Cruise Jitter: Normal?
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Freshman User
Joined: Dec 2002
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From: Midland MI USA
Cruise Jitter: Normal?
My '99 F-350 PSD Automatic has a interesting personality when using cruise control. When pulling uphill or pushing the engine downhill (un-loaded) the cruise does a nice job of maintaining proper speed. However, when rolling down a slow grade near the 'break point' where the engine does not need to pull but also does not need to hold the vehicle back, I feel a miss, light buck, chug, jitter (call it what you like). I compare this to driving a stick shift near the same point with the clutch out but wiggling the accelerator to make the car buck.
On cars with automatics, the transmission disengages on downhill slopes to allow the car to coast. The diesels don't disengage, to allow the engine to help slow the truck on downgrades. Thus, the same program for cruise control may not work properly.
My question is, is this normal? I have never felt this miss, buck, chug, etc when modulating the speed myself...only when cruise control is on. Thus I doubt it's a real engine miss nor a transmission problem.
Thanks,
On cars with automatics, the transmission disengages on downhill slopes to allow the car to coast. The diesels don't disengage, to allow the engine to help slow the truck on downgrades. Thus, the same program for cruise control may not work properly.
My question is, is this normal? I have never felt this miss, buck, chug, etc when modulating the speed myself...only when cruise control is on. Thus I doubt it's a real engine miss nor a transmission problem.
Thanks,
Elder User
Joined: Aug 2002
Posts: 656
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From: South of you
Cruise Jitter: Normal?
Originally posted by mediumfry
My '99 F-350 PSD Automatic has a interesting personality when using cruise control. When pulling uphill or pushing the engine downhill (un-loaded) the cruise does a nice job of maintaining proper speed. However, when rolling down a slow grade near the 'break point' where the engine does not need to pull but also does not need to hold the vehicle back, I feel a miss, light buck, chug, jitter (call it what you like). I compare this to driving a stick shift near the same point with the clutch out but wiggling the accelerator to make the car buck.
On cars with automatics, the transmission disengages on downhill slopes to allow the car to coast. The diesels don't disengage, to allow the engine to help slow the truck on downgrades. Thus, the same program for cruise control may not work properly.
My question is, is this normal? I have never felt this miss, buck, chug, etc when modulating the speed myself...only when cruise control is on. Thus I doubt it's a real engine miss nor a transmission problem.
Thanks,
My '99 F-350 PSD Automatic has a interesting personality when using cruise control. When pulling uphill or pushing the engine downhill (un-loaded) the cruise does a nice job of maintaining proper speed. However, when rolling down a slow grade near the 'break point' where the engine does not need to pull but also does not need to hold the vehicle back, I feel a miss, light buck, chug, jitter (call it what you like). I compare this to driving a stick shift near the same point with the clutch out but wiggling the accelerator to make the car buck.
On cars with automatics, the transmission disengages on downhill slopes to allow the car to coast. The diesels don't disengage, to allow the engine to help slow the truck on downgrades. Thus, the same program for cruise control may not work properly.
My question is, is this normal? I have never felt this miss, buck, chug, etc when modulating the speed myself...only when cruise control is on. Thus I doubt it's a real engine miss nor a transmission problem.
Thanks,
FTE Leadership Emeritus
Joined: May 2001
Posts: 13,653
Likes: 6
From: Cabot, Arkansas
Cruise Jitter: Normal?
Since I drive the handshaker I normally don't pay much attention to the auto tranny threads, however, in reguards to this:
I'm pretty sure I've read that the 4R100's have a coast clutch. I'm not sure what that is, but folks have had problems with them before.
On cars with automatics, the transmission disengages on downhill slopes to allow the car to coast. The diesels don't disengage, to allow the engine to help slow the truck on downgrades. Thus, the same program for cruise control may not work properly.
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Freshman User
Joined: Dec 2002
Posts: 46
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From: Midland MI USA
Cruise Jitter: Normal?
Thanks for the replies so far. I experience the issue only with cruise control on and this is usually at 60-70mph. I have no issue with gear hunting or Torque Converter lock-up (that I know of). I did have the torque converter replaced less than 2000 miles ago, however. I never noticed the problem before the TC replacement, but I also only had the truck for ~500 miles before the TC was replaced. To my knowledge I hadn't driven with the cruise on beforehand.
As for the coast clutch, I'll have to look into that. I can definitely feel that the tranny stays engaged when letting off the accelerator, thus allowing the engine to slow the truck down. This has been the case with every diesel/automatic I've driven. Thus the situation only seems to occur near the point where the cruise is trying to apply zero force to the truck (neither pull it forward nor push it back). As soon as the truck moves onto a steeper downgrade (where the cruise asks for steady drag) or turns uphill (where the cruise asks for steady pull) the jitter disappears.
Later,
Medfry
As for the coast clutch, I'll have to look into that. I can definitely feel that the tranny stays engaged when letting off the accelerator, thus allowing the engine to slow the truck down. This has been the case with every diesel/automatic I've driven. Thus the situation only seems to occur near the point where the cruise is trying to apply zero force to the truck (neither pull it forward nor push it back). As soon as the truck moves onto a steeper downgrade (where the cruise asks for steady drag) or turns uphill (where the cruise asks for steady pull) the jitter disappears.
Later,
Medfry
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