Front End Noise

I have a noise thats sounds like grinding. Happens mostly when excelerating, then letting off on highway. Seems to be vacuum system trying to engage something in front end. It also happens if I turn the truck off, then re start a minute or so later. Pulling away you will hear it..... It stops when you hit 4H then 2H control buttons on dash. Advice appreciated. J

When you get your hoses replaced and in good order unhook your Neg battery terminal for 30 min and let Com reset.

Jdmccright you’re right. It was the vacuum switch on the top left side of the transfer case.

I wanted to understand why it was that before I changed it out. I did some more trouble shooting. While under the truck checking the vacuum hoses, I noticed that the front drive shaft was turning freely. Made me wonder why the grinding, how could that be if its free. I checked the boot on the vacuum switch on the trans case, it was only half way on. Well, I’ll take it for a drive and see if that was it. Got up to 70 and let off, the grinding started again. Hit 4h and it quit, hit 2h and it again remained quiet. I pulled off the road and got out, truck still running. I reached under the truck and the front drive shaft would not turn. I reached in and turned the truck off. 5 seconds later, I heard something actuate under the truck. I reached under and the shaft was free again. I wondered if it was the encoder motor in the trans case acting up. I had read this in some of the posts and also had a mechanic tell me recently he thought that was what it was. So, I turned the key on (engine off),,, reached under and the shaft was still free. I pressed 4h and the case engaged the shaft, it would not turn. Pressed 2h and it released. It’s the first time I felt relief that it was not the encoder motor. I started the truck in 2h, reached under and the shaft would not turn, it was engaged. It all made sense to me now. The shaft has been engaged in the front axle or hubs (how ever it works up there) since I bought the truck 12k miles ago. Normal driving maintained enough vacuum to keep the front end engaged. When I excelerated to pass, naturally I lost vacuum. Front end would dis engage, and the front shaft would go free,,, letting off the gas, creating vacuum again, the front end would try to re engage the shaft with out synchronizing first, thus the grinding. When I hit 4h (the components I assume with sync capabilities) the trans case would sync the front shaft, spin it up to speed and the front end then could re engage the shaft. When I hit 2h, the trans case then would release leaving the front end locked in, all would remain quiet until next time.

So, to check the switch,,, While in 2h, truck running, reach under the truck and see if the shaft is engaged. I suppose if the switch failed in the other position than it did me, this may not be an indicator. But, if in 2h and the shaft is engaged and won’t turn, then its looking like its bad. Turn the truck off. Turn the key to on. See if the shaft is free. It should be. Then press 4h to see if the trans case engages the shaft, as it should. Then press 2h to see if it releases. If it releases then its looking even more to be the vacuum switch on the trans case. Also, the other night, I took the battery out. Removed the battery pan. Then hooked up the battery to start the truck. After started took the battery back out. Then actuated the switches on the dash. At that time I noticed that the vacuum diaphram located under the battery pan never released vacuum. That’s another way to see if its failing. It should release vacuum when in 2h. Doing this also gives you the opportunity to move the cable from the diaphram to see that it is not corroded and seized. It should move freely. Mine did.

Hey jdmccright I would have swapped that switch sooner if I’d known it was only 15 bucs at autozone, and that it would have only taken me 5 minutes to put it in. Also, I’m difficult to except solutions when I don’t understand the whys. Thanks for the intuitive input. Kept me interested. What a laugh. Was so simple too.