This coveted option adds a fifth gear, lowering the engine rpm to about 3000 at 70mph. 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) by quieting the operation of the MG. FOREWORD This Manual has been compiled with a view to familiarising our Agents and other Repairers, throughout the World, with the operation and repair of the Laycock de Normanville Overdrive Unit. We supply a full and comprehensive parts list for all Laycock de-Normanville overdrive units. We have the largest stock of original, obsolete Laycock parts in the world and re-manufacture all consumable parts. We offer a full re-conditioning service on either your unit or on an exchange basis. We also offer competition upgrades on all units for.
MGB
From MGB Driver - The North American MGB Register publication
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The cone clutch is moved forward by hydraulic pressure in the operating cylinders, so that the outer friction lining of the clutch comes into contact with the stationary brake ring. As the cone clutch is attached to the sunwheel, both come to rest and the sunwheel becomes the reaction member for the planetary train. Since the planet carrier is splined to the input shaft and driven by it, the planet wheels orbit round the stationary sunwheel and in so doing, rotate the annulus and output shaft at a speed greater than that of the carrier and input shaft. The UDC enables the output member to rotate faster than the input member.
The forward drive is transmitted directly through the uni-directional clutch (UDC). The cone clutch is held rearwards by spring pressure exerted through a thrust ring and ball race, loading the inner friction lining of the clutch in contact with the outside diameter of the annulus on the output shaft (the loading is further increased by the reverse thrust of the helical sunwheel). Thus the gear train is locked and overrun or reverse torque is taken by the cone clutch, without which, the UDC would give a freewheeling condition.
The overdrive gears are epicyclic (planetary) and consist of a sunwheel, three planet gears and an internally toothed ring gear or annulus. All gears are in constant mesh. The planet carrier is attached to the input shaft. Carried on an extension of the sunwheel, a double sided cone clutch engages with a stationary brake ring or with the outside surface of the annulus. A uni-directional clutch (UDC) or freewheel connects the input and output shafts.
Oil is drawn from the sump through the suction filter by the reciprocating pump and delivered to the pressure filter. It then passes through to the relief valve and acts upon the solenoid valve which is closed. Oil passes through the relief valve spill port and is conducted into the main drive shaft to lubricate the running gear. Excessive pressure build up in the lubrication pasages is prevented by the lube relief valve which operates at a predetermined pressure.
Since oil supply to the dashpot is withheld by the solenoid valve, the dashpot piston is held down by its spring(s). Movement of the relief valve piston is resisted by the spring, the strength of which determines the residual pressure within the system when the unit is in direct drive. It is necessary to maintain this residual pressure in order to initiate action of the system when engagement is signalled and for lubrication.
When the solenoid is energised, its valve opens and oil - at residual pressure - is allowed to enter the passage which leads to the dashpot. An increase in system pressure results from the 'out-of-balance' between the dashpot piston and the relief valve piston due to their differing areas. This causes the dashpot assembly to compress the residual spring. The dashpot then continues to move compressing the dashpot spring until the dashpot piston contacts the dashpot cup.
The reaction of the increasing load of the relief valve spring being applied to the relief valve piston, progressively increases the system pressure. The dashpot piston continues to its stop by which time the relief valve spring has been compressed to its working length, thus giving full system pressure.
When the solenoid is de-energised its valve is returned by a spring allowing oil to exhaust from the dashpot. The relief valve spring is able to gradually relax towards its direct drive position. Progressively, the system pressure will drop and eventually allow the cone clutch to leave the brake ring and move gently into contact with the annulus. The supplementary dashpot spring provides additional load upon the piston and allows relative rotation between the cone clutch and the brake ring as the engine speed rises and the UDC takes up direct drive.
Smooth overrun disengagements are obtained by progressively increasing the load on the inner clutch linings, compared with that provided by the free action of the clutch springs by varying the hydraulic pressure which off-sets the clutch spring load and determines the net loading at the clutch face. This is accomplished by controlling the diminution of pressure determined by the action of the dashpot piston, exhausting oil through the discharge orifice. When the contents of the dashpot have been fully discharged, the system pressure has fallen to its original residual value.
Failure To Engage
Low oil level
No electrical feed to solenoid
Solenoid valve sticking due to dirt
Blocked oil flters
Dirt in pump non-return valve
Sticking relief valve piston
If these items check out OK, engage the clutch fiercely with overdrive switched in on the overrun in third gear at 40/45mph.
Failure to Disengage
Check electrical system for 'closed circuit'
Remove solenoid and check for seized plunger
Check relief valve for sticking components
Check relief orifice for blockage
Check for sticking clutch by giving brake ring several taps with a hide mallet.
- Overdrive Repair Services Ltd. Unit 1 50 Rother Valley Way, Holbrook Industrial Estate Halfway, Sheffield, S20 3RW Phone: 0114 248 2632 Fax: 0114 248 2786 [email protected]. Company Registration Number: 8146837. VAT Number: 457735807. SPARES & GEARBOX SPARES LH Type Spare Parts List.
- May 05, 2011.
From MGB Driver - The North American MGB Register publication
Overview on Overdrive
Overdrive was fitted to a very small percentage of the MGBs sold in North America, and certainly not to 'all' MGBs 1975-80 as erroneously reported by some authors. This coveted option adds a fifth gear, lowering the engine rpm to about 3000 at 70mph. 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) by quieting the operation of the MG. Sales literature during the Seventies suggested fuel savings by using the overdrive-although I have always doubted this as the energy necessary to propel the MGB along the highway remains constant, no matter what the gear or engine speed. Perhaps there is some marginal mpg savings by allowing the engine to turn at 500 rpm less, but pushing the air out of the way in front of the MGB is the dramatically significant function of the engine.
