It's snowing and I need 4wd to get around! I try to shift (push button) into 4wd and the 4hi light will blink and blink and then automatically go into auto4 and my service 4wd light comes on. I had difficulty telling if my 4wd kicked in on auto or if it came on at all as the roads were not too awful. At first I thought this was a vacuum line, my old man and myself checked the line by the battery but nothing. In a last attempt, my dad unplugged the battery for a few minutes and when we turned the truck back on, I was back in 2hi. Our conclusion; it's electrical, but what? We're both at a loss and we're not mechanics. I had big tranny work last April but being that the problem is electrical, we aren't sure the shop is at fault. I'm getting desperate and really don't want to run back to the tranny shop. I'll try anything, man.

 

Behind the pass kick panel is the tccm or 4x4 computer...the connections can have corrosion on them...take a look and clean up the connectors with some electronics cleaner. Also the vac line under the battery tray goes to an acctuator...see if that has a tear in it...happens sometimes.

 

Vac lines are good and I'll check the contacts. Hopefully that's it, more snow is coming tomorrow.

 

those vacuum lines go all the way to the transmission have you checked them from front to back. there is a big one about the size of a gas line the splits into three vacuum lines and I had the same problem and tried everything and it was that big vacuum line. I started by getting all four ties off the ground and put it in four wheel drive the drive shaft was spinning but the 4 wheel wasn't working and it was just a worn vacuum line.

 

I didn't know they went that far back. It would make sense that they do. I'll check that out for sure. It feels like it's such a simple problem, I just can't pin point it! I'll look into that tomorrow when the truck isn't snowed in.

 

Make sure you lift all four tires off the ground put it in drive and the press you four wheel drive button and look under and see if the front drive shaft is spinning, If it is shut it down go to the actuator under the battery unplug the vacuum line. I tied a string to it and got under the truck and pulled it all out the vacuums all come off the top/back part of the transmission it goes to a separater three lines come off that. that's attached to one big well not big but about a 3/8 vacuum line that's the line that usually goes bad replace any bad vacuum lines and pull it back thru with the string so it comes back thru the same way it was be very care full that it doesn't touch the manifold after or you're going to end up with the same problem if you have questions or need help you can email me my email is shwnbrun71@gmail.com it goes right to my phone and I'm not working at the moment and have no problems trying to talk you thru it.

 

I was having similar issues as you, I had already replaced all vacuum lines and the actuator was working fine for me. What I did was pulled the tccm out and found a bunch of corrosion in there. Cleaned that all up and noticed my lights were doing things different than I had seen them do before, they still were not working "right" tho. So I took the 4x4 buttons (mine is the 4 button) out of the dash and opened it up, I noticed some of the solder joints in side the switch were cracked, so I re-soldered all the joints and put it back in. That seemed to have fixed all my problems.

 

Swells 87, yeah, Ive taken apart many electro- mechanical subassemblys apart for intermitant problems and with the magnifying glass found primarily the ground traces are cracking and creating problems. I assume the manufactures have complied with a lower lead/different temper as low heat solders go, and these traces are makeing for annoying intermittant failures. check it out.wiper motor trace looseing ground.If you get a rebuilt unit this was not discoverd. If you get a new unit you are on barrowed time until that solder trace fails. Thank you guys for the blogs. 10 inches of snow half inch ice sunday to monday, 5 more inches snow tuesday, thats when I discoverd 4wd not working. Thanks to you guys I found a vacuumn leak and restored 4wd and the selector for the heater. Thanks again.

 

Shwn, I appreciate it. I'll look at that TCCM panel and the solder in the switch before I start yanking lines. But if it comes to that, I'll keep you informed. Thanks for the electrical tips Swells... We'll see what the outcome is. More snow is coming this weekend... Pain in my butt.

 

Well, it's a Christmas miracle. The 4wd is A Ok and I didn't do anything. We think it's the vac lines and then the snow and ice has formed over them creating a temporary seal. So, that will be a job for another day and a warmer climate.