Lock-up torque converter will not lock intermittently

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I have a 1996 Pontiac Grand AM SE with the 2.4L twin cam engine. It is a four speed automatic trans. with overdrive (IE..lockup torque converter). Here recently I have been having a problem with the torque converter not locking when it is supposed to at speeds upwards from 50-70 MPH. The transmission will run through all four forward gears and then you will feel the torque converter lock like it should until you get what seems like some miles on it and the trans. temp up. I am not sure if I can 100% for sure say that this will only happen when hot, but that has been the case thus far. Once the transmission has gotten hot and this could be 20 miles or 60 miles down the road, all of a sudden the transmission will not engage the torque converter to lock up so it stays in fourth all the time. This means that when I depress the gas, I have instant power and acceleration and I should not, I should have to punch the gas a little to knock out the locked torque converter so that I hit drive range 3. This is causing the CHECK ENGINE light to come on and poor gas milage. I am trained in automobiles and have worked on them for years but my knowledge of auto transmissions is not as detailed. Now, you can watch the tachometer but I just know what I am looking for and know what is supposed to happen and for some reason the torque converter decides to take a break after a while. When the CHECK ENGINE light comes on every time this happens, the error is an OBD II code J1870 "Tranmission component slipping." I do all the PCM scans to the computer myself and that is the repeating code. I took the car on a trip this weekend and I had to clear my check engine MIL 5 times. J1870 everytime. I predicted the light coming on each time before it occured because I noticed the loss of overdrive in every case. Someone help me. Is my torque converter clutch (TCC) solenoid messing up?? Fluids are fine, that is the first thing I ALWAYS check. Where is that solenoid, in the valve body? What could this be? Should I drop the trans pan and drain the fluid and refill it and try that? I drove the car 70 miles home from a friend's place tonight and it had screwed up all day, she was fine the whole way home. I could not get it to mess up. Could this problem, whatever it is, be correlated to how many times I knock the torque converter in and out of locked/unlocked status with the gas pedal? For example, letting it slip into locked status for highway cruising, you are not going to get the TCC Solenoid hot because you are keeping it locked whereas, if I was punching the gas every minute and jumping into passing gear back and forth, then I am constantly putting the transmission in a position where it needs to re-lock with the torque converter, thus causing something to heat up. This seems electrical to me, I know that some things that are electrical start messing up when they are hot. Please advise me. Thank you so much.

posted by  cmeseadoin

First I'd like to say thank you for providing enough info about your car when asking a question. Most people do the "my car is broke, how do I fix it?" routine then get pissed when I ask them questions.

The old 1870 is kind of a catch all code when the PCM is not really sure what the problem is. As with any auto trans start with the basics, fluid condition and filter condition. You've already covered that.

If they check good next step is to look at the MAP and TPS. These both read to the PCM and if malfunctioning may cause the problem you describe. They may or may not set a DTC. You don't say what type of scan tool you have but if it has a datastream function have somebody else drive your car so you can ride along and see what they're doing. Also if your scan tool has a freeze frame function this may help you to isolate the root problem.

All of that being said let me give you my gut feeling on this. The trans in your Grand Am is famous for having torque converters go away in the 100-125k area. This is especially true of cars that have been under maintained, which I assume yours was if they ran it out of oil. The good news is it's a fairly quick swap. The bad news is it's not cheap. Good luck and let me know what you find.

posted by  vwhobo

In all honesty it would take more typing than my fingers could stand to guide you through the diagnostic process for this.

I've never personally used the scan tool you have but understand it to be a comprehensive tool so you should have no problem going through the steps with it and a DVOM. I'm also going to assume you work in a shop or have access to one so the next question is do you have All-Data or Mitchell On-Demand? My All-Data has a really good step by step procedure for checking this out, I'm sure Mitchell has the same.

At any rate I'm not trying to put you off, just guide you to where you can get the info your looking for. If you don't have access to either of the above let me know and I'll see if there's some way to get the info to you. Good luck and let me know how it goes.

posted by  vwhobo

Dude, shoot me an address and I'll mail you testing and inspection procedures for P1870.

posted by  vwhobo

It's in the mail.

posted by  vwhobo


Here is something else that is now happening with the Grand AM. As you know, I have been having intermittent problems with the lock up torque converter not looking up when it should causing a loss of overdrive situation. Well, I am still in the process of trying to get to changing the trans fluid to see what happens. Well, yesterday I get in the car to go home from work and I notice that when the converter is locking up when it should, it is basically slam shifting into lock-up. It was very harsh and totally not like usual. Other than that, it seemed ok. Well I noticed shortly after this started occuring that the ETS (Electronic Traction System)had gone out. The light will not toggle on or off via the toggle switch on the automatic gear stalk and the "low trac" light on the dash does not illuminate when the key is turned to the accessory position in start-up. I have disconnected the battery and reconnected it and that made no difference. I think that something major is going on with the trans and I am going to have to put it in the shop I think. Have you ever heard of this and do you know what controls the "low trac" system? Is there a main fuse somewhere or is this being caused by something else? Thanks for your help again. :-)

posted by  cmeseadoin

check your boost valve in the valve body for the transmission. the valve is wearing downand fluid is getting by it causing a high line pressure. a company called sonnax makes a kit to repair this so you do not have to buy a new valve body. good luck!!!

posted by  problemchild

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