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

Ok back with another update. I got a completely different ECM and EPROM chip (complete unit) from a different vehicle, with the BPAA chip so this time it was compatible. However this did not cure it.

The shifts may be a little different now but fundamentally the pressure drop is still there. I still get a significant drop in pressure on the 1-2 shift, down to about 50-60psi, and still only making 90psi line pressure while in 4th cruising, which still seems low to me? however does shift all gears and locks up the TCC still.

On a 1-2 shift pressure still drops to about 60psi or a little less. Also had a couple times where the trans went into limp mode and got code 87 which i think is a slow shift / ratio error, which makes sense because at these low pressures i dont think the shifts are very crisp at all and seem rather delayed. Also i still get a good whiff of hot fluid/burnt clutch type of smell when i get out of the truck, so something is still not right here.
 
THEFERMANATOR said:
Those numbers are not correct. Pressure numbers for shifts top out at 90 as the epc runs a max of 90 psi. So your xdf must have a multiplier in it to convert eoc pressure to line pressure which will not be accurate. And those are the pressures it shoots for while in gear, not during a shift. I haven't been able to find my old files yet to see if I can locate the TAPS for the actual shifts, but TAPS are just base numbers that will vary based upon what the PCM sees during shifts, and the adapt value it sets.
Click to expand...
I think there was a multiplier in the xdf. not sure where I picked up this xdf from. I can play with the multiplier and see what it changes. I have two different xdf files for tuner pro, one of them doesn't show these tables at all. I do have the one from Lyndon for the old tuner cat program but havn't looked at that one to see what tables are there. maybe once you are able to find your info and what it should show I can get the multiplier correct. I also have one for a stand alone tranny controller but not sure where it's from ether or which tranny controller it was made for, I can take a look and see if it even shows that info.

the good news is I think @brhynard has ruled out the PCM as a suspect now.
 
Yes thats true i believed i have ruled out bad PCM and EPROM chip since both have been changed and issue persists. Unless BPAA is the wrong one however i know BPAA has been in the truck a long time, even during its good running days, so i dont think thats any issue.

as for the grounds, i have removed all and cleaned them and also done a voltage drop test across them and am not finding any issues. So far the grounds seem Okay but i will keep digging there.

As for the internal trans harness, i have swapped the pass thru back to an old one (the one i installed in the rebuild was brand new) and again no change. So i dont think pass thru harness.

One thing i did notice with the TDSCAN tool is a dropout in both the input and output speed signals. Especially the input. Which you can see here during a slow drive to about 12mph: https://www.youtube.com/shorts/SDJDytDhBi4

I am going to try to buy an oscilloscope to read the speed signals and figure out what could be going wrong with them. Both sensors have been changed and ive checked the wiring at the pigtails, nothing seems off. I will install new pigtails/connectors and i have already ran some external wiring to the ISS but no success there on fixing anything. So now i need an oscilloscope to read the signal and try to find out why its not right.
 
When you run into problems with signals from 2 wire magnetic pulse speed sensors, ALWAYS make CERTAIN that the speed sensor wires are twisted pairs. If you run 2 straight wires through the harness you WILL get magnetic interference. I believe GM called for a minimum of 10 twists per foot of wire to avoid EMI interference.
 
@brhynard I need to hook up my lappytop and see if I get the same results on the brake torque and such. I have not ever messed with the tranny other than replacing the TCC solenoid due to a code I was getting before.
 
yeah i will be curious to see if you get anything similar. Consider yourself lucky! I am running out of things to try with this truck and i am somewhat afraid i will have to give up soon
 
I looked through the .bins I found saved from when I dabbled in 6.5 tuning and none of them had the shift timing or desired pressure tables mapped out. I do have multiple tunes for 6.0l & 8.1l's backed by a 4l80e and what your seeing during shifts is pretty close to what they would run. Up to 1/4 throttle or so most of the shifts should drop down to around 40-60 psi of line pressure from the base pressure tables.
 
brhynard said:
yeah i will be curious to see if you get anything similar. Consider yourself lucky! I am running out of things to try with this truck and i am somewhat afraid i will have to give up soon
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Don't give up! sounds like from Ferm, your on the right track and seeing pressures within spec. maybe it's just something simple. I don't know how many times I have racked my brain trying to figure out something and come to find out it was staring me right in the face the whole time in plain sight where I couldn't see it!!

I will compare your readings to my truck, but I won't be able to do anything for a few days. my truck is going into the shop today to have the ring gears replaced and the rear end gone through. not sure if I will have it back by the weekend.
 
Ok wow, well if 40psi is normal during a low throttle 1-2 shift maybe I don't have any pressure problem at all then. I just figured it was abnormally low considering it's 150psi while driving in first under the same throttle level, however maybe that is just how they have them tuned for whatever reason. Thanks for looking that up.

