I just recently tried to put my truck into 4WD while rolling, and when i flipped the switch to 4 high, it makes a horrible grinding/clunking noise by the front tires. It will lock into 4wd if the truck is perfectly still, and doesnt grind when driving in 4wd, but when i try to switch on the fly (at only around 10 mph, with my foot off of the accelerator) it does this horrible grinding and clunking sound, and i can actually feel in interfering with the truck going down the road as well. I havnt tried shifting on the fly after confirming that my truck was doing this for a second time, but i just wanted to know what was wrong. What do you guys think?

 

read the owners manual... this is normal when engaging on the fly

 

I saw that, but this noise doesnt stop untill the truck comes to a complete stop, and it doesnt engage untill i come to a complete stop. i drove for 10-15 seconds with it grinding and clunking and it still didnt engage untill i stopped, i just figured there had to be something else wrong with it. Its quite loud too... is this really normal for shifting into 4WD?

 

Yeah... a long grinding and clunking noise is definitely not normal. You might get a slight sound while shifting into 4WD on the fly, but nothing alarming. It sounds like you're not getting enough vaccum while driving to disengage the actuators which is odd considering you should have more pressure while the engine is under load...

 

That is exactly the opposite of what actually happens. Under load, the throttle blade is open wider and you have LESS engine vacuum.

If you lift off and coast with a manual trans, you might briefly get more vacuum during the coast down, but it would be temporary. Given the the way engines work and the limits of atmospheric pressure, vacuum has a maximum value.

OP, if you are having this kind of trouble, Check the usual failure point of the vacuum actuator.

 

Disregard. Irish, you are 100% correct. I meant at IDLE. I believe the owners manual says to stay off the throttle while engaging the 4WD for this exact reason. I know the vacuum box is designed to help engage the 4WD while under load and at lower vacuum. I would get a vacuum pump and start checking for leaks in your system. Work from the solenoid down to the actuators, that should help you narrow it down.

 

This is the exact issue my f150 is having it will go into 4wd with no problem at a stand still, but makes a loud grinding noise if shifting into 4wd on the fly. I have already replaced the iwe solenoid. Any thought on what would cause this or how it could be resolved.

 

Check the vacuum lines for a leaks first going to the hubs and going to the solenoids. Depending on how often you shift into 4wd on the fly your front hubs may be worn causing it not to engage

 

Maybe try and disconnect your vaccum line at the check valve and plug both ends of the line and see if your hubs will lock in and the noise stop.