Faulty ZF4HP500.
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Having given some thought about ZF buses that lost its lock-up, here's what I can think of, would like opinions if possible because this is just my hypthesis.
First we need to understand these buses have torque converters. A torque converter is something like a link between the engine and the driving axle. Not sure of the full explaination, but as I understand, the torque converter allows the bus to come to a halt while still engaged to a gear, without causing the engine to cut. This will also explain the reason why the buses crawl when the brakes are released. For the full explaination, please find out what's a torque converter because I really do not know how to explain.
As far as I understand for gearboxes fitted with lock-up clutches, the function of the lock-up clutch is to link up the engine and driving axle once the bus gets up to speed, so that the engine and driving axle will now be directly linked.
Having explained to a lousy extent to my fullest knowledge what's a lock-up clutch and a torque converter, now let's touch on ZF4HP500 buses whose gearbox no longer engages the lock-up clutch.
Worn-out ZFs can be identified by a horrible crunching sound where the lock-up is presumed to have occured as the locking engages without the engine cutting for a short while causing a clash between the driving axle and engine.
Worse would be those ZFs that do not even perform the lock-up act.
Easily identified by the lack of a pause where the lock-up whould have engaged, and more noticeable in heavier buses or those buses whose power to weight ratio is not so high, draggy gear transitions, and a general sluggish performance. Also, when the pressure on the pedal is lifted, there is a very noticeably faster than usual drop in rpm which indicates that the engine is definitely not connected to the driving axle. A normal ZF bus whould at this point still have both connected which causes the rpm to drop at the same rate as the axle until the gears change or the lock-up disconnects. Again, this is more noticeable in heavier buses because of the generally higher strain on the engine.
The general explaination for the characteristics of such buses follow. And since such effects are felt most in the VO3X and LO3X, their characteristics will be explained, here goes:
1) The pick-up speed is still the same generally.
Pick-up speed is generally decided by the torque. Mordern torque converters are so efficient that they can actually multiply the torque from the engine. As a result, the pick-up in general for each gear is about the same until the higher rpm regimes. As the engine reaches higher rpm, the power loss in the torque converter grows by greater than proportionate amounts due to friction. As a result, for each gear, pick-up speed is still the same until the higher rpm regimes, which lead on to point 2 and 3.
2) Draggy gears.
As the bus prepares to change gears and the engine reaches higher rpm, as we understand above, the power loss increases more proportionately than the power delivered by the engine. This tapers off the pick-up speed eventually and the bus is held in high rpm limbo until the gearbox decides that it has no choice but to change gears.
3) Generally lower top-speed or noiser than usual high-way cruising.
Noticed that VOs and LOs with this problem usually have problems with the highway? As explained in point 1, the pick-up speed for each gear diminishes as the engine rpm climbs. The same goes for gear 4. There is very much less power loss for a normal ZF bus because the engine is connected to the driving axle. But for a faulty one, the power loss due to the torque converter increases more than proportionately as the rpm climbs. Eventually, no matter how much fuel you inject, you'll be stuck at that speed simply because even if you inject more fuel, there'll come a point where the power increase delivered will always be less than the subsequent power loss resulted.
4) The engine pitch falls far sharper when the foot leaves the pedal.
Because the engine is not connected to the driving axle. Notice that for normal VO/LOs, the rpm falls proportionately as the turning rate of the wheels as long as the gears do not switch? And that for faulty ones, the rpm just slides to idling?
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Someone please correct me if I'm wrong.
If the whole thing is wrong, then I would have wasted 15 min of my time.
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Red : Not exactly. The torque converter connects the flywheel (which basically , is part of the engine) to the set of gears . Take for example , an airplane propeller . A gentle breeze can move the propeller . But if eu use your hand and hold it still , the propeller will stop moving. Same goes to the torque converter. Instead of using air , they use fluid dynamics , which in this case they use hydraulic fluid. Hence , the power of the brakes is enough to prevent the vehicle from rolling off . The torque converter consist of some turbines , so when a lock up occurs , the turbine is actually linked directly to the gearset , so no fluid dynamics is involved. For more detailed and comprehensive explanation ,Originally posted by SBS3625D:Having given some thought about ZF buses that lost its lock-up, here's what I can think of, would like opinions if possible because this is just my hypthesis.
First we need to understand these buses have torque converters. A torque converter is something like a link between the engine and the driving axle. Not sure of the full explaination, but as I understand, the torque converter allows the bus to come to a halt while still engaged to a gear, without causing the engine to cut. This will also explain the reason why the buses crawl when the brakes are released. For the full explaination, please find out what's a torque converter because I really do not know how to explain.
As far as I understand for gearboxes fitted with lock-up clutches, the function of the lock-up clutch is to link up the engine and driving axle once the bus gets up to speed, so that the engine and driving axle will now be directly linked.
