My 97 K3500 6.5L TD with 80kmi was not shifting out of 2nd gear. Replacing the rear Vehicle Speed Sensor (VSS) fixed the problem! Cheap, easy fix.

When I first started having shifting problems I figured the transmission was gone for sure, needed rebuilding, and that’s just what I get for buying a plow truck! Fortunately and after much reading, I now have a new appreciation for and a smooth shifting 4L80E transmission..
The 4L80E is dependent on the Engine Control Module (ECM) for shifts. If confused the ECM puts it into “limp mode” giving 2nd gear only. The speed sensor can cause this type of confusion. FlashOffRoad has lots of great information, this one is exceptional on “Output Speed Sensor and Various Other Sidetracks“ (http://idiventure.com/flashoffroad/Maintenance/SpeedSensor/SpeedSensor.htm). FlashOffRoad is for HUMVEE owners, there is lots of 4L80E and 6.2L/ 6.5L Diesel info like field fixes, like removing speed control fuse (humvee only?) and swapping flakey rear VSS with front one to get home “Get Home Fix” (http://idiventure.com/flashoffroad/Maintenance/Transmission/Transmission.htm).

The Automotive Service Association “Making Sense of Sensors” (http://www.asashop.org/autoinc/feb2004/mech.cfm) gives detail on sensor drop out found under Variable Reluctance Sensor section.


With the truck on jack stands it was obvious that the rear VSS was showing sensor dropout, where it couldn’t get a reading at low speed. The wheels were spinning 5mph but the speedo was at 0mph. Off idle at higher rpms showed at 15mph but by then the ECM had already put trans in “limp mode” about 20mph max.

http://i51.photobucket.com/albums/f364/lcgarage/Truck/060107_005.jpg (http://i51.photobucket.com/albums/f364/lcgarage/Truck/060107_005.jpg)
(Image has been resized. Click it for full size.)


This link shows the anomaly (?? need to play twice to see?? Sorry) it’s good enough to show speedo response with bad VSS (http://s51.photobucket.com/albums/f364/lcgarage/Truck/?action=view&current=BadSensor01.flv) and response with the new VSS (http://s51.photobucket.com/albums/f364/lcgarage/Truck/?action=view&current=NewVSS.flv), where wheel spin at idle now shows at 6mph. You can also see the 6.5L diesel, at 2000rpm in 4th? (wasn't warmed up, maybe it was just 3rd) gear gives 50mph! That’s 4:10 gears, not exactly a highway cruiser.

This show the old and new VSS. There was a lot of metal particles stuck on the bad sensor, but cleaning a replacing showed sensor was still bad.
http://i51.photobucket.com/albums/f364/lcgarage/Truck/060107_012.jpg
On mine the rear VSS is located on transfer case since it's 4x4, very easy to get too.

http://i51.photobucket.com/albums/f364/lcgarage/Truck/060107_013.jpg (http://i51.photobucket.com/albums/f364/lcgarage/Truck/060107_013.jpg)
(Image has been resized. Click it for full size.)

The transmission fluid looked great and the shift solenoids also new….because that was the first thing I replaced….thinking it had to be the shift solenoids. Nope.

So how good is the 4L80E? Real good, not for a Dmax but…..It is the transmission of choice for guys building hp and proving they got it by blowing up 4L60E’s.
Here’s some great links with swap and more 4L80E details:
Super Chevy, Super OD (http://www.superchevy.com/technical/engines_drivetrain/transmission/0409sc_hughes)
4L60E to 4L80E Swap Page, 2003 4wd Chevy. (http://www.neufamily.org/4l80e/4l80swap.html)1500
4L80E in a Range Rover / Landrover (http://www.mez.co.uk/4l80e.html)
Hot Rod, The GM 4L80E Automatic for Max Power and Good Cruisin’ (http://www.hotrod.com/techarticles/gm_4l80e_automatic_transmission_review/index.html)
Complicated? "Mission: Transmission" (http://www.asashop.org/autoinc/may2001/techtotech.htm)

Thanks for the info, buy I'm losing faith fast. My truck won't shift from first or second or even third unless I let off the pedal.

I was searching the site and found that the Engine Coolant Sensor could be a problem, replaced it , and no change.

This isn't normal right? Thanks again.

I would start a new thread on your shift problem.
DO NOT lose faith! Give your miles too.
Saw previous posts, you don't have SES light at key on.....could be a grounds issue? Wires and connectors? The 6.5L and 4L80E is 100% dependent electronics.
Pretty sure the Temp sensor (ohms vs temp) (http://i51.photobucket.com/albums/f364/lcgarage/Truck/TempVsRes_1.jpg) only effects shift into 4th.

This is the old Input VSS, mounted towards the front on the trans.
http://i51.photobucket.com/albums/f364/lcgarage/Truck/061407_003.jpg