Transmission on hill climbs…. Normal?

[ramdough]

All,

I have notice behavior with my transmission that is different than what I am used to on civilian AT cars.

When I am climbing a hill and I want to go faster, but the transmission is already loaded and I am starting to slow down…. If I push the pedal to the floor, I am expecting a down shift….. but instead, it either won’t down shift or it will, but the TC unlocks, I lose torque, and revs jump…. Then I slow more.

Yesterday, I went from 50-40 and felt like I should have had enough torque to push me up higher

Is that normal?

Truck is a 1999 1083A1 with 4,700 miles. Stock gearing.


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[Ronmar]

The Throttle Position Sensor input plays a big roll in how the transmission responds IRT up and downshifts. If it is not inputting proper info about how you are commanding the pedal(desire to accelerate or decelerate), all the trans can do is wait till a hi or low RPM is reached to up or downshift…

The TC remains 100% locked in gears 3-7… it only unlocks in 2nd(and 1st) as the vehicle can be stopped in those gears. Lockup comes on as soon as you hit 3rd. I think that is why these things shift so harshly 2-3 when lightly loaded, because the programming cannot pulse/roll-on the TC lockup any softer.

On an automatic all the gears are fully meshed all the time, so in 3rd and above, the transmission shifts by relaxing one set of clutches and engaging another set(pulse width modulates the electric current feeding the solenoid valves). It learns or optimizes these shifts by monitoring RPM And the time it takes to get from one ratio to the next.

There are 3 RPM sensors. Input or engine RPM at the TC housing, turbine or midsection RPM(input to gearbox from TC) and final output RPM. The TCU is expecting to see proper ratios between these RPM inputs. Any slippage should show as an incorrect ratio which should store a fault code.

if you are getting slip above 2nd gear, it could be pressure, it could be worn or failed clutch friction material or a valve problem, but the controller should record it.

pushing the up and down arrow simultaneously will put you in diagnostic mode and the display will flash D1, followed by 2 sets of numbers. This cycle will repeate. Pressing mode will step thru the 5 memory locations D1-D5. Pressing both arrows will exit diag mode, pressing and holding both arrows for 10 seconds will clear all stored codes…

 

[GeneralDisorder]

Torque converter is not supposed unlock unless it is downshifting to 2. Doesn't sound like that's the case at 40 MPH......

If it downshifts and RPM climbs but you lose speed..... sounds like engine power is down or transmission is slipping in the lower gear. Has the transmission and filters been serviced?

Wouldn't be a bad idea as Ronmar suggested to check what the Allison computer is seeing for TPS and what the CAT computer is outputting for TPS to ensure they are in agreement. Also scanning all the computers for codes and looking at live data could be useful.

 

[DeMilitarized]

Torque converter is not supposed unlock unless it is downshifting to 2. Doesn't sound like that's the case at 40 MPH......

If it downshifts and RPM climbs but you lose speed..... sounds like engine power is down or transmission is slipping in the lower gear. Has the transmission and filters been serviced?

Wouldn't be a bad idea as Ronmar suggested to check what the Allison computer is seeing for TPS and what the CAT computer is outputting for TPS to ensure they are in agreement. Also scanning all the computers for codes and looking at live data could be useful.

Transmission pump is probably getting weak and may need to pull the transmission if all else fails.

 

[MrMikey4026]

Make sure you are actually getting full throttle. When the pedal is all the way down, the throttle lever on the governor should be lightly on the stop.

 

[GeneralDisorder]

Transmission pump is probably getting weak and may need to pull the transmission if all else fails.

These trans like to throw the C6 thrust bearing and plug up passages and filter screens also. I doubt the pump is worn out at 5k miles but a C6 failure is definitely possible. One of the benefits of the ECO hubs is dropping the transmission output speed by half for any given road speed. Less RPM on things like that C6 means less heat buildup.

The transmission computer can detect slipping by comparing it's multiple speed sensors. Best to drive it with live data and watch input and output speeds. Pretty easy to tell if there's slippage from the data.

 

[GeneralDisorder]

Make sure you are actually getting full throttle. When the pedal is all the way down, the throttle lever on the governor should be lightly on the stop.

The 3126b engine in the A1 has no throttle lever. Pedal is electronic.

 

[MrMikey4026]

The 3126b engine in the A1 has no throttle lever. Pedal is electronic.

Your right!

 

[ramdough]

Isn’t there a throttle pedal relearn procedure or is that for an A0?


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[ramdough]

The Throttle Position Sensor input plays a big roll in how the transmission responds IRT up and downshifts. If it is not inputting proper info about how you are commanding the pedal(desire to accelerate or decelerate), all the trans can do is wait till a hi or low RPM is reached to up or downshift…

The TC remains 100% locked in gears 3-7… it only unlocks in 2nd(and 1st) as the vehicle can be stopped in those gears. Lockup comes on as soon as you hit 3rd. I think that is why these things shift so harshly 2-3 when lightly loaded, because the programming cannot pulse/roll-on the TC lockup any softer.

