'86 Buick Century, torque converter not locking up

Car is a '86 Buick Century, 4 cyclinder engine, 3 speed tranny (th125?). Speedo shows 80,000 miles I think, kinda thinking not 180,000 because its not to rusty for a Minnesota car. From what I can tell driving it the torque converter isn't locking up. Would like to fix problem to increase fuel milage. Blue plugin on front of tranny is plugged in. Trans fluid is nice and red. Thats basically all I've checked so far. Car had been sitting probably 6-8 years and started once a year just to circulate fluids.

I drove a '85 Century up until a few weeks ago when it died. Had this 86 as a back up so am retiring the '85 and going to use this one. The '85's TC wouldn't lock until about 3 miles after leaving home and that was after warming up a bit in the yard. This would make me think to check temp senders on the '86. Also on the '85 cruising along 55mph slowly put on the brakes the TC would unlock. Maybe on the '86 the brake switch is mis adjusted or bad. Does this same switch unlock TC and put on brake lights?

Any fuses or relays to check? Thanks.

Did it work this past summer? I used to have an '86 Celebrity with same engine/trans combo with the same problem of TC not locking up. It only happened in cold weather, and temp gauge would not move much, which led me to find out the thermostat was installed upside down. Went ahead and installed new one (the correct way) and it fixed the problem. Not saying that's what you have here, just throwing out another possibility.

Oh, and if the head bolts haven't been replaced, beware. the original ones were prone to breaking, expecially when the thermostat starts working correctly!

The solenoid that controls lock-up is probably lacquered up from the car sitting so much and sticking. You can try running some Trans-X or Seafoam conditioner through it that may help.

I got the car 6 years or so ago give or take a couple years, and just parked it because I was driving a '85 century at the time.

But here is an update:
While doing a test run I decided to push on the plug in on front of tranny really hard and lockup worked for less than a mile then kicked off, pulled over, pushed on plug, worked for less than a mile, so thats where i'm not now. somthing around the plug area isn't making a good connection.

Before the "lucky" test run it had a newer TPS from working car, all 3 temp senders from working car, brand new TCC solenoid, new brake switch. I should have tried ohming and doing some tests when I had the side cover off replacing the solenoid, which would have showed me that somthing still wasn't right between the plugin and new solenoid, but it was getting dark and cold so wanted to hurry, most everyone I talked to said its probably the solenoid which even further boosted my confidence to just replace and put back together.

Check for corroded or damaged terminals on both the plug and the tranny.

Another update. it might have somthing to do with temperature rather than the plug in.

So after pushing on plugin it would work for a mile I decided to put a ratchet strap around the whole tranny around the plug in to keep permanent pressure on the plug in and it still came out of lockup. So now it seems to be that its just the routine of pulling over, stopping for a minute and getting back into car and taking off it works for a mile or so. That might mean that during the pulled over part the coolent heats up. Take off coolent is still hot and tells TCC to lock, but after a mile of 55mph it cools it down enough to unlock TCC. In my trouble shooting attemps I spliced in a test light into the ground wire going into the tranny to see if it was getting the signal to lock up tranny. When I did this lock up worked and stayed in forever, BUT it would lock up at 25mph instead of 45. Thats where I"m at now, but instead of a test light I have the wire going to a switch, and the wire out of the switch going to ground. So when switch is off I have 1st, 2nd, and 3rd gear and no lockup (well read below). when I get up to speed I flip the switch and get lockup. Just have to remember to turn switch off going through town and at stop lights, or after braking for a deer.

Update again: with switch off after climbing a long hill lock up works, this also might point to it being a temp issue since I'm sure the coolent gets hot climbing the hill, no idea how long it would stay locked up after the hill climb because by that time I'm to the next curve or somthing and am below 45mph.

I'm a little scared to dig into the temp problem after reading what an above post said about head bolts breaking.

I had a problem with the TCCS in a few of my cars, unplugged them and they worked fine. Put 15k miles on my century that way before it was totaled and never had a problem. But if it is bad and causing that much of a problem, you should get it replaced.