Having explained to a lousy extent to my fullest knowledge what's a lock-up clutch and a torque converter, now let's touch on ZF4HP500 buses whose gearbox no longer engages the lock-up clutch.
Worn-out ZFs can be identified by a horrible crunching sound where the lock-up is presumed to have occured as the locking engages without the engine cutting for a short while causing a clash between the driving axle and engine.
Worse would be those ZFs that do not even perform the lock-up act.
Easily identified by the lack of a pause where the lock-up whould have engaged, and more noticeable in heavier buses or those buses whose power to weight ratio is not so high, draggy gear transitions, and a general sluggish performance. Also, when the pressure on the pedal is lifted, there is a very noticeably faster than usual drop in rpm which indicates that the engine is definitely not connected to the driving axle. A normal ZF bus whould at this point still have both connected which causes the rpm to drop at the same rate as the axle until the gears change or the lock-up disconnects. Again, this is more noticeable in heavier buses because of the generally higher strain on the engine.
The general explaination for the characteristics of such buses follow. And since such effects are felt most in the VO3X and LO3X, their characteristics will be explained, here goes:
1) The pick-up speed is still the same generally.
Pick-up speed is generally decided by the torque. Mordern torque converters are so efficient that they can actually multiply the torque from the engine. As a result, the pick-up in general for each gear is about the same until the higher rpm regimes. As the engine reaches higher rpm, the power loss in the torque converter grows by greater than proportionate amounts due to friction. As a result, for each gear, pick-up speed is still the same until the higher rpm regimes, which lead on to point 2 and 3.
2) Draggy gears.
As the bus prepares to change gears and the engine reaches higher rpm, as we understand above, the power loss increases more proportionately than the power delivered by the engine. This tapers off the pick-up speed eventually and the bus is held in high rpm limbo until the gearbox decides that it has no choice but to change gears.
3) Generally lower top-speed or noiser than usual high-way cruising.
Noticed that VOs and LOs with this problem usually have problems with the highway? As explained in point 1, the pick-up speed for each gear diminishes as the engine rpm climbs. The same goes for gear 4. There is very much less power loss for a normal ZF bus because the engine is connected to the driving axle. But for a faulty one, the power loss due to the torque converter increases more than proportionately as the rpm climbs. Eventually, no matter how much fuel you inject, you'll be stuck at that speed simply because even if you inject more fuel, there'll come a point where the power increase delivered will always be less than the subsequent power loss resulted.
4) The engine pitch falls far sharper when the foot leaves the pedal.
Because the engine is not connected to the driving axle. Notice that for normal VO/LOs, the rpm falls proportionately as the turning rate of the wheels as long as the gears do not switch? And that for faulty ones, the rpm just slides to idling?
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Someone please correct me if I'm wrong.
If the whole thing is wrong, then I would have wasted 15 min of my time.
you can visit
http://auto.howstuffworks.com/torque-converter.htm
FYI , lock-up purpose is to save fuel. Heres why :
"Modern torque converters can multiply the torque of the engine by two to three times. This effect only happens when the engine is turning much faster than the transmission.
At higher speeds, the transmission catches up to the engine, eventually moving at almost the same speed. Ideally, though, the transmission would move at exactly the same speed as the engine, because this difference in speed wastes power. This is part of the reason why cars with automatic transmissions get worse gas mileage than cars with manual transmissions. "
Thats why you notice that when the gearbox (with no lock up that is) changes gears , the RPM stays almost the same . -
Ah....I see, thanks!Originally posted by Scania N113CRB luver:Red : Not exactly. The torque converter connects the flywheel (which basically , is part of the engine) to the set of gears . Take for example , an airplane propeller . A gentle breeze can move the propeller . But if eu use your hand and hold it still , the propeller will stop moving. Same goes to the torque converter. Instead of using air , they use fluid dynamics , which in this case they use hydraulic fluid. Hence , the power of the brakes is enough to prevent the vehicle from rolling off . The torque converter consist of some turbines , so when a lock up occurs , the turbine is actually linked directly to the gearset , so no fluid dynamics is involved. For more detailed and comprehensive explanation ,
you can visit
http://auto.howstuffworks.com/torque-converter.htm
FYI , lock-up purpose is to save fuel. Heres why :
"Modern torque converters can multiply the torque of the engine by two to three times. This effect only happens when the engine is turning much faster than the transmission.
At higher speeds, the transmission catches up to the engine, eventually moving at almost the same speed. Ideally, though, the transmission would move at exactly the same speed as the engine, because this difference in speed wastes power. This is part of the reason why cars with automatic transmissions get worse gas mileage than cars with manual transmissions. "
Thats why you notice that when the gearbox (with no lock up that is) changes gears , the RPM stays almost the same .