• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

95 4L80 Low Line Pressure and No Line Rise under throttle

Yeah, you are right. I think i was incorrect in my diagnosis. I drove a year with a great working trans however without a locking torque converter, so i removed the torque converter to replace it with a rebuilt one because i figured the internal piston broke or something on the original. When i did so i disassembled the pump to inspect and file the halves for better sealing. I thought i was doing something good. However upon reinstallation of it all, i immediately (within 10 miles) had a lack of line pressure and burned up another converter. So i did the full rebuild thinking i screwed up the pump by filing it and so that is why i included a rebuilt pump in my full rebuild, thinking i would fix the issue with the new pump.

However after all this, it just seems this trans has a history of burning up TCCs, and possibly low pressure as a result of that contamination. I cant find anything wrong with the TCC valves in any of the pumps (new or old) and i tried using brake clean to see if it would leak across the TCC reg valve in the VB but i was not able to make that happen. Valve seems tight to bore. I also asked a valve body rebuilder to test it for me to see if anything would explain TCC issues, but i was unable to get confirmation on this yet.

So i will admit, i still dont know why these torque converters keep failing and why the rest of the trans works. And i honestly am not sure i will ever know, unfortunately. I am just going to keep trying to make it work until i finally do, somehow some way.
 
brhynard said:
Yeah, you are right. I think i was incorrect in my diagnosis. I drove a year with a great working trans however without a locking torque converter, so i removed the torque converter to replace it with a rebuilt one because i figured the internal piston broke or something on the original. When i did so i disassembled the pump to inspect and file the halves for better sealing. I thought i was doing something good. However upon reinstallation of it all, i immediately (within 10 miles) had a lack of line pressure and burned up another converter. So i did the full rebuild thinking i screwed up the pump by filing it and so that is why i included a rebuilt pump in my full rebuild, thinking i would fix the issue with the new pump.

However after all this, it just seems this trans has a history of burning up TCCs, and possibly low pressure as a result of that contamination. I cant find anything wrong with the TCC valves in any of the pumps (new or old) and i tried using brake clean to see if it would leak across the TCC reg valve in the VB but i was not able to make that happen. Valve seems tight to bore. I also asked a valve body rebuilder to test it for me to see if anything would explain TCC issues, but i was unable to get confirmation on this yet.

So i will admit, i still dont know why these torque converters keep failing and why the rest of the trans works. And i honestly am not sure i will ever know, unfortunately. I am just going to keep trying to make it work until i finally do, somehow some way.
Click to expand...
Biggest thing is, dismantle each and every clutch pack and pull the pistons, thoroughly clean and flush every component. Compressed air is Your friend here. Blow out every rifling and galley as You go. After assembling the new seals and pistons into the clutch pack housings, use compressed air, with a pressure regulator, and check to make sure there are no leaks around the seals and pistons, then install that clutch pack.
Again, compressed air is Your friend, blow out veins, drillings and riflings, rinse them some more with solvent and B&P cleaner the blow them out again. Repeat as necessary until You know that there is no debris left in any of the components.
 
Ok so i have rebuilt this Godforsaken trans and changed out all the clutches, seals, gaskets, etc again! Got a new RECON converter. I did do the dual feed mod to the direct clutch while i was at it....... by removing the center support sealing ring and direct clutch seal, and plugged the hole in the case, just as everyone instructs to.

Put it all in and test drove it shortly today. I get good pressure in Park, Neutral, Reverse, and first. Reverse goes up to 230+ under throttle and never drops out while moving. First gear pressures are great and will get to 150+ under throttle, never dropping. However when this trans shifts to second pressure drops like a rock, to like 60 psi and doesnt recover. Then if it shifts to 3rd gear it will drop even more, to like 40psi. Dangerously low while driving, so its like it has a big leak that comes on in 2nd+???

I only ran down the dirt road a mile or so to check this, and i'm not going to run it much more because i am afraid it will slip at these low pressures. The pressures do not rise with throttle while driving either, once they fall down they don't really recover, until the trans shifts back down to first. If i unplug the trans and put into default, pressure is good at 150-180psi in forward, much more than that in reverse.


So obviously the pump is working well, and reverse works great. I wouldnt think there is a leak with the forward clutch as that is applied in 1st gear. And the direct clutch doesnt come on until 3rd...My first thought is something is leaking with the intermediate clutch (which comes on in second gear and up) but i pulled the VB and airchecked it through the feed bolt and no leakage. So now I'm extremely confused at what could be happening. I definitely have not seen this before.
 
that was my first thought as well, however i cant find any cracks or any concerning areas in the valve body passages or any of the case passages! and i've looked through them extremely closely. Also checked 2nd accum bore and seals for possible leaks. Went back through the valve body to make sure everything was correctly installed. Nothing explains any sort of leak at any time, let alone just 2nd gear+.

I have never been this stumped on anything in my life!
 
