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1994 Thunderbird LX 3.8
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Discussion Starter #1
I've read a lot here about turning off the overdrive in town to prevent frequent shifting. How frequent is normal? If I'm going 35 up a hill (for example), and it's just the right grade, is it ever normal for the transmission to immediately switch back and forth between 3rd and 4th gears-pretty much continuously for the few seconds going uphill? (I'm shutting off the overdrive until after that part of my commute now, obviously...)

My somewhere-between-stock-and-just-plain-original '94 3.8 has already had the MLPS replaced (which seems to have put an end to the shifts into 3rd at 50 on flat terrain ?). It's never had any codes (I bought it in December with under 25k miles). I'm a nervous driver coming off two years of driving a CVT, so I'm having to re-acclimate myself to a traditional automatic transmission, and I've taken the car to a couple of mechanics who can't get it to misbehave for them, so maybe I'm the problem?
 

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Stupid question, but how sure are you that the trans is actually shifting back and forth? The early transmissions are known for torque converter shudder when the converter locks up under moderate load. This will usually feel like driving over rumble strips, and won’t last very long. If that is the issue, then changing the fluid with fresh MerconV might help, but it isn’t going to hurt anything.
 

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1994 Thunderbird LX 3.8
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Discussion Starter #3
It doesn't feel rough exactly, just like the clutch engages, the tachometer moves a bit (nothing extreme), and it happens a few times in a row. I have had the fluid changed (not probably real Mercon V... "Universal full synthetic"). I try to drive gently... Maybe my indecision is contagious ???
 

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If it's shifting back and forth, there might be an issue. These cars are a little clunky when going from 4-3 while on the throttle a little, but shouldn't shift back and forth. When my car downshifts to 3rd going uphill, there's the shift, then the converter locks up. So there is 2 events.

If everything is working normally, there should be no reason to turn off O/D, unless you plan on doing some WOT pulls. Or if you get stuck in city traffic with a 35 speed limit. My car clicks into O/D at 35, and stays in it 90% of the time. If you're turning off O/D, your gas mileage will be pretty rough.

Al
 
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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Based on your description it sounds like it could also be the torque converter locking and unlocking. A complete shift from 4th to 3rd would include an RPM increase of 35% but if the converter unlocks, it may jump as little as 50-100 RPM. There is logic in the PCM to unlock the converter before downshifting to take advantage of the torque multiplication at a given speed and load, so if it's "riding the line" it might unlock, then think it's appropriate to lock again, etc.

There's a possibility the TPS is playing a part in the odd behavior as well (speed vs. TP are the two main factors that determine shift/lockup schedule under normal conditions). If you want to check it, use a multimeter and measure voltage across the entire range of operation. It should start at under 1V for idle, and progress smoothly and evenly (with no sudden spikes or sags) all the way to WOT of at least 4.5V. If it has sags/spikes at the TP you're at while this condition occurs, that would explain the odd behavior.

Otherwise if the car truly does have under 30k, it's a good idea to replace the fluid with Mercon V (not Mercon LV) anyway as what's in there may be the 25+ year old original!
 

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1994 Thunderbird LX 3.8
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Discussion Starter #6
It doesn't feel rough exactly, just like the clutch engages, the tachometer moves a bit (nothing extreme), and it happens a few times in a row. I have had the fluid changed (not probably real Mercon V... "Universal full synthetic"). I try to drive gently... Maybe my indecision is contagious ???
 

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I like everything theterminator93 said. Sounds like the converter is locking and unlocking. It will also unlock the converter if you come off the throttle or tap the brake pedal.

You can test the unlocking by cruising at a steady state when it gets to OD and the converter locks and then tap the brake with your left foot. the RPM should jump a couple hundred RPM then it should lock again a few seconds later after you release the brake.

See if that feels like what is happening at 35 up the hill.
 
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The only time I disable overdrive is going up a 2 - 3 mile hill with a 50mph speed limit thru some state parkland. RPMs with OD enabled at 50MPH going up this hill with the 4.6 v8 seems to be way off the power/torque curve of the engine. Feels like I'm driving a 1965 40hp Volkswagon. With OD off going up this hill RPMs are around 2k and the engine seems to perform better. - Steve
 

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1994 Thunderbird LX 3.8
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Discussion Starter #9
Thank you all! I appreciate all the advice. I sent a link to my big brother, who can hopefully translate ? He could probably even walk my 15-yo through the TPS test. I'm the German/econ major in a family of EEs ? Thankfully my oldest son takes more after my family than after me!
 

