However, after a few minutes of running, the oil pressure dropped to 300 and when shifting, to 200. When spun on the test bench, initially it tested fine. We then drained the oil and replaced it with 10W30 multi grade oil. All was right in the world of overdrives. When shifting the pressure dropped to 300 PPSI and quickly recovered to 400 PPSI. When spun up on our test bench at 1,000 RPM, it reached a normal pressure of 400 PPSI. We rebuilt an A type overdrive unit and initially ran it with 30 weight non detergent motor oil. Many years ago (in a land far, far away (OK it wasn’t that far away or even that long ago)) we ran an experiment on overdrive oil. Some say to use multi grade oil (MG manuals), some hypoid oil (Triumph manuals), some non detergent oil and some even recommend automatic transmission fluid! No wonder everyone is so confused! Historically even the recommendations from various car manufacturers are confusing. This month’s discussion will invariably get some interesting responses as it seems to be a rather emotional (and not necessarily logical) discussion item for British car overdrive owners. We constantly get into discussions( and sometimes heated debates!) with British car owners on this subject. I want to express our experience and opinion on the topic of the proper oil for use in the Laycock de Normanville overdrive units. These later "LH" model overdrive units are larger and more robust than the earlier ones so they can stand a bit more abuse. The instructions you quoted above are obviously for a later car since A) the OD switch is on the steering column and B) there is no concern for the safety of the OD clutch by shifting down during overrun.
In fact, these model cars came with a vacuum inhibitor switch which prevents the overdrive from disengaging while the engine is in overrun, or engine braking. This is especially important on D-Type overdrive units as found on Mark I MGB. The same goes for overdrive, only instead of avoiding lurching, you're reducing the shock on the unit from slamming into a lower gear ratio. When downshifting with the stick, you give it more gas to match the RPMs and avoid "lurching" the car. It's generally good practice to adjust the throttle when shifting in and out of overdrive the same way you would if shifting through any other gears - lighten up the throttle to up shift (engage overdrive) and give it a bit more gas to downshift (disengage overdrive). Just like the clutch however, the overdrive's clutch can suffer damage or premature wear from continuous abuse. You can engage or disengage at any throttle just like you could engage or disengage the clutch under any load. If for any reason the overdrive does not disengage, do not reverse the car otherwise extensive damage may result.Īny thoughts on the throttle position the handbook espouses?
In certain driving conditions while traveling in third gear, the overdrive can be switched in to provide a top gear ratio or out to provide third gear acceleration without the necessity of manually moving the gear-change lever. Do not 'switch out' the overdrive when traveling at speeds exceeding normal third or top gear road speeds.
If increased acceleration is required the overdrive can be 'switched out' without alteration to the throttle setting. Overdrive can be engaged at any throttle opening when in third or top gear. Accelerator pedal pressure should be maintained and it is not necessary to depress the clutch pedal during engagement or disengagement. To engage overdrive move the switch lever towards the steering-wheel to disengage move the lever away from the steering-wheel. 1971 MGB Tourer and GT Handbook wrote:The overdrive unit, controlled by a switch on the steering-column, provides a higher driving ratio for use with third or fourth gear.
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