blackwolf
Member Since: 03 Nov 2009
Location: South West England
Posts: 17443
 
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The difflock light on the dash is not controlled by the ECU, it is an old-fashioned light operated by a switch on the transfer box. The switch itself is operated by the selector fork which moves the difflock dog clutch, and therefore the switch is only activated when the selector fork moves to the engaged position. The slector fork, for obvious reasons, is spring mounted on the selecotr shaft, so does not necessarily move when the lever in the cab is oeprated.
If the lever in the cab is moved to the difflock position at a moment when the dog clutch splines do not line up, the selector shaft moves, compressing the "engage" spring which is fitted between a collar on the shaft (which has moved) and the selector fork (whcih slides on the shaft, but which cannot move since the splines are not aligned). As soon as relative movement between front and rear outputs takes place, the splines will align and the dog clutch will spring into the locked position, driven there by the compressed "engage" spring. At this moment, the movement of the selector fork should activate the warning light switch and the light will turn on.
When you move the lever to the disengage position, the reverse happens. The shaft moves, and a collar on the shaft compresses the "disengage" spring. The dog clutch may not be free to move, however, because transmission wind up may be resulting in sufficient friction on the dog clutch teeth to mean that the spring is insufficient to overcome the friction and disengage the clutch. The driver then takes the usual steps to unwind the transmission, and once the friction has reduced to an overcome-able level, the spring pops the dog clutch out of engagement. At this moment the warning light switch is deactivated by the movement of the selector fork and the light goes out.
It is not unusual for the light switch to "gum up", since it is in a fairly hostile environment, especially if the the difflock is seldom engaged, and this may cause the light to fail to illuminate, or more commonly, to fail to extinguish. It also sometimes happens that the switch will operate, but sluggishly. It is also important that the switch is properly adjusted on installation and checks are made that it does turn on and off correctly.
The most common reason for a delay with the light coming on or going of when engaging or disengaging difflock is the time it takes for the dog clutch to engage or disengage. This is however a normal characteristic of this type of mechanism.
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13th Sep 2016 6:27am
