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Discussion Starter #1
1996 T-bird 4.6L with a newly remanufactured transmission.

At cruising speed (doesn't matter if 45 or 65, just constant speed) the TC will randomly unlock and then lock up. I see the RPM's increase by about 1K then go back down. Any idea what is causing this? The car runs great, I don't have any codes popping beyond some environmental codes, and the transmission is fine otherwise. Cruise control works fine (also does this while in CC mode) and I don't have any problems with unlocking when I brake.

I should note that it did this with the old transmission, and was one of the many reasons I used to justify the new tranny. So I doubt it's a transmission issue. I am thinking electrical or ECM related. I am often known to be wrong, though...
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Test the TPS. Make sure the voltage is less than 1 at idle and over 4.5 at WOT - and a smooth increase as you go (no spikes or dips).
 

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Discussion Starter #3
Ok, so I replaced the TPS, mostly because it was cheap and my DVOM isn't going to catch any nuances in the voltage very well, not like the analog meters. Did not fix the problem. I did notice that the plug shell is falling apart, so that's going to need replacing, but the plug fit is fine and is not moving.
One other thing I noticed while doing all this is that my A/C lines are frosted over. I have been using heat only, no A/C (it's winter...). Is this normal? I don't mean to distract from my TC unlock problem, just though I'd throw that out there...
 

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no it shouldn't be frosted over, thats indicative that the a/c clutch is engaged, verify the clutch is disengaged while the engine is running and the a/c if off, could possibly be a failed a/c relay

the brake circuit can also unlock the converter, so if you have wiring issues with the brake lights in the trunk area it can cause it

another possibility is that you have the wrong tcc solenoid for your ecu, the 95 tbird uses a 1 ohm the 96+ uses a 10 ohm, but if it did it before thats unlikely

are you sure you don't have corrupt tune on the ecu ?
 

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Discussion Starter #5
To the best of my knowledge nothing has been done to the ECU. I will check the AC relay. How much damage other than to my nerves am I allowing to happen letting the TC continue to unlock this way?
 

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IIRC, the stock tune will do this on the interstate, with the ac on, as egr or something cycles. :zdunno:

Problems are likely related. :)
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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The A/C lines should never be cold enough to be frosted over. Check to be sure the low side cycling switch isn't closed when it's removed from the accumulator (there is a schrader valve under the switch - you can remove it safely to test it).

The stock program will allow some TC slippage to cushion the feel of engagements as the A/C clutch cycles, but not so much as to completely unlock.

Also... assuming a stock TC... you should not be seeing an RPM variation of anywhere near 1000 RPM if the TC is unlocking while cruising. 200-300 RPM sure, but 1000 RPM is more indicative of the trans dropping into neutral completely. Even a high stall converter will only cruise ~500 RPM higher unlocked than locked.
 

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Discussion Starter #8
I like looking for common solutions. Definitely going to look down the A/C route till I have another lead to go on. Thanks!
 

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Discussion Starter #9
Pulled the a/c clutch fuse before I left for work and voila. Now gotta figure out why the clutch is engaging when I don't have the a/c on
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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A/C engages in defrost, floor/defrost, panel/floor and the A/C positions.
 

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Discussion Starter #11
So pretty much Every position but panel? Wow. Did not know that. Ok so in that case I need to figure out why my AC lines are frosting up.
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Every position but panel and floor, yes; temperature permitting. Below a certain temperature (usually between 40 and 50 degrees) the pressure in the lines decreases and the low pressure switch opens, preventing the compressor from engaging. If your compressor is still engaging in near or sub-freezing temperatures, there is a problem.
 
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