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Discussion Starter · #1 ·
Hey all, I'm new to the forum so bear with my stupid if I come across that way.
Currently I'm having some 4l80e issues on a 96 Burban 2500. So let me lay out the issue and what I've currently done.

The current problem I'm having is the truck will not move in reverse. I've searched thru the forum and have found great info, just nothing I wanna hear...

So: I just had a rebuild done and put it back in and it moves and shifts great in all forward gears including 1st - 3rd and engine brakes appropriately, but no reverse. Trans pressures are at 75psi at idle in all forward gears and 175ish at 1500 rpms . Reverse is at 100psi at idle and 280ish when I "blip" the gas...so maybe 1100 rpms or so. I am also getting about 1-2 gallons per minute of fluid flow. Just before I put this back in the truck, I replaced the radiator and the add on Transcoolers along with the cooler lines.
I've also replaced the rear servo and pin, epc selonoid, new torque converter.

I did notice that the rear servo looks to be out of a late model 4l80 and I thought that a1996 model would still be using the early model version.(flat cover vs rounded cover) . Could that be my issue?
 

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Did you do the rebuild or did you get some shop to do it?

If some shop, why not drag it back to them to get reverse working?

If you did it yourself, did you get the right length reverse pin (the pin that clamps the reverse band)?

Did you test all the electrical stuff (that the solenoid works, and that the internal harness has continuity and the right pins are getting powered on the plug to the transmission for reverse? Any codes?
 

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Discussion Starter · #3 ·
Rebuilt 1996 4l80e - No reverse

Davester, Thanks for the reply...in response to your questions:

I had a shop rebuild it but they are 100 miles away, so before I yank this pig back out I want to make sure I'm covering any issues I can resolve without pulling it.

I've checked all the electrical connections and harness's. I did get the correct reverse Servo pin, but the piston looks the same as what comes out of the newer model 4l80E...I had thought the 96 model had a different servo casing and the piston was flat instead of rounded. If the rebuilder had put in a newer version of the servo, the pin would be shorter than the older, right?

I had also heard that if the PCM/TCM was getting input from the Vss incorrectly (over 8mph) that it would not allow the reverse to engage. But I'm getting the correct pressures when I move the selector to reverse(290psi) and when I unplug the harness to bypass the PCM/TCM, all line pressures go high (380psi) and I still get no engagement. So, short of maybe a Center bolt being bad or a Reverse band incorrectly installed, I don't know what else to look for??? Maybe a Torque converter not seated properly?? But wouldn't that show by having a low fluid flow or low pressure??
 

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Discussion Starter · #4 ·
Oh, I also probably need to mention that there is nothing electronic involved when you are in reverse(solenoid wise) on this year 4L80E. Reverse actuation is mechanical which is why when you unplug the Trans Harness the system defaults to high pressure and reverse and 1st should work...or maybe it's reverse and 3rd?
 

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From what I understand, after rebuilding my 4l80e and reading several pdf's on rebuilding it (mine is an '06, so there are some internal bits slightly different from yours), that the pin that presses against the reverse band needs to be just the right length (it comes in multiple lengths). There is a special tool for measuring what length it should be, and there is at least one other way to at least ballpark the measurement if you don't have the tool. My reverse band wasn't particularly worn and reverse was working fine, so I reused both the band and the pin.

Having only worked on the one transmission, I haven't seen the bits for older transmissions like yours to compare.

Short of the torque converter outright failing, it's unlikely to be the problem. If it's not inserted correctly into the transmission, you generally have to bust things to install it into the vehicle, as the converter winds up sticking out too far, and presses against the flex plate before the transmission touches the engine.
 

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Discussion Starter · #6 ·
K...I'm pretty sure the torque converter was installed properly as I had about a 3/16 gap between it and the flexplate. One more question: I've noticed the gear shift selector(in the cab) will not move down to 1st or even 2nd...it seems to bottom out at 3rd. I'll be replacing that, but I assume that since the tranny is actuating right as I select P-R-D-3 that this is probably not the actual issue...Just one more thing to get fixed.
 

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Depends on where the problem is. If you take off the shift cable from the lever on the transmission, can you move the lever between all the positions (PRND321)?

Because it's possible to install at least one of the rods in the valve body incorrectly, so the shift lever doesn't have the full range of motion, as well as also preventing it from shifting properly.
 

