What sensors does the computer use to tell torque converter to lock?

I have a recently rebuilt transmission (700R4) and torque converter that is locking up at inappropriate times. This was happening before the rebuild too. AAMCO says the computer is telling the torque converter to lock but doesn’t yet know why it does this at the wrong times. I have verified that it is the computer by unplugging the computer from the transmission and the problem stops. I have seen discussions of this issue in this forum but no discussion on what sensors are used by the computer to tell the torque converter to lock.

Does anyone know what information the computer uses to decide to lock the torque converter? I’m trying to diagnose if it is a computer problem or a sensor problem.

How does it act if you lock out the overdrive?

Running in D3 (I assume that is what you mean by locking out overdrive) does not change the problem. So far only unplugging the TCC wire relieves the problem.

www.alldata.com or www.mitchels.com have the troubleshooting charts which you can use. All you have to do is buy a subscription ($15 - $20) for your vehicle from their Web site; or, it’s free at the public library Web site.

The troubleshooting charts tell you what sensors are involved in TCC and what to do to check them.

First of all, what year is this Suburban?? Second, tell me EXACTLY what the converter clutch is doing. I need more info than locking up at inappropriate times. When you are driving down the road, what exactly happens.

One of the main problems here is that usually AAMCO, or at least the ones I am personally familiar with dont like to troubleshoot transmissions, they like to sell overhauls…Pan gasket leaking-overhaul transmission. Speedo not working- time for an overhaul. Is this the reason why the trans was overhauled?? We’re going to figure this out but there is certain info I really need to make it happen.


AAMCO = All Automatics Must Come Out!


Oops! This is a 1989 1/2 ton 4WD suburban. And yes, I may have been “taken” by AAMCO going for the rebuild right away instead of diagnostics. This was the original problem I was having when I took it to AAMCO.

The exact problem is that the transmission seems to want to drop in the highest gear very quickly and then it would jump back to the lower gear and continue hunting. This was on gentle acceleration. Also at speeds of 35 to 40 mph while going up a slight grade (and sometimes when on flat ground) the transmission seemed to be hunting for the right gear–the up/down/up/down shifting.

When it was still doing this after the rebuild (only much less noticeable) I took it back to AAMCO and got the explanation that the computer (didn’t know I had one) was telling the TCC to lock up when it shouldn’t be. They did not know why yet. I have the burb home now and am supposed to take it back to them on Tuesday for more diagnostics. I am researching the problem so I don’t get taken again.



 From the way I am reading your response, it sounds like what you are saying is that the trans is shifting from 1st thru 4th very quickly then after it hits 4th (O/D) it drops back down to 3rd then hunts back and forth from 3rd to 4th.  Is this correct??  The shifting rapidly through the gears is what we call "Stack shifting".  The most common causes of the stack shifting is either #1. A mis adjusted TV (Throttle Valve) cable.  It sounds like the cable is adjusted too loose.  #2.  A sticking governor.  Shift points in this transmission are controlled by a "Governor" Simply put, the faster the vehicle goes the more line pressure is directed to the shift valves through the governor.  The line pressure forces each shift valve against a calibrated spring causing the upshifting or downshifting.  If the governor is sticking wide open, the shifts are going to occur very rapidly.  This is where I would start.  TV cable then governor.



I did try adjusting the TV cable before I took it to AAMCO. I read about that solution online too. It may have been my incompetence but it didn’t help anything. I tried to “push the button” on the side of the TV cable and move the throttle through full range and release the button. The button was very hard to push. What happened didn’t seem like an adjustment. It just let the TV cable housing move in its mount. Is that what was supposed to happen?

I would have hoped that AAMCO did this kind of adjustment when they reinstalled the transmission.

Also, I read about the wires that control the TCC so I disconnected them and the problem went away so I thought it was a TCC problem. That is why I was asking about the sensors.

Shouldn’t AAMCO have also fixed a sticking governor with the rebuild? I guess it could be that they messed up.