TCC code P0741 - need advice

[mandkole]

Ok, Ive made comments a few months ago about a random converter lock unlock/lock in top gear. Over the past couple weeks, its become much more frequent and has now thrown a p0741. I know that's a general TCC code and could mean many things. The driving observations;

-trans shifts fine at all fluid temps
-converter locks on time in 3rd like it always had (at 35mph).
-converter locks firmly as normal (no strange behavior)
-converter function at slow speeds seems very normal
-trans shifts to 4th as normal/on time (50mph, with light throttle) with TC locked, but if you hold throttle position and continue accelerating, it will begin to unlock/lock (I have a light wired showing TC lock and it will blink off at same time it unlocks)
-lighten throttle position and it will stay locked, but as you tip in, it will unlock then lock
-Have TC lock override switch. It will lock it if unlocked at slow speeds and it will stay locked at any speed when engaged.
-When the switch is on, you can see the converter lock light blink every time it tries to unlock it
-trans temps have gotten slightly higher during last few weeks, but not out of control. Still no more than 100-105F over ambient. Ive discovered if I leave the lock switch on, it controls temp much better. The lock/unlock definitely heats the fluid up.

This issue 'seems' to have gotten worse since the 20s were installed. (Have not reset speedo.) Im viewing it as it takes more throttle input/load with the taller/heavier tires.

In reading on the web, some have said it could be VSS related but no VSS codes. It seems that my choices are to check the wiring, replace the solenoid, check line pressure. What input drives the solenoid to lock/unlock? Im developing an opinion but want some other thoughts.

Its due for a filter anyway, so perhaps I'll take a look around.

What else can I check?

 

[Mark Kovalsky]

You should check the TCC solenoid for an open or short. You should also check the wiring harness between the PCM and solenoid for open circuits, short to ground, or short to power.

 

[mandkole]

Thanks Mark

 

[mandkole]

After reading much more on this, it seems that this current issue could be very related to the issue that I complained about 6 months ago with the converter drifting on in reverse (when fluid cold) and killing the motor. Strangely, about 2 months ago it just stopped doing it and now operates fine. However, at about the same time, I had the first issues with the converter unlocking like described above.

When I get a few minutes I'll probe the control harness connectors on the outside of the trans first.

Mark or someone, do you have a resource for the service manual data for testing these components??

 

[Mark Kovalsky]

When I get a few minutes I'll probe the control harness connectors on the outside of the trans first.

OK, what will that do for you?

Mark or someone, do you have a resource for the service manual data for testing these components??

www.helminc.com

 

[mandkole]

my thought was to check the solenoid for shorts/opens, but Ive realized that the plug is not very accessible. Is the best way to check the solenoids with a plug/repair harness? At least the plug would be much easier to check.

 

[Mark Kovalsky]

It's very hard to check the solenoids electrically without a test harness.

 

[mandkole]

I toured the wiring of the TC lock switch put in by the PO and looked at the harness for any unusual damage or other tampering. In the way its wired, the PCM signal for the TCC has to go through this 9pin, 2 position switch before making its way to the trans. Its a very simple wiring similar to what is described by DIs old information (unlike the Guzzle box). It could stand to be rewired, but its very functional.

I then pulled the solenoid pack plug and found it full of ATF. May have found the issue. I blew it out good on both sides and removed about a 1/2 thimble full of fluid. Went on test drive-- started very normal until I encountered a short little hill after slowing for a car that put me at about the slowest speed possible in top gear before it would want to backshift to 3rd. I accelerated and it commanded an unlock, then relocked. To me it felt like it should have commanded an unlock but it was very brief and similar to the others.

Got out to the interstate and it never attempted to unlock on the on ramp like many times before, acting very normal. Ran fine all the way back to the house. Fluid temp at 100 over ambient after drive. Since the larger tires, the fluid definitely heats quicker.

I don't know how the solenoid pack is built, but it seems that the pack may be compromised. Im also still wondering about the calibration of vehicle speed to throttle position and the impact that the 10% larger tires may have and the speedo was not reset. Im hopefully getting more confirmation that the trans is not having mechanical issues.

 

[Mark Kovalsky]

Im also still wondering about the calibration of vehicle speed to throttle position and the impact that the 10% larger tires may have and the speedo was not reset.

There is no calibration between vehicle speed and throttle position. I think I know what you mean, and this doesn't exist. At any road speed you can be at any throttle position.

10% larger tires makes NO difference in what the vehicle speed sensor sends to the PCM. There is a difference made from engine RPM to actual road speed, but the VSS and PCM don't know that, and it also doesn't matter to them at all. It will NOT affect the transmission shifting, it will only affect the reading on the speedometer compared to actual vehicle speed. Actual vehicle speed doesn't matter, only the rotational speed of the axle matters. And compared to engine speed, that doesn't change no matter what size tire you install.

 

[mandkole]

It still seems like the solenoid pack, but Id like better information.

