Overdrive disengaging intermittently

[mattski]

My overdrive disengages, but it seems only partially because the engine speed increases only by about 200 RPMs. This happens only when I drive at highway speeds for a while. I will be doing 75 with RPMs at 2000, and then it will jump up to 2200 for a few seconds, and back down to 2000. Some days it will be fine, other days it will do this repeatedly. I have a cooler that was installed at ~60k and have been running Mobil 1 ATF, and changing it every 15k via a drain, not a flush. Changed the screen about a year ago. Car has 115k miles on it now. Any ideas would be greatly appreciated. Thanks in advance.

Matt

[oab_au]

Originally Posted by mattski

My overdrive disengages, but it seems only partially because the engine speed increases only by about 200 RPMs. This happens only when I drive at highway speeds for a while. I will be doing 75 with RPMs at 2000, and then it will jump up to 2200 for a few seconds, and back down to 2000. Some days it will be fine, other days it will do this repeatedly. I have a cooler that was installed at ~60k and have been running Mobil 1 ATF, and changing it every 15k via a drain, not a flush. Changed the screen about a year ago. Car has 115k miles on it now. Any ideas would be greatly appreciated. Thanks in advance.

Matt

Hi Matt, I think it would be more like the converter unlocking, than the box going back to 3rd. The revs would rise a lot higher than 200 if it was going pack to 3rd.
When it happens, try giving it a little bit more throttle, to see if the rpms rise straight away, or the rpms slowly rise, with road speed. If the converter is unlocking the rpms will rise straight away.

Harvey.

[mattski]

Originally Posted by oab_au

Hi Matt, I think it would be more like the converter unlocking, than the box going back to 3rd. The revs would rise a lot higher than 200 if it was going pack to 3rd.
When it happens, try giving it a little bit more throttle, to see if the rpms rise straight away, or the rpms slowly rise, with road speed. If the converter is unlocking the rpms will rise straight away.

Harvey.

Harvey, it is definitely the converter unlocking, but it seems that it is not completely unlocking because the RPMs are only going up about half of what they should. And it is happening under a "no load" condition while cruising at a steady speed.

[oab_au]

Originally Posted by mattski

Harvey, it is definitely the converter unlocking, but it seems that it is not completely unlocking because the RPMs are only going up about half of what they should. And it is happening under a "no load" condition while cruising at a steady speed.

OK Mattski, lets look deeper.
So you notice a rise in rpms, under no load conditions. Just looking at the possibility's, the rise is not enough to be a complete unlocking of the converter. It is not enough for it, to be going back to 3rd, so that really leaves the possibility of something slipping.

The no load condition, is also a low line pressure condition, so there is the chance that either the converter clutch is slipping or the brake band is slipping, due to the low line pressure.

The faults that could cause the low pressure are, a fault with the dropping resister, or the Throttle Position sensor, or a restricted pickup screen in the pan. As it is a late model the pump should be OK.

I would get the line pressure checked by a transmission specialist, to check it first.

Harvey.

[Earthworm]

Normally a dirty TPS will send mixed signals fooling the ECU/TCU into thinking the throttle is being pressed more than it actually is. If you're not accelerating you're not going to notice much of an RPM change when the TC unlocks.

[mattski]

Thank you Harvey and David. I will check these things out.

Matt

[rayford]

I was having a problem with the car coming in and out of overdrive well at least the rpms would go up some and I can tell it is dropping down but not all the way to 3rd gear,, Well, I changed the TPS I think Harvey suggested that, and it isn't doing the downshifting thing now but it now idles too fast most of the time, not always but more times than not. I might not still have the TPS adjusted right even after two tries at it.. and there is a chance I might have cracked a vacume line when I was working on it.And too but it has done this before I changed the TPS.and it acts like the carburated cars did when the accellerator pump wasn't working, it hesitates some at a stop sign and when I give it a little gas but that seems to be more when it isn't completely warmed up. There is no codes showing now, I was getting a code for the left knock sensor and the left O2 sensor, I changed the oxygen sensor and now I am not getting any light, I bought a new knock sensor but since the light didn't come on , I haven't put it on yet.. anyway, does anyone know why it is idling too fast it goes up to about 1500 after I go about 5 miles to work and put it in Park, it used to not do that, might be the TPS??? .. Thanks.. Vernon.

[Earthworm]

The idle speed takes about 3 cycles before it resets to a proper RPM.

1 cycle = start, engine warmed up, stop, engine cool down

If the idle doesn't adjust then there's something else wrong.

[rayford]

well, it has been about a week since I did the work and I drive about 10 miles each day, I would think the computer would have relearned the settings by now??? But maybe it hasn't went through all the right sequence yet?