The Technical Archives

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

The Austin-Healey Overdrive System

A Revealing Expose!

Care and feeding

As is the case with most of the mechanical components of the Healey, the overdrive unit is of very heavy-duty construction and certainly up to the task demanded of it, given reasonable care. At least annually, drain the transmission and overdrive units and refill with non-detergent oil if available. Note that there is a drain on both units, although they share a common oil supply. A filter is incorporated in the drain assembly of the overdrive unit, and should be cleaned at this time. Depending on the model, there may be a series of magnetic rings below the filter to catch metal particles and these should be thoroughly cleaned before replacing the drain and filter. The capacity of the transmission and overdrive oil sump is 3.75 US quarts. The overdrive unit will retain approximately 1/6 of a quart after draining, so fill with about 3.6 quarts. On center shift models, a dip stick is incorporated with the fill plug, so after refilling, drive around the block and recheck the level.

Theory of Operation

The majority of problems experienced with the overdrive unit are electrical in nature. This is especially true if the unit will not engage. Fortunately, the electrical circuit is pretty simple, consisting of only 5 components. These are:

1. The drivers switch (A), mounted on the dash.
2. The throttle switch (B), mounted on the firewall.
3. The relay (C), also mounted on the firewall.
4. The gear switch (E), mounted on the transmission.
5. The solenoid (F), mounted on the overdrive unit.

The letters, A, B, C, E, and F correspond to the accompanying diagram, stolen from the Bentley manual.

The operation is as follows. Turning on the drivers switch (A) energizes the relay (C) and applies voltage to both the throttle switch (B) and the gear switch (E). Lets ignore the throttle switch (B) for a moment, and go on to the gear switch (E). The gear switch has only one purpose in life; to make sure the transmission is in either 3rd or 4th gear before allowing the overdrive unit to engage. If this is the case, the solenoid will energize, moving a lever which opens the hydraulic operating valve inside the overdrive unit. Thats all there is to engaging overdrive. But what about the throttle switch we ignored? It is part of a self-holding circuit for the relay. It's purpose is to not allow the overdrive to dis-engage under a deceleration condition which would cause a shock load to be felt by the transmission. When you turn off the drivers switch, the overdrive should not disengage unless the throttle is at least 1/5 open, if the throttle switch is properly,adjusted. That, my friends, is all there is to the entire system.

Testing

You will need a voltmeter and have the transmission tunnel removed.

1. With the ignition on (engine not running), turn on the drivers switch. You should hear the relay click. (Make sure your stereo is off) If you hear no click, measure the voltage at terminal A3 of the fuse block. It is very likely that you will have voltage at this point, since if the fuse were blown the fuel pump would not work and the overdrive operation would seem insiqnificant. Actually, this just makes sure you know how to use your meter. Next, measure the voltage at terminal Wl of the relay. If you read zero, either the drivers switch is bad, or the wiring going to it or coming from it is bad. If you read 12 volts, measure terminal Cl of the relay. Zero here means a bad wire from the fuse block. If you read 12 volts, measure terminal C2 of the relay. Zero means a bad relay; 12 volts mean it clicked and you weren't paying attention.
2. If the relay is ok, go back inside the car and check the gear switch. With the transmission in 3rd or 4th, you should find 12 volts on both terminals of the gear switch. If you find zero on both sides, check the wiring from the relay. If you find 12 volts on one side only, you have a bad gear switch. You also have a bad gear switch if you have 12 volts on both sides in 1st gear. (Switch shorted).
3. If the gear switch is ok (12 volts on both terminals), the solenoid should now be engaged. If not, examine the wire from the gear switch. If it checks out alright, you have a bad solenoid.
4. To check the throttle switch, it is best to use the ohm function of your meter. Turn off the drivers switch and the ignition switch. To extend the life of your meter, leave it on the volts DC function and measure both terminals of the throttle switch to make sure there is no voltage present. This step is especially important if you have borrowed my meter. Now change the meter to the ohms function and measure across the throttle switch. With the throttle against the idle stops, you should read a short (zero ohms). Leave the meter attached and slowly open the throttle. At about 1/5 open, the meter should swing to an open (infinite ohms).

Adjustments

There are 2 adjustments to the electrical circuit; the throttle switch opening position, and the solenoid lever position.

First, the throttle switch. If an adjustment is required, loosen the clamp nut on the lever on the side of the throttle switch. Move the throttle linkage to the 1/5 open position and block it. While measuring the ohms across the throttle switch (again, with everything turned off!), place a screwdriver in the slot cut into the end of the throttle switch shaft and rotate slowly until the it is right at the point of the meter trying to make up its mind which way to go. Carefully lock the nut on the clamp and remove whatever you blocked the linkage with. Remove the meter.

Secondly, the solenoid lever. Locate the alignment lever on the opposite side of the overdrive unit. This lever is attached to the shaft which passes straight through the overdrive from the solenoid lever. The other end of the alignment lever has a 3/16" hole drilled in it. This hole should line up perfectly with the 3/16" hole behind it on the overdrive housing. (note: if there are 2 holes in the housing, use the top one). If adjustment is required, loosen the clamp bolt on the solenoid lever and move the alignment lever until a 3/16" rod will pass through the hole in the lever and into the hole in the housing. With the solenoid energized, lock the clamp bolt and remove the rod from the alignment lever.

If the electrical system checks out, and the adjustments have been set properly, the overdrive unit will probably function satisfactorily. If not, first rent a porta-crane....

Turn on ignition, O/D dash switch and have car in 4th gear.

1. Overdrive works fine? Yes - Have a beer and relax!
2. No - 12 volts at A3 of fuse block? No - Blown fuse.
3. Yes - 12 volts at W1 of relay? No - Bad dash switch or wiring.
4. Yes - 12 volts at C1 of relay? No - Bad wire from fuse block.
5. Yes - 12 volts at C2 of relay? No - Bad relay.
6. Yes - 12 volts at supply side of gear switch? No - Bad wire from relay.
7. Yes - 12 volts on both sides of gear switch? No - Bad gear switch.
8. Yes - 12 volts at solenoid? No - Bad wire from gear switch.
9. Yes - Solenoid energized? No - Bad solenoid.
10. Yes - proceed to the next section.

Check adjustment

To check operation of throttle switch:

Ignition switch on, car in 4th gear, flip on O/D dash switch. Solenoid should energize (audibly detectable), if not go back to top of this sheet! If it does, turn off O/D dash switch while placing no pressure on gas pedal. Solenoid should not de-energize. Now slowly depress gas pedal. At about 1/5 throttle, solenoid should de-energize. If so proceed to step 1 (Overdrive works fine), if not refer to throttle switch check-out and adjustment in the text.

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

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