4l80e code 85 hard shifting

[bckslyr2003]

Hello all thanks for the add. i recently swaped a 7.4 enginge and a 4l80e from a 95 suburban to my 93 k3500 engine runs fine but i have a code 85 and it shifts hard which it did not do when it was in the suburban any ideas im thinking wiring or the vss buffer any insight would be greatly apriciated

 

[ak diesel driver]

Gas to gas vehicles?

 

[bckslyr2003]

yes same engine same trans

 

[ak diesel driver]

Hard shifts usually indicate limp mode. Go over all the connectors also check your TPS adjustment

 

[bckslyr2003]

im gonna check all connectors and grounds if the vss buffer in the suburban is the same im gonna swap it to be safe if all else failes im pulling the wiring harness and inspecting it all

 

[THEFERMANATOR]

Fyi, 93 and 95 are not the same 4l80e trans. There are MANY differences in them, but what is getting you is you have no output speed sensor in the trans. 91-93 4x4's used both the input AND output speed sensors in the transmission. 94+ 4x4's did not use the output speed sensor in the trans, nor did they even have a reluctor wheel in the output of the trans. You have done an incompatible swap.

Your options are you can swap to a 94 harness and all the wiring changes for 4 wheel drive that entails, or pull the 95 trans and convert it back to a 93 compatible style. You need to swap the valve bodies, I'm guessing you already swapped the manual shift shaft to connect the linkage, I'm guessing you already switched to the late model harness, and pull the trans COMPLETELY apart to install a sonnax reluctor for the output speed sensor. There is no easy way out of this one unfortunately.

As to the engine, did you swap tbi units? If not be prepared for a BURNT engine from running it SCARY lean. Up to 93 the 454 ran at 8-13 psi of fuel pressure, 94-95 ran at 30-38 psi of fuel pressure. The early fuel pumps tend to max out at 13-15 psi of pressure, so it will run and open the injectors, but it will be SCARY lean and BEGGING to melt a valve or piston.

 

[bckslyr2003]

the harness is the same as the 95 both transmissions had input and output speed sensors both towards the top of the transmission 1 towards the fron 1 towards the back tbi's were swapped

 

[bckslyr2003]

well i guess i will be manually shifting this for a while

Vehicle Speed sensor Circuits As of start of production 1994, all 4L80E models underwent changes to their vehicle speed wiring circuits. These changes included dropping the transmission output speed sensor and toothed wheel on 4WD units, as well as significant wiring and circuit changes in 2WD and 4WD applications. All 4WD units now use only output speed sensor (located at the transfer case) rather than sensors at the transfer case and the transmission output shaft. Since the output toothed wheel was also dropped (Figure 3), care must be taken not to interchange output planetary carrier assembles between output VSS and non output VSS equipped units. Attention must also be given when replacing a unit with a rebuilt transmission to assure the transmission is properly equipped with or without a toothed sensor wheel. If the wrong transmission part number or a carrier unit without a toothed wheel is installed in a 1991-93 vehicle, late/no upshifts may result. NOTE: Late upshifts may result due to the sensor picking up a 21 pulse peer rotation signal from the machined slots cut in the output planetary assembly, rather than the normal 40 pulse per rotation signal the PCM/TCM experts to see. Both 1994-95 4WD and 2WD vehicles use the same wiring layouts for their output vehicle speed sensor circuits. (Figure 4)

 

[bckslyr2003]

would ussing the vss buffer from the 95 fix the confusion with the 21pulse vs the 40 pulse ?

 

[THEFERMANATOR]

No, the ecm is looking for a 40 tooth signal from the trans, not a vssb conditioned signal like a 2x4 would. And you better address the tbi units if you haven't already, otherwise you're going to melt a piston skirt. You could convert to the 95 wiring which isn't to bad. You'll have to repin the ecm plug to read the vssb instead of the output speed sensor, install a 4 low and 4x4 trigger wires to the ecm, and swap the fuel pump in your tank to work with the high pressure tbi unit.