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 confusedwhether 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 simplywish to smooth out the gear change.
FAULT DIAGNOSIS
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-wayclutch. The problem may be electrical (switch has gone continuous) orhydraulic (dirt jammed in solenoid). This situation is extremely rare, but must be repaired immediately.
In standard or direct drive-overdrive shifts in and out: Thissituation 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 ignitionon, 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. Artboard 2 1 1 download free.
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 faultin the sliding member (cone clutch).
RULES
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 wiringfaults 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.
John Twist
Questions?
Call John Twist during his technical hour: 1-2 p.m. EST, Monday-Thursday at 616-682-0800, or contact him by e-mail: [email protected], or visit the website: www.universitymotorsltd.com. Adobe incopy cc 2020 v15 0 1.
Call John Twist during his technical hour: 1-2 p.m. EST, Monday-Thursday at 616-682-0800, or contact him by e-mail: [email protected], or visit the website: www.universitymotorsltd.com. Adobe incopy cc 2020 v15 0 1.
Made in England
HOW AN OVERDRIVE WORKS
The forward drive is transmitted directly through the uni-directional clutch (UDC). The cone clutch is held rearwards by spring pressure exerted through a thrust ring and ball race, loading the inner friction lining of the clutch in contact with the outside diameter of the annulus on the output shaft (the loading is further increased by the reverse thrust of the helical sunwheel). Thus the gear train is locked and overrun or reverse torque is taken by the cone clutch, without which, the UDC would give a freewheeling condition.
Laycock Lh Overdrive Service Manual
The overdrive is operated by an electric solenoid controlled by a switch, usually mounted on the steering column or fascia panel. An inhibitor switch is invariably fitted in the electrical circuit to prevent engagement of overdrive in reverse and some or all of the indirect gears.The overdrive gears are epicyclic (planetary) and consist of a sunwheel, three planet gears and an internally toothed ring gear or annulus. All gears are in constant mesh. The planet carrier is attached to the input shaft. Carried on an extension of the sunwheel, a double sided cone clutch engages with a stationary brake ring or with the outside surface of the annulus. A uni-directional clutch (UDC) or freewheel connects the input and output shafts.
Oil is drawn from the sump through the suction filter by the reciprocating pump and delivered to the pressure filter. It then passes through to the relief valve and acts upon the solenoid valve which is closed. Oil passes through the relief valve spill port and is conducted into the main drive shaft to lubricate the running gear. Excessive pressure build up in the lubrication pasages is prevented by the lube relief valve which operates at a predetermined pressure.
Since oil supply to the dashpot is withheld by the solenoid valve, the dashpot piston is held down by its spring(s). Movement of the relief valve piston is resisted by the spring, the strength of which determines the residual pressure within the system when the unit is in direct drive. It is necessary to maintain this residual pressure in order to initiate action of the system when engagement is signalled and for lubrication.
When the solenoid is energised, its valve opens and oil - at residual pressure - is allowed to enter the passage which leads to the dashpot. An increase in system pressure results from the 'out-of-balance' between the dashpot piston and the relief valve piston due to their differing areas. This causes the dashpot assembly to compress the residual spring. The dashpot then continues to move compressing the dashpot spring until the dashpot piston contacts the dashpot cup.
The reaction of the increasing load of the relief valve spring being applied to the relief valve piston, progressively increases the system pressure. The dashpot piston continues to its stop by which time the relief valve spring has been compressed to its working length, thus giving full system pressure.
When the solenoid is de-energised its valve is returned by a spring allowing oil to exhaust from the dashpot. The relief valve spring is able to gradually relax towards its direct drive position. Progressively, the system pressure will drop and eventually allow the cone clutch to leave the brake ring and move gently into contact with the annulus. The supplementary dashpot spring provides additional load upon the piston and allows relative rotation between the cone clutch and the brake ring as the engine speed rises and the UDC takes up direct drive.
Smooth overrun disengagements are obtained by progressively increasing the load on the inner clutch linings, compared with that provided by the free action of the clutch springs by varying the hydraulic pressure which off-sets the clutch spring load and determines the net loading at the clutch face. This is accomplished by controlling the diminution of pressure determined by the action of the dashpot piston, exhausting oil through the discharge orifice. When the contents of the dashpot have been fully discharged, the system pressure has fallen to its original residual value.
Laycock Lh Overdrive Manual Transmission
TROUBLESHOOTING GUIDE
Low oil level
No electrical feed to solenoid
Solenoid valve sticking due to dirt
Blocked oil flters
Dirt in pump non-return valve
Sticking relief valve piston
If these items check out OK, engage the clutch fiercely with overdrive switched in on the overrun in third gear at 40/45mph.
Failure to Disengage
Check electrical system for 'closed circuit'
Remove solenoid and check for seized plunger
Check relief valve for sticking components
Check relief orifice for blockage
Check for sticking clutch by giving brake ring several taps with a hide mallet.
![Overdrive Overdrive](https://www.picclickimg.com/d/l400/pict/122532648267_/MGB-MGB-GT-Overdrive-Gearbox-chrome-bumper-model.jpg)