My 99 with the 4 button 4wd takes a while but it will go into 4wd on the auto and hi range setting. Goes in and out of 4low just fine. I put a new switch in the dash and that stopped the service 4wd light. I almost positive now its a vacuum issue cuz it will pop out of 4wd on acceleration. Anyone else have this issue?

 

Well yes I did but I'm 3 button, it would come out with a bang and it was vacuum. Not sure how it relates to yours.
When yours comes out do you have a bang in the front diff.
I'm not going to come back cause that's all I can really say, I know nothing about the four button set-up. Good luck!

 

Thanks for the info. I figured it was vacuum it does bang but not real bad.

 

Probably a vacuum leak in the HVAC or 4WD system(s), or possibly just a bad vacuum check valve. The valve keeps vacuum in the systems when the engine vacuum is low, (acceleration) to prevent the vacuum actuators from reverting to their default positions. 4WD goes to 2WD and HVAC goes to defrost.

 

OK thats kinda what i was thinking. I plan on changing all the vac lines once i get time. The vacuum valve is this the one mounted at the firewall? I may as well replace it while im right there. Thanks for the info

 

Follow the 7/32" vacuum line from the supply port. #4 in the image below:




It will connect to the vacuum check valve, (might not look exactly like the one in the image below). This image might help with vacuum line routing:

 

works for me thanks