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Basically the car is a 94/95 GT, it's a 91 Thunderbird 5.0 with a 4R70W conversion (from AOD) and an 94/95 GT Auto EEC (U4P0).

Let me give you a run down on what I've diagnosed so far.

Well the car was all together Saturday night and has ran perfect (despite tune) until tonight. Tonight we put about 10-15 miles on the car with no problems. Suddenly while leaving a stop sign the car bogs down really bad while accelerating, and goes nowhere! I unplugged the wiring from the EEC to the transmission and I have 2nd and reverse which works fine. Mechanically it sounds fine, no noises or anything, and the fact that it was so sudden, we really dont think its the transmission, but something electrical. So we limped it home 10 miles in 2nd gear, with 4.10's out back, that wasn't much fun. Checked all the obvious, fluid level is fine, no burnt smell or color, trans cooler is working, heats up/cools down. We're out of ideas and time. Unless we can fix it soon there's no dyno for me. I am already trying to figure out how much I can get by parting it out.

Update #1: Hadn't messed with it any yet, went out to start it up and now it starts right up and dies right away, unless you unplug the harness going to the trans then it will idle fine. So I checked all the circuits, all had continuity and none were shorted to ground. So I dropped the pan, fluid was clean, nothing on the magnet. All the valvebody bolts were tight and EPC hadn't popped out, but I went ahead and replaced it with a new one I had laying around. Still same problem, won't run with trans plugged in, but if you unplug it it will, it has to be electrical but I'm lost at this point.

Update #2: I've further narrowed down the problem, I went under the car and left the MLPS, VSS, and OSS plugged in while unplugging just the transmission connector for the solenoids, car would start and idle fine. So I pulled the single EPC signal pin from the harness out of the connector and plugged it back in and the car idled fine, so I've narrowed it down to the EPC circuit (EEC, wiring, solenoid) the thing is I replaced the solenoid this morning and checked the circuit for short to ground. Is it possible the EEC just went belly up for no reason? Maybe something with the valvebody? At least I can see some light at the end of the tunnel now.

Anyone have any suggestions?

-J
 

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I'm a bit confused by the symptom, but here goes nothing.
The EEC could be dead, but i'd want to be sure before sending you out to spend $50.

Did you pull any of the other pins individually and check the results? Make sure the wiring harness isnt somehow grounding or shorting out as well. If the solenoid itself is within resistance and specs, then i'd pull the EEC connector and check the circuit from there (if you havent already). If the resistance is way off, then theres the problem. EPC solenoid "Resistance should be between 2.48 and 5.66 ohms." Add about 5ohms for the wiring to and from... as a rough estimate from what my CD manual seems to say.

I'm thinking along the same lines you are... you're on to it IMO.
 

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I wouldn't add 5 ohms to the reading for wiring losses, however, more like .5 ohms.
Sounds suspiciously like a wiring short or a bad EEC to me...
Maybe a harness pinched around the recent work?
 
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