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Ever since I owned my 928, and then my 90 Ford Bronco, I've followed the same routine: Overdrive off around town, and on when on the highway. I might suffer a MPG or two for a short time, but it is easy to justify to cause less wear on the hardware, and easier driveability. If you're solely interested in MPG, you're not driving these cars anyway. I drove my '66 mustang for almost ten years, with a final drive of 3:1, and never felt there was any problem not having an overdrive.
 

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1994 Thunderbird LX 3.8
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Discussion Starter #11
Yes, I only turn it off until I get on the back roads. If it is the TC lock up issue, it should only act up right at the shift point? I need to do some more research on that. My brother reminded me that our sister had an MN12 Cougar back in the day and that it had the TC problem the whole time. He also said the TPS problem would be more likely on a dewy morning like when it acted up.

You all are the best!
 

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The TC issue does not necessarily happen only at the time of the shift. Generally the shift will be completed, and then the TC lockup will be commanded shortly afterward.
 

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1994 Thunderbird LX 3.8
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Discussion Starter #13
Ok, but it's not going to do weird unlocking/locking maneuvers when I'm sailing down the highway at 70, right? It did do some weird things like that before I had the MLPS replaced (downshifting (?) at 50 mph for no apparent reason =-O), but hasn't done it since. I've done some reading, but it's just confusing my humanities/social science brain (_)
 

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I realized today that setting my Rev limiter on Lazarus to 5500, and the shift point at wot 2-3 at 4000 is a problem.

I nailed it today, and bounced off the revlimiter and did the 2-3-2-3-2-3 several times, based on revlimit and shiftpoint.

It's amazing how long it takes to get your foot out of there, lol.

Adding a new tune tomorrow. :)

Lazarus is over 300k, and has a PI intake; I'm gonna say F'it and move the rev limit to 6k.

:D
 

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No, it should not unlock at 70mph, unless of course you gave it more gas, but then it should unlock, and stay unlocked until you let off. The MLPS could have caused that issue though, so if it hasn’t done it since replacing that, I would call that problem fixed.
 
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Discussion Starter #16
Hopefully last question on this one... We got in a new TPS from Rock Auto. Son is doing resistance tests (because he couldn't get the sewing needle trick to work... I guess wire-piercing leads are going onto the wish list!) and discovered a significant disparity in total range. The one in the car is 3.3, new one is 4.6 (what do you measure resistance in, ohms? He went inside!). Did Rock Auto send us the wrong part, or is this evidence of a problem with the old one?
 

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Resistance is in ohms or k Ohms. 1000 ohms is a k(ilo) ohm.

The TPS is a potentiometer, from what I've seen it should be about 2-4 kohms. (I've measured these on a lot of cars, I don't remember what ours is offhand.)

With the key in the on position (KOEO, Key on, engine off), There's 5V power on one lead, the other end is ground, the middle leg is the signal we're interested in.

It should be the same voltage KOER, Key on engine Running; but don't jack the throttle around like running; hitting the revlimiter is bad, M'kay?

The important part is what it's telling the computer; The signal lead should be ~.99V at rest, and ~5V at WOT. (pushing the throttle blade full open. Don't do that with the engine running, lol.)

The sewing needle trick should work; I've used it a bunch.

This is overkill, but it checks all the boxes you would ever need:

 

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Discussion Starter #18
Well... I thought turning off the overdrive in town was going to be a reasonable fix, since the mechanics kept telling me the car was fine. It wasn't. Yesterday I had to brake fairly hard on one of my country roads to avoid a vulture collision, and that was apparently the last straw. It acted very strangely, even with the overdrive off, but got me to work. I was so desperate I called poor famous Rob in Carthage out of the blue, but he thought it sounded like a misfire until I described what could only be the transmission not engaging. I pulled out of the parking lot onto the road, and immediately knew I wasn't driving the rest of the way to the transmission shop, turned off the state highway, pulled over, and called a tow truck. Still waiting to hear the verdict. So thankful for friends who'll let a girl into their car in these virus-consumed times.
 

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Braking hard and turning leading to problems makes me think it's an electrical problem.

I noticed recently that my Tbird, who has problems in the rain, also doesn't like hard bumps; I'm going to look at the connectors this weekend.

I can say, if you take your car to Rob, it will get fixed. He's really good, and a great guy as well.
 

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1994 Thunderbird LX 3.8
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Discussion Starter #20
I hope you figure yours out quickly. I hope Don figures mine out in the morning. I was really hoping to hear something today! It cost enough to have it towed 10 miles... I'm really counting on not having to take it to Carthage (+_+)
 
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