1996 4L80e: Have 2nd and reverse only no codes

[RedVenom48]

Hi, I have a new to me 96 GMC K2500 Suburban with the 454 and 4L80e 195k on the clock. Carfax shows a decent amount of service, including trans fluid service ~30k miles ago. Currently, the 4L80e is in limp mode with only 2nd gear and reverse. I have NO DTC codes stored, active or pending. Actually just passed emissions last week like this. Im familiar with the solenoid and manual valve function and why its only got 2nd and reverse. What stumps me is the lack of failure codes to get me started in my diagnosis. Ive got a Chiltons manual and will start perusing through the transmission section.

Ive seen some videos on youtube describing the same issues Im having and the fix was replacing the Input and output Speed sensors. I crawled under the truck and didnt really see anything abnormal with the wiring, everything looks connected and wiring doesnt look damaged.

Anyone have any tips or start here?

 

[1997]

Checked fuses?

also you can download the service manuals from here, they are much better than haynes/chilton stuff.

 

[618 Syndicate]

Checked fuses?

This.

 

[NickTransmissions]

Hi, I have a new to me 96 GMC K2500 Suburban with the 454 and 4L80e 195k on the clock. Carfax shows a decent amount of service, including trans fluid service ~30k miles ago. Currently, the 4L80e is in limp mode with only 2nd gear and reverse. I have NO DTC codes stored, active or pending. Actually just passed emissions last week like this. Im familiar with the solenoid and manual valve function and why its only got 2nd and reverse. What stumps me is the lack of failure codes to get me started in my diagnosis. Ive got a Chiltons manual and will start perusing through the transmission section.

Ive seen some videos on youtube describing the same issues Im having and the fix was replacing the Input and output Speed sensors. I crawled under the truck and didnt really see anything abnormal with the wiring, everything looks connected and wiring doesnt look damaged.

Anyone have any tips or start here?

Start probing for power (volts) and continuity (ohms). Fuses like the others have said then look for bad grounds, feayed/exposed or cut wires.

You typically wont see a code unless one or more trans electronic parts are bad (solenoids, pressure switch manifold assembly, etc)but that's not always the case, just most of the time. Loss of signal from the VSS (output speed sensor) shows up as no upshifts from first gear while input speed sensor can result in limp mode which what you have now.

What scan tool are you using?

 

[RedVenom48]

Ill have more time next weekend to dive into it. I checked the 10A Trans fuse in the drivers side dash fuse box. Intact and ok as the gauges died when i pulled the fuse and worked again once reinserted. Ill have to check for available voltage on the main plug itself.

Current scan tool is just my generic Matco OBD2 code reader. I was going to go get a HF Zurich ZR8s. Claims to be able to look at live data as well as read DOM, Euro and Asian ABS codes. Hoping a 1996 GMC Suburban is compatible to read live trans data.

Did the initial ownership oil change this morning and of course took a peak at the trans area. I didnt see any abnormal condition with the harnesses, in fact for 195K miles it look relatively tidy under there. Trans fluid looked ok, level looked ok at op temp. Feels pretty stout even if onl 2nd gear. Manual shifting to 1st gear detent still 2nd only.

I never believed in shot gunning parts at a car I was diagnosing when I was a Lexus tech. But with no DTCs to point in any direction and for only $18 bucks shipped I can get a genuine Delco ISS.. I may pop that in and see where that gets me. Worse comes to worse, I at least have a new ISS sensor lol.

 

[RedVenom48]

Quick update: replaced the Input speed sensor with a Genuine GM sensor. No dice, still no gears but second and reverse. Though of note, there were metal particles on the old ISS I pulled out. My guess is Ill need a rebuild sooner rather than later. Ill have more time this Saturday to really get into it. Just thought Id share. Ill be looking for the factory service manuals here to maybe get a head start.

 

[618 Syndicate]

Mechanical failure won't put it into limp mode.
This is most likely a power issue.

 

[NickTransmissions]

Quick update: replaced the Input speed sensor with a Genuine GM sensor. No dice, still no gears but second and reverse. Though of note, there were metal particles on the old ISS I pulled out. My guess is Ill need a rebuild sooner rather than later. Ill have more time this Saturday to really get into it. Just thought Id share. Ill be looking for the factory service manuals here to maybe get a head start.

Have you dropped the pan to ohm out the solenoids and test (or simply replace) the internal transmission harness. Ive mentioned this in other threads but the internal harness is often the source of limp mode issues.

How big were the metal particles on the SS? Anything larger than fine dust-size?

 

[RedVenom48]

Hi all. Little bit of an update regarding my issue. Im only just now getting around to diving into this (life, work, kids).

Ok, so I verified my 10A fuse to the trans is good, Ive got source voltage at the trans connector pin when key on. I took a guess and decided to buy an internal trans harness and I guessed right. it was every bit the 25 year old harness you though it was going to be and seeping ATF into the pin side. Probably a lot of my issues from that. As for dropping the pan, I was shocked to see clean, unburnt fluid in the pan. Ive got about 50 miles driving this trans since I bought the Suburban so its had plenty of time to circulate through the system. Pan has a few sections of "dirt" and the magnets had what Id consider a normal amount of sludge on it. Its entirely possible theres a section of clutches that are gone, but at the moment no "well, thats the problem right there" evidence. The small slivers of metal on the previously replaced ISS must have been normal things that collect on a 195k 4L80.

Was going to just replace the harness and see where I was at with he repair process. I decided to use this opportunity to Ohm check with the new harness my solenoids. Shift 1 was open, no continuity. Shift 2 was half the specified Ohms. TCC was in spec as was the Pressure control solenoid. No idea on the pressure switches. Decided to replaced both shifts, the pressure switches and the TCC solenoid. Genuine GM parts ordered except for the TCC which is a Carquest premium (lifetime warranty, works for me). As the PCS Ohmed in spec and I have no way to set pressures on a replacement, Im opting to roll with that as is for now.

All the parts came in and Ill be installing everything Saturday morning. Wish me luck. If this doesnt "fix" my issues, I hope it at least gets some DTCs to pop up showing issues elsewhere to finally figure this 4L80 out.

 

[tayto]

did you put a load on the power pin @ the trans connector? jist because you have correct voltage doesn't mean its passing amperage