I drove the truck again yesterday into town and immediately had the TCC act up again, turning on/off as soon as the trans shifted to 4th on the interstate. After the first couple of on/off/on sequences, I turned on the lock switch and again drove with no additional unlocking issues-- trans temp never exceeded 155F. Just like before, due to PCM command activity that continued, the trans went into limp after about 3-4 lock/unlock events. The trans functioned fine (except with a firm default shift).

Very similar to the old Diesel Innovations instructions, I use a green light to show when the PCM locks the TC. There is a red light to show when the override switch is activated.

My issue is with the TC lock in 'on' position (grounded control wire overriding the PCM), according to the lock/unlock light the PCM continued to want to unlock the TC with any application of throttle from a cruise state. How does the feedback loop work that controls the PCMs need to run the TCC? With the solenoid pack plug full of oil, it seems the issue could be there, but the grounded signal needs to go through there as well, so Im not sure. What is the PCM detecting that causes it to want to unlock/lock the TCC even though it cant due the override switch being on?

I want to be sure Im troubleshooting this properly but Ive had other trans experts tell me that this is one of those situations where I need to just replace the solenoid pack. I was hoping to understand this a little better to know for sure.

Who is the best source to get the pack from?

 

[Mark Kovalsky]

The PCM looks at engine speed and turbine speed to measure slip across the converter. It's really easy to tell that it's locked when the PCM wants it unlocked.

By the same method it can tell that the converter isn't locked when it should be. It will try several times to lock, and if it doesn't, it unlocks and sets a code.

The PCM can detect the switch being there. There is an electrical short to ground test run on that wire.

 

[mandkole]

I drove the truck briefly with the chip unplugged. The trans shift points were a little earlier, 3rd gear TC lock on time, but in top gear/TC lock, the PCM signal would erratic unlock/lock like before. Its not the chip.

The day I blew the ATF out of the plug and inspected the harness, then drove it for 10 miles or so it performed correctly under all conditions. The next day I drove it, back to the incorrect behavior immediately after getting on the interstate. This isn't high quality troubleshooting, but it makes me think its the pack.

There is nothing in the lock switch wiring that appears failed. Its been there for years and the harness is in excellent condition. The switch performs correctly. Im thinking now I may need to verify if the TC is fully locked when the switch is on. Given that it runs at 150-155F on the interstate, it seems that it is. The heat builds quickly if its having trouble locking.

 

[mandkole]

Im getting closer. Ive removed the lockup switch from the circuit and acts the same. Its also now thrown a P0743 . With the TC lock switch on, it no longer kills the motor at a stop in gear. It just puts a strong load on it. Ive tested the switch before (within the last 6 months) and it would kill it immediately when coming to a stop or stopped in gear.

I've not plugged in a test harness to ohm it, but seems that the solenoid is locking the clutch but not completely.

 

[mandkole]

Im not sure what Im looking at now... I got a service harness, tested the pack and found nothing out of spec.

Dont know how to better describe than to say this is a vehicle speed vs. throttle position vs. load issue that the PCM is trying to manage within a specific window. Since the tire size increase its become more noticeable. (I may put the stock tire size back on) I can go WOT and the PCM holds the TCC locked the whole time and it stays locked. The situation that causes the PCM to turn the lock off/on is being locked then increasing speed/load up a hill and gradually increasing throttle position. On the interstate at 60-65 and accel to 75 with firm throttle, its fine.

Its concerning that the manual lock switch no longer kills the motor in gear stopped with the brake on. Would seem that the TCC is slipping.

I need get with some of the local guys and check TC lockup and slip on AE.

 

[Arisley]

I agree, seems like a slipping TC, and not by a bad TC, but by bad info to the PCM. Can you try it with other tunes? Can you try it with a different PCM. That would eliminate PCM and tuner issues.

 

[Tom S]

I agree, seems like a slipping TC, and not by a bad TC, but by bad info to the PCM. Can you try it with other tunes? Can you try it with a different PCM. That would eliminate PCM and tuner issues.

My thoughts as well. This might not be the fix but it would help to narrow it down. I would also want to watch for slip across the convertor if that is possible on a SD.

 

[mandkole]

Agreed- some decent datalogging is needed now. A good excuse to get AE finally. The scan gauge has been great but its package for the PSD doesn't provide any trans function data except temp.

 

[Tom S]

I spend all winter watching some data with a Dashdaq/DP-tuner infinity. I found it pretty darn useful for watching data as I could have it up mounted at eyesight while driving.

 

[mandkole]

I pulled some data but Im not really sure what Im looking at. I was struggling to get AE to datalog more than 3 parameters at a time. I really wanted to see throttle along side the TCC information. It seemed like slip rpm was high, but it never gave anything for slip%. It was always zero.

Basically the TCC was locked or unlocked. If it unlocked, it was on those real low speed transitions. What was strange about today, it that it was not predictable. I highlighted moments were it commanded unlock.


If you have some parameters that should be used, then let me know and I'll do another drive loop.

 

[Mark Kovalsky]

Each of those 0 duty cycles in the middle of the 100% duty cycles means the PCM has recognized that the converter hasn't locked when it should. It resets and tries again.

You could have a sticking TCC solenoid, or a sticking TCC valve. Those are the two most likely in my mind.