I drive a 95 pickem' up truck. I can check later after work. check to make sure your dash is grounded good. may also be a faulty ignition switch. will a "95" schematic help? this link is for a tahoe but should be virtually the same for the trucks


and another I found online under a quick search

wirings-diagram.com

This domain may be for sale!
wirings-diagram.com
 
Wow thank you dbrannon, that wiring diagram helps a ton! So upon further investigation, it appears my line pressure solenoid may be commanding a low pressure once the vehicle shifts into 2nd, even though throttle is steady or increasing. Not sure why this is, but almost appears its a vehicle command issue. I am going to have to dig deeper to find out what is going on here.
 
ok sorry it took so long, had to run into town after work. I checked fuse 20 marked on my inside panel as trans with the key on and it does have ignition power. clamped the ground on the test light to the bolt head holding the dash just under the parking brake pedal and probed both sides of the 10 amp fuse. #20 is the second fuse from the far side on the bottom row. I hope this helps.
 
a couple of questions. what scanner are you using and are they any codes that are showing up? I'm wondering if there may be a loose ground somewhere. there is a ground eyelet on the trans for the PCM, others might know where exactly it is but I seem to recall on one of the bell housing bolts. also double and triple check that a wire or two didn't get sandwiched between the block and bell housing too.
 
no scanner, its OBD1 so i am pulling codes with the paperclip and flash-count method. there was a ground strap attached to one of the bell housing studs, i recently removed that to see if it made any difference with what was going on, no change. I did check the wires on the trans housing to make sure nothing got pinched and so far so good. I am going to see if i can try a different PCM and also investigate the sensors that feed info to the PCM to see if that could be something screwing things up. Your idea to check grounds is a good one, and maybe i can run a manual ground to the PCM just to try it. Now that you sent me that wiring diagram, i feel like i can really navigate this stuff better now
 
for the obd1 diesel rigs, a good scanner to invest into is a program on the laptop called GMTDscan, the paid version is $100 but the free one will do everything except for setting the timing and a few other test functions. I made my own cable with at device from amazon for around $10. together it does just about everything a tech 2 does plus gives you the ability to run some data recording with other programs and such. if interested I can link where to get GMTD software and how to build the cable, it's fairly easy to do.
 
With the help from dbrannon I was able to get a scan program attached to the truck today. And verified just about everything was right where I would have expected it, except for PCS current during a shift. Anyone have any ideas of what checks I should do with this thing connected to the laptop ? Next step for me is to try a different PCM as I'm not sure what else I can try at this point. Current commanded to line pressure solenoid is obviously wrong but I have no idea why.
 
brhynard said:
With the help from dbrannon I was able to get a scan program attached to the truck today. And verified just about everything was right where I would have expected it, except for PCS current during a shift. Anyone have any ideas of what checks I should do with this thing connected to the laptop ? Next step for me is to try a different PCM as I'm not sure what else I can try at this point. Current commanded to line pressure solenoid is obviously wrong but I have no idea why.
Click to expand...
Corroded connection in that circuit ?
 
I have seen PCM's where the programming has experienced what they call "bit rot" but it's very rare. if you have or can find a used PCM from maybe one of those pick a part yards to try it and see if there is any differences. were you able to isolate the no power issue on fuse #20?
 
brhynard said:
With the help from dbrannon I was able to get a scan program attached to the truck today. And verified just about everything was right where I would have expected it, except for PCS current during a shift. Anyone have any ideas of what checks I should do with this thing connected to the laptop ? Next step for me is to try a different PCM as I'm not sure what else I can try at this point. Current commanded to line pressure solenoid is obviously wrong but I have no idea why.
Click to expand...
Is that commanded or actual? What were you seeing for amps to the EPC?
 
Both, so far as i know.
If i remember correctly there are two channels, reference current and feedback current to PCS. These channels match very well, as i monitored both. I did not notice any discrepancy between the two. Amperage during the 1-2 shift is around 1.04amps. Before and after the shift its around 0.84. In 1st gear only under similar accel its about 0.2amps.

I can visibly watch pressure drop from around 145psi to 45 psi (while amps increases from 0.2ish to 1ish) as the 1-2 shift happens. I have done this same test with a DVOM on the wire and it yields similar results as what i saw on GMTDscan.

In a rest position like park, starting from idle, i can increase throttle and amps decreases as you would expect. Starts around an amp and decreases close to 0 under heavy throttle.
 
Under load the amps should drop down to around .2-.4 amps, depending on how hard you're into the throttle will dictate how many amps it sends to drop pressure during a shift. When I get home I can try and find some TAP tables for a 4l80e to see what the PCM should be shooting for during shifts. The EPC current will vary between roughly .08 amps to 1.2 amps, the lower the amps, the higher the pressure.
 
Right, that makes sense to me, but i cant imagine that 1amp would be commanded in the middle of a shift, thats basically idle pressure at around 1800 or 2k rpms which is right around peak torque for this engine, and in 2nd gear, under throttle, the pressure rise is completely different than 1st gear. You basically have to punch it to get any increase in pressure, because steady throttle keeps a low pressure around 45-60psi. Which seems very odd to me.

Is there a good way to attach a video here? i tried to take a video of the shift but my phone records in MP4 which it says it cannot be uploaded as that file type.
 
You must log in or register to reply here.
Back
Top