D-Type (MkI) |
Hover over a wire to confirm the colour
Schematic:
When the manual switch is closed the relay is operated, and the relay contact energises the solenoid via the gearbox switch if closed, and overdrive is engaged. At this point the condition of the vacuum switch - open or closed - is immaterial.
Assume now that with overdrive engaged the engine is doing high rpms but the throttle is closed i.e. it is on the overrun. This will create a high vacuum in the inlet manifold which will close the vacuum switch. If the driver now opens the manual switch, the vacuum switch being closed will continue to maintain a 12v supply to the relay winding from the relay contact, independently of the manual switch, so the relay remains operated, the overdrive remains engaged, regardless of the fact that the manual switch has been turned off.
Imagine now that either the speed of the car has slowed so that the engine revs are closer to idle, or the clutch is dipped so allowing the revs to fall to idle, or the throttle is opened again. In all cases the vacuum in the inlet manifold will reduce, allowing the vacuum switch to open. This causes the relay to release (the manual switch is already open) and its contact disconnects 12v from the overdrive solenoid so disengaging overdrive.
Update January 2008 I was under the impression that the vacuum switch was to prevent OD being disengaged under conditions of "high manifold vacuum" (Leyland Workshop Manual) i.e. to prevent high reverse torque from damaging the OD. But info from Bruce Cunha indicates that the vacuum switch only opens at manifold depressions lower than 7 in. Hg., and so OD could only be switched out if accelerating significantly, i.e. a bit like a kick-down on an auto box. But the Workshop Manual talks in terms of it delaying the change "until the engine takes up the drive", which could still be significantly above 7 in. Hg. Subsequent discussion with the designer of the transmission system for the MGB confirmed that the vacuum switch was indeed designed to prevent disengagement unless the car was accelerating, but to give a smoother disengagement rather than to prevent damage. However they found the vacuum switches were unreliable and so deleted them opting for 'driver education' instead, however my 73 Drivers handbook says it can be engaged and disengaged accelerating or decelerating, and not to depress the clutch while doing either.
Of course, if the gearbox should be taken out of an overdrive gear the gearbox switch will ensure that overdrive is disengaged instantly, regardless of the position of the manual or vacuum switches. 'Normal' gear changes, say from 3rd to 2nd, will usually allow the overdrive to disengage safely and not encounter the mechanical stresses that the vacuum switch and relay are designed to avoid.
The vacuum switch on its own cannot operate the relay and so cause overdrive to be engaged, the manual switch must be closed first.
The other thing to be aware of is that the manual switch will operate the relay, and the vacuum switch will keep the relay operated under conditions of high manifold vacuum even if the manual switch is turned off, when the gearbox is not in an overdrive gear. All this means is that when an overdrive gear is selected the solenoid will be energised and overdrive engaged as normal.
The later LH-type overdrive does not have this vacuum switch and relay, presumably the designers feel it is strong enough to take disengagement under conditions of high manifold vacuum without damage.
It should be noted that if you intend to fuse the overdrive circuit to protect the wiring, both the white feeds from the fusebox to the manual switch and the relay must be fused.
The location of the overdrive and reverse light switches:
