Down Hill Speed ?
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Joined: Nov 2004
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Down Hill Speed ?
Hi to all,
Took a road trip the other day. Truck ran great. But I did notice some thing that I think is a little odd, and would like to know if this is normal.
Truck was empty, except passengers, not towing, transmission in normal mode.
The thing I noticed was that on down hill grades with out touching the brakes the truck would either slow down requiring me to get on the throttle or just maintain speed (65mph) on 5-6% grades. After noticing this on a long 6% grade, I decide to see if some thing in the driveline might be holding it back, (i.e. hubs still engaged). So I put the trans in N. The truck shot up to 80 mph (<900rpm’s) in no time, so I put it back into D, rpm’s went up to around 2200 rpm’s and the truck started to slow back down until it reached 60-65 mph (1700 rpm’s approx.) without ever touching the brakes.<O
Now I would have figured with the trans in normal mode that is truck would be rocketing down a 6% graded especially since it’s a diesel or has Ford programmed in some engine braking even during normal mode.<O
If this is not normal then I’m guessing excessive exhaust backpressure from either bad sensor, sticking turbo, or plugged cat. Truck has 9000 miles on it.
Thanks to all that reply
Took a road trip the other day. Truck ran great. But I did notice some thing that I think is a little odd, and would like to know if this is normal.
Truck was empty, except passengers, not towing, transmission in normal mode.
The thing I noticed was that on down hill grades with out touching the brakes the truck would either slow down requiring me to get on the throttle or just maintain speed (65mph) on 5-6% grades. After noticing this on a long 6% grade, I decide to see if some thing in the driveline might be holding it back, (i.e. hubs still engaged). So I put the trans in N. The truck shot up to 80 mph (<900rpm’s) in no time, so I put it back into D, rpm’s went up to around 2200 rpm’s and the truck started to slow back down until it reached 60-65 mph (1700 rpm’s approx.) without ever touching the brakes.<O
Now I would have figured with the trans in normal mode that is truck would be rocketing down a 6% graded especially since it’s a diesel or has Ford programmed in some engine braking even during normal mode.<O
If this is not normal then I’m guessing excessive exhaust backpressure from either bad sensor, sticking turbo, or plugged cat. Truck has 9000 miles on it.
Thanks to all that reply
Senior User
Joined: Jul 2002
Posts: 364
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From: Redding, CA
This happens to me also. I think it's normal given the compression in the PSD. I haven't done the "testing" you have so I don't know the speeds. But it makes sense to me. Of course, I really notice it with T/H on.
Mike
Mike
5th Wheeling
Joined: Mar 2006
Posts: 38
Likes: 1
Happens to me too... I also have a Jeep Grand Cherokee (it's for sale by the way if anyone might be interested) and when I'm going down a hill or whatnot and let off the gas the transmission will "disengage" and the tach will go down to idle speed and then when you get back on the accelerator it'll reengage. The truck definately just stays in gear and the RPMs stay up.. I wonder if maybe this is better for the transmission? Kind of annoying though when you're not towing anything.
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Ok, if this is normal, then Ford must have programmed the computer to keep the TC locked with 0 throttle input. Can someone tell me it this is correct and at what speed the TC unlocks?
Mwpierson, I would expect the truck to act this way when in T/H, and true the PSD, in fact most diesel engines have high compression, compared to gas engines. But my understanding of diesel engines is that a diesel engine has little inherent compression braking, like a gas engine does. A diesel engine requires either an exhaust brake or Jake brake to slow a vehicle down with the engine. Ford uses the variable vanes in the turbo to restrict the exhaust flow, which in effect makes it an exhaust brake. What I don’t understand is why are they doing this when the trans is in normal mode?
The reason I bring this up is because; I suspect fuel economy would greatly improve on our diesel engines if what appears to be a good amount of exhaust backpressure was removed during normal operation.
Mwpierson, I would expect the truck to act this way when in T/H, and true the PSD, in fact most diesel engines have high compression, compared to gas engines. But my understanding of diesel engines is that a diesel engine has little inherent compression braking, like a gas engine does. A diesel engine requires either an exhaust brake or Jake brake to slow a vehicle down with the engine. Ford uses the variable vanes in the turbo to restrict the exhaust flow, which in effect makes it an exhaust brake. What I don’t understand is why are they doing this when the trans is in normal mode?
