Hey all,

My 4wd system doesn't engage at all. Going from 2wd to 4hi, there is an immediate sound coming from under the car but the light just keeps blinking and the 4wd doesn't actually set in. I tried this on reverse, neutral, and drive.

When going from 2wd to 4lo it doesnt make any noise at all, just blinking.

The vacuum lines looked good so I took out the battery and looked at the actuator. The diaphragm or whatever didnt have any tears in the rubber but I did notice that I could push it in back and forth and when I did, a hissing sound came from the hose that goes from the solenoid above the distributor cap. I'm not sure if this is a sign of a leak because the hoses look perfect and are fit onto the solenoid all the way...

Any pointers?

This is a 2001 4wd 4 door blazer.

 

Best way to test the vacuum is with a hand vacuum pump like a MityVac. Failing that, you might be able to suck on the end of the hose with your mouth and see if it holds vacuum.

A visual inspection on these is likely not sufficient as there can be hairline cracks or the hoses might not fit snugly on the nipples.

Also know that the transfer case control module (TCCM) can fail on these. This is module with several relays that you should be able to hear clicking. If it is like my 1999, it is located under the passenger side kick panel.

Also, do you have "4-button" or "3-button" 4wd switch. This can matter with regard to diagnosing the problem.

 

I hear the TCCM clicking when pressing on 4wd hi..
I have never opened and visually checked the TCCM, but can those relays be replaced or the whole unit if there is a problem?

Also, i have the 4 button 4wd switch.

 

I think in principle, a person could replace the relays but I'm pretty sure they are soldered in. I had to replace my TCCM. I got a good price on a NOS one on eBay. Shop around and ask for advice on where to get one.

Also you have the same 4wd system as I do so what I can contribute should be at least somewhat relevant. LOL

Do you have a vacuum valve hanging on a bracket above the distributor? This should be leak tight from the supply side when engaged. On the side that goes to the axle actuator, I think it should be vented when not engaged. This valve is electrically controlled and therefore like any electrical component, it can fail.

 

Well the TCCM is fine because it clicks everytime I press the 4wd high. I took out the battery tray tho and found out when I do press the 4wd button, nothing moves. I tried pressing on it by hand and it moves freely so the cable that runs down is not stuck. The vacuum line that hooks up to that valve above the distributor seems to be the issue. I am however unable to find that part to buy since I do not know what its called.. any help?

 

There is 2 little hoses on that vacuum switch on the firewall , if you hook them direct bypassing the switch ...do you get 4 wheel drive ?...this will only test the vacuum system... if you can hear the encoder and tccm working then we will assume for now they are ok....also check the line from the vacuum canister ball in the drivers fender it will go to the firewall switch as well as t off to the hvac....REMEMBER which line goes where on that firewall vacuum switch, if they are backwards the front diff will stay engaged.

 

I'm not sure whether You're asking for a vacuum hose... If so, I initially took a silicone one (sold on meters for $15 at a tuning shop), and later on I've found a quality & budget one sold as a "fuel hose" at a tractor suppliers ($2 for a meter). It's a diameter that matters and has to be oil/temp proof.

 

No, the black thing where the two vacuum hoses connect.
See the picture https://pasteboard.co/HpCI7O6.jpg

 

Oh, I got it. My '00 does not have that one. It seems to be called "front drive (axle) vacuum actuator solenoid valve".
On the picture You've posted, one of the wires (a white one?) seem to have an insulation damaged. If it's a positive terminal and it got grounded to the chassis, You may have a solenoid driver busted.

If You have a multimeter You may:
- check if the power goes to the solenoid on 4wd request
- measure the coil resistance on the solenoid terminals (should not be infinitive nor zero)

I think You might also try bypassing the solenoid by coupling the two hoses together (yet again my '00 does not have that solenoid) and see if 4wd starts working.

Brgds,
Mike

 

This is the Dorman part number . Vacuum Actuator Solenoid Valve Dorman 600-104 - 1AFWA00011