4wd diff lock not engaging

[idosubaru]

1989 XT6 manual

 

Push center diff lock button and center diff light in dash flickers but nothing happens.

 

Swapped solenoid in engine compartment.

 

Almost positive I manually put it in 4wd last year.

 

How do I test the big diaphragm on the side of the trans?

 

Is that easily replaceable, looks tight

[TOONGA]

The big diaphragm on the side needs full vacuum from the inlet manifold to operate. the easiest way to test it  would be to bypass the solenoid. With the engine running have a hose run from the diaphragm to the inlet manifold, if it is working the diff lock lever and cable will move if it isn't the lever and cable will not move.

 

Check the switch for the diff lock on the side of the box it could be that it is faulty.

 

With the stater motor out the  diaphragm is easier to get to.

 

TOONGA

Edited February 29, 2016 by TOONGA

[idosubaru]

Thanks Toonga.

 

One thing I'm uncertain about: if the lever is "already in" the position the diaphragm makes it move to - then it won't appear to move it, even if it attemps too?

[Numbchux]

There are a lot of different components to that system, first step is to isolate them as best you can to figure out the problem.

 

 

There's an electric switch, that operates those solenoids, that routes engine vacuum to a diaphragm, which pulls a cable, which moves a lever on the side of transmission, which moves a fork in the transmission, which engages the diff lock, and trips a switch, which lights the indicator light.

 

 

 

I would start by making sure it's actually not locking (you may have done this, but I'll say it just in case). Push the button, and make some tight turns to see if you can feel it binding. There's a chance the switch, or wiring for it, for the indicator light is failing.

 

Next thing I would do is bypass the solenoids. Connect the engine vacuum directly to one of the hoses that goes down to the diaphragm, and drive it a bit (shouldn't take much...IIRC my RX box would usually engage at idle), if that doesn't work, try the other hose. (Also, start the car with that disconnected, and put your thumb over it and see if there's good vacuum getting to the solenoids)

 

If that works, then check for voltage going into the solenoids (testing the switch, basically). If you've got power there, the solenoid(s) isn't working, if not, you have an electrical problem.

 

If connecting vacuum to it doesn't work, then I would jack the car up, and climb underneath, and see if I could get the locker lever on the side of the transmission to move by hand, that would tell you if it's a transmission problem, or the diaphragm/cable.

[idosubaru]

Chux - does the diaphragm move the lever both ways?   

I can see how giving it vacuum makes it move one way - but how does it go back the opposite way?

 

I forgot this:  the DIFF light in the dash will sometimes blink rapidly (randomly) while driving and then go off - like it's trying to engage. 

What actually triggers the DIFF light to turn on - does anyone know?  Because that might point to the problem - it's like it's trying to turn on but not quite?

 

It's not the switch in the center console...

[Numbchux]

Yes, there are 2 vacuum hoses from the solenoids to the diaphragm. Only one should get vacuum at a time, as they connect to opposite sides of the diaphragm, therefore pushing and pulling on the cable.

 

 

There is a switch on the transmission that is activated by the center diff lock mechanism (looks and functions almost identically to the neutral and reverse switch). That's what activates the indicator light. It is certainly possible that this switch is failing, or the wiring to it, which could cause all of the symptoms you're describing.