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

[WarWagon]

and maybe i can run a manual ground to the PCM just to try it

NO!!!

The ground from the PCM goes to the engine at a specific area, period. You put the ground anywhere else and you risk a ground loop: massive wire melting current attempting to find a ground using this wire and anything attached to it as a ground path. Hit starter = instant Bad PCM and smoking burnt wire. ECM case as part of the ground loop with random location ground wire can burn a trace clean off the motherboard.

You can "replace" the PCM (ECM/Computer) ground wire completely, but, it needs to go to the engine directly. Same location with same other grounds it had from the factory or by itself without other "noise" from other ground wires bolted to it. The small voltage difference you can have from the body to the frame and/or frame to engine is enough to throw sensors out or ground loop as noted above. 0.5V on a TPS indicating a closed throttle is thrown off by 0.1V as an example.

The engine is the ultimate ground point. Most people want to use the negative battery terminal as a ground because they are staring at it in plain sight: "Why not?" ... well the above is Why Not!

In the "scanner" software you have a way to read voltage that the PCM is seeing. This can let you know if you have a problem with low voltage. As noted grounds or positive supply side via ignition switch or other corroded connection. One example: Remove and clean every single fuse...

You have somewhat of a good ground to the ECM otherwise it would not enter diagnostic mode and flash codes. There were three corroded crimp connectors on the ECM ground for one mess I worked on that would not flash codes or communicate with scanners. They declined to continue to pay me to keep fixing the electrical mess.

As this turned electrical recall "oil" is an insulator. Oily connectors, as some years are noted for leaky transmission connectors, can be a problem.

I would carefully inspect the harness where it joins the main harness from the transmission connector. I have had speed sensor wires loose their insulation, flakes falls off, and short out in that area.

 

[brhynard]

NO!!!

The ground from the PCM goes to the engine at a specific area, period. You put the ground anywhere else and you risk a ground loop: massive wire melting current attempting to find a ground using this wire and anything attached to it as a ground path. Hit starter = instant Bad PCM and smoking burnt wire. ECM case as part of the ground loop with random location ground wire can burn a trace clean off the motherboard.

You can "replace" the PCM (ECM/Computer) ground wire completely, but, it needs to go to the engine directly. Same location with same other grounds it had from the factory or by itself without other "noise" from other ground wires bolted to it. The small voltage difference you can have from the body to the frame and/or frame to engine is enough to throw sensors out or ground loop as noted above. 0.5V on a TPS indicating a closed throttle is thrown off by 0.1V as an example.

The engine is the ultimate ground point. Most people want to use the negative battery terminal as a ground because they are staring at it in plain sight: "Why not?" ... well the above is Why Not!

In the "scanner" software you have a way to read voltage that the PCM is seeing. This can let you know if you have a problem with low voltage. As noted grounds or positive supply side via ignition switch or other corroded connection. One example: Remove and clean every single fuse...

You have somewhat of a good ground to the ECM otherwise it would not enter diagnostic mode and flash codes. There were three corroded crimp connectors on the ECM ground for one mess I worked on that would not flash codes or communicate with scanners. They declined to continue to pay me to keep fixing the electrical mess.

As this turned electrical recall "oil" is an insulator. Oily connectors, as some years are noted for leaky transmission connectors, can be a problem.

well i ran a manual ground anyway and it had no effect. I took a lead and went from the existing ground location on the engine to the existing ground pin in the PCM. It did not have any effect.

 

[WarWagon]

well i ran a manual ground anyway and it had no effect. I took a lead and went from the existing ground location on the engine to the existing ground pin in the PCM. It did not have any effect.

^^^ That's the way to do it. Note edited post above while you replied.

 

[dbrannon79]

as to the possible failing PCM, on my truck, I was getting a code 78 ( I think) for the boost solenoid. but I noticed when connected the solenoid, it would allow vacuum to the actuator. disconnect and it would release vacuum! that got me to try the solenoid test that's in GMTD to try and see if the solenoid was working. with the OEM (95) PCM the solenoid would not respond to anything the PCM was commanding. I just so happened to had pulled a PCM from a 94 truck in the junkyard just to have as a spare prior to seeing this issue and for shits n grins I connected it and retested the solenoid. that fixed the problem. I did try putting my OEM one back in to make sure it wasn't just a bad connection but nope it was the PCM.

these old PCM's can do some weird things when they begin to fail. back in my wrench turning days I had seen one like this that would randomly stumble and misfire for no apparent reason. we found that whacking the PCM with the handle of a screwdriver made it go bananas!

look around for a used PCM with a prom chip in it to try and see if things change. at least where you can try and possibly eliminate the PCM as a suspect without spending $$ that you can't recoop. maybe someone here in the forum can possibly lend you a working one.

I have the one from my 95 stashed under my seat for a "just in case spare", but I know it won't control the waste gate solenoid. I also can burn you a prom chip from the latest GM update they released in 96 for the 95 vin f model, but would need to order a blank chip since I used the one I have in my truck.

 

[dbrannon79]

keep in mind that if you do swap in another PCM, you will need to run the TDC offset relearn procedure for the PCM to learn what your engine timing. youll need the paid version of GMTDscan or a GM tech 2 scanner for that.

