The McNamara vacuum diff locks use a cabin mounted L.E.D. (light omitting diode) system. The L.E.D. omits two colors - red & green. When the diff is in the unlocked position the L.E.D. is green. When the diff lock is selected the L.E.D. will change color to red. Only when the diff lock mechanism is fully engaged will the color red appear. Also the green light will only appear when the diff lock is fully unlocked. Limit switches positioned at each end of the vacuum actuator reverse the polarity of the electrical currant through the L.E.D. to change color. By observing the L.E.D. the operator is unlikely to ever cause damage to the entry of the locking dog teeth. Full power or cornering can safely be negotiated once the L.E.D. light is either red or green and working inconjunction with the vacuum switch. This system gives the operator total control when selecting or unlocking the differential. The electrical control box is mounted under the bonnet. In the box there are 4 terminals +, -, D, D. The + terminal uses any 12 Volt DC connection that is fused and is switched off with the ignition, (for example - cigarette lighter). In the box, the + is connected to both D terminals by resistors (560 ohm's). Outside the box, both the separate ends of the L.E.D. connect to the separate D terminals, as do the limit switch wires. The - is an earth wire which can be connected under the pop rivet that holds the control box in place. From the differential and earth wire connects to this outside terminal. Warning: Never test the L.E.D. by using the +12 Volt terminal, this will blow the L.E.D. If testing is needed, use either D terminal inconjunction with the - (earth).