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. If not, test the circuit, connection to connection, The problem is most often the 3rd/4th lockout switch. 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. If the overdrive surges on and off, low oil in the gearbox could be the 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. In overdrive – Overdrive works intermittently: There may be an electrical problem or an hydraulic problem. The solenoid assembly requires three O-rings – change all three. 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. This situation is extremely rare, and must be repaired immediately.
The problem may be electrical (switch has gone continuous) or hydraulic (dirt jammed in solenoid). 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. 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. The later models (1977-1980) restricted use to fourth gear only. While it is possible to engage overdrive in third gear on the earlier models (1963-1976), there is just no point in doing so. Overdrive is designed for constant load, high speed driving. A cursory inspection of the underside will quickly reveal the truth – you have overdrive or you do not. The standard tailshaft begins at the main case and slowly tapers, over about 18”, to the rear flange.
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 ¾” hex plug is fitted (1968-1980). The overdrive gearbox is significantly different at the tail end than the standard box. In 30 years of working with MGs, this latter case has occurred only half a dozen times. 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?!!”). Owners are sometimes confused whether their MGB is fitted with overdrive. Some of these MGBs had overdrive wiring problems and later were fitted with an aftermarket gearknob. Some MGBs 1977-1980 were fitted with overdrive, and these have a switch on the gear knob. These do not have the overdrive function. Midget wiper switches can be and are sometimes fitted to the MGB. If overdrive is not installed, this switch makes a wonderful anti-theft device (see end of article). A fore or aft motion of the wiper switch stalk engages or disengages the unit. Of course, turning the engine requires some power – and turning it 500 rpm less in overdrive certainly offers some savings.Īll MGBs 1968 – 1976 were fitted with the overdrive switch and wiring. Pushing air out of the way of the front of the MGB is the significant function of the engine. This is doubtful as the energy necessary to propel the MGB along the highway remains constant, no matter the speed of the engine.
Sales literature during the Seventies suggested dramatic fuel savings by using the overdrive. While the durable B-series engine can and does power the MGB at continued speeds of 80 mph without overdrive, the Laycock overdrive offers “POM” (Peace of Mind) and slight mileage savings. This coveted option adds a fifth gear, lowering the engine rpm to about 3000 at 70 mph. Overdrive was fitted to a very small percentage of the MGBs sold in North America, and certain not to “all” MGBs 1975-1980 as erroneously reported by some authors.
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