From MGB Driver - The North American MGB Register publication
All MGBs 1968-76 were fitted with the overdrive switch and wiring. A fore or aft motion of the wiper switch stalk engages or disengages the unit. If overdrive is not installed, this switch makes a wonderful anti-theft device (see end of article). Midget wiper switches can be and are sometimes fitted to the MGB and these do not have the overdrive function.
Some MGBs 1977-80 were fitted with overdrive, and these have a switch in the gear knob. Some of these MGBs had overdrive wiring problems and later were fitted with an aftermarket gearknob. Owners are sometimes confused whether their MGB is fitted with overdrive. The more common error is believing that it is fitted when it is not ("I have the switch for it...") while the opposite is rarely true (I have overdrive?!"). In 25 years of working with MGs, this latter case has occurred only half a dozen times.
The overdrive gearbox is significantly different at the tail end than the standard box. The overdrive unit, about six inches in front of the rear flange, has a flat bottom to which one square and one rectangular plate, along with one 3/4" hex plug is fitted. The standard tailshaft begins at the main case and slowly tapers, over about 18", to the rear flange. A cursory inspection of the underside will quickly reveal the truth - you have overdrive or you do not.
USE OF OVERDRIVE
Overdrive is designed for constant load, high speed driving. While it's possible to engage overdrive in third gear on the earlier models (pre-76), there is just no point in doing so. The later models (post-77) restricted use to fourth gear only. It is not necessary to disengage the clutch when entering or leaving overdrive, unless disengagement is harsh and you simply wish to smooth out the gear change.
In standard or direct drive-overdrive is on all the time: This situation is dangerous to the gearbox and expensive if not repaired immediately, as reversing the car while in overdrive will burst the one-way clutch. The problem may be electrical (switch has gone continuous) or hydraulic (dirt jammed in solenoid). This situation is extremely rare, but must be repaired immediately.
In standard or direct drive-overdrive shifts in and out: This situation is caused by the failure of the tiny O-ring at the top of the solenoid piston. The solenoid assembly requires three O-rings-change all three. This situation is extremely rare.
In overdrive-overdrive works intermittently: There may be an electrical problem or a hydraulic problem. Try moving the gearlever to the right and/or to the rear-if this allows the overdrive to work, then the problem is the 3/4 electrical lockout switch-this is not uncommon. If the overdrive suggest on and off, low oil in the gearbox could be the problem.
In overdrive-overdrive does not work: Test for an electrical problem by piercing the solenoid wire with your test light with the ignition on, the overdrive switch on, and the gear lever should be hot. If not, test light with the ignition on, the overdrive switch on, and the gear lever in fourth. The solenoid wire should be hot. If not, test the circuit, switch to switch-but the problem is most often the 3/4 lockout switch.
In overdrive-overdrive does not work: Open the bottom of the overdrive unit and clean all the parts-the solenoid, piston, ball, and housing; the oil pressure relief valve; and the oil pump. The most common fault is a sticky oil pump which has stuck down.
In overdrive-overdrive freewheels on deceleration: In this condition, the MGB will accelerate but the free wheeling clutch can allow the engine to return to idle speed while the car is still running down the highway at 55 mph. This is caused by dirty hydraulic components or a fault in the sliding member (cone clutch).
There are always a few exceptions to all these rules:
Always fuse the overdrive circuit. This is an unfused circuit which, when it dead shorts, will burn up the wiring from the overdrive to the engine bay, and from the engine bay across to the left side of the dash. Fit an in-line fuse (available at auto parts stores) to the Yellow circuit (1968-76) or to the White circuit (1977-80) at the junction of the main loom to the gearbox loom at the rear of the right front, inner fender.
Solenoids do not fail. The number of owners who purchase a new solenoid to correct their overdrive problems is unbelievable! I have encountered one faulty solenoid in 25 years! And that one I simply unwound, repaired the wiring, and rewound on the lathe.
Overdrive gearboxes require 20W/50 engine oil. They do not use 80/90 hypoid gear oil. As an aside when changing engine oil this engine oil drain valve makes very easy work of the normally messy process. When changing oil you might like to consider Mr Moly oil supplements.
All overdrive faults can be fixed from underneath the car. Repairing wiring faults and cleaning the hydraulic components (and fitting new O-rings) always repairs the units.
The thrust washer fitted to the later gearboxes breaks into several pieces because it is too thin. Replace this with a bronze thrust washer from the top of the kingpin (machine the inner diameter to fit around the mainshaft).
Top fill overdrives drive the speedo at 1280 turns per mile; side fill overdrives at 1000 tpm. A mismatch will cause the speedo to read incorrectly by 25%.
Fitting an overdrive gearbox to an MGB requires a complete overdrive gearbox, a longer speedo cable, and the necessary gearbox wiring. No other components are necessary.
The 3/4 lockout switch is the least accessible electrical part of the MGB. To change or adjust this switch requires removal of the console arm rest, the elliptical plate underneath, and dropping the gearbox cross member. Attempting to work with the switch without removing these items is futile.
USE OF THE OVERDRIVE SWITCH AS AN ANTI-THEFT DEVICE
If you do not have overdrive fitted to your 1968-76 MGB, you can use the overdrive switch to turn the fuel pump off and on. Find the fuel pump wire (White) in the rear loom, and connect it to the Yellow wire in the main loom. These wires are located at the junction of the main, rear, and gearbox looms at the rear of the right front, inner fender. Now the fuel pump will operate only when the overdrive switch is pulled rearwards.
Call John Twist during his technical hour: 1-2 p.m.. EST, Monday-Thursday at 616-682-0800, or contact him by e-mail: JohnTwist@UNIVERSITYMOTORSLTD.COM , or visit the website: www.universitymotorsltd.com.