200R4 won't shift to 4th, TC not locking

Car is an '81 Cadillac deVille diesel. The transmission won't shift into overdrive and the torque converter won't lock up. This started very recently; before that it worked fine. Any ideas what exactly is causing
this, what it'll take to fix it, and how likely it is to leave one stranded (local, mostly non-highway driving)?
Would a 700R4 be a drop-in replacement for the 200R4? Bell housing, driveshaft, shift linkage, etc.? If I'm looking at major work on the 200R4, this would seem like a good option.
Thanks for any replies.
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snipped-for-privacy@yahoo.com wrote:

IIRC, the 200R4 has the BOP bolt pattern (Buick-Olds-Pontiac) whereas the 700R4 has a Chevy bolt pattern.
I also seem to recall that the 200R4 is the stronger of the two in factory form, although both can be custom built to handle any reasonable amount of power you want to throw at them.
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Steve wrote:

4th and lockup not working is a result of the 4th gear clutch pack or band getting toasted because the lockup wasn't working, in the first place ( to keep things cool) . It's only controlled by a solenoid, which is fairly easy to change, but probably wont make any difference now. I've worked with cars that use both transmissons. They, both, bolt right up to the same bell housing. Out of the box, the 700 is the stronger. You can build both to handle a lot of HP. The obvious differences are the 200 is narrower in the pan area, so changing might induce problems with the exhaust routing. The big difference is in the gearing. If you had a 200 you wouldn't like a 700..
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On Sat, 8 Mar 2008, BobJ wrote:

So, what caused it in the first place, and how do I tell whether it's still happening because of that, or because something's toasted?

Well, this is a 5000 lb. car with a 130HP engine; it doesn't do a whole lot of drag racing so gearing differences won't matter much. :)
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Anyone have any ideas on this? I'm trying to learn enough to decide whether it makes more sense to fix the 200R4, or swap it for a 700R4.
Thanks.
On Sat, 8 Mar 2008, snipped-for-privacy@yahoo.com wrote:

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You haven't seen the many replies to your question?
--

-Mike-
snipped-for-privacy@alltel.net
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On Wed, 12 Mar 2008, Mike Marlow wrote:

Besides mine and yours, I've seen 2 replies. Have there been others? I was kind of hoping BobJ would expand on his answer a bit because it sounded circular to me, but maybe I didn't understand him correctly.
In the meantime I've gotten hold of a factory manual for the car, and according to the 200R4 diagnosis section, it doesn't look like there's a single cause that'll take out both 4th and lockup, which was my initial assumption. They both stopped working at the same time; if there was a time when 4th worked and lockup didn't, it was very brief.
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snipped-for-privacy@yahoo.com wrote:

Well, I'm here... How could you tell your lockup wasn't working beforehand. It's entirely independent from the 4th gear circuit and generally, engagement is very subtle. The lockup is there to prevent the TC from slipping and causing excessive heat, especially when in 3rd and 4th. 4th is worse because it is OD and the strains on the TC are immense. The oil temp can climb over 260 degrees in a very short time and cook your 4th gear clutches, which is probably what happened. In a car with an trans Control unit, there should have been a code once the TC lockup solenoid malfunctioned. The lockup can be applied in 3rd or 4th. Once the temp got into the destruct range, the trans should have gone into 'Limp" mode, with lights flashing, etc.. A lot of things could have caused the problem. When was the last time the trans fluid was serviced...
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On Wed, 12 Mar 2008, BobJ wrote:

I usually know from the pitch of the engine when power is applied or removed. I suppose that's easier with a diesel.

This car rarely goes more than a few miles at a time on the highway, so it must've happened fast.

This is an '81 diesel car; there's no computer.

I changed the fluid last summer. It's now a bit burned. :(
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