4WD Vacuum Actuator question

tylernt · #1 03-04-2006, 03:18 PM

I just found out that the P.O. of my S-10 Blazer had left his 4WD vacuum actuator totally disconnected and the hose plugged with a golf tee.

I just replaced the hose, but I'm afraid to drive it until I know that it's really out of 4WD. When I start the car, the actuator "actuates", but when I shift into 2H, 4H, Neutral, and 4L the actuator does nothing. It continues to do nothing until I shut off the engine.

Is that the way it's supposed to work, and there is some other signal to the transfer case to shift in/out of 4WD? Or is my actuator borked?

Here is why I'm afraid to drive it: http://www.automotiveforums.com/vbul...87&postcount=7

Thanks!

herkyhawki · #2 03-04-2006, 06:07 PM

If the actuator "actuates" as you say, then it is not the problem. If vacuum is always present at the actuator, then the transfer case switch is stuck open. Put the golf tee back where you found it until you get new switch.
Change the transfer case switch. It is on top drivers-side of transfer case and has 3 vacuum hoses connected to top of it. It is a $15-20 part at any parts store.

herkyhawki · #3 03-04-2006, 06:16 PM

Quote:

Originally Posted by tylernt

Is that the way it's supposed to work, and there is some other signal to the transfer case to shift in/out of 4WD? !

The TC switch is controlled by the transfer case. It does not shift the TC case, it is switched by the TC.
The vacuum actuator only engages the front axle after the transfer case is shifted into 4WD by the encoder motor, and causes the TC switch to send vacuum up to the actuator.

The reason this confuses so many people is that the part named "transfer case switch" is not a switch at all. It is a mechanically operated vacuum valve.

tylernt · #4 03-04-2006, 07:45 PM

Quote:

Originally Posted by herkyhawki

the transfer case is shifted into 4WD by the encoder motor, and causes the TC switch to send vacuum up to the actuator.

Ok, this is starting to make more sense. I think I screwed up, then. The hose that was plugged and that I just piped to the actuator, goes to the carburator. I guess I need to run it down to the TC switch and from there up to the actuator? This Chiltons I have is completely useless.

I should have mentioned this before, this is on an '87. I thought this website listed that below my name but I see now that it doesn't, sorry.

tylernt · #5 03-04-2006, 08:42 PM

So I went out and tried to figure this out. After the vacuum actuator, I traced the cable down under the engine and across the engine bay to what I assume is the transfer case (this metal box that both front driveshafts connect to). At the point that this cable goes into the transfer case, there are no vacuum lines that I can see but there is an electrical connection. I assume this is because I have an older floor-shift model rather than the newfangled dash button TC? I am now more confused than ever.

herkyhawki · #6 03-05-2006, 11:04 AM

[quote=tylernt] After the vacuum actuator, I traced the cable down under the engine and across the engine bay to what I assume is the transfer case (this metal box that both front driveshafts connect to). QUOTE]
That is not the transfer case. It is the front differential. The cable is pulled by the actuator to engage the front axle. Simply put, this connects the front wheels to the front drive shaft. The front drive shaft is only driven when the transfer case is shifted to 4WD.

The transfer case is behind the transmission.

tylernt · #7 03-05-2006, 06:45 PM

Quote:

Originally Posted by herkyhawki

The transfer case is behind the transmission.

Ok, thanks for educating me. So at this point I'm going to replace the switch on top of the transfer case (which I have since read elsewhere is under the drivers seat or thereabouts), run vacuum to the TC switch, then run vacuum from the TC switch to the actuator and see what happens.

Rmbodie · #8 03-06-2006, 05:57 AM

I just replaced mine yesterday .
It is located on the drivers side on top of the transfer case . You will see where the the shifter come thru the floor just above it. The switch was $14.88 at Advance auto on the HELP product section . Be AWARE , when you remove it , fluid will start flowing out the hole . have a finger ready and the replacement to fill the hole. Park it downhill if posible first .
First , my vacuum harness was on wrong when I bought my blazer . It has 3 vac lines and an alignment pin . Check if it is on correct first ! Rob

tylernt · #9 03-09-2006, 11:02 PM

Thanks again for the help everyone. I found the switch, and I also got this diagram from a dealership. I'm posting it here in case it helps someone else in the future:

muddog321 · #10 03-10-2006, 04:54 AM

What years is this switch on 97 and earlier or 1st gen? My 99 and 01 have a vac electrical switch on the firewall to the front actuator but my 98 does not - need to look under the 98 for this. Anyone know? 98 appears to be a mix of 4wd from 1st to 2nd gen till GM used all the old parts up!

POST REPLY TO THIS THREAD