Hello all,

My transmission (th700r4) blew up a month ago and I had it rebuild. When the shop wanted to test the newly rebuild transmission in my car, they noticed that my lockup was always 'on'. As I have little knowledge of USA cars and automatic transmissions (I am Dutch, sorry :P ) I never noticed this. But it explained why it shifted so rough always. I thought it was normal. It also may explain why the transmission gave up working properly (some input drum was warped like a pretzel). They did not understand how and why the lockup was always engaged so they simply disconnected it so I could drive home.

Now my question: how does the system (ECM) normally decide when the lockup should engage or not? is it the speed of the vehicle, the RPM of the engine? maybe other sensors? My Tachometer is broken, it pretends there is a turbine engine installed and displays 5000 rpm at stationary and over 8000 (I have the H.O. engine with 8k max tachometer) on the freeway.

If the computer uses the tachometer to decide when to engage the lockup, this might explain why this was not working in the first place. But if its the speed sensor 9I believe I have a mechanical one) then what could be the problem with the lockup being always on?

I understand that I can simply make a switch to manually override the lockup, or even some advanced kits are available to do this automatically. But I rather keep the system original if possible. I found a tachometer that has a max of 8000 on e-bay and will install it and try to reconnect the lockup wiring the shop disconnected. I hear many different stories about driving without lockup. Some say its ok, just a little hotter. Some others say that without lockup some valve blocks and the transmission cannot circulate the oil properly and will go boom.

Thanks for reading, I hope some can help me. Those cars are rare here in Urp.

Some info on the car: 1984 Trans Am with the L69 H.O. engine and electronically controlled quadrajet. th700r4 transmission, cowl induction hood (non functioning because some previous owner replaced the aircleaner housing with some cheap tin can)

Kind regards,

Robert

 

The ECM needs to see engine temperature high enough and a relatively stable TPS value to switch lockup on. Sometimes it consider whether a 4th gear pressure switch is on to.

Wiring in the trans varies vehicle / year too.

 

Hi, thank you for your reply.

I tried googling TPS but I reckon you mean Throttle Position Sensor and not Toyota Production System . So basically I have to check temp sensors (is it the same as the temp sensor for the dash gauge?) and look for some wires connected to the carburetor or something. It does not use RPM or speed information?

I was told that the 1984 th700r4 just has the lockup solenoid wires, a ground and a positive.

Any idea what could have caused the lockup to be 'on' all the time? I was hoping I could fix this just by replacing the tachometer

Kind regards,

Robert.

 

On an LG4/L69 engine the temperature sender that the ECM "sees" is threaded into the thermostat outlet / water neck.

The temp. sender the dash gauge sees is threaded into a cylinder head.

If the lockup solenoid positive wire was connected to 12v when the engine is running and the negative wire is grounded, it will lockup in all gears except first. First gear cannot lockup due to the hydraulic routing within the transmission.

It would be nice if you would complete the fields "Car, Engine Transmission and Axle/Gears" here: https://www.thirdgen.org/forums/prof...do=editprofile

 

I was googling to try to find a cause of my original problem: lockup was always engaged.

I read somewhere that the ground wire of the lockup is switched by the ECM. That would mean that if some turnip connected this to ground directly, I would always have lockup... Is this correct?

I have tried searching this site for schematics but I was not able to. I am sorry if I have many questions but I just happen to like this car and I am not a mechanic

Kind regards,

Robert

 

Thanks again for the answer, i filled in what I know, I have no idea what axle ratio I have.

The thermostat is on the radiator right? I will check it out once we have some daylight again. Sad thing is I do not have a garage.

I was writing the previous message while you posted so there is some overlap. I read about ground to ECM and positive to brake switch. Some loon could have just wired them directly to ground and positive in the past. I have to find some friend who has a car lift and see how all wires go.

Thanks for the support, if you or someone else has more information about schematics (what wire should connect on which terminal on what device) I would greatly appreciate it.

Kind regards,

Robert

 

Yes, stock the ECM switches the negative side of the solenoid.
So, turnip-modified, grounding that wire would cause the always-locked condition.

The thermostat is bolted under the water outlet neck.


There are a number of ways the TCC circuit can be wired.

You may want to unplug the TCC wiring plug from the transmission and determine which is switched by the ECM. That wire will go into the transmission and one possible way to route it is to a two-pin, normally open 4th gear pressure switch. The second pin of the switch will go to the TCC solenoid. This prevents lockup until the trans. is warmed up. The positive wire will go out of the transmission to a positive 12v source.

L69 had 3.73 rear gears. A fine combination.

 

Hi again, thanks for the information

Today I tried to locate this sensor but I failed. My engine looks a little different or maybe I look at it at the wrong angle.

I did manage to remove the gauge cluster (with lots of @#$% flying around to get the speedometer cable loose) and install a new tachometer, so this works now. I hope the speedometer still works though as it was a pain to get the cable back in too (again lots of @#$% and possibly some *&@% involved too).

I am still trying to find schematics and diagrams of the original wiring but I am not successful with this. Also my Haynes manual has nothing about this. Bleh

I am sorry I don't know even how a 4th gear pressure switch looks like. All info I can find says the ground on the TCC connector of the transmission is switched by the ECM and the positive is routed on the brake switch, but I don't know what connector pin on the ECM is the correct one. I found a scan of an 1981 ECM, not sure if this is correct for 1984 too.

Again, thanks for the help. I am relatively new in automotive repairs, so please excuse me if I don't understand all very well. Also sometimes I confuse terms because my brain is wired for Dutch and we discuss in English here

Kind regards,

Robert

 

Only when your foot was off the brake pedal
Coming to a stop with convertor locked is the same as coming to a stop in a manual car and not putting your foot on the clutch
If the lockup was engaged while the car was stationary ,then the engine would stall

Typical GM TCC circuit on right ( for a later ECM so the ECM terminal probably different )
Purple wire from the brake switch to trans - tan /black from trans to the ground at the ECM

 

Hi all, thanks for the schematic

For now I just checked out the temp sensor. It reads about 4k ohm when cold. But the wires that are connected to it (a black and a yellow one) also have a resistance of about 5k ohm. Does this mean the ECM has a short circuit? Or is this normal?

I checked for turnip wiring on the ALDL but the lockup pin is not grounded. This does not rule out there is some short circuit to ground somewhere but impossible to get to the transmission now, i do not fit under the car to test this. I have to wait till i find someone with a carlift.

Kind regards,

Robert