OK, I have read and read threads on 4WD issues and I didn't want to start another one, but here I am..
2001 - 4 button switch (assume it's the 236 from what I've read)
Service 4x4 light is not on (was on previously, new 4 button switch in dash fixed that)

When I push a 4WD mode all it does is flash.

I have verified vacuum lines are good from manifold to tee located just above EBCM. (there are 2 lines from manifold to this area). It's really hard to follow lines from this point as they appear to go inside driver's side fender.

I know there is NO vacuum at the vacuum switch located on the firewall. I have replaced this switch with a new one and still nothing.

The actuator under the battery appears to be in good shape, but obviously vacuum is not being sent to it, since the switch at the firewall is not sending it to it.

So, where does the vacuum lines go, so I can follow them to find out why there is no vacuum at the switch on the firewall. I am thinking once I verify vacuum at the switch, I can verify the actuator is working or not.

Am I on the correct patch with the vacuum verification?

I welcome your thoughts/suggestions. Any help is appreciated.

 

If the switch on the firewall does not have manifold vacuum at idle, and send it to the axle actuator under the battery tray when you select Auto 4WD, 4HI, or 4LO, the lights on the switch will flash, and 4WD will not engage. Vacuum for the switch comes from the vacuum lines over by the EBCM.

 

There is one vacuum line that connects to a very small, almost impossible to see, difficult to feel, hose nipple in the driver's inner fender. You are going to have to follow the vacuum lines from the engine to the T, then to each location to find where the leak is at. Only way you are going to get 4WD to work.

Good luck.

 

I just fixed my 4x4 and it was the actuator, so I want to just throw this out there. You say that the actuator diaphragm "looks" fine, but are you sure? I thought mine looked fine as well, but I wanted to be sure. So I took it out. Come to find out, there was a small tear on the bottom of the diaphragm. Went to my local auto parts store, paid $50 for a new actuator, I installed it myself in under 1/2 hour and now the 4x4 is good as new.

I have read posts all over the internet, but noone ever mentions checking for tears, so I just wanted to throw that bit of information out there. A tear = no 4x4. Good luck

 

BlazerMan, thanks for the input. I have not removed the actuator to give it a good inspection yet, because I know that I am not getting vacuum to it at this time. As soon as I can get the vacuum to it I will definately check for tears.

Lanny, this info really helps... I see the line going into the fender, however I could not find it (or know where it came out) from there. I plan to try to locate the nipple in the fender today and see if there is vacuum there. Hopefully once I find that nipple I can follow the hose out of the fender and find the next location to check prior to the switch on the firewall. Thanks again for your input.

Captain, this is exactly what is happening. I have verified there is vacuum at the t's by the EBCM, but none at the switch located on the firewall. I am trying to find the hose routing between the EBCM and the switch on the firewall to identify the area that I am losing vacuum. Thanks for your input as well.

 

The illustration below should give you an idea of how everything is routed.
I created it a while ago for a tech article.

As you can see, the line running from the check valve to the fender does not go anywhere else. The other line from the check valve should run to another 'T' and split from there to the actuator on the firewall as well as into the firewall for the HVAC.

 

Swartlkk... Thank you so much, that is exactly what I needed. i know I have vacuum at the tee where the red line starts, however I am not getting any at the bottom plug on the solenoid. (I assume it should have vacuum at all times) So I am losing it somewhere in the red line on your drawing. The solenoid would then send the vacuum to the actuator.
I did pull the actuator out and check it today, it is good, however the cable was stuck and still does not move as freely as I think it should. So I will replace that once i get vacuum to the actuator, and it should be good to go.
Thanks again, a great help, and I feel like I am getting closer to a resolution.

 

Are you saying that you have vacuum at the T where the red line connects, but then when you connect the red line you do not have vacuum at the other end...which would connect to the solenoid....is this correct?

If so, simple fix....replace the vacuum (red) line between the T and the solenoid.

Of course the next test will be if the solenoid switches and provides a vacuum to the cable actuator under the battery tray.....

 

Does the power wire to the selonoid go through a fuse? I bought one of these for my '95 S10 and when I wired it up, I hooked it to a 20 amp fuse and it blew the fuse right away.....

 

The feed for the switch is fused, (everything is fused except the starter). There are two terminals in the switch, one fore positive, one for negative. If it pops a fuse, the switch, or your connections to it, are bad.