Hello guys and gals;

I have a 96 4 door Blazer LT.

Issue: Front axle will not engage.

What I've done:
1, Replaced the small gear and thrust washers on the front pass. side axle.
2, Replaced EVERY vacuum line.
3, Replaced the 4wd actuator under the battery.
4, Replaced the t-case vacuum switch.
5, Replaced the T-shaped vacuum check valve by the vacuum reservoir.

Now, what happens when I push the 4 Hi button:
1, T-case engages, i.e., front drive shaft will not spin by hand, but will in 2wd.
2, I can manually pull the cable and it will engage at that point, but as soon as I release the cable, it's no longer engaged.

The problem is I'm only getting 5 psi of vacuum on the line that goes to the actuator. everywhere else in the system, I'm pulling over 40. Even at the 3-way plug on the t-case connector, I'm pulling 40. So, is it possible that I just got a dud switch right out of the box from Dorman? If not, I'm at a loss....

Thanks for everyone's time who reads or responds.

Here's what the original gear on the axle looked like before replacement.

 

Nothing special about this issue. It is a vacuum leak, plain and simple. Either a dud switch, a leak in the line running from the switch to the actuator, or a torn actuator diaphragm. You can rule out the switch by placing only a short piece on the connector where the vacuum line to the actuator hooks on and run that to your vacuum gauge. You can check the line and diaphragm by using a hand vacuum pump to pull a vacuum on the line & actuator at the switch connector. You can test the actuator by connecting your vacuum pump directly to the actuator.

 

Thank you for your reply, but there are no vacuum leaks. As I said in my post, I replaced EVERY vacuum line, the actuator and the switch. I'm only getting the 5 psi at the outlet nipple on the switch...I did check it right at switch to rule out the new line that runs up to the actuator before I posted this.

I guess my real question is; could whatever it is that makes contact with the plunger on the switch have worn to the point of being ineffective? Can't tell from the exploded view of the 233 what exactly makes contact with the plunger.

Again, thank you for your reply.

 

You sure that's PSI? Shouldn't be able to get over 14PSI vacuum as that is what atmosphere is at sea level. You aren't reading positive PSI right? Or is it a different measurement? Just checking to make sure it is actually at vac and not above atmo.

 

You're right.... Lol. Gauge is in centimeters of mercury.

 

I simply listed possibilities for the source of your problem. Replacing vacuum line is all fine and well provided that it is not leaking at the ends or somewhere between. No sense chasing other potential problems until you confirm that what you have done is in proper order rather than assuming that it is because you replaced it. Of course, confirming that what you replaced wasn't working is better...

I did leave out an improperly sealing connector at the switch as one of those potential sources. You can rule this out by using some vacuum grease on the connector and the nipples on the switch to promote a better seal. If that is found to be the issue and vacuum grease does not work long term, a new connector can be purchased and installed.

A problem with the cam inside the transfer case would cause the switch to simply not work. Even a partially engaged switch that is capable of pulling 5 cm-mercury of vacuum should draw down to a constant vacuum consistent with the other readings provided it is allowed to pull for a time against a properly sealed system. It shouldn't result in a constantly low reading.

 

pinched or collapsed hose maybe? Maybe you should consider going to the manual cable conversion.

 

So, did some more things today;
1, totally bypassed the HVAC system while performing the following....
2, disconnected the line at the actuator and ran vacuum down it from the reservoir. Pulled measurement at the 3-way fitting at the switch and have 40 cm of mercury.
3, pulled t-case switch out and put PTFE paste on the threads thinking that maybe the switch is leaking there...it wasn't.
3, pulled the switch out and applied vacuum to the ports directly and upon depressing the button on the switch, I get 40 cm of mercury on the actuator side!!!!

So, I have come to the conclusion that either the Chinese didn't make the plunger part of the switch long enough, or whatever pushes on the button inside the t-case is worn to the point of being useless.

Does this sound logical to you gents?

Thanks for your replies.

 

So, for my immediate needs, I've put a T into the line that goes to the vacuum reservoir and ran another length of hose over to the actuator, so when I want to engage my 4wd, I pull the vacuum cap off the T I put in and connect the line running over to the actuator. It works, but I'd much rather be lazy and have the system work by me pressing a button.