Hi Dutch Rutter,
Jon here in the UK. I rebuild classic American transmissions for a living here. I specialise in 700R4's. Everyone is trying their best to help with advise but there is a mix of truth and untruth being batted around. This could lead to money and time being wasted so I thought I would chip in with what I know as fact here.
Fact 1. The 700R4 is plagued with slipping issues over time and high wear and tear internally. Its a medium duty trans with issues from new. The age of your transmission is ripe for a full rebuild. Seals are hardening with age, clutches are tired. It is highly likely to be slipping on both the 2/4 band and third gear clutches. I have never taken one apart that has not had a burnt 3rd and burnt band and something else worn or broken. All this generates excess heat. It is very logical that when you changed the fluid for fresh when you did the oil leak - that caused it to play up. Why? Because the dirty old fluid was 'grippy' the new fluid is clean and slippy. If you have something that is hanging on by the skin of its teeth, making it slippy kills it. I have had this many times from customers who say 'I changed the fluid but now it doesnt work at all' The dirty fluid you saw again was because the converter holds about 4 quarts and its very difficult to totally drain. This half fresh and half old mixes again when you restart making it look shagged.
Fact 2. The lockup clutch in the converter is just a paper lined plate. It too fails just like the other clutches, thats why when overhauling you should replace it with a recon, preferably carbon. This large single clutch tries to lock up but cant, its metal to metal causing huge amounts of heat. I have seen evidence of raging infernoes of friction welded particles come out of the pump area of faulty converters. Converters are the main cause of why trans gets hot. its the way fluid transmits power. Its utter garbage that having the lockup not connected will cause overheating. Both the th350 and th400 dont have lock up, they happily slip all day long so why would a 700 not be happy. The clutch is there for Economy purposes to increase efficiency, not reduce heat. Slight heat reduction will occur as a bonus but not as the primary need.
Fact 3 Lockup control is a series of events that MUST all take place in order to get lockup. GM built into the circuit many safety features to ensure both you and the transmission dont get harmed. The lockup clutch isnt a performance device so must be backed out under hard throttle conditions to protect it. That is the purpose of the vacuum device - it measures engine load.
The brake switch disconnects it to back out lockup if you need to hit the brakes. With lockup enabled you have full mechanical connection between engine and Trans like having the clutch pedal up in a stick shift car, its dangerous if you cant stop. You dont want to be hunting for a toggle switch as you run that old lady down.
Inside the transmission is usually a thermal device - it prevents lockup from operating until the fluid is hot enough. This is a bad component that fails all the time and is recommended to be bypassed completely in all good rebuild manuals. Along with that you have a control valve in the pump body, the lock up solenoid itself and a pressure switch on the 4th gear oil circuit. So the lockup works only like this:-
Hot fluid - cruising throttle - in 4th gear - no foot on brake - assuming all components are funtioning OK and all the wiring is sound. All this can be retro ripped out and just done with a toggle switch but bear all I said in mind. Some models had further lockup in 3rd as well but all controlled via the ECU.
I hope this helps - I totally recommend you excersise caution in what you further spend on it. Recon converters can be had for just $109 and rebuild kits for not much more. Tear it down and fix it properly all the parts are on your doorstep in the USA, I have to wait 2 weeks for everything and it costs me double.
Jon
