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

[brhynard]

Hello! I have a trans issue that is really throwing me for a loop. Hoping to get some help from anyone with a good knowledge base on the 4L80. I have a 1995 silverado w/ 6.5 diesel. I recently did a full rebuild on my 4L80 trans (new clutch frictions, seals, filter, bearings, bushings, gaskets, Torque Conv, and Reman Pump assembly). It ran great for 1200 miles or so. Shifted great, TC locked up, everything worked great for the past couple of weeks. Then one time on the freeway the torque converter failed to lock up, so i got off the freeway and slowed down some, and eventually it locked up. A handful of miles later, had lockup issues again where the TC just wouldnt lock, followed by some slipping when starting from a stop. The truck did not want to get going in drive from a stop without slipping. Moments later, truck wouldnt move, first it was reverse only and then it was neither. I let it cool off and then it would drive/reverse OK and i could limp it home. I hooked a gauge to it and its showing 60psi at idle (P,N, or D), cold or hot doesn't seem to really change much. 90 psi in REV idle. However when i go full throttle line pressure only rises to about 65 in D and 100 in reverse. So no real increase in pressure.

I am struggling to determine the following, and i am hoping someone can help me.
a) why the pump seems to be pumping but the line pressure is not really rising with throttle apply?
b) why the trans worked for 1200 miles but then failed, as opposed to just not working right away from a build error etc?
c) what to check for to root cause lack of pressure? stuck PR valve, etc seems unlikely considering the pump is 'remanufactured' but i am not ruling it out

Thank you in advance for any advice you can offer or things to check!

Other info. 1) trans never really got hot. when TC wasnt locking and/ or trans started slipping i felt the pan and the fluid and it was very much a reasonable temp (200 or less). Fluid is plenty full (1" over fill mark when hot and running in Park).
2) reason for rebuilding the trans was because previously, it was running for thousands of miles w/ no TC lockup, so i pulled trans to swap converter, and disassembled the pump to inspect it when i did this. After swap of TC, the TC locked for a short period of time but the trans immediately failed (~10miles) for slipping clutches. I figured i screwed something up with the pump when i took it apart, so thats why on this rebuild i put in a reman pump.
3) in this rebuild i did change force motor solenoid and PWM TCC solenoid with new OEM parts. Also replaced the wiring harness.

 

[Stoney]

Welcome to TTS..be patient one of the guys will chime in.... I personally don't know much about transmissions but some of them are experts

 

[dbrannon79]

Welcome to the forum! Glad to have you aboard @THEFERMANATOR is the most knowledgeable person here on transmissions. I'm sure he and others who know more on pressures n such will chime in soon. there's lots of good info scattered about TTS and in the stickies threads.

 

[brhynard]

Well just to make this more interesting, I pulled the pan today and here is what I found. Filter seems nice and connected and doesn't appear to be clogged with crap. There is some clutch material in the fluid. Now it's 20 degrees here and I'm doing this outside, keep that in mind. But in the bottom of the pan there are ice chunks, as well as very fine dots of glycol. I did a little taste test and the green along the pan rail and some dots inside the pan, is no doubt antifreeze. But totally separate from the water /ice chunks at the very front of the pan. Obviously got some contamination and I don't know if that explains my problems or just makes this whole thing even more confusing. Overall the fluid looks OK but not perfect, considering it only has 1k miles on it.

 

[brhynard]

Well just to make this more interesting, I pulled the pan today and here is what I found. Filter seems nice and connected and doesn't appear to be clogged with crap. There is some clutch material in the fluid. Now it's 20 degrees here and I'm doing this outside, keep that in mind. But in the bottom of the pan there are ice chunks, as well as very fine dots of glycol. I did a little taste test and the green along the pan rail and some dots inside the pan, is no doubt antifreeze. But totally separate from the water /ice chunks at the very front of the pan. Obviously got some contamination and I don't know if that explains my problems or just makes this whole thing even more confusing. Overall the fluid looks OK but not perfect, considering it only has 1k miles on it.

 

[Will L.]

Radiator cooler loop inside the tank.
Check antifreeze, it is probably contaminated with atf as well

 

[Stoney]

I grenaded a transmission back in October... That debris looks like a very mild amount of what I had in mine when I pulled the pan... I can't for the life of me think how antifreeze is in your transmission...

 

[Will L.]

I grenaded a transmission back in October... That debris looks like a very mild amount of what I had in mine when I pulled the pan... I can't for the life of me think how antifreeze is in your transmission...

