The accumulator piston actually bottomed out in the sleeve (similar to coil bind on valve springs). So even though the accumulator piston had passed the oil hole relief position, the pressure continued to build up because the oil could not leave the system as fast as it was being pumped in. The 90 weight oil was so heavy it could not escape from the accumulator chamber as fast as the oil pump could pump new oil into it. The oil holes in the accumulator sleeve are very small. After running a number of these test with the same result we found what was happening. A tap on the brake ring with a hammer (the universal overdrive release tool), shifted it back into the direct drive position. The higher pressure had driven the sliding clutch member so hard into the brake ring that the clutch return springs could not return it to the non overdrive position. Even though the overdrive unit was now in the non overdrive position (solenoid disengaged), the overdrive was now stuck in overdrive and would not come out. We finally shut it off at 750 PPSI as we did not want to damage the unit. (Which made no sense since we thought the oil would thin out and the pressure would drop). After a few minutes of running the oil pressure actually began to climb even higher. This time the oil pressure jumped to 600 PPSI! When shifted, the pressure dropped to 450 PPSI, which made the shift immediate and harsh. We then drained the oil again and replaced it with 90 weight hypoid oil. We surmised that the detergents in the oil were causing the oil pump to cavitate, and develop air bubbles as it pumped. Upon observation of the internals of the operating overdrive we found bubbles developing in the oil pump body and oil pump output passage. 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. Gang – I apologize for not writing an article for some time, but between the rough winter and everything else going on, it was impossible! Topic of the month for June 2005 – Overdrive oil recommendation I "borrowed" this thread from the TR Forum, an article written by the quantumechanics guy, the overdrive "guru's" on this side of the pond.
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