On an automatic all the gears are fully meshed all the time, so in 3rd and above, the transmission shifts by relaxing one set of clutches and engaging another set(pulse width modulates the electric current feeding the solenoid valves). It learns or optimizes these shifts by monitoring RPM And the time it takes to get from one ratio to the next.

There are 3 RPM sensors. Input or engine RPM at the TC housing, turbine or midsection RPM(input to gearbox from TC) and final output RPM. The TCU is expecting to see proper ratios between these RPM inputs. Any slippage should show as an incorrect ratio which should store a fault code.

if you are getting slip above 2nd gear, it could be pressure, it could be worn or failed clutch friction material or a valve problem, but the controller should record it.

pushing the up and down arrow simultaneously will put you in diagnostic mode and the display will flash D1, followed by 2 sets of numbers. This cycle will repeate. Pressing mode will step thru the 5 memory locations D1-D5. Pressing both arrows will exit diag mode, pressing and holding both arrows for 10 seconds will clear all stored codes…

Can this be done power on but engine off, or do I need the engine running?


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[ramdough]

Torque converter is not supposed unlock unless it is downshifting to 2. Doesn't sound like that's the case at 40 MPH......

If it downshifts and RPM climbs but you lose speed..... sounds like engine power is down or transmission is slipping in the lower gear. Has the transmission and filters been serviced?

Wouldn't be a bad idea as Ronmar suggested to check what the Allison computer is seeing for TPS and what the CAT computer is outputting for TPS to ensure they are in agreement. Also scanning all the computers for codes and looking at live data could be useful.

Filters and oil were done by seller a few years back. Only has a few hundred miles on it since then.


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[Ronmar]

Can this be done power on but engine off, or do I need the engine running?


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Checking codes? Anytime power is applied...

 

[Skyhawk13205]

All,

I have notice behavior with my transmission that is different than what I am used to on civilian AT cars.

When I am climbing a hill and I want to go faster, but the transmission is already loaded and I am starting to slow down…. If I push the pedal to the floor, I am expecting a down shift….. but instead, it either won’t down shift or it will, but the TC unlocks, I lose torque, and revs jump…. Then I slow more.

Yesterday, I went from 50-40 and felt like I should have had enough torque to push me up higher

Is that normal?

Truck is a 1999 1083A1 with 4,700 miles. Stock gearing.


Sent from my iPhone using Tapatalk

Is this a new problem? Or has the truck always done this?

torque converter lock is 3 and higher. I think your C4 clutch is for range 3 and 7, C1 is for range 2,3,4,5. If you are concerned about clutch slipping, check the clutch pressures, and do a stalls test for the C1 and C4. If your clutch pressures are low you might have an internal module leak. If you pressures are good you might have a worn clutches. From watching the forms it seems that clutch failures seem pretty rare, I think most clutch failure are due to the transmission filling with glycol from the heat exchanger. You can also check you fluid quality, see if the fluid is aerated or dirty.

you can also check the TPS, with CAT ET you can see if the TPS has full range, Allison doc can show if the transmission ECU is getting full range.

 

[ramdough]

The Throttle Position Sensor input plays a big roll in how the transmission responds IRT up and downshifts. If it is not inputting proper info about how you are commanding the pedal(desire to accelerate or decelerate), all the trans can do is wait till a hi or low RPM is reached to up or downshift…

The TC remains 100% locked in gears 3-7… it only unlocks in 2nd(and 1st) as the vehicle can be stopped in those gears. Lockup comes on as soon as you hit 3rd. I think that is why these things shift so harshly 2-3 when lightly loaded, because the programming cannot pulse/roll-on the TC lockup any softer.

On an automatic all the gears are fully meshed all the time, so in 3rd and above, the transmission shifts by relaxing one set of clutches and engaging another set(pulse width modulates the electric current feeding the solenoid valves). It learns or optimizes these shifts by monitoring RPM And the time it takes to get from one ratio to the next.

There are 3 RPM sensors. Input or engine RPM at the TC housing, turbine or midsection RPM(input to gearbox from TC) and final output RPM. The TCU is expecting to see proper ratios between these RPM inputs. Any slippage should show as an incorrect ratio which should store a fault code.

if you are getting slip above 2nd gear, it could be pressure, it could be worn or failed clutch friction material or a valve problem, but the controller should record it.

pushing the up and down arrow simultaneously will put you in diagnostic mode and the display will flash D1, followed by 2 sets of numbers. This cycle will repeate. Pressing mode will step thru the 5 memory locations D1-D5. Pressing both arrows will exit diag mode, pressing and holding both arrows for 10 seconds will clear all stored codes…

No error codes. I am guessing it would throw an error code if input and output RPM is not as expected?


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