4l80e no 4th unless commanded, no codes

[travmac79]

I think you need to redo the tests in post #83. Anything that doesn't match that chart needs to be remeasured until you are 100% sure that you have accurate readings. Actually anything that returns a zero/low voltage needs to be confirmed 100%.

You're 100% right. I will redo them tomorrow when my minds fresh. I disconnected trans grounded out each of the 3 pins a, b and c, measured voltage and got incorrect readings this time....
Ground r pin
A 12v
B 0v
C 12v

Ground p pin
A 12v
B 0v
C 12v

Ground n pin
A 12v
B 12v
C 12v

Again, I'll try again tomorrow. I also have different driving conditions that seemingly change at random. So I don't know if its likely that I'm not insane and some intermittent electrical bs is happening, causing the changing readings or not. Anythings possible. I'm not too stupid. I have a bachelors in mechanical engineering. But that just taught me how much i don't know . And this is also the first auto trans I've ever worked on. So bear with me. Thanks again for all the help.

[gtstorey]

Disconnect both ends of harness and check continuity between pins r and p. If your readings are correct then they are acting like they are connected together. And not connected to C.

[TransGo Robert]

You're 100% right. I will redo them tomorrow when my minds fresh. I disconnected trans grounded out each of the 3 pins a, b and c, measured voltage and got incorrect readings this time....
Ground r pin
A 12v
B 0v
C 12v

Ground p pin
A 12v
B 0v
C 12v

Ground n pin
A 12v
B 12v
C 12v

Again, I'll try again tomorrow. I also have different driving conditions that seemingly change at random. So I don't know if its likely that I'm not insane and some intermittent electrical bs is happening, causing the changing readings or not. Anythings possible. I'm not too stupid. I have a bachelors in mechanical engineering. But that just taught me how much i don't know . And this is also the first auto trans I've ever worked on. So bear with me. Thanks again for all the help.

Man this thing keep moving the goalposts on us... Now it looks like the pink wire for for TFP A has an open circuit and that the red wire for TFP C is shorting to the blue wire for TFP B and open after that on it's way to pin 18 of the C1 connector.

[gtstorey]

I’m thinking at this point that maybe he’s using crappy grounds or has problems with the voltmeter or leads. The trans disconnect test needs to ge reran using ground wires ran from negative battery post to ECM and trans connector. Until consistent readings are achieved. There is no way to determine the problem.

This is why some of us where reluctant to help the guy recently that was asking how to measure voltage trouble shoot an electrical accelerator throttle body issue.

[kingtal0n]

This is why I said just replace the pressure switch then check for continuity between wires in the harness with it unplugged from everything else for shorts. If the wires are unplugged they should not touch each other or 12v the only thing they can reach is ground through an actual ground wire and that will be obvious 004 002 001 000Ohms plus it can be seen physically grounded to the engine block for example and will be marked as a ground in schematic.

Rule out switch and shorts that leaves only ECU and transmission internal issue. Mechanics love to change a good alternator without diagnosing the issue but when it comes to this sort of problem sometimes changing a good pressure switch is easier and can speed things up than going through this mess.

[travmac79]

Solved! B signal wire had been pushed back in the blue(c1) pcm connector. It was sitting flush with the clip thats supposed to lock it in so either I accidentally pushed it in and didn't see it when I pinned my efans in, or it happened one of the times I had the pcm unplugged. This is with the original pressure switch in it as well. Either way, its fixed. I will remember to check all connections more thoroughly and frequently when diagnosing intermittent issues lol. Thanks for all yalls help and suggestions

[TransGo Robert]

Solved! B signal wire had been pushed back in the blue(c1) pcm connector. It was sitting flush with the clip thats supposed to lock it in so either I accidentally pushed it in and didn't see it when I pinned my efans in, or it happened one of the times I had the pcm unplugged. This is with the original pressure switch in it as well. Either way, its fixed. I will remember to check all connections more thoroughly and frequently when diagnosing intermittent issues lol. Thanks for all yalls help and suggestions

Glad to hear it, good job sticking with it adn figuring it out!

