4wd lever back and ease forward. The front gears fall in place and your good 2 go.

When it tries to shift the indicator light will flicker ass well.

Actuator is part of front pumpkin? hmmm … sorry, no experience with that 4wd actuation method. Hopefully somebody here has some experience. It seems like the first priority is to figure out if the gear-grinding noise is in the transfer case, the front pumpkin, or the front wheel hubs. If it makes this noise at parking lot speeds, maybe a helper drives slowly through empty parking lot and you walk along the side of a front wheel, listening if the noise is coming from that.

Like I said above, the primary source of 4WD-associated weird noises w/my truck’s configuration is if I accidentally engage the transfer case in 4WD without having first engaged the front hubs.