Cooling loop fails. Atf higher pressure when operating so it spits atf into radiator. Engine coolant under normal operating pressure when you shut it off and coolant now at say 14 psi pushes coolant back into trans cooler.

after people experience this they often eliminate radiator from atf cooling loop. If they use a pan heater all the time for atf warm in cold weather and have a fan on the new atf cooler- great.

hummers don’t have in radiator atf loop. Just a separate cooler. But slow offroading heate trans up more because no slow speed cooling so shorter life. 4l80e is good for 200,000 miles almost no matter what you do. But my hummer is at 150,000 miles and I expect to need full rebuild before 200,000 miles because of it.

 

[Stoney]

So why not eliminate the loop out of the radiator... Run your transmission lines into an external cooler... And put a pan heater on the transmission?

 

[Will L.]

So why not eliminate the loop out of the radiator... Run your transmission lines into an external cooler... And put a pan heater on the transmission?

Because money and time.
Every time it is cold you want to plug in the pan heater before starting? (Some believe)
then the cost of the external cooler with it’s own fan is expensive. Plus have to find where to put it. Now one more electric thing to fail.

When a person is weighing buying a new radiator, they could eliminate the loop and just put in plugs if it is leaking between the two. That might save some of the cost. So at that point, ok. Buying one too small is a painful mistake.

Heating up atf by the antifreeze is an argument many people have. Think:
Cold day, no block heater needed or used but needed two cycles of glow plugs. You let the truck warm up a minute and drive to work. The engine takes 2 or 3 miles before it is up to 180° and you can start to warm your hands in the vents. That radiator heat still hasn’t done a darn thing to heat up the atf yet. All the heat generated by the atf is from it working itself. So, many people are up to hiway speed by time the thermostat opens the first time.

How does atf loop inside the radiator help at this point? You can guess what side of the fence I am on with this idea of warming up the atf with cooling loop.

How it helps 100% is driving slow. the fan isn’t on, the converter isn’t locked. That trans is building heat from the converter slip. The trans can be at 220 while you are putting around in rigs without the radiator loop. But if your thermostat is a 195- that coolant is not cooling the atf until it is over 200. And on rigs with a problem getting rid of heat already- any transmission heat added to the radiator work load isn’t nice. Being able to put the transmission cooler anywhere you want it in pickups definitely helps. Unfortunately hummers space is so tight there are limited options.

 

[dbrannon79]

Just curious on the coolant getting into a fresh tranny. how much damage can that do and how quickly does it get to where it's at the point of no return?

 

[Will L.]

Complete rebuild. Longer ran the worse, more contamination worse.
There is no exact amount that is safe- I’ve seen tiny amounts destroy new transmissions

 

[brhynard]

Ok update, i pulled the valve body off and so far all i have found is a bunch of clutch material in the AFL screens (both screens have ripped allowing crap thru them) and also clutch material plugging up the PCS solenoid which would explain the lack of line pressure response. Not sure what clutch has failed though however.

Antifreeze looks great and no sign of contamination in the overflow tank. I also made a mark of coolant level before truck hit the road and the coolant level does not seem to have dropped. Talking to the other person with me at the shop that day, there may have been trace amounts of antifreeze in the container that we used to put the ATF in and pour it into the trans. That may explain the small droplets of antifreeze that were in the pan.

 

[MrMarty51]

Dang.
So, with the ripped screens and such as You mentioned, will You be having to again rebuild the transmission ? Or just replace the screens and such ?
After You get it all back together, maybe put a few miles on it then drop the pan once again and check for antifreeze.
Welcome to the truck stop.
I am sure that theferm will be through and He will have a whole bunch of knowledge.

 

[brhynard]

The first thing i am going to do is put a remanufactured valve body in (replacing the screens also) with a new filter and fluid, and a cleaned and working pressure control solenoid and button everything back up to see if i get my line pressure back up. If so then i know the problem was in the valve body or due to the crap clogging up the solenoid. Not sure if the trans will work at that point, as i am guessing i lost the TC clutch and maybe more clutches, but that will at least show i solved the pressure issue and then i can pull trans a rebuild. I dont want to rebuild trans again though without knowing what happened here because this thing has bit me twice now!

 

[MrMarty51]

The first thing i am going to do is put a remanufactured valve body in (replacing the screens also) with a new filter and fluid, and a cleaned and working pressure control solenoid and button everything back up to see if i get my line pressure back up. If so then i know the problem was in the valve body or due to the crap clogging up the solenoid. Not sure if the trans will work at that point, as i am guessing i lost the TC clutch and maybe more clutches, but that will at least show i solved the pressure issue and then i can pull trans a rebuild. I dont want to rebuild trans again though without knowing what happened here because this thing has bit me twice now!

It’ll be interesting. Please post up the results.
I am one that is hoping it all will be just fine with new components and fluid and filter.

 

[THEFERMANATOR]

So why not eliminate the loop out of the radiator... Run your transmission lines into an external cooler... And put a pan heater on the transmission?