The reason I bring this up is because; I suspect fuel economy would greatly improve on our diesel engines if what appears to be a good amount of exhaust backpressure was removed during normal operation.
Elder User
Joined: Apr 2004
Posts: 797
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From: Wickenburg, AZ
If you're off the gas enough with a diesel, fuel flow is cut off completely and the thing just deadrolls. It's been that way with diesels for as long as I can remember. Remember, the only variable you have to control engine speed on a Diesel is the amount of fuel you put through it.
On the plus side, if you do run up the backpressure while no fuel is going in, there is no consequence in terms of emissions or fuel consumption.
As to the Torque Converter, the computer is going to keep it locked in unless a load comes along that is too heavy for the engine to efficiently pull through.
On the plus side, if you do run up the backpressure while no fuel is going in, there is no consequence in terms of emissions or fuel consumption.
As to the Torque Converter, the computer is going to keep it locked in unless a load comes along that is too heavy for the engine to efficiently pull through.
Senior User
Joined: Jul 2002
Posts: 364
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From: Redding, CA
Originally Posted by Cowboy Brett
As to the Torque Converter, the computer is going to keep it locked in unless a load comes along that is too heavy for the engine to efficiently pull through.
Thanks,
Mike
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Elder User
Joined: Apr 2004
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From: Wickenburg, AZ
I'll try to elaborate as best as I can. A torque converter, of course, involves what amounts to two turbines immersed in transmission fluid. One turbine is connected to the engine, while the other is connected to the mainshaft in the transmission. The spinning turbine on the engine side transfers energy to the transmission side turbine through the fluid, much the same way that if you stacked two box fans in front of each other but turned only one of them on, the other fan would start to spin as well.
There are a number of advantages to the hydraulic drive that makes the torque converter work, but efficiency is not one of them. The efficiency lags behind that of a manual transmission.
Lockup torque converters are a clever way to make automatics more efficient. Once the vehicle has reached a steady speed and the load on the engine is constant, the hydraulic drive is no longer really needed to keep it going. So, when the computer feels those conditions apply, it fires a solenoid in the transmission that engages splines that make a direct connection between the two turbine sets. With a solid connection, more energy gets transferred across the torque converter.
You can't use lockup all the time, of course, because it basically makes your automatic like a stick shift without the clutch! Obviously as you come to a stop, the converter is unlocked. The other time that you don't want the torque converter locked up is when you place a large load on the engine. That usually occurs when you accelerate from stops or climb a hill.
The computer, of course, is programmed to minimize emissions, maximize efficiency, and keep the engine running. If the computer thinks it needs the hydraulic amplification from the torque converter, it unlocks it. It is going to make that call based on how far down you have the gas pedal, what the exhaust backpressure is, where your boost is, and a number of other sensor inputs it has available.
By "load", I mean how much of a demand you are placing on the engine. The engine is going to need to work a lot harder to make the truck increase speed by 25 MPH while climbing a hill than it will to do the same thing cruising across the plains. When the torque converter is locked up, it is just like a manual. There is no traditional "clutch", so in order to gear down or even just use the hydraulic advantage it has to unlock first.
I hope this helps out.
There are a number of advantages to the hydraulic drive that makes the torque converter work, but efficiency is not one of them. The efficiency lags behind that of a manual transmission.
Lockup torque converters are a clever way to make automatics more efficient. Once the vehicle has reached a steady speed and the load on the engine is constant, the hydraulic drive is no longer really needed to keep it going. So, when the computer feels those conditions apply, it fires a solenoid in the transmission that engages splines that make a direct connection between the two turbine sets. With a solid connection, more energy gets transferred across the torque converter.
You can't use lockup all the time, of course, because it basically makes your automatic like a stick shift without the clutch! Obviously as you come to a stop, the converter is unlocked. The other time that you don't want the torque converter locked up is when you place a large load on the engine. That usually occurs when you accelerate from stops or climb a hill.
