Hi all,

I consulted the 4wd no worky thread, and it helped me understand how the system is supposed to work, but didn't mention the situation we have on this blazer. It's not my blazer, but a friend's, who luckily has a heated shop with a lift to work in.

Here's the deal: He has push button 4wd. When you push any button, you hear an immediate solenoid-type noise from under the truck, and the light for whatever drive mode you chose comes on pretty much immediately, with no flashing involved.

Looking under the truck with it up in the air, both driveshafts turn when 4 low or 4 hi are selected. The front wheels do not turn. There is 18 inches of vacuum at all times applied to the vacuum actuator underneath the battery. (We pulled the battery and tray and used jumper cables to start and run the truck.) Note, I said there is vacuum applied AT ALL TIMES to the vacuum actuator, no matter what drive mode is selected on the dashboard. However, at NO point does the front axle ever engage, even when trying to "help" the actuator by pushing the diaphragm in further. There are also no grinding noises from the front axle indicating partial engagement. The diaphragm on the actuator is not leaking, as shown by me hooking a hand vacuum pump with a gauge to it and testing it.

Going underneath the truck, I removed the 3 bolts that hold the cable to the front axle, and managed to pull something partly out of the axle, simulating what the cable does when vacuum is applied to the actuator. None of this engages the axle. The thing I pulled partly out doesn't come very far out, maybe 3/4". While I had this part unbolted and loose, I moved the cable sheath towards and away from the axle, verifying that the cable slid easily in and out of the sheath.

I think there are at least two problems here-constant vacuum applied to the actuator, maybe caused by a bad vacuum switch on the transfer case, and whatever is causing the front axle to not engage. I asked the guy that owns it if it's possible it has been stuck in 4wd because of the constant-vacuum issue and maybe broke something in the front axle. He said that he knows what it feels like in 4wd, and that he can definitely tell when it's in 4wd going around a turn. The guy is no dummy, so I doubt that constant vacuum caused the non-engagement issue due to axle damage.

I'm kinda stumped. Any ideas?

 

There is a gear on the front axle that is engaged when vacuum is applied to the actuator under the battery.
This gear can get stripped. It could be this, especially if it had been stuck in 4wd all this time.

 

Possible, but I don't think the guy would have failed to notice constant 4wd. He's pretty perceptive mechanically, and mentioned how it immediately feels different when it's in 4wd. In case it is that gear, how does one go about replacing it? Also, roughly how far should the part on the axle moved by the cable move when pulled?

 

Hmmmm. If there was constant vacuum to the front axle, then something could have broken due to it being if 4wd and with that broken part, it can no longer engage even with the vacuum applied.

Can the yoke for the front axle engagement break without the engagement gears not first grinding away?

 

Are you talking about the pinion yoke? By the way, even with constant vacuum applied to the actuator, the transfer case will disengage the front driveshaft in 2wd, so it wouldn't actually be in 4wd all the time.

 

Anybody got any ideas? Still haven't figured this one out.