Overdrive Operation and Repair
Overdrive Operation and Basic Trouble-shooting BN4-BJ8

With some assistance from our schematic circuit diagram, we can examine how theoverdrive electrical circuit looks under various scenarios. This is also a good procedureto use for checking operation of the electrical system in the event the overdrive isn‘t working.

As shown in Diagram B, with the ignition on, the engine not running, and the gear leverin third or fourth ,when the dash switch is moved from NORMAL to OVERDRIVE, you should hear a distinct click from the overdrive as thesolenoid is energized and the plunger is pulled up. If you don’t hear the click, then some component of the electrical system is faulty, and you’ll need to check each component in turn as discussed below.

If you do hear the click, but the overdrive doesn’t work when the car is driven, then the problem is in the overdrive itself, which will have to be checked by a qualified specialist.

Diagram C illustrates what happens when the dashboard switch is turned off while the throttle pedal is less than one-quarter pressed. The throttle switch points are closed, keeping the solenoid coil energized, maintaining contact between the points on C1 and C2 so that current continues to flow to the solenoid. The overdrive will stay engaged
until such time as you depress the throttle pedal enough to open the throttle switch points, thus de-energizing the circuit - as shown in Diagram D. You can use this feature to anticipate down-shifting out of overdrive. For example, if you are driving on fairly level ground, you can flip the dash switch to NORMAL and still maintain your speed
using only a very light throttle. The overdrive will remain engaged. When you are ready to shift out of overdrive, merely depress the throttle about one-quarter and the transmission will immediately down-shift out of overdrive. This procedure can be handy when exiting a freeway, as it only takes a light “blip” of the throttle as you enter the off ramp to drop out of overdrive.

To check the throttle switch adjustment -  with the engine off and the ignition turned on,flip the overdrive switch on. Then, with your foot off the throttle, flip the switch to NORMAL. Now flip the switch back on again. You should not hear the overdrive solenoid click, as it should not have been de-energized when the switch was flipped off.
Next, repeat this above sequence, except this time depresses the throttle about one- quarter before flipping the switch on for the second time. You should now hear the solenoid energize, as depressing the throttle should have opened the throttle switch points.

A more accurate and direct way to check the throttle switch setting is to disconnect the two wires and put an ohmmeter across the terminals. The reading should go from zero ohms (throttle less than one-quarter depressed switch points closed) to infinite when the points open.

To adjust the throttle switch, loosen the clamp bolt on the lever arm. The throttle switch has an internal spring which helps in setting it. With the shaft free to be positioned by this spring, the cam should be oriented so that the points are closed.
You can check this by using a screwdriver to rotate the shaft by its slot. Turning an equal amount clockwise and then counterclockwise you should find a position in each direction where the resistance becomes infinite - the points are now open. Position the shaft about half way between these extremes, tighten the arm clamp, and use this as a
starting point for adjustment. Observe the throttle position when the resistance changes from zero, and make adjustments to the shaft position until this occurs at about one- quarter throttle.

Diagram E illustrates what happens if the transmission is shifted down into second or first, as for example if you have to slow down on the highway in a traffic jam. In this case, there is still power flowing through relay coil and consequently through the C1 and C2 points but the overdrive solenoid circuit is open at the gear shift switch,
preventing the power from getting to the solenoid coil. As soon as you shift back intothird, the overdrive will re-engage.

Further Electrical Trouble-shooting

The gear shift switch operation can be checked as follows. Disconnect the ground wire from terminal W2 on the overdrive relay and connect a jumper wire (with alligator clips at both ends) from terminal A3 on the fuse block to C2 on the overdrive relay.
Switch on the ignition (but do not start the engine) and move the gear shift lever back and forth from the three-four slot to the first-second slot. Each time the lever is moved over to the three-four position, you should hear the solenoid click as the plunger is pulled up. If you don’t hear that click, then the throttle switch is faulty. Before continuing with the following additional tests,  remove the jumper wire and reconnect the ground
wire to W2. If the gear lever switch is working properly, to check the overdrive relay, turn on the ignition and shift the transmission into third or fourth gear. In the engine compartment, attach one end of a short jumper wire from terminal W1 of the relay and touch the other end to fuse post A3. If you don’t hear the relay respond, the internal coil is bad. You can also sense operation of the relay by feeling the cover for vibration. If you verify the relay is working, then go listen for the overdrive solenoid to click while someone else touches the jumper wire to the terminals. If the solenoid doesn’t operate, then the relay points could be dirty or damaged and not making good electrical contact.
Once you have verified that all the other components are working properly, you can check the dash toggle switch. With the engine off, ignition turned on, gear lever in one of the top two gears, and throttle pedal released, flip the toggle to OVERDRIVE. You should hear the solenoid click. Flip the switch back to NORMAL and turn the ignition
switch off (to de-energize the solenoid even if the throttle switch is not set correctly). Re-test by turning the ignition switch on followed by the toggle switch. If the overdrive should stop operating while you’re out on the road and you can determine that it’s an electrical problem rather than a problem within the overdrive itself, it may be tempting to bypass the offending component(s).
However, since these components are designed to protect the overdrive, this may notbe a good idea, since it can be considerably more expensive to replace the overdrive should that fail as a consequence of one of the override switches not working.
Your engine can quite effectively handle highway speeds even without overdrive. Though the higher rpm may be a little wearing on driver and passenger, the car will getyou to a place where you can trouble-shoot the system properly and properly replace a failed electrical component.

                                         Overdrive Throttle Switch Adjustment

With the summer season in full swing and everyone is out enjoying their Healeys on long summer drives, the overdrive becomes very important. Without it, highway driving becomes noisy and very fuel consuming.  Correct adjustment is critical.  One often overlooked switch in the overdrive system is the throttle switch.
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.

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 carefully 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.
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.