2000 gmc Jimmy SLT. Vehicle is crab hopping on tight turns even though selector says 2WD. I recently replaced vacuum lines in engine compartment (completely shot) to get HVAC working including supply off the manifold under air intake, to the reservoir and up to the firewall mounted 4wd vacuum switch.

I did not replace any lines leading out of the vacuum switch that go down to the actuator or pull battery yet so I have no idea yet what condition any of that is in. I'm suspicious that the vacuum switch is broken and allowing some vacuum to get through to the actuator causing 4x4 engagement. I don't think 4X4 could have been working before due to the condition of the other associated vacuum lines off the manifold and to reservoir etc..

What is a reliable way to disable the system so it goes to 2WD until I can investigate? Can I pull and cap the firewall vacuum switch line that goes to the actuator or do I have to go after the TCCM (wherever that is)>

 

Flip the hoses on the front axle vacuum switch ports. It is trapping vacuum when you go from 4WD to 2WD. To prove it, pull the hose to the actuator off the vacuum switch.

 

Even if you have vacuum to the front axle actuator and the axle is engaged, it shouldn't really feel much different unless the transfer case is in 4WD. It sounds to me like you may have another problem.

 

Thanks Les. I saw that you recommended switched lines in other threads. IDK what transfer case I have (vehicle brand new to me) and/or if there are different switches used but it may be that they're on wrong. I read through Christine's vacuum hose thread ( good post/thread) and see the way the lines are supposed to go. Can someone confirm that the colored hose diagram in Christine's thread is the right orientation?

 

So I searched the forum to find as many example pics and diagrams of the hose configuration to the vacuum switch as I could and mine were reversed. Changed hoses around and I don't feel any more binding on hard turns so chalk up another one to LesMyer, thanks!

One thing I don't understand, while making the change I did a little checking. I felt good vacuum at the source hose going to the switch and once installed to the correct port switch (lower) I get vacuum through the switch at the other port. Temporarily put the source hose to the wrong (upper) port and got vacuum to the lower. So if now matter where I put the hose I get vacuum to the opposing port then how does that solve the problem and how does the switch work?

I also notice that what I believe to be the vent on the switch (dead center on top of cylinder) also pulls vacuum. I ask because I have not yet started to check if 4WD system is working and it would help to understand how it all works.

 

Do you have 3 or 4 buttons on the dash?

 

This is the 4 button. I have read on the forum that only the 4 button has the firewall switch and that the 3 button has the three hose vacuum switch on the T-case. Pretty sure that's correct but I've been wrong before!

 

That is correct, that's what I was looking at on the firewall. Was wondering lol

I have thw 3 button and don't have that. Is what I was getting at lol

 

So my front wheel binding issue is back. I currently have the vacuum hose from the firewall actuator to the diaphragm off and capped. Still feeling a bind so either that's not enough to disable the 4x4 or it's stuck.
can anyone confirm that when the system is set for 2wd that the firewall actuator should not be applying vacuum? The problem came back after a long drive. The little bit of vacuum passing through the actuator may have slowly pulled the diaphragm but that's a guess and should have gone away after I disconnect it.

 

Well I found a problem that will probably be of no help to anyone but zombie threads are even more useless so here goes.

As I said above I removed the hose from the firewall actuator to 4wd diaphragm to no effect, still getting wheel hop/binding on tight turns. Coincidentally I threw a code for the air pump (secondary air P0410). Decided I needed to check the diaphragm and cable so I pulled the battery/tray and after much tracing and checking I found the vacuum hose that was supposed to be going to the diaphragm from the solenoid was actually connected directly to manifold vacuum and the vacuum hose for the air pump was connected to the actuator instead. This of course explains why removing the hose from the actuator had no effect and may solve my p0410 code problem as long as the air pump is supposed to be connected to manifold air.

I'll take the rap for possibly switching the hoses on the actuator but not this so somebody was monkeying around. It was amazing to me how well the system held vacuum (car off for at least an hour) as the diaphragm would not release the cable even with vise grips until I pulled the hose off the diaphragm. I began to assume to cable or diaphragm was rusted in place but now that the vacuum has been broken it moves freely

So in any case the diaphragm relaxed and the cable is free. There's no way the system was working at all before I got the truck due to the condition of most of the hoses so it wasn't driven like that for too long. I just hope I didn't do too much damage having the system connected to a direct source of vacuum for about 150 mi on and off over the last couple days. I'm leaving the diaphragm hose disconnected from the actuator until I can confirm the actuator is functioning.

Anyone please chime in and let me know if your firewall actuator pulls a little vacuum on the diaphragm port.even in 2WD.