4wd Actuator working, still no 4wd, what next?

Crystal Jackson · September 3, 2020

Hey all!

1990 GMC K1500

I'm working on my old farm truck and sprucing her up. Since we bought, it the 4wd has never engaged. It wasn't the biggest deal. Now, I'm looking at putting some money into her and getting some things fixed up.

The actuator is getting power and I have pulled it out and shifted to 4-HI. The pin/plunger comes out the full length and i cant physically push it back in. I have ordered a new one since they are cheap and its a 5 min replace.

Since it seems to be in good condition and working, what should be my next step in figuring out why the 4wd wont engage?

davester · September 3, 2020

Put the truck in 4wd and lift up one of the front wheels.

Reach under the truck and try rotating the front driveshaft. If you can rotate the driveshaft, then something is wrong with the transfer case, broken chain, shift fork problem, gear problem.

If the driveshaft doesn't rotate, try spinning the lifted wheel. If the wheel doesn't spin, you are in 4wd mode.

If it does spin, and you've already verified that the actuator pin goes in/out as you shift in/out of 4wd, then bits in the front axle are busted. The locking ring (this is what the actuator moves to lock the front axle) or the passenger side stub axle are common points of failure in this case.

Crystal Jackson · September 3, 2020

Great! I'll have to check it out tomorrow and let you know! I'm also slowly picking through some quirky wiring issues with turn signals and maybe the ignition switch from what I've been reading.

Its doing the random won't crank but fuel pump is going when starting (new starter) and other times works just fine. At least parts are cheap and I enjoy the work, lol.

Crystal Jackson · September 5, 2020

Ok, I put her up on the jacks. In 4wd, the front driveshaft won't turn and on both sides the front tires lock up as well. I shifted it into 2wd to confirm that the tires release and lock up again when put back in 4wd.

I have put it in 4wd when it's on the ground multiple times including a day or two ago and the front wheels just won't spin while the back are turning(spinning) under weight.

davester · September 5, 2020

If, in 4wd, you can't rotate the lifted wheel but you aren't getting any power to the front wheels, then something isn't fully engaging. There is no clutch to slip.

You might re-verify the front axle actuator extends about 1" or so, when you are trying to hold it back with your hand and someone shifts the truck into 4wd, to make sure it does fully extend under some pressure.

If you are certain you aren't getting power to the front wheels in 4wd, then I would suggest splitting the front diff or pulling it, and then checking out the front axle locking setup (locking gear, passenger side stub axle, other stub coming out of the diff on the passenger side).

Then, if all those look good, then it's crack open the transfer case to check it out.

Crystal Jackson · September 21, 2020

Just wanted to update on this issue. With the old actuator in place and after 10 years of having the truck, it finally just locked into 4WD. I have installed the new actuator which is much faster than the old one. I wanted to make sure it wasnt a fluke so I have been testing every day or so. So far it seems be working on command.

I had it shifted into 4WD and drove it around for a while to let things try to move around. I went to turn in a parking lot and it must have locked in. I am in the process of changing the transfer case fluid and front diff fluid. So far so good.

Next project is looking at replacing the valve seals.

Thanks for the help!