Problem with '91 4L80E

[1998_K1500_Sub]

Solved the shift problems suggests perhaps the issue may have lied in the ECM?

Well, the "ECU", or some sensor providing input to, or device receiving output from, the ECU.

I'm wondering if OP's 4L80 problem isn't some manifestation of failsafe mode and, if so, what has "failed" to cause it to operate this way.

I might be dreaming but I think I read someplace, recently, that a failsafe mode for the 4L60E (with certain ECU's or certain years perhaps) was to operate in only a few gears. (edit: Is this what happens with a MAF failure on 1996+? Darn, I wish I could remember...)

 

[tayto]

Hi folks,

Update:
The US shift stand alone controller solved all the shifting problems. I still have a couple other issues to overcome however. The US Shift talks about connecting their device to a speed sensor coming off "an" electronic speedo. Since hooking it up I have no speedo output from my stock speedo, so I need to know, wire for wire, what to hook up to. Does anyone know? The tech help from US Shift is falling short. Also, I need to find a "when ignition is switched on and remains on during cranking" wire that feeds the PCM. US Shift steered me to an "always on" connection. Don't want that , I need an "only when ignition is on" connection. Hopefully both connections I seek are near the PCM as my new controller is mounted in the glovebox, and the PCM is directly behind it.

Anyone have any thoughts? Reminder, this is for a 1991 GMC 3500 with a 4L80E, completely stock.

Again, as always, thanks for any and all help. No bad answers here.

Thanks, Gary

get a wiring diagram (found on this site) and a test light or DMM. get a helper to turn the key, find something that the light stays on in the ON and START key position and turns off in the OFF position.

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

EDIT: i assume you cut the wires to the drac from the speed sensor. you will need to hook that back up so you can feed the speedometer the 2000 PPM signal it needs.

 

[AuroraGirl]

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Number: 92-272-7A Section: 7A Date: AUGUST 1992 Corporate Bulletin No.: 277129R ASE No.: A2 Subject: NEW WIRING HARNESS ASSEMBLY WITH IN-HARNESS TEMPERATURE SENSOR INFORMATION Model and Year: 1991-92 C/K, R/VP AND G TRUCKS WITH 4L80-E/4L80-EHD AUTOMATIC TRANSMISSION Transmission Applications: 1991-92 HYDRA-MATIC 4L80-E/4L80-EHD (MT1) Transmission Models: All Models Subject: New Wiring Harness Assembly (In-harness Temperature Sensor New Service Package) Vehicle Applications: C/K, R/V and P-Trucks, G-Van

Number: 92-236-6E Section: 6E Date: JULY 1992 Corporate Bulletin No.: 236509R ASE No.: A1, A8 Subject: LACK OF POWER/TRANSMISSION HOT MODE Model and Year: 1991-92 C/K, RV, G AND P TRUCKS WITH 4.3L, 5.7L, AND 7.4L ENGINES Condition: Some owners of 1991-92 trucks with 4.3/5.7/7.4L engines and 4-speed automatic transmission (4L80E) may comment on the following conditions: 4.3L (RPO L84 with MT1 transmission) 5.7L (RPO L05 with MT1 transmission) 1.Transmission converter clutch (TCC) is locking too early in 2nd, 3rd, and 4th gears and may be perceived as lack of power at highway speeds. 7.4L (RPO L19 with MT1 transmission) 1.Transmission converter clutch (TCC) is locking too early in 2nd, 3rd, and 4th gears and may be perceived as lack of power at highway speeds. 2.Tach needle flutters on decelerations. 3.Catalytic converter odor. Cause: 4.3/5.7L with MT1 transmission 1.Transmission hot mode, which causes TCC lock-up in 2nd, 3rd, and 4th gears, entry temperature is too low. 2.Thermac door in air cleaner may not be fully opening. This may cause the engine to operate on warm air, lowering power. 7.4L with MT1 transmission 1.Transmission hot mode, which causes TCC lock-up in 2nd, 3rd, and 4th gear, entry temperature is too low. 2.Engine fuel calibration is not optimal. 3.At idle, the engine goes into an open loop condition which does not provide precise fuel control, possibly leading to catalytic converter odor. 4.Thermac door in air cleaner may not be fully opening. This may cause the engine to operate on warm air, lowering power.

 

[AuroraGirl]

Well, the "ECU", or some sensor providing input to, or device receiving output from, the ECU.

I'm wondering if OP's 4L80 problem isn't some manifestation of failsafe mode and, if so, what has "failed" to cause it to operate this way.

I might be dreaming but I think I read someplace, recently, that a failsafe mode for the 4L60E (with certain ECU's or certain years perhaps) was to operate in only a few gears. (edit: Is this what happens with a MAF failure on 1996+? Darn, I wish I could remember...)

2nd gear. 4l80e 2nd gear is failsafe.

 

[Supercharged111]

Well, the "ECU", or some sensor providing input to, or device receiving output from, the ECU.

I'm wondering if OP's 4L80 problem isn't some manifestation of failsafe mode and, if so, what has "failed" to cause it to operate this way.

I might be dreaming but I think I read someplace, recently, that a failsafe mode for the 4L60E (with certain ECU's or certain years perhaps) was to operate in only a few gears. (edit: Is this what happens with a MAF failure on 1996+? Darn, I wish I could remember...)

MAF failure = full line pressure for both 4L60 and 4L80.