For the majority of people this is a bad idea. The radiator cooler is there for slow speed cooling, unless you have a trans cooler with it's own electric fan on a tstat switch, you will get overheating at low speeds. Gmt400 trucks have horrible airflow through the cooling stack, below 40 mph you get very little ram air through your cooling stack. So you have the radiator cooler to do the cooling at lower speeds, and your external cooler to shed heat once you get moving.

As to the OP, you always unplug the trans plug when you have low line pressure. This takes the electrical out of the equation and sends it to max line pressure. You should always renew or replace your EPC on a build. And anti-freeze, even in small quantities is a DEATH sentence for an auto transmission. Since your clutches run in oil, they cannot use a petroleum based adhesive, so they use a water soluble adhesive, and anti-freeze, even SMALL amounts will ruin a transmission. It may have just taken the 1200 miles before your rebuild failed. Why/how did the original trans fail? Was the root cause of the failure found and corrected before installing the rebuilt trans? Did you put in a afl valve kit into the rebuild?

Most trans failures don't just happen, almost all of them have a root failure, and if not found & corrected, the replacement will fail a rapid repeat death.

 

[MrMarty51]

I have seen what prestone does to babbitt in main and rod bearings. That aint a pretty sight.

 

[brhynard]

Thanks for the help! here are some answers to your questions:

As to the OP, you always unplug the trans plug when you have low line pressure. This takes the electrical out of the equation and sends it to max line pressure.

yup i tried this. Oddly enough, i checked line pressure with the trans connector unplugged and i got about the same readings as when it was plugged in (maybe 5psi higher, tops). 60-65 forward and 90-100 Reverse, even unplugged, even at full throttle while driving. Upon inspection of the VB, the inlet ports of the PCS are covered in clutch material (presumably from the TCC, i am not sure) , which may explain why it cannot provide a signal to the PR valve to up the pressure.

You should always renew or replace your EPC on a build.

The EPC is new. The old one i couldnt find anything wrong with it but i did replace it anyway. The PWM and harness was also replaced and are new.

And anti-freeze, even in small quantities is a DEATH sentence for an auto transmission. Since your clutches run in oil, they cannot use a petroleum based adhesive, so they use a water soluble adhesive, and anti-freeze, even SMALL amounts will ruin a transmission. It may have just taken the 1200 miles before your rebuild failed.

I agree.

Why/how did the original trans fail? Was the root cause of the failure found and corrected before installing the rebuilt trans? Did you put in a afl valve kit into the rebuild?

Here is the detailed history. I had been running the truck at low speeds for hundreds (maybe thousands?) of miles with a TC that would not lock up. I ran the truck like that for the better part of a year, everything worked great but no lockup. Not sure when/why it quit working as i didnt notice it when it quit, but i kept driving on it that way for a while just didnt go on the highway and made sure to keep the temps down as best i could. It was my spare truck so this worked okay.

However eventually i got sick of being limited so i pulled the pan and swapped the EPC and PWM solenoid w/ brand new OEM parts thinking that might solve it. No change. So Then i pulled the trans and put a reman converter in from a trans shop, and when i did this i was trying to be ambitious, so i took just the front pump apart and inspected it (looked good), filed down the halves slightly because i thought it would ensure a good seal, and then put it all back together. Took it for a test drive and the TC locked up and everything shifted but something seemed off, like a pulsing/ticking noise or something. Maybe 5 or 10 miles later the trans started slipping heavily and shortly after it didnt move. So i took the whole thing back out and did a full rebuild, bushings, seals, TC, and all. Only thing i found to explain any issue was a damaged forward piston seal. All the clutches were seemingly in really good shape, but the AFL filter screen was plugged with crap (so i have to assume it was the TCC ). I figured when i took the pump halves apart i must have screwed something up that caused pressure to be low, and thats why it failed, so this time i put in a reman pump. After all that, i Installed it, worked great and nothing seemed off, flawless for 1200 miles, and then here we are

I did not change the VB (its been same as all along) and i did not put in an AFL kit. I did inspect everything in the VB and change solenoid seals, etc, but i couldnt find any valves that didnt move well and i couldnt find any indications of wear (but i also just crudely checked) . Looking at it now and putting brake cleaner in the ports, i still cant get a leak around the AFL valve. It could be a number of things but i cant easily find a smoking gun, and one common theme on this trans is it keeps eating torque converters first.

 

[brhynard]

Update: put the reman valve body in (comes with new solenoids) and a filter with some fluid and everything shifts good, TC even locks up. Gotta test more hot and I'm not sure it will hold up for any real mileage but everything is making good pressure now. Pressure jumps immediately with throttle , 150+