I had a dragging rear brake shoe so I realized today that the funny smells I was getting after getting out of the truck was just that, and not the trans, so that was good news. I put new shoes and a wheel cylinder on today and drives much better. I'm sure having that extra load was not helping things with my test driving.

Tomorrow I will go for a drive and check out that speed signal some more and check for torque converter slippage, now that I am less worried about breaking anything. And then if I get a chance I will try to get a gauge on another 6.5 rig at some point soon to see if it's shift points match for pressure, etc. Definitely still interested in seeing the force motor current during light throttle if you get a chance in the near future when your trucks back dbrannon.

Thanks guys!
 
glad you found where that burning smell was coming from! a brake drag can cause a real hardship for the tranny! I talked to the guy doing my rear axle and he said hopfully he will be done sometime today! keeping my fingers crossed that all goes well and I will have it back for the weekend :) Ill get you a comparison reading on my truck from GMTDscan.
 
Awesome. When you get it back and have the scan program hooked up, if its not too much trouble, i would be curious to also see what kind of TCC slippage you are getting when cruising at 50-60mph with TCC locked. I went for a drive today and my truck did well, and i found always 1-3 RPM slippage indicated during lockup. Assuming that is normal but would be curious to see what another member finds.
 
1-3 rpm is really good! iirc I remember seeing mine up around 100 - 500 depending on my foot and the ever so slight incline of the road / wind drag

anything over 500 for a period of time will set a code on the PCM
 
brhynard said:
One thing i did notice with the TDSCAN tool is a dropout in both the input and output speed signals. Especially the input. Which you can see here during a slow drive to about 12mph:
Click to expand...

Have you physically inspected the vehicle wire harness from the connectors? Where the two wires go into the main bundle is where I find the insulation flakes off the wires and they short out. Seen this on three 1993 2500 pickups out here.
 
I have tried but maybe i need to take another look. I have not found any indication of any wire damage anywhere really.

I was able to test the 6.5TDscan program on another 6.5 diesel that wasnt having problems, and the speed signal dropout appeared to be completely normal (the other vehicle did the exact same thing). So now i am convinced that i may not have issues with speed signal after all.

As of now i have put about 1000 miles on the trans, and just hauled a small 2,000lb tractor with it last weekend. Only issue is one time driving through town in some stop/go, after getting off the highway, a high ratio code was set (DTC87). So maybe not out of the woods yet. After keying on again and driving the couple hundred miles back home, the code never set again.
 
Man I completely forgot to go get you the readings from my truck with the laptop, my apologies. I'm glad you were able to do a hands on comparison with another truck. if you need it still I will get it this weekend when I start toying with my ac.

on that code, I have had odd ball codes popup up before for no apparent reason, cleared them and they never returned. those codes set only because the PCM doesn't see what it wants in the time frame it wants to see it happen. I have seen codes set like that when cruising at a speed where ether it's shifting in and out of OD or just a hair under the time for a shift and the PCM thinks it should have, but doesn't command things properly (glitches) one of them is the infamous APP (accelerator pedal position) code that can be set just by a quick blip of the pedal where you let off before the PCM could respond to the blip!

something on the pressures, if the owner of the other diesel truck will allow you to connect a gauge for you to compare actual pressures on yours, that might be a good thing, not that ether are correct, but to see how one does with no issues ruling out the thoughts and mind games we all sometimes get into convincing our selves we didn't do something right or a major event will happen when later we find everything is right on par! lol I've been there many times!
 
Yeah thats a good idea, i did check PCS command, which is what sets the pressures, and looked about the same on that other vehicle. So reason to believe pressures should be comparable. But i could do some more comparison.

At this point ive got about 1k miles on the trans and everything seems to be mostly going pretty well. I have even pulled a trailer several hundred miles. Its working good but i have set this DTC87 High ratio error a few times now. So i have gone out and checked shift times and my 2-3 shifts do seem to be a little slow. I'm thinking that could be causing it to set the code. 2-3 shifts around .300 seconds.

I did a dual feed mod on this trans to dual feed the direct, however i didnt modify any orifices or leave out any checkballs when i did, and now i am thinking maybe there is slow oil flow to the direct piston and thats whats causing the slow shift and the code to set. I think this could be because my reverse engagement is noticeably slower now since i did the modification.

Next time i pull the VB out i will open up these orifices or leave out a checkball or two to try to remedy this, but hoping it wont cause any damage in the meantime.

If anyone knows of their typical 2-3 shift times during light/heavy accel, i would be interested to hear them.
 
Got over 2000 miles now on this latest rebuild, and its been at least 500+ miles since its set a DTC87 code. I am not out of the woods yet but things seem to be trending in a good direction at least. Im gonna keep driving it and hopefully she keeps working.

Thanks again for the help everyone!
 
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