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Jaguar XJ-S Bellcrank Microswitches

Jaguar XJ-S V12

Throttle Bellcrank Microswitches

There are two microswitches on the Jaguar V12 throttle bellcrank pedestal:

The red microswitch at the right in this picture is connected to the EFI ECU.  Its roller rides on a cam on the bellcrank itself, and it is actuated as the throttle passes somewhere around the half throttle point.  This takes the EFI out of "closed loop" mode and puts it into "open loop" mode.  Closed loop uses the oxygen sensors to hold the fuelling very close to stochiometric, but stochiometric is not rich enough for high throttle operation; pistons may be damaged.  So, with the EFI operating in open loop mode, it operates on a built-in fuelling map -- which is deliberately rich enough to operate safely at full throttle.

The other microswitch operates the kickdown on the GM400 automatic transmission, forcing it to downshift.  This microswitch is under that metal cover on the left with the green wire connected to it -- there's supposed to be two wires connected to it, but this is Kirby Palm's car with a 5-speed in it so the switch is nonfunctional.  You can see the same two-screw mounting as on the red microswitch.  This microswitch is operated when the throttle pedal is pushed to the floor with conviction.  As the throttle cable is pulled, it rotates the bellcrank until the protrusion at the right side in this picture comes around and hits the stop screw in the center of the picture.  As the pedal is pushed farther, the center of the cable cannot move any more -- so the housing of the cable moves instead.  The end of the housing, coming in from the left in this picture, pushes against a stiff spring and operates this microswitch.  The spring in the end of the cable housing is stiff enough that the driver can choose whether or not he really wants the transmission to kickdown; he can press the pedal down and apply full throttle without engaging the kickdown if he so chooses.

It's a fairly simple matter to make sure microswitches work as intended.  You can check their operation with a VOM or even with a battery and light bulb.  If they aren't clicking right, they can usually be enticed to move by loosening the screws, fiddling with their position, and tightening the screws back down.  On microswitches such as these, with attached levers with rollers at the ends, the levers can be bent a little one way or the other.  Finally, if the microswitch is toast, they are easily replaced; microswitches with the same type roller levers are available at any electronics supply outlet.

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