I totally agree on the seal but, the image in post #12 shows the vacuum switch is well below the top of the case, and the vent connector. When the fluid is at the proper level, the switch is only slightly above the fluid. It doesn't take much over full for the switch to suck it in, (when it leaks). Once fluid enters the vent line, (or the source line), at the switch, the fluid will restrict the flow to atmosphere and allow the negative pressure to increase. The more fluid in the line, the easier it is to suck more in... viscous cycle. Between us, we've covered all the bases

 

Attached to the hose next to the tranny dipstick tube is what doesn't appear to be a "filter" but rather a black plastic little check valve thing- this allows free flow out but limited flow in--am I looking at the right thing? it appears to function so I guess I should check for restriction in the hose itself?Thanks

 

Kelani and Psoni,
I don't know if you have seen these links yet, but someone shared them with me and they were very helpful. Since then I have been sharing the links like mad. Very useful, but you want to read them in order:

GMC Sonoma & Chevy S-10 Transfer Case Vacuum Switch

GMC Sonoma & Chevy S-10 Transfer Case Vacuum Switch

 

Here's the progress, or lack there of, from last night. I removed the heater core hoses from the firewall, and sure enough contaminated coolant began to spill out. I forgot about my previous problems with a clogged heater core (I've flushed it once before) and the possibility of these problems being related. Do you think they could be? Anyways, I flushed the heater core.

Next, with the heater core lines removed for easier access to the torn vacuum line, I replaced the bad section with new line. I definitely plan on replacing all the lines, but I needed to have the problem fixed until I have the time to do so. I did inspect the rest of the vacuum lines, and they appeared to be in good condition. With the car running, I again removed the battery and tray to gain access to the actuator. I then engaged 4WD, and sure enough the actuator engaged and stayed engaged! Finally progress, and I thought I may have solved my problem. A quick test drive seemed to verify the fix, but upon driving to work this morning the problem returned. While in 2WD a pretty quiet, inconsistent humming could be heard from the front end, which would not go away until in 4WD. This hum is a LOT quieter than the original sound.

I did not pull the transfer case fill plug before replacing the switch on the case, as I wasn't aware of that whole issue until after. I tried to remove the plug again with no luck. I tried every trick in the book to remove a seized bolt. Starting to think a trip to the dealer might be necessary... Ouch, my pride...

I have also not yet blown out the vacuum system, or tested it with a vacuum tester. I wanted to fix the obvious issue first (the torn vac line). I believe I saw writeups for both of these on here somewhere, which I need to find again.

Thanks again for everyones help so far.

EDIT: Based on what I've found, in order to blow out te vacuum lines I need to remove the 3 lines that plug into the switch, and the lines to the actuator, source, filter, and line on the top of the transfer case. Then simply blow them out with a compressor. What I can't find info on is how the HVAC system ties in to this, and if I need to or should be worried about blowing them out.

 

Yes, thank you gentlemen very much for the great help here--- One last question? I just havent quite figured out the vacuum switch though-I understand the source and actuator vacuum at the switch but what is other vacuum line for? it goes to the transfer case or transmission? WHY? and it tees off to up by the dipstick--again Why? what is the function of that third vacuum line? if it is a vent why does it need vacuum? why not just a straight vent up by the dipstick?

 

when vacuum is switched off at the diaphram it needs a vent to return to normal, and the transfercase needs venting for heat expansion...both run up to the dipstick to keep water from entering the system.

 

Not related to the 4wd issue. The two main causes of contaminated coolant are: 1) Neglect, (not flushing often enough) and 2) adding ethylene glycol, (green) coolant.

Could be you got a faulty switch. Easy way to check it is remove the vacuum line from the actuator and connect the vacuum line to a vacuum gauge. Start the engine and select 2WD, vacuum should be zero. Then select 4WD, should show manifold vacuum. Then select 2WD again, vacuum should drop immediately to zero. Next, remove the filter from the vent line on the transmission dipstick tube, you should be able to blow and suck on the line with little or no resistance. Next, post your results.

If trans fluid did not drain out when the switch was removed, it's not overfull, it might be low, but it's not overfull (normal fluid level is slightly below the switch).


Correct, a shot or two of brake clean in each line will help remove the oil without harming the rubber lines.

HVAC lines, and the vacuum reservoir, should be checked for oil and cleaned out if necessary. Now, as you can see, I'm not an artist, never claimed to be, so bare with me on this This shows the 2 vacuum systems and how the tie together:



This diagram shows the HVAC actuators and lines in the dash:

 

Last night, I removed the 3 vacuum lines from the switch on the transfer case, and the line on the actuator. I blew out that line and there wasn't any fluid. I left the lines removed, so the actuator couldn't have vacuum. I drove to work this morning, and didn't hear anything until I was above 55 MPH, about 15 miles into the drive. Then the faint humming returned, which led me to believe that it has to be something regarding the front axle engagement, the wire, or actuator. BUT, when I pressed the 4HI button, the sound stopped. What does pressing that button still do when my vacuum lines are all removed? It seems thats there current problem.

I don't have a vacuum gauge handy, but I'll post the results once I can.

I gave in and went to the dealer and had them loosen the fill and drain plugs, and the fluid was normal.

Thanks for the diagrams Capt! You might not be an artist, but they look like they were done by Picasso compared to what I could do!

 

The t-case shift motor will still try to shift when you push the 4HI button, vacuum or no vacuum. What I believe is supposed to happen when you push the button is that a sensor detects whether or not the front axles are joined (via the vacuum actuator under the battery tray). If there's no vacuum and the collar doesn't join the axles, then the TCCM is supposed to tell the encoder motor to shift back to 2HI. However, in my experience with my Blazer, I was able to shift into 4HI without vacuum to the 4x4 actuator. So the front drive shaft was receiving power from the t-case and the dash indicator said I was in 4HI, but the front wheels weren't turning. This may be what's happening to you. Don't know if that means the sensor is bad or what.

Anyway, it would seem that having power applied to the front drive shaft is what makes the noise go away...but not sure why.

 

Beat to death would be another way to put it!