zakbradshaw

I have heard that the 81 transmission torque converter lockup is controlled by the computer. I will be swapping to an Edelbrock Performer EPS intake, Barry Grant 750 Speed Demon, and HEI distributor. What exactly do I need to keep the torque converter lockup?

Thank you.

jaybird81

It's not. Don't worry, I replaced my computer distributor and carb off my 81 a while ago and didn't have to do anything with the transmission. You will be fine

jim-81

I was under the impression that the 81 ECM 'DID' control the auto lockup. And if you remove the ECM it just wont lockup. So some guys put in a separate switch to manually lock it up on the freeway. Mine did not have such switch so I'm guessing it never locked up. But it drove fine. (now it's manual).
Jim

7T1vette

This could get a bit tricky, because the existing trans has the features built into it that allow the lockup to engage/disengage as required.

There is probably a way to leave the '81 ECM in place, and with the right sensor inputs connected, allow it to manage the new lockup clutch; but I don't know the details around that adaptation.

But, I would suggest that you purchase a ready-made lockup clutch controller, rather than to just put a simple switch in place. The problems come when you forget to turn it off when braking or when going up hills. The lockup clutch is not made to handle full torque from ANY engine (let alone a Corvett engine) and it can be damaged pretty easily if left on under those conditions. A lockup controller will prevent that damage.

jaybird81

not to beat a dead horse, but My 81 was completely stock never been altered until I got my hands on it. I disconnected the computer and removed the carb and distributor and put in a standard Quadrajet from a 79 and an after market HEI. Now you could be right that it worked but never locked up, and I kinda thought maybe that is what was happening, until I swapped the automatic for a 4 speed. When I removed the transmission there was nothing connected to the automatic that would cause it to lock up. So I don't think the computer controls the transmission at all.

RC81

Should be a harness going into it.

7T1vette

Jaybird81...

You could be right. I'm certain that the '82 tranny lockup was controlled by the ECM. But, after thinking about your post, it could be that lockup on the '81 350 tranny was self-contained. If so, the OP's "issue" would disappear.

More info from others with '81s ????

wilcar

You can drop the trans pan, install a normally open 4th gear pressure switch, and then connect the tcc solenoid ground wire to this switch. This will allow lock up in 4 th gear only. You can also install a toggle switch in the tcc brake switch circuit to turn off the lock up in 4th gear. If you are not concerned with losing a few miles per gallon you can just run with no lockup feature at all, will not harm anything.

David Mc

The 700R4 wasn't introduced until 1982

A TH350 doesn't really care if the lockup works or not

DaveL82

My 80 with turbo 350 trans was not computer controlled but had TC lockup. It was controlled by engine vacuum switch, brake switch, and trans governor if I recall. I would think it you want to keep you could copy the 80 model setup.

Before going to the 5 speed I disabled mine by pulling the solenoid from the trans body inside the pad. Solenoid closed closes off bypass gallery causing lockup but with solenoid removed fluid flows into pan so no lockup.

wilcar

Correct, my mistake, 82 first year for 700r4 in the C3. My post would apply to the 700r4 for an 82 and 84 crossfire vette, unsure about later models. I have a carbed 84 and did the mod to my 700r4.

maxmachz

81 is a TH350 lockup transmission. The ECM monitors and controls the lock up function. Under normal situations it locks up too early, by design, to save fuel.

You can install a simple switch to choose automatic lockup as designed, or to bypass this by wiring the switch to the brake switch which unlocks the trans when you tap / use the brakes. This makes for MUCH nicer in town driving, then when out on the highway engage it an get the lower rpm and better fuel economy.

spikebot 81

This was the best mod for th350C trans lockup and worked so well I swaped the normaly open preasure switch setup to the 2004r trans as well. Lockup in 4th gear only.

David Mc

But a TH350 doesn't have a 4th gear

spikebot 81

I didn't catch the "4th gear " part of wilcar's statement. I thought all the 4th gear preasure switchs in the 2004r and 7004r trans are normally closed like the one used in the th350c trans in 81 cars. I used a switch from a th400 witch is normally open.

Moser28

I had an idea that I presented on another thread in regards to a switch for the 81 lockup but didn't hear anything. Could you not get a male and female 3wire connector and place a 3 wire switch in between? That way the original wiring is not affected. I'm not that good with the electronic stuff and I could be missing something. When I disconnect the plug from the transmission harness to the engine harness everything works great and doesn't cause any problems so why not put a switch in line with that?

81C3BUILT

On the gm schematic the ecm has one lead that goes into the th350c trans, it is a ground, the other lead is the 12 volt power that comes from the brake pedal switch that disengages lock-up by cutting 12volt power to trans.! What you need to understand is not what parameters the ecm needs to engage lock-up, but how you need to get THAT ECM WIRE to the trans to give the th350c trans solenoid a ground to complete the circuit. if the computer is gone from the car, you need to tap the wire that is a ground to the trans plug on the side of trans feed it a constant ground from a vacuum operated switch off of the port vacuum on the carb. This way when low vacuum is seen at the carb port vacuum, like when you accelerate, (vacuum decreases at the port vacuum as engine rpm increases) it will cut the ground to the trans and disengage the the 12 volts to the trans and stop tc lock up function. Then when vacuum increases it sends the ground to the trans that is already getting 12volts at the brake switch.....You will never have lock up in 1st gear because the pressure switch is designed for 2nd and 3rd internally only!!!!. This same exact trans was used on the 1980 computerless corvette. you need to recreate the same environment on your 1981 as the 1980 by getting ground function to operate converter lock-up because you have removed the ecm from the loop and now lost ground function. Don't be stuck on closed loop functions to operate your lock-up. Think in terms of what exactly that ecm wire to the trans is actually doing...answer, providing ground to complete lock-up circuitry!!! The added benfit from this setup is you will not feel that lock-up lug feeling you get waiting for the archaic computer to send the ground after this lazy early computers says OK all parameters are there for lock-up, engine temp, 02 sensor reading nominal, engine speed, all that stuff is gone. your lock-up is now controlled by engine vacuum, trans pressure switch, trans internal solenoid,and brake pedal application...Hope this helps

doorgunner

81C3BUILT.....

............good info/easy to understand!

But he probably has it solved by now. I get bitten by an old thread often, myself.