So my 4WD is not working, I troubleshoot and it looks like the actuator is getting no vacuum. Well at least that's what I'm thinking atm since it looks fine. So I read all I could find about the problems with the system, trying to figure how to fix it, and the vacuum switch located at the xfer case struck me as a particularly horrible engineering idea. I like the posi-lock cable eliminating all this crap but I don't want to spend the money for it so I was thinking about a cheap way to at least eliminate the problematic switch. Then I remembered a while back when I had a turbo project car I repurposed stock boost solenoids for a similar purpose (controlling dual runner intake valves). So I realized this would be a very simple solution. Just get a cheap vacuum solenoid that's normally closed, put it somewhere on the firewall close to the T that feeds the switch on the x-fer case, run the vac line from the T to feed it and then the output to the actuator, and run 2 wires to the interior where you'd mount a simple electric switch to activate it. Or if there's a relay that's activated when you press the 4hi/4lo buttons then the positive could just be fed from that and it would work like stock. Sure that still leaves the actuator as a failure point but it's easy to replace and your whole cost is just the solenoid. And you eliminate the possible contamination of the vacuum system with ATF. Sounds like a fairly simple (if partial) solution to something that's been a problem for a lot of people so I wonder if somebody already thought of it? Anybody can think of any potential problems?

EDIT: for example this should work

 

Why not just use the electro-vacuum switch that is used on 99 and newer Auto 4WD? Two vacuum ports, normally closed, with two electrical terminals. That was GM's answer to eliminate the mechanical vacuum switch on the transfer case😊

 

Is this what you're talking about?


probably would work too, but the OEM equivalent part would be more expensive and maybe it works a bit different since there are only 2 ports and you need a solenoid that switches between vac_source->actuator and vent->actuator so that the actuator would disengage quickly. Or maybe this part has a built-in vent? I don't really know how it works.

 

on my old 83 i got sick of changing that vacuum switch every years so i actually just ran a valve inside the cab, you turn the dial to engage 4x4 and you just push the button to release the vacuum to disengage.

on my 85 i just put a pair of vice grips on the cable to the front end, it was a offroad truck so it stayed in 4x4, never failed

 

GTTK, Yes that's the switch that's used on Auto 4WD. Two terminals: One positive, one negative, doesn't matter which way you connect them. You could probably go to a boneyard and clip the connector, then solder it on yours. A SPST, (on/off) toggle switch on the dash would do the trick. Not sure if/where you'd find a circuit that is hot only in 4WD. If you can find one, switching would be automatic and you wouldn't need the toggle switch. Possibly at the TCCM???

 

Today I ran a new vac line from the actuator to the T on the driver side, disconnected the line that goes to the t-case and plugged the new line there, and my 4wd works like that. Once I get the solenoid and attach it I'll let you know how it works.

The vac lines were somewhat wet, I assume it's ATF...

 

Applying manifold vacuum to the actuator under the battery will keep the front differential engaged all the time. Might want to put a check valve in the line. On hard acelleration, manifold vacuum drops which will cause the actuator to release, possibly damaging the clutch gears in the front axle.

 

I did not connect it to the manifold. The T I mentioned is after the check valve near the driver side fender. I disconnected the line that normally goes the the t-case and plugged my new line in the same nipple. Actually it stays engaged even after I turn off the engine. At any rate I'm not driving it until I fix some other things and I should have the solenoid in a couple days.

 

Got it. Provided the entire HVAC system, the line to the actuator, and the actuator are leak free, the differential should remain engaged indefinitely. It won't cause any problems other than fuel mileage will suffer slightly.