Does anyone actually know how it works? Owners manual says OD off simply prevents it using the OD gear - it affects the whole gearbox range so that ain't right. The condescending repair manual (which doesn't even have a photo of it), says it stops the torque converter slipping so much - that ain't right either because the motor revs faster with OD off even down hills. My guess would be a planetary system devoted soley to OD and controlled by a hydraulic solenoid, but I've never seen inside one. Scarey the manuals simply don't know... what other errors are in them?
========================== Not sure I understand your question, but basically, you seem to be mixing two concepts. Overdrive is simply a gear ratio of less than one. It means if the input shaft turns once, the output shaft turns something more. There is also a locking torque converter that improves fuel economy by removing the normal slippage of the torque converter drive at low throttle positions by mechanically holding the torque converter so that on the highway, it's like you had a standard transmission with the clutch engaged - no slippage. If you're going up a hill on cruise control, and watch the tachometer, as the transmission downshifts, first you'll see a slight rise in RPM - that's the torque converter lock up disengaging, then you see a larger jump in engine speed as the transmission down shifts to the next lower gear. If you are in overdrive, and the lock up torque converter is engaged, you should see the same engine speed at the same road speed, regardless of whether you're going up hill or down hill, but it doesn't take much throttle for it to automatically disengage. The actual fourth gear is a planetary system with a hydraulic clutch, the lock up in the torque converter is a separate hydraulic clutch. If you block over drive, it's like you have a three speed transmission. I don't know if the lock up converter works in that event.
Thanks, that explains how the lockup works. The only thing is, the motor seems to rev consistently higher in all gears & gradients (there's no tacho) with OD off and I'm fairly sure there are still 4 ratios with it off.
========================================== The torque converter lock up is primarily designed to work with the overdrive gearing to conserve fuel on the highway. When you say there are four gear ratios with the overdrive off -- I don't see it that way. There are a total of four gear ratios, and in fourth gear, (over driven) the torque converter lock up function is also available which gives that fourth gear ratio kind of a dual function by reducing engine speed at low throttle openings when all slippage is mechanically removed from the torque converter. Toyota Transmission Gear Ratio's ... Automatic Transaxles, 1st, 2nd, 3rd, 4 th. Ratio. A140E, 2.810, 1.549, 1.000, 0.706 As you can see, if you take away the fourth gear, (overdrive at .706 ratio) there are only three gear ratios remaining. The A140E is for my '94, so your '91 may have a slightly different transmission but the principle is the same.
Just feels to me like there are still 3 distinct changes with OD off (1st gear making 4 ratios), and they all seem different to with OD on. And though the manual agrees with what you say about the lockup, there doesn't seem to be much point since there is effectively no slip in the torque converter at low load anyway.
there is effectively no slip in the torque converter at
====================================== There's always going to be more slippage in a fluid coupling than a direct mechanical connection. Think about the torque converter at idle - that's a "low load" condition - yet you know there is slippage because the engine is idling with the wheels not moving and the car in gear. Some cars have a button to change the shift pattern in the electronically controlled transmission - aka "ECT" switch. That one would change the behaviour of the shift pattern for every gear, but "overdrive" is just the name of the top gear by virtue of its low ratio. If you have a four speed transmission and take away the fourth gear - how can you still have four gears? When I went to school, 4 - 1 = 3, not four. ========================================= Here's another way to look at it. I have a Toyota truck with the manual transmission - 5 speed. The fifth gear is like an overdrive in your automatic transmission. It is the "taller" "high" gear. I once drove with someone who had a Toyota with a 4 speed manual transmission. His top gear was 4th. So the engine was revving higher on the freeway at the same speed. That would be like driving the automatic with the overdrive switched off. You have locked out access to high gear, so you would always be in the next lower gear - with the four speed automatic, that would be third gear, according to my records, a 1:1 transmission gear. ======================================== For all practical purposes, you just leave the overdrive on all the time. That's the benefit of an "automatic" transmission, it automatically selects the most appropriate gear for all conditions using a sophisticated array of sensors - even retarding the ignition timing under most conditions to smooth upshifts. The only recommended time to turn off overdrive is towing uphill, or when the transmission is at an awkward speed continually shifting between gears under rare conditions. ======================================== Best idea, to borrow a phrase from Ron Popeil - "you just set it and forget it." ======================================== When I used to experiment with the overdrive off button, the only real use I found was in city traffic, around 40 - 45 mph, when I wanted to waste fuel, and have more power for passing temporarily, I would switch off the button, then press down on the throttle to save the millisecond of time it would take the transmission to automatically downshift, but even that really isn't worth the bother.
Yes I understand the theory, but it didn't seem to be what happened in practice. Actually I very recently discovered the kickdown wire was way too loose (don't know why) and it spent a great deal of time in 3rd or 4th depending if OD was on just as you say, so it seemed to be always at a higher or lower ratio. The torque converter fooled me into thinking it must have changed gear. Centrifugal force "hardens" a torque converter so that much less slip occurs at revs than at idle, but I guess the engineers have determined there is still enough to warrant locking it. My manual makes it sound like the locking is soley responsible for creating an overdrive effect. Wish they had included a drawing.
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