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1996 K3500 6.5L TD dually F
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Oh, I also probably need to mention that there is nothing electronic involved when you are in reverse(solenoid wise) on this year 4L80E. Reverse actuation is mechanical which is why when you unplug the Trans Harness the system defaults to high pressure and reverse and 1st should work...or maybe it's reverse and 3rd?
So if a 1996 and no reverse, its either the pin’s too short or I *must rebuild the direct pack?
 

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Does it engine brake in manual first? do you have 3rd gear?

also, limp mode is 2nd gear, and 2nd is activated by both solenoids being off. In order to get forward 2nd gear, you need forward clutch and 2nd clutch engagement.

Reverse is different. IIRC solenoid A is off and solenoid B is on, which applies only the 3rd clutch. in order to get reverse, you also have to have band engagement, and since there is no reverse solenoid, i assume that band engagement is done with the manual valve. Just throwing this all out there in case someone needs it.

so to answer your question @Art4med, if you have 3rd gear in forward, and a no reverse condition, you have a rear band issue. either it's too worn or the pin is too short. HOWEVER, if you have no 3rd gear, and no reverse, the direct is screwy. Do you have any codes? start there because codes will really help narrow this down as well. If you do have codes, please report them back here.
 

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1996 K3500 6.5L TD dually F
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@johnH123 @davester : I have no codes stored. I have no reverse nor third in D, tho checking MANUAL3 this morning. There is noise and oil in the thru connection. Only 53K. Have not checked pressures.
The tranny shop [in high regard but 40 miles away] sez “must rebuild” by phone. Of course.
While I was *considering doing this rebuild myself (despite my low competence), I need dependability here. So I’m leaving it to the shop while I’m away for ten days. ??
 

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So what did the shop find as I am having the same problem on my 98 dully. Rebuilt a little over a year ago and do not use reverse much for I park on the street. Had to use it daily for about 2 weeks, it acted up like jumping in and out a bit, not slipping. than just quit working. Any ideas what could cause this. Called the people that rebuild and it was 2 days after the warranty lapsed. Said it was maybe a valve sticking.
 

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I ran into this problem maybe a month ago.A guy I know had weak reverse on his 4L80.I checked the pressure in reverse at idle and it was delivering almost 300psi so I knew the oil circuit was decent.I removed the valve body and used new acdelco gaskets and new separator plate from rockauto.I also bought a new accumulator gasket and servo piston seal.I checked piston travel with a modified servo cover and came up with .309” which was way too much so I welded on almost 1/4” to the servo pin and grinded it down nicely and came up with .160” travel after that which did the trick perfectly.Reverse now works as it should.

There are two steel check balls in the valve body but just ensure to use brake parts cleaner and wash everything out with that followed by compressed air.The check balls should roll back and forth when you rotate the valve body in your hands.
 

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Another problem with the reverse might be a broken servo piston but those are easy to access with the valve body removed.
 

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1996 K3500 6.5L TD dually F
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Rebuilt the 4L80E in 2019. Ran fine for 6k miles.
But now (9/21) I’ve lost reverse and the rear band won’t compression-brake in ManualFirst gear. That means my rear band is fried, right? I dare not do the pin length adjustment, because the rear band is not working? Is that correct?
I’m looking for the two anchor pins (“on top that can sometimes back out“; no luck. I’m assuming that a rear band replacement requires total takedown, right?
 

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1996 K3500 6.5L TD dually F
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This found in YT video noted upper right in photo:
If your 4L80E was built before 02/14/96, there’s a correction needed in the VCM for a “bad/weak internal ground” affecting 1-2 upshift and/or 3-4, or no 3rd or no 4th; launching shudder,
Handwriting Gesture Finger Font Line
possibly with a code P1820.
“Corrections” were begun after that date, note the Service# sticker on the VCM itself (behind glove box, some underhood).
Corrected Service# 16244210 on VCMs have already been done.
 
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That is for gassers , not diesels.
Gassers have a VCM, diesels have a PCM.
 

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1996 K3500 6.5L TD dually F
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Whew, thought I wuz a goner; keeping this straight is tiresome.
So, I had the trans rebuilt in 2019, and *now reverse is out AGAIN, tho it’s not the direct pack this time, but the rear band.
Started a thread on this, and as soon as the weather allows, I’ll drop the pan, etc., to extend the servo pin with a kit…..
 

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1996 K3500 6.5L TD dually F
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Asked the trans mechanic who rebuilt my 96 4L80E, and he said, “unless the rear band is outright broken, it’s usually the servo is cracked.” So, with dropping the pan and servicing, I’ll replace the rear servo altogether.
Then re-assess.
 
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