Syncro Locker Knob: Explained

When I pull my locker knob, why is there a delay with the "on" indicator light? Is it broken?

The knob for the rear locker (and decoupler and front locker, if you also have those) and the indicator light are on two completely different systems. That is why the light does not necessarily come on/off at the same time the knob is pulled out or pushed in. The knob is part of a purely pneumatic system, and the light is part of a purely electro-mechanical system.

When you pull the knob, you are pulling on a slide valve that diverts vacuum from one side of a diaphram servo on the transaxle (or front differential) to the other side. Once the servo gets the "lock up" signal, it provides force to the engagement fork inside via a push/pull rod. If the internal locking parts are not lined up, it will not lock immediately, and the light will stay off. The servo will simply keep pressure on the push/pull rod until things line up internally. As long as there is no relative motion between the two side-to-side wheels (in the case of front or rear locker) or the front-to-rear axles (in the case of the decoupler), engagement will never take place—as long as the internal locking parts are not lined up. When on dry pavement, there will be no slippage happening between the tire and the pavement. So, you can go on down the road in a straight line forever and never experience engagement. NOTE: This isn't necessarily the case with the decoupler, because the internal locking parts have much finer increments. Thus, lock-up is almost instantaneous. But with the differential locks, as soon as you steer the vehicle into a constant turn in either direction, the wheels start to move relative to each other, the internal locking parts eventually line up, and engagement takes place. 

Once that happens, the aforementioned shaft and fork move a little bit further and the internal parts engage. There is a spring-loaded electrical "engaged" switch located perpendicular to the shaft. Once it has engaged, and the switch moves far enough, the tip of the switch drops into a notch in the shaft. That completes the circuit inside the switch, and the light turns on.

Once this is engaged—and you are still turning—the forces on the locking parts are pretty high. Be careful if you are trying this on dry pavement. You can damage an axle... or worse. When you push the knob in, assuming you're still turning, the servo cannot produce enough force to overcome the frictional forces between the internal locking parts to disengage the locker. That is, the servo applies constant force to the shaft in the "pull" direction until the load is removed. In this instance, the reverse takes place: the "engaged" light stays on after you have pushed the knob in. As soon as you steer the vehicle the other way, the forces on the parts relax, the servo can move the shaft and fork attached to it, the switch comes out of the notch, and the light goes out. 

Looking to read more about differentials? We recommend reading our "Differentials" article.

 

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