 

[brhynard]

so today i got an ECM in with the exact same PN. I plugged in that ECM and tried it and the trans goes immediately into limp mode upon startup, however no codes, so i will run the scan program and see if i can find any issues. However so far i dont notice anything being off.

I swapped the PROM chip from my old ECM into this one, and the vehicle came out of limp mode but does the same thing before (line pressure drops out). **This is with the new ECM but old PROM chip--No change**

 

[dbrannon79]

what is the bbc code on the prom chip? should be the 4 letters. also the code on the replacement ECM I may be able to lookup the specs and differences as to why it went to limp mode and lookup your line pressure specs

 

[brhynard]

Old ECM 16212488 , BPAA, 86BPAAM252690210.
Chip says BPAA 16216871



New one: ECM 16212488 , 862488W252860754. Chip says 1621 BNTL 5282

 

[dbrannon79]

Old ECM 16212488 , BPAA, 86BPAAM252690210.
Chip says BPAA 16216871



New one: ECM 16212488 , 862488W252860754. Chip says 1621 BNTL 5282

ok I will look up the info after I get home from work. BNTL might be for a manual trans but I will verify that after work.

 

[brhynard]

Yup I'm thinking it's for a manual trans, I should have done my homework. So I am not sure if my PROM chip could be causing the line pressure drop out and I need to try a different one....or if something else is going on still. Obviously not the ECM itself causing the problem, unfortunately.

Anyone got a BPAA chip they want to loan me to try out? I'll pay you $20 and shipping both ways!

 

[dbrannon79]

if you order a blank chip and the adapter from Moates I can flash one for you free , even the updated BXPY which has some updated specs for the trans pressures. there is a TSB out for it. I'll post the TSB and info here for you after work. not sure if the TSB covers the issue your having, but I remember it was something to do with the trans.

 

[dbrannon79]

ok I hope this info will help. I know Ferm will have a vast better knowledge on this than I. I have never messed with the trans.
Here is was I was able to find in the stock programming for BPAA for the line pressure. I couldn't find BNTL in the list but I will do some more looking.



For BXPY there are a few differences along with some other things they updated.




and the TSB for the update...

 

[brhynard]

Thanks. So what's that telling me? Temperature on the left axis -40 to 80 Celsius and the top axis is pedal position %??

Or am I reading it backwards? Regardless line is pretty steady at 130 or so if that's in PSI so much much higher than I'm seeing

 

[dbrannon79]

when I looked at the table specs it does show temps in Celsius and pedal position as percentage but doesn't show what the output is in. I'm wondering if the pressure is in Kpa, but that wouldn't make sense. I can't get to my laptop to verify it. the question would be honestly what the PCM is commanding compared to what the actual pressures are showing. Ferm would be able to confirm but I think only the GM tech 2 scanner would show what the PCM commands

 

[brhynard]

I'm assuming the reference current and feedback current channels in GMTD scan is telling you amps going to the line pressure solenoid and amps coming back, so basically desired(commanded) vs measured I would think...but I'm no expert.


I can get my hands on an PCM for a 94 blazer w/ 6.5 diesel (PCM code BJAW) and a 95 suburban w/ 6.5 2wd, to try out, however not sure if the PROM in either of these is compatible with my truck. And its a 2 hr drive w/ $4.50 fuel to go get them

 

[dbrannon79]

I would think the suburban would be compatible. I just reached out to Lyndon at Westers Garage who sent me the BXPY prom file realizing after I was reading the TSB, seeing that BPAA was updated to BXPW in 2003 and not the one I have! Hopefully he responds soon and will let me know what the differences are and if he might have that prom.

there are a few sites online offering these OEM proms for a decent price shipped. that might be a good route and much cheaper than the cost of fuel traveling that far for a used one not knowing if they will work.

But I remember ferm saying something about disconnecting something on the trans that would force the trans into full pressure to see if there is something internal rather than something electronic giving you the issues.

 

[brhynard]

I'll do whatever it takes at this point. Money im not too worried about just wanna get this thing back on the road and working sooner than later. Thanks for checking that online. If you are referring to putting the trans in limp mode by unplugging it so that it defaults to full pressure, i have already done that and confirmed that hydraulically, the trans works good and the pump is capable of making pressure.

 

[MrMarty51]

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Можно оставить заявку по телефону +7(495)128-45-21, вас проконсультирует менеджер и подскажет подходящую машину.
Вывоз мусора кроме стройки, ремонта, отходов производства и т.д. соответственно всей Москве и области.
Для больших объемов делаем скидки, можем заключить условие и вывозить мусор в определенные даты и время.
Непомерный, собственный, автопарк под любую задачу. Звоните будем рады.

I did a translation of most of that, from the phoney numbers down.
Something about hauling out the trash and garbage.
I got a wife for that chore.

 

[brhynard]

Its interesting that with everything going on over there our friend took such an interest in a 4L80E with line pressure dropping out unexpectedly. Although i cant say his comments really aided the discussion.

 

[THEFERMANATOR]

ok I hope this info will help. I know Ferm will have a vast better knowledge on this than I. I have never messed with the trans.
Here is was I was able to find in the stock programming for BPAA for the line pressure. I couldn't find BNTL in the list but I will do some more looking.

View attachment 72844View attachment 72845View attachment 72846

For BXPY there are a few differences along with some other things they updated.

View attachment 72847View attachment 72848
View attachment 72849

and the TSB for the update...

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.