It's not an easy thing to check as access is very limited. It could be the switch or the wiring to it. If it does then the inhibitor switch is not working and there is the danger of reversing with the overdrive engaged. Accelerate slowly then switch the overdrive in. It re engages when third is selected if not cancelled by the dashboard switch.Ĭheck whether the overdrive operates in first. As reverse is the same side of the gate as first and second then the overdrive cuts out when the gearlever moves to that side because of the inhibitor switch for first and second and of course reverse irrespective of the dashboard switch. There is no reverse gear overdrive inhibitor switch as such. It has a unidirectional nu clutch and reversing destroys that clutch and the overdrive then spins in its housing and there's no drive You couldn't have reversed with the overdrive engaged.
#Laycock overdrive a type vs d type full
On my previous MGB GT which had a vacuum switch the overdrive wouldn't disengage on full throttle even though I had switched it off with the dashboard switch until I lifted off the throttle. Mark, when you disengage the overdrive do you have to lift off the throttle? You should do to operate the vacuum switch. You have to remove the engine and gearbox as a unit. This is because the overdrive is too big to fit over the fixed crossmember. It is worth remembering that you can't just remove the gearbox and overdrive as a unit on the MGB. See, and go down the page until you find 'MGB - MGB GT – Overdrive problems'. To change the switch is not easy because of where it located on top of the gearbox but there note on Brown and Gammons website might be useful guidance. What ever it is it needs investigating urgently as you will (as Vic says) destroy the Overdrive unit if try to reverse with the overdrive engaged. The plunger can be stuck because it is rusted up or has become bent for some reason. If it has failed because the operating plunger is stuck up inside the switch the overdrive will be operating all the time, so in theory it will work on all gears including reverse. Turning now to the isolation switch failing. There is no similar protection for engaging reverse on the MGB. On the TRs reverse is to the right of 3rd and 4th gears and you have to lift the gear leaver to engage reverse. I always thought the overdrive only worked of 3rd and 4th gears because of the torque of the B-Series engine, but interestingly, the Triumph TRs used the same overdrive units but it worked on 2nd, 3rd and 4th gears and the TR engine develops more torque, so I now think it is more to do with the location of reverse gear also being on left of the gate. It points forwards on the 3 synchro box and rearwards on the 4 synchro box. For both types the isolation switch is located on the nearside of the gear lever remoted control housing, which is bolted to the top of the gearbox and this switch prevents the overdrive being activated in 1st, 2nd and reverse gear. Thus the J Type needs a special fabricated mount if fitted to earlier cars. For simple identification, looking at the rear mounts, the A Type O/D had a horizontal mount surface, but the J Type had a vertical flange. The main distinguishing difference is the type has an external solenoid and the type LH has an internal solenoid. The J Type O/D was introduced for 1973 model TR6's, so it follows that all the earlier cars had the optional A Type O/D. Early 3 synchro cars used the type D and later 4 synchro cars used the type LH. Firstly all MGBs used a Laycock Overdrive.