4WD not engaging

[Mick]

I'm pretty much at wit's end. On my '90 Dodge, the 4wd won't engage. The dash light doesn't come on, it sounds like it's trying to engage, sometimes it does go into 4wd, but then jumps out. I've had it to the mech twice. He's cleaned out the rear vacuum pump and line. Each time, he'd get it into 4wd and we'd think it was fixed. Then it usually happens when I'm going uphill and applying power, it'll jump out and won't go back.

Any ideas how I can fix this for good (buying a different truck is not an option I am considering at this point)?

[DT366]

have you checked or even replaced the vacuum hose that run to the front end. If they are dry rotted they wont hold a vacuum. Its only a guess. good luck!

[BWhite]

Check out this link it should be really helpfull
Bill

http://dodgeram.org/tech/repair/4X4/4x4_engagment.htm

[Mick]

Thanks lots, woodnazi and Bill. No I hadn't done anything myself. I was relying on him. Bill, that tech site is great - going into my favorites list. Thanks. I'm going to try fixing it myself. I usually don't do much mechanical stuff, but I was a mechanic many years ago so I at least have the fundamentals and can figure it out. I've just never done any repairs to any 4wd system. The diagram doesn't look too complicated, even for me.

[BWhite]

It is a good site for tech info . I know that the vacuum switch on top of the trans is a weak link and easy to change . I am assuming the setup for a 90 is the same as a 94+

[John DiMartino]

Mick I dont know if you fixed it yet,I never seen this post in my view new posts. If you havent is there any chance the one way check valve isnt working,going uphill under power,there isnt a lot of vacuum left,and thats when it pops out.I thought the load on it would be enough to keep it in.The actuator could have a small leak,eough to engage the axle with full vaccum but as vacuum drops it falls out.Ive replaced a few vacuum canisters. Im 99% sure that posi-lock has a kit for the truck,that would convert it to a stainless steel cable,this would end the problems and for cheaper than replacing the vacuum actuator. i found it here ,http://www.4x4posi-lok.com/dodge_page.html

[Mick]

Haven't even looked at it yet. Waiting for warmer weather to be crawing around under it even though I've got a garage. With the lack of snow, it hasn't made much difference, since I'm using it as a backup.

Thanks for the info and sites.

[Crimedog]

I had the same problem on my truck this winter. It is a very similar system, if not identical. You could go underneath, and jimmy the lines around and then the light would come on. Get back in and go, and then the light goes out. Finally we took it into a heated garage, just warmed it up, and it worked like it should. Got out and the CAD was stuck in 4wd. Not a big problem, so after all the plowing was done, we figured it was the switch. I think what happened is moisture got into the lines somwhere, and eventually worked its way into the vacuum switch and froze it up. replaced the switch, and then the rubber block that connects to the switch was worn out and leaked. So, got a new block and plastic lines and it works like it should, no matter what temp it is.

BTW, you should have around 20-25 mm-Hg pulling at the front.

Here is what you could try, if you are getting a vacuum leak, take the rubber block off of the switch at the T-case, and pull out the black and grey plastic lines, and connect them together using a rubber boot. This is "hard vaccume", and bypasses the switch, so your front CAD is always locked in. Do this and you can see if you have a faulty switch, and if you do you can leave it since it is a backup truck and you will always have 4x4.

Good Luck

[Lawngodfather]

My jeep has the same setup, had to many vacuum leaks, I put on a posi-lok.