Hey all, I've got a 1999/L 7.3L F250 , 4wd CCSB 4r100 Auto. The problem I'm coming to this forum to help solve is related to the torque converter coming unlocked. There are two situations in which this happens. The most common occurrence is while I'm cruising in lockup, 3rd gear or OD, no matter what speed or throttle position, it will just unlock and my Rpms will stay high until it locks back up on it's own a few minutes later, or if i tap the brake pedal lightly it will lock back up every time without fail. The other occurrence happens on the 3rd gear to OD shift. It will have been locked up in 3rd, and comes unlocked at the shift. Other times it will not even lock up in 3rd when it's supposed to. Then as stated before, if i tap the brake pedal or wait a minute it will lock back up. Most of the time it acts normal, but it does this probably at least 5 times during a 20 minute outing.

Little backstory: purchased the truck about 1 year ago with 370k. Motor and trans was in surprisingly good shape for the asking price and mileage. Bought it, later found out the motor and trans were replaced or rebuilt in 2017-2018. Unknown miles on them. Estimating around 100-150k Only problem i noticed was it shifts pretty tight, not neck-breaking, but definitely harder than normal. That part has stayed the same 35k miles later. Truck is lightly modded(tune, HPOP, intake and exhaust, little upgrades to the turbo, etc.) She's on bigger tires if that makes any difference. The TCC lockup issue just started about a month ago. I have religiously kept up on maintenance. No change after trans fluid and filter change, cleaning shift solenoid pack connector, ensuring functionality of all 3 brake lights. The only codes on my scanner are p0472(EBPS circuit) and p0236 (i broke the nipple off my MAP sensor). OD light does not flash. Trans fluid stays at 140-170 degrees. Never hunts for a gear, slips during shifts, or anything like that. One thing i notice is it unlocks when going over a certain section of road almost every time. Another thing, since I've owned the truck, I can't do a KAM reset on my scanner or via Forscan. The PCM just doesn't respond to it. I am leaning towards PCM just based off of that fact. Any ideas?

 

The 1999 7.3L/4R100 did not store anything in KAM. That's why you can't clear it.

I'll have to think about what could cause this. With Forscan can you see if the PCM is commanding the lockup?

 

Hey Mark. Thanks for the reply. Do you remember what Metric on Forscan would show me whether or not the PCM is commanding lockup? I have the laptop in the truck with me most of the time.

 

I only knew the engineering term which was different from what you see in Forscan.

 

From what i remember, There's a metric for TCC Duty Cycle %, one for desired slippage RPM, and one for actual slippage RPM. Any time it unlocks, including when it isn't supposed to, the Duty cycle % metric drops to 0.00. I haven't paid attention to the other two. I will look into it again when I'm driving tomorrow to see what else I can figure out.

 

The TCC duty cycle is the one that I was thinking of. That shows what the PCM is telling the solenoid to do. If you feel the converter unlock while there is still more than 50% duty cycle the problem is inside the trans or torque converter. If duty cycle goes to 0 as the converter unlocks, it is the PCM telling the converter to unlock. That pretty much rules out the converter as the problem. It then could be a bad sensor sending bad info to the PCM, or a bad PCM, or what we used to say, the dogs don't like the dog food. In other words, it's working as designed, but you don't like the way it was designed.

 

I had Forscan up on my drive this morning, and i monitored all the metrics that i thought might affect trans/TC function. I noticed the BRK (brake pedal) metric showed ON under some situations even when the brake pedal wasn't depressed. I thought i had found the problem, then i realized even when when lockup was functioning normally, it would still sometimes show the wrong brake value. Lifting up on the pedal doesnt fix it, but tapping the brake a few times does. Strange. I will be looking into buying a replacement switch. Do you know if there are any other conditions that would cause it to command unlock? I am not opposed to replacing the PCM, as it could likely be causing the hard shift as well. I just don't know what's involved with doing that. Is it plug and play, or does it have to be programmed to my truck?

 

The brake will only unlock the converter when the pedal position is less than 10%. When it is above 10% the PCM ignores the brake input.

Get this fixed before doing anything else.

 

Hey thanks for getting back to me. I'm going to go ahead and order an OEM replacement brake pedal switch, although I'm a little confused with the way your reply was worded. It seems like the PCM would ignore the input if it was less than 10%, not above. Perhaps you meant the pedal being physically above the 10% mark. I appreciate the help, sir.

 

I meant the PCM ignores the input if the accelerator pedal is more than 10% depressed.

 

Gotcha. So what's the next thing to do? Are we looking at a PCM issue here?

 

The next step is to fix the brake switch problem and see how that affects this.

 

The new switch is ordered. I will update you as soon as it comes in and I install it. Thanks again

 

Just a secondary question in order to acquire as much information as possible, for thread contributors and for your own diagnostics....is it possible to check your brake lights when the pcm is showing that it is depressed?

 

Installed it last night. Drove for about 100 miles collectively, lockup was functioning as normal. It looks like our problem is solved. Thanks for your help guys. I also want to try to figure out the hard shift if anyone's had experience with that. Should i just start a new thread for it? No other threads on here were able to give me much insight.