Interesting circumstances surrounding a friends '01 A4 with a P1870 code which is Slipping Transmission Component/s. In this case it's the torque converter failing to lock up or intermittently unlocking usually at Hiway speeds. Most P1870s point towards a sticking TCC (torque converter control) valve or torque converter PWM solenoid. Without going into all the conditions that must exist to set a P1870 and clear it I'll get to the issue.

First the car is an '01 A4 with 22K on it. Before engine and trans reach operating temps lockup works as designed then after a 30 plus mile Hiway drive in which the converter will unlock sometime during the drive the car will throw the P1870, and max out line pressure which results in very harsh 1-2 shifts. Let the car cool down for around 30 minutes and all is good until the return trip and the same issue replays itself. Research seems to point towards a very common sticking TCC valve or a worn TCC bore. However the worn bore is most common on much higher mileage cars.

So before condemning the valve, solenoid or blaming the recent tune we did some testing on the road and found this during a Hiway test drive; when the converter unlocks, which you can feel, hear and see on the tach we pulled the codes and there was a current P1870. We cleared it and within 2-5 seconds the converter locked up. Shortly after, while going up a grade the converter unlocked as it should but didn't lock back up at the crest or down grade. Pulled codes and the P1870 returned. Cleared it and the converter locked. Did this several more times during the drive. Last time it unlocked we left it that way and exited the Hiway. At the stop we pulled the codes to see if P1870 returned and it had. We cleared it and took off from a stop. Half expecting the 1-2 shift to be harsh we were somewhat surprised it was all normal, without any cool down period.

So at this point I'm not sure what to blame for the malfunction. If the TCC valve was sticking or had a worn bore, clearing P1870 thus resulting in the PCM recommending lockup shouldn't work, but it does. Next I would suspect the PWM solenoid but if it was faulty why would it function correctly after clearing P1870 again causing the PCM to recommend lockup and have it work each and every time. I will add that the unlock condition only occurred during loading the car such as a hill or accelerating to pass but was somewhat intermittent. It would unlock/re lock 2 or 3 times in a row then unlock and not re lock until we cleared the P1870.

The TCC valve and solenoid are not pricey parts and the job is easy but we don't want to throw parts at it if they are not the cause. Kind of out of ideas at this point. Don't know if anyone has experienced the same issues but would appreciate any input.

Sorry for the long read. At least I used paragraphs.

 

I wonder about the tune myself. Did this happen right after tuning it? If someone changed the gear ratio, the computer will be looking certain rpms versus mph. If its off, it assumes torque converter issue. I changed gears in my car without tuning the computer to the new gear ratio. Even though changing gears doesn't alter the speedo, it does alter the rpms versus mph for shifting. Just a thought. Especially if some just copied over a canned tune file they found.

Other than that, a sticky tcc solenoid maybe. But cjearing code, it should come right back.

 

Thanks for the input. Did not surface until after the tune. Tuned by reputable supporting vendor. No gear change or tire diameter change. Funny thing about not coming right back. After clearing, it won't throw the code until an incline or acceleration causes the PCM to command unlock. Clear the code after the hill and it locks within 2-5 seconds and stays until the next hill. I think we'll do a fluid and filter change and replace the TCC solenoid. If that fails we'll go back to the tuner. For now they claim it's not a tuning issue. We'll see.

 

The converter/transmission is designed to allow a limited amount of converter clutch slip when it is locked. If the PCM detects more than the allowed amount it will set the code and while the code is current will quit trying to lock the converter. Since you've had the car tuned I'm assuming that it isn't stock and it may be putting out more torque where you are seeing the code pop up (going up a hill or accelerating) than what it could do stock, and hence slipping more than it would with a stock engine.

A couple things you can do:
- increase the PWM duty cycle in the computer tune
- in the computer tune have the converter unlock with less throttle for given speeds so that it doesn't see the higher than stock torque for a given speed/rpm

 

That's worth looking into. The tune was performed after adding LT headers, high flow cats, CAI and cat back exhaust. Not big power adders but maybe enough to throw the code. I'll talk to the tuner and see what they say.
Thanks!

 

I agree. It's handy to have a plot of your TCC lockup points, both apply and release, in hand when diagnosing this. Make sure you include vehicle speed, throttle position, engine speed, OSS, TCC slip, TCC apply and maybe engine torque in your scans and you should be able to quickly spot whether or not it's the programming. The fact that you can clear the code and behavior appears normal for a while (during the time the next round is maturing) certainly points to that. I'm sure you know this, but the test is designed to apply only under conditions when you'd expect slip to be small.