Recently my old man's truck wouldn't lock into 4wd. Transfer case working. And replaced the actuator on axle. Now it works when it wants but not when you need it. So... reading thes forums and thanks to all who have ever answerd someones question, cause you have no ideah how many times these blogs have taught me how to or specifics... anyhow I ended up putting a vacume guage on the lines at the axle and had nothing in 2 or 4 wheel selection at either line until my pops revs the engine. I then get 23in hg... and 4wd locks, and amazingly the mode doors in dash start working again. So I pulled the line at the vacume pump. Shut the truck off, restarted it, and again had no vacume. Once pops revs the engine again I now have 25 in hg of vacume. And even when engine returns to idle I continue to have vacume. But only after the truck is in park and revved once does it start making vacume. So we figured out when he drives it down to his new property before he can go up the driveway in 4wd he has to stop put the truck in park or neutral and rev the engine, then enguage the 4wd. This never used to be. Does anyone have an answer on why this vacume pump would be acting like this? And is the fix to replace the pump, or is there something little I'm missing? Thanks for any input.

 

the vacuum pump is bad. they make rebuild kits for them. cummins sells them and a couple places online. dodge dealers will usually tell you that you have to buy a new or reman one. rebuild kit is around $20 iirc

 

ok thanks i will check into the re-build kit for sure...