Comment on Overdrive Behavior

[jonroe]

Just a quick report that I was very pleased in the behavior of the MM using overdirve in the mountains.

Drove from DC to north central West Virginia and back this weekend (500 miles) across I-68 through the Appalachians. Set my speed control at 75 most of the way and noticed that the MM NEVER pulled out of overdrive climbing ANY of the long hills on I-68. I've driven many cars which hunt in and out of OD going up and down long mountain hills like these. I fully expected some of the behavior and saw none of it.

Now, on to fixing the around the town shift patterns with tuning the PCM.

Jon

[CRUZTAKER]

Be aware that the OD clutch plate is the first to go. Not wise running hard in OD. :nono:

[Haggis]

It does not sound like he was running it hard to me, 75mph is a pretty common speed to run in OD, set it and forget it. If he had a problem with the OD clutch he ran the perfect test for it by going up hills in OD with the cruise control on. If the OD started to come off and on on him he would then have had a problem with the OD clutch or C-clip. Mac had posted a Thread about this a while ago on how to test the OD and C-clip, I will try and find it and post it here.

[TripleTransAm]

I think 75 mph in overdrive puts the 4.6 close enough to its power band such that the economy-minded tranny programming will resist downshifting. I think the increased wear come as a result of downshifting or upshifting under power (ie. the 3-4th upshift or downshift is the thing that causes most damage under power since it's factory-designed to be slushy, but things hold up okay as long as the tranny doesn't have to do anything...) but what do I know...

Has anyone realized (on the stock program) that most of the time you get a 'downshift to 3rd' when opening the throttle at highway speeds (not WOT), you're actually just slipping the torque converter while the tranny stays in 4th? In other words, it seems to take a hell of a romp on the throttle to really get it to both unlock the torque converter to 0% AND get the downshift to 3rd. Most of the times, the TCC just smokes down to 50-55% slippage rate from 100% lockup and one *thinks* that the tranny has downshifted for passing! Romp on it a bit more and you get 3rd finally. Hit WOT, and you probably will grab 2nd depending on your cruising speed (haven't tested this scenario with Autotap yet).

I wonder if they did this on purpose...
(ie. "we know the tranny is weak on the 3-4 or 4-3 under power, so let's make the b*tch stay in 4th as long as possible even when the driver demands passing power - in that case we'll just slip the TCC a bunch to raise revs, but really try and keep a 4-3 downshift from happening unless we REALLY have to..." )

[TripleTransAm]

Here's an interesting test to prove this tendency to hold 4th gear as long as humanly and/or mechanically possible...

With the stock programming, accelerate to the speed where both 4th and TCC=100% is reached (on my car, about 38 mph). At this point, letting off the gas will unlock the TCC to 0% and you will decelerate. If you reapply throttle at say 31 or 32 mph, you will see the RPM raise to higher than the RPM you had in 4th at 40 mph, so you figure the tranny downshifted to 3rd because your speed decreased below the downshift threshold.

Wrong: the tranny is still in 4th, and the TCC is slipping at 50-55%. To prove this, maintain your speed below the original 4th gear threshold (in my case, lower than 38 mph). Disable OD with the button. Wow, the RPM has increased yet further! NOW you got your 4-3 downshift! The TCC immediately goes to 100% once a second has passed in 3rd.

So everytime you let the car go into 4th (on stock programming, possibly on some modified programs too, who knows) and let it fall below the threshold where it upshifted to 4th and reapply throttle to hold that speed, you're not in 3rd with TCC=100%. You will cruise in 4th with TCC = 50-55% as long as you don't cross above the original 3-4th upshift point (38 mph in my case). And you will be BEATING the living *****e out of your TCC and tranny fluid as long as you cruise in that speed.

No wonder the damned TCC shudders at such ridiculously low mileage!

I really gotta find some cash to get that tuner s/w...

[SergntMac]

Zack adjusted my shift program and OD just last night. I haven't that much seat time with it, but so far I like it. The TC lock up in 3rd gear occurs at 50 MPH and the shift to OD occurs at 60 mph. My drive to work this AM kept me at these speeds and it was very comfortable. I need to do some more testing, but so far, so good.

My test for good/bad OD is as stated above. Drive up a steady incline with the cruise control and OD turned on. If the incline isn't too steep, OD should pull you up the incline. If it's blown, the tranny will slip and you'll lose power. If the tranny downshifts, the incline is too steep for the test.

[cyclone03]

Here's an interesting test to prove this tendency to hold 4th gear as long as humanly and/or mechanically possible...

With the stock programming, accelerate to the speed where both 4th and TCC=100% is reached (on my car, about 38 mph). At this point, letting off the gas will unlock the TCC to 0% and you will decelerate. If you reapply throttle at say 31 or 32 mph, you will see the RPM raise to higher than the RPM you had in 4th at 40 mph, so you figure the tranny downshifted to 3rd because your speed decreased below the downshift threshold.

Wrong: the tranny is still in 4th, and the TCC is slipping at 50-55%. To prove this, maintain your speed below the original 4th gear threshold (in my case, lower than 38 mph). Disable OD with the button. Wow, the RPM has increased yet further! NOW you got your 4-3 downshift! The TCC immediately goes to 100% once a second has passed in 3rd.So everytime you let the car go into 4th (on stock programming, possibly on some modified programs too, who knows) and let it fall below the threshold where it upshifted to 4th and reapply throttle to hold that speed, you're not in 3rd with TCC=100%. You will cruise in 4th with TCC = 50-55% as long as you don't cross above the original 3-4th upshift point (38 mph in my case). And you will be BEATING the living *****e out of your TCC and tranny fluid as long as you cruise in that speed.

No wonder the damned TCC shudders at such ridiculously low mileage!

I really gotta find some cash to get that tuner s/w...

Perfect disciption,T3.
You should try the same test with a PI converter,in OD when the converter unlocks the PM comes up but the ground speed stays the same,switching off overdrive the RPM holds but the car accelerates.
Everytime that happened all I could think of was the trans fluid temp. climbing.
AT MMII I had the computer reflashed and all is well.My set up locks the converter at about 43mph in OD and it stays locked if a downshift is called for it quickly unlocks in OD drops to 3rd the relocks in third.The sensation from the seat is the car feels like a stick.MY car even "burbles" when costing.

Some may not like my set up but it works for me.

[jstevens]

This is really good information especially for us new people who are not exactly gearheads.
Please confirm. I thought someone said if you get on it on the freeway and make it downshift to go like hell that its bad for the trans. True

I have 4.10's and usually leave the OD on as I thought you shouldn't run the engine with high rpms.
Of course, I really don't know what high rpms are for a 4.6 liter.
Thanks,