Captian Hook; Great post / info... I've been able to trace the vacuum lines from up by the drivers side firewall to the transfer case vacuum switch...another goes from that switch to the actuator under the battery and there is a third from the switch that I haven't located the other end of yet...it gets lost in the cluster coming up between the firewall and the engine near the transmission dipstick tube with the first two vacuum lines... It actually looks like it might terminate with a plug...plumbed right along side the transmission dipstick tube?

I'll check the vacuum on the line by the driver side hinge and see what that gets me when I actuate the 2 to 4 wheel drive switch...

Looking at the vacuum switch on top the drivers side of the the transfer case...that looks like it may be very difficult to.

 

At the transfer case vacuum switch:
One line comes from the t's and supplies manifold vacuum to the switch.
One line runs from the switch to the axle actuator under the battery tray.
The third line T's to the transfer case vent pipe on top of the transfer case.
The other side of the T runs up along the transmission dipstick tube and terminates with a filter, not a plug. If it has a plug, the axle actuator will not be able to vent vacuum to atmosphere when in 2WD. It's just a black plastic jobbie that fits in the end of the vacuum line. If you pull it out, you should be able to blow through it very easily. In this image you can see the vent pipe on top, (line still attached) and the vacuum switch below the vent pipe. The encoder motor is between the vacuum switch and the output shaft to the front differential.


The vacuum switch is under $20 and VERY easy to replace. On 4 door models you can get right at it. On 2 door models you need to reach forward over the cross member from behind. A 15/16" deep socket, 3/8" drive, and a ratchet works for both 2 door and 4 door models.

 

Interesting...

Trying to blow into the vent line that terminates by the tranny dipstick tube...there is apparantly blockage as it only pressurizes...will not pass air.

Also...putting a vacuum guage to the line that leaves the manifold and heads for the transfer case switch shows no loss of vacuum whether the vehicle is in 4x4 or 2x4.

This makes me think the problem may be the switch and I will try to replace it...and check on the vent tube to see if it got kinked or blocked by something.

Any ideas?

 

also...when changing the transfer case switch...does it matter which vacuum line is hooked to which of the three nips on the valve?...I don't have the old one out yet and will mark them somehow to know the difference if it makes any difference.

 

The switch is probably stuck. If you select 2WD while the engine is at idle, and remove the filter, if you apply vacuum to the line, the actuator should pull in.

The vacuum line connector at the switch is indexed, you can't mess it up only goes on one way.

With the switch connector unplugged from the switch, remove the filter and you should be able to blow through the line with very little resistance. If not, look for restriction in the line.

 

This is the problem that just keeps on giving...

So...I've changed out the transfer case vacuum switch and yes the old one was very sluggish...almost holding open the whole time...but while it was in the front wheel drive line seemed to not want to disengage...the transfer cas would but the axles stayed engaged. Also, there was never any change in the vacuum line from the manifold to the switch.

Now with the new valve in...and yes the vacuum line connecter keeps you from mixing the lines up...I still get no change in vacuum from the manifold to the switch, the tranfer case works like a snap and engages the front drive shaft but the front axles don't engage!!! I took out the actuator and checked it...it holds vacuum fine and the cable seem to move ok...so I reinstall everything and cut a tee into the vacuum line from the actuator back to the transfer case switch...and voila...no vacuum...any time!!! So, using the tee I supply vacuum to actuator and the front axles engage fine!! So...the problem seems to be a loss of vacuum between the actuator and the transfer case!!??? I can't pull a vacuum on that line...now I'll trace it out to see if it is broke somewhere...but that isn't likely because it was apparantly pulling vacuum before I changed the transfer case switch... ????? Hhhhhmmmmmmmm.

Any ideas? Could the vacuum line to from the actuator to the transfer case be connected to the wrong port, that previously with the old switch pulled vacuum constantly, and now with the new switch is not getting any vacuum...like maybe it is supposed to be the vent??? Incidently...with the truck off, I can pressurize the vent line...if its a vent, shouldn't it be free flowing?

 

Pull the vacuum connector off the switch, start the engine, there should be vacuum on one of the three lines.

As you can tell, I'm not an artist, I never claimed to be one. I think you'll get the idea:



In 2WD, the axle actuator and the vent are connected together in the switch. In 4WD, manifold vacuum is connected to the actuator in the switch.

 

Yup...that is what I came up with. I was able to pull a vacuum at the transfer case switch connector in the line from the manifold. I was also able to pull a vacuum on the actuator line...and verified that the new switch is working so that only leaves the transfer case motor that should be sliding a part across the bottom of the switch to push the "ball" up and transfer the vacuum to the actuator line. I think. There doesn't seem to be a way to test the motor and taking it out seems to require dropping the front drive shaft. At this point I now know the front wheel drive assembly works if it gets vacuum and it is not currently engaged and the transfer case sends power to the front drive shaft with no problem if I engage the 4x4 button. So...if I run it as is right now I'll know I'm in full clear 2 wheel drive and can check the MPG problem again...that is what started this whole process. If my MPG has drastically improved back to what it was then I have at least solved that problem and I can then work on getting 4x4 working again...which looks like it will require a new transfer case motor...yes? Open to thoughts...it's been a long night.
Many thanks. And the diagram worked out great too !!

 

When the transfer case is in 2WD, the axle actuator vacuum line is connected to the vent line. When the encoder motor performs a range shift, (2WD HI to 4WD HI) the range shift fork contacts the ball in the vacuum switch and pushes it in. The switch then closes off the vent line and connects manifold vacuum to the axle actuator. If your transfer case is in fact performing the range shift, rest assured that the ball is being pushed in and the problem is somewhere in the vacuum lines.


Here's a way to check the integrity of the vent line from the filter to the switch, the vacuum switch, the line to the axle actuator, and the actuator all at one time: Make sure 2WD is selected and turn the engine off. Remove the vent line from the transfer case vent pipe and plug the line. Remove the filter from the vent line and apply ~20" of vacuum to the line. The actuator should pull in and vacuum should hold.

Here's some other stuff you can try:
When the engine is at idle, unplug the vacuum switch 3 way connector at the vacuum switch. Do you have manifold vacuum at one of the vacuum lines in the connector? If so, next step. If not, trace it back to the source and repair it.

At the vacuum connector, one of the remaining lines goes up a few inches to the Tee for the transfer case vent. You should be able to see or feel it from under the vehicle. You should be able to blow through the line at the 3 way connector with very little resistance, and air should come out the vent filter on the trans dipstick tube. If so, next step. If not, find the restriction or leak and repair it.

The third line goes to the actuator. When you apply vacuum to it, make sure the actuator pulls the cable in and the vacuum holds. If not, trace the line and repair either a restriction or a leak.

 

So Captain...if I understand this correctly, the test of the transfer case motor is that if the transfer case / front shaft is engaging when I activate the 4x4 switch then the motor is good, and, it must also therefore be activating the vacuum switch leaving only a vacuum line issue?? I can (somewhat) easily change out the entire length of vacuum line to fix that issue. Thanks.