[1badc10]

Solved! B signal wire had been pushed back in the blue(c1) pcm connector. It was sitting flush with the clip thats supposed to lock it in so either I accidentally pushed it in and didn't see it when I pinned my efans in, or it happened one of the times I had the pcm unplugged. This is with the original pressure switch in it as well. Either way, its fixed. I will remember to check all connections more thoroughly and frequently when diagnosing intermittent issues lol. Thanks for all yalls help and suggestions

I've had that happen before. LOL Glad you found the issue. ;-)

[gtstorey]

Solved! B signal wire had been pushed back in the blue(c1) pcm connector. It was sitting flush with the clip thats supposed to lock it in so either I accidentally pushed it in and didn't see it when I pinned my efans in, or it happened one of the times I had the pcm unplugged. This is with the original pressure switch in it as well. Either way, its fixed. I will remember to check all connections more thoroughly and frequently when diagnosing intermittent issues lol. Thanks for all yalls help and suggestions

So it's fixed 100% for the original problem or just why the trans harness disconnect test isn't giving consistent results?

[travmac79]

So it's fixed 100% for the original problem or just why the trans harness disconnect test isn't giving consistent results?

Its mostly fixed lol. I drove for the past 4 days and all was good. Normal shift feel, 4 gears, lockup and no codes. I even hauled my camper around some of our backroads(not highway) for 15 miles to see if that would make the code come back. I drove on the highway at about 80mph for a couple of hours and still no codes. I then drive with a 20ft car trailer into the mountains of VA to pickup some project parts, and halfway there, the code is thrown and it shifts hard again. that was about 162 miles of mostly highway driving, one way. I cleared the dtc when i got gas and added some fluid since it was on the bottom of the crosshairs. Threw the code again on the highway but it wasnt that noticeable since i stayed in 4th gear most of the time. when I turn the truck on after loading the parts, it still had the code(current and history) but it didnt max out the line pressure. Ive been driving it on hilly backroads and even the highway today and the line pressure acts like i think it should. shifts smooth. code is still there. Attached is a log from this morning. If i need to start another thread since this is a different problem, just let me know. Thanks again

11223truck.hpl

[TransGo Robert]

Its mostly fixed lol.

Your lock-up clutch is not able to hold under load. You either have a worn out valve body or a cracked lock-up piston in the converter. I would start with the valve body, we have a drop in valve to fix it if the bore is worn out. Here's a link to it: https://transgo.com/product-details/4l80-tcc-regulator/

[travmac79]

Your lock-up clutch is not able to hold under load. You either have a worn out valve body or a cracked lock-up piston in the converter. I would start with the valve body, we have a drop in valve to fix it if the bore is worn out. Here's a link to it: https://transgo.com/product-details/4l80-tcc-regulator/

Thanks for the quick response! I replaced the valve body with a sonnax unit over the weekend so it looks like Im gonna be buying a converter? Any specific ones I need to buy or is a parts store replacement fine? I do tow a small camper and car trailers pretty frequently. But not every week. The engine is, and will stay stock most likely

[TransGo Robert]

Thanks for the quick response! I replaced the valve body with a sonnax unit over the weekend so it looks like Im gonna be buying a converter? Any specific ones I need to buy or is a parts store replacement fine? I do tow a small camper and car trailers pretty frequently. But not every week. The engine is, and will stay stock most likely

Hopefully that valve body is good, it is always a bit hit and miss with reman valve bodies. Too bad we don't have a log of before and after replacing the valve body that would have made it easier. Now if it needs to come out I recommend you let a transmission shop handle it. The converter will have to be replaced but other things in the unit need to be checked at the same time as well to ensure it does not damage the new converter in the not so far future.

[travmac79]

Just checking back in. I replaced my torque converter with a reman unit from oreilly. While I was in there, I planned to change the pump bushing but the one I purchased was too large to press in. It started collapsing as I pressed it in... so I put the old one back in out of necessity, and restaked it. ill replace it eventually. I have put 3k miles on it thus far, with no issues or codes. I've taken 3, 500+ mile trips to the NC, VA and GA mountains. One towing my 4000lb camper. It's been excellent and consistent the whole time. Just wanted to let yall know it's working well and that I appreciate all the help.

[RDF1]

Thanks for the update!!!

[TransGo Robert]

Hopefully that valve body is good, it is always a bit hit and miss with reman valve bodies. Too bad we don't have a log of before and after replacing the valve body that would have made it easier. Now if it needs to come out I recommend you let a transmission shop handle it. The converter will have to be replaced but other things in the unit need to be checked at the same time as well to ensure it does not damage the new converter in the not so far future.

Glad it is working good, thanks for taking the time to post an update! Man, you are one brave man putting that old bushing back in. Never heard of someone taking a chance like that before, I sure hope it does not end up spinning in there taking out your new converter and the pump.