1995 GMC Jimmy 4wd not working.

[rickyd123]

It has a 4.3 liter engine and an automatic transmission. I've replaced the vacuum actuator and that didn't fix it. I've traced the vacuum lines. They are in good shape. I got a paper from this site from a friend of a friend of a friend it said to unhook the battery for 30 minutes to test the TCCM ( Transfer Case Control Module) I did that. Nothing changed after. Then the paper said it's probably the encoder motor. Encoder motors are very expensive. I thought I should ask the forum and see if you all agree or have other ideas as to what my problem is.

[Blue Bowtie]

Is the actuator under the battery tray getting vacuum when the 4WD function is selected? The vacuum valve is attached to the transfer case, so if the actuator motor is not moving the shift lever, the new vacuum actuator is never going to work.

The first step in diagnosing this is to determine if the actuator motor (on the side of the transfer case) is even trying to move.

[rickyd123]

It isn't doing anything when I try to put it into 4wd so I'm assuming it isn't getting the vacuum.. When I move the lever inside the Jimmy the actuator should show movement. It doesn't. Is the acuator motor different from the encoder motor

[Blue Bowtie]

If the vehicle has a lever for engaging the transfer case, there is no encoder motor. The motors are used on vehicles with a push-button transfer case select switch. The motor basically does what the manual lever does in moving the transfer case linkage.

Both type systems used a vacuum switch (valve) at the transfer case, however. When 4WD is selected the valve is actuated and ports vacuum to the front axle actuator under the battery tray. Of course, vacuum must be present at the switch/valve in order for that to happen. It is not uncommon for the vacuum line to the transfer case to deteriorate. There is also a vacuum reservoir bolted to the underside of the hood on the driver side, and it has to have vacuum as well, since that serves the transfer case as well as the HVAC controls and cruise control.

[rickyd123]

It has a lever to engage 4wd. No push button. I've inspected the vacuum hose. It all seems in good shape. I didn't know about the reservoir, so I need to check the part of the vacuum line under the firewall, but what I can easily see from it looks good.
Could the vacuum switch valve be bad? Is it on the transfer case?

[Blue Bowtie]

The vacuum valve is mounted onto the transfer case, and is basically a plunger actuated by a cam inside the case. The valves do fail, and there is a history of the Dorman replacements not lasting very long in service.

https://www.youtube.com/watch?v=YJx93-Af0t8

[rickyd123]

Blue Bowtie mentioned my vacuum reservoir, so I took a closer look at it. One of the vacuum hoses isn't connected to anything. I can't see where it was supposed to connect. It looks like it goes under the fire wall and on top of the tranny a very short distance. I can't see any ports up there for it to plug into, but I can't see much. Anyone have any ideas where it goes. I tried to post a picture of it, but couldn't figure out how to post it from my desktop.

[Blue Bowtie]

It connects to the valve on the top of the transfer case, not the transmission. The video linked above shows it pretty clearly.

[rickyd123]

If when you say "It connects to the valve on the top of the transfer case, not the transmission." you are saying it connects to the vacuum valve switch, The vacuum valve switch has all its hoses at the plug, and they feel firm and connected. The hose coming off the reservoir is to short to reach the switch. I guess one of the hoses coming off the switch could have a "T" junction or something that shorter hose ties into? I'll try and look when I replace the switch, but it's probably isn't going to help until I get that hose hook up to the switch.