The Technical Archives

This page was last updated on $Date: 1999/01/07 01:51:02 $.

Overdrive Throttle Switch Adjustment

By Roger Moment
rmoment@aol.com

Problem:

My O/D engages fine, but doesn't want to disengage either for several minutes after the switch is flipped or until I cut the ignition for a second.

Answer:

Your overdrive may be working exactly right!!!!! The purpose of the overdrive throttle switch is to PREVENT the overdrive from shifting out unless you have about 1/3 throttle. Thus, if you flip the O/D switch with your foot off the throttle, the O/D should stay engaged. As you depress the throttle you should have it kick out. If you bypass or misadjust the throttle switch so that you kick out of O/D with your foot off the gas (i.e. with the engine braking using compression), you risk breaking the internal unidirectional clutch in the O/D and, because this consists of hardened steel rollers and bits, if it breaks the metal bits will thoroughly trash out much of your O/D internals including the transmission/O/D drive shaft, gears, rear O/D output rotating gear/shaft etc.

Read the shop manual carefuly about how to adjust the throttle switch. An ohm meter helps, along with a good ear and patience. The switch box actually contains a cam which is rotated by the lever that shows outside the switch box. You can rotate the cam too far so that its action is reversed from what it should be.

If you loosen the lever clamping screw and turn the shaft with a screwdriver (I strongly suggest turning the battery switch off since power to the ohmmeter will trash it and you will be working close to the fuse block and risk shorting to it with your screwdriver) you should see the resistance go from infinite to zero to infinite as you turn the screw counter clockwise. With the throttle off, and the lever at its top position, you should have low resistance (actually zero if you disconnect the two wires to the switch). As the lever moves downward (in response to applying throttle) there will be a point where the resistance jumps to infinite indicating breaking of an internal contact. This position should coincide with about 1/3 throttle and the clamp should be tightened onto the shaft.

You can check the setting by having someone depress the throttle and note the amount of travel when the ohm meter indicates breaking of contact.

I would guess that I have seen only one in ten O/D switches set correctly and that almost all failures of internal parts can be traced to cars shifting out of overdrive under engine braking because the switch was set wrong.

Please send comments, additions, and errors for the SOL Healey web pages to Bob Haskell.

[SOL Web ][ Scions of Lucas Home Page ][ SOL British Car Clubs Page ]