The computer, of course, is programmed to minimize emissions, maximize efficiency, and keep the engine running. If the computer thinks it needs the hydraulic amplification from the torque converter, it unlocks it. It is going to make that call based on how far down you have the gas pedal, what the exhaust backpressure is, where your boost is, and a number of other sensor inputs it has available.
By "load", I mean how much of a demand you are placing on the engine. The engine is going to need to work a lot harder to make the truck increase speed by 25 MPH while climbing a hill than it will to do the same thing cruising across the plains. When the torque converter is locked up, it is just like a manual. There is no traditional "clutch", so in order to gear down or even just use the hydraulic advantage it has to unlock first.
I hope this helps out.
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Brett Foote
Senior User
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Originally Posted by Cowboy Brett
I'll try to elaborate as best as I can. A torque converter, of course, involves what amounts to two turbines immersed in transmission fluid. One turbine is connected to the engine, while the other is connected to the mainshaft in the transmission. The spinning turbine on the engine side transfers energy to the transmission side turbine through the fluid, much the same way that if you stacked two box fans in front of each other but turned only one of them on, the other fan would start to spin as well.
There are a number of advantages to the hydraulic drive that makes the torque converter work, but efficiency is not one of them. The efficiency lags behind that of a manual transmission.
Lockup torque converters are a clever way to make automatics more efficient. Once the vehicle has reached a steady speed and the load on the engine is constant, the hydraulic drive is no longer really needed to keep it going. So, when the computer feels those conditions apply, it fires a solenoid in the transmission that engages splines that make a direct connection between the two turbine sets. With a solid connection, more energy gets transferred across the torque converter.
You can't use lockup all the time, of course, because it basically makes your automatic like a stick shift without the clutch! Obviously as you come to a stop, the converter is unlocked. The other time that you don't want the torque converter locked up is when you place a large load on the engine. That usually occurs when you accelerate from stops or climb a hill.
The computer, of course, is programmed to minimize emissions, maximize efficiency, and keep the engine running. If the computer thinks it needs the hydraulic amplification from the torque converter, it unlocks it. It is going to make that call based on how far down you have the gas pedal, what the exhaust backpressure is, where your boost is, and a number of other sensor inputs it has available.
By "load", I mean how much of a demand you are placing on the engine. The engine is going to need to work a lot harder to make the truck increase speed by 25 MPH while climbing a hill than it will to do the same thing cruising across the plains. When the torque converter is locked up, it is just like a manual. There is no traditional "clutch", so in order to gear down or even just use the hydraulic advantage it has to unlock first.
I hope this helps out.
There are a number of advantages to the hydraulic drive that makes the torque converter work, but efficiency is not one of them. The efficiency lags behind that of a manual transmission.
Lockup torque converters are a clever way to make automatics more efficient. Once the vehicle has reached a steady speed and the load on the engine is constant, the hydraulic drive is no longer really needed to keep it going. So, when the computer feels those conditions apply, it fires a solenoid in the transmission that engages splines that make a direct connection between the two turbine sets. With a solid connection, more energy gets transferred across the torque converter.
You can't use lockup all the time, of course, because it basically makes your automatic like a stick shift without the clutch! Obviously as you come to a stop, the converter is unlocked. The other time that you don't want the torque converter locked up is when you place a large load on the engine. That usually occurs when you accelerate from stops or climb a hill.
The computer, of course, is programmed to minimize emissions, maximize efficiency, and keep the engine running. If the computer thinks it needs the hydraulic amplification from the torque converter, it unlocks it. It is going to make that call based on how far down you have the gas pedal, what the exhaust backpressure is, where your boost is, and a number of other sensor inputs it has available.
By "load", I mean how much of a demand you are placing on the engine. The engine is going to need to work a lot harder to make the truck increase speed by 25 MPH while climbing a hill than it will to do the same thing cruising across the plains. When the torque converter is locked up, it is just like a manual. There is no traditional "clutch", so in order to gear down or even just use the hydraulic advantage it has to unlock first.
I hope this helps out.
