The overdrive gearbox is significantly different at the tail end than the standard box. The overdrive unit, about six inches in the front of the rear flange, has a flat bottom to which one square and one rectangular plate, along with one Ύ” hex plug is fitted (1968-1980). 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 is possible to engage overdrive in third gear on the earlier models (1963-1976), there is just no point in doing so. The later models (1977-1980) restricted use to fourth gear only. It is not necessary to disengage the clutch when engaging or disengaging overdrive, unless disengagement is harsh and you simply wish to smooth out the gear change.

FAULT DIAGNOSIS

In standard or direct drive – Overdrive remains 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, and must be repaired immediately.
In standard or direct drive – Overdrive shifts in and out by itself: 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 an hydraulic problem. Try moving the gearlever to the right and/or to the rear – if this allows the overdrive to work continuously, then the problem is the 3rd/4th or 4th electrical lockout switch – this situation is not uncommon. If the overdrive surges 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, ignition on, overdrive switch ON, gear lever in 4th. The solenoid wire should be hot. If not, test the circuit, connection to connection, The problem is most often the 3rd/4th 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 hydraulic fault is an oil pump that is stuck
In overdrive – overdrive freewheels on deceleration: In this condition, the MGB will accelerate and run at constant speed. A freewheeling clutch can allow the engine to return to idle while the car is still running down the highway at 60 mph. Dirty hydraulic components or, more commonly, a worn sliding member (cone clutch) causes this.

RULES

There is always an asterisk next to any rule – there are always exceptions!
ALWAYS fuse the overdrive circuit. This is originally 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 dashboard. Fit an in-line fuse (available at auto parts stores) to the YELLOW circuit (1968-1976) or to the WHITE circuit (1977-1980) at the junction of the main loom to the gearbox loom at the rear of the right front, inner fender. This rule has no exceptions.
Solenoids NEVER fail. The number of owners who purchase a new solenoid to correct their overdrive problem is unbelievable! I have encountered only ONE faulty solenoid in 30 years! I repaired that solenoid by unwinding and rewinding the magnet!
Overdrive gearboxes require 20W/50 Castrol GTX engine oil. They DO NOT use 80/90 hypoid gear oil.
All overdrive faults can be repaired 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 MGB 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 3rd/4th lockout switch is the LEAST accessible electrical component of the MGB. To change or adjust this switch requires removal of the console, 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 1968-1976 MGB, you can use the overdrive switch to turn the fuel pump off and on. Find the fuel pump wire (WHITE) from 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 is pulled rearwards.

CONCLUSION

Overdrive is a wonderful option! It reduces engine wear, reduces noise, and increases mileage. Once you have experienced the joy of an overdrive, you will want one, too! Expect to pay $1200 for a guaranteed, rebuilt overdrive gearbox; or nearly $2000 to have one fitted to your MGB.

© University Motors Press 1st 02281998 11112000 jht
MG – F – Overview on Overdrive