4L80E question

[gordr]

I have a '97 GMC 3500 4X4 Suburban, 5.7 liter engine, and 4L80E transmission. It has been very reliable since I have owned it; bought it in '07.

Recently, the transmission has been throwing hard shifts, especially the 3-4 upshift. Did this for about 2 days, then went into limp-home mode, and only went forward in second gear. It was idle for two days, and on Wednesday last, I started it up, and for the first mile or so, it shifted perfectly, all four gears, then went directly to limp-home. I searched on the internet, and one suspect was the vehicle speed sensor, located on the output shaft end of the transfer case. Changed that, and woo-hoo! I had four gears again, and smooth shifting. That lasted a whole 5 miles, and crap!, back to limp-home.

Took the tranny pan off; extremely clean inside, zilch for sludge or metal trash. Checked resistance on all the solenoids, and on the tranny fluid temp sensor; all within spec. Cleaned the filter with the solvent that dare not speak its name, and buttoned it all back up, and refilled it. Drove it ten minutes ago. Limp-home mode right from the get-go.

Where do I go from here?

 

[gordr]

Update: It's healed. Or is it just in remission? I put it on the hoist this afternoon, and decided to check the front and rear shaft speed sensors on the outside of the case. The front one measured 1338 ohms on my digital multimeter, a little shy of the spec, 1420 ohms. I pulled the same part out of a working 4L80E that I pulled from a parted-out '95 Diesel pickup, and that one measured 1387 ohms, a lot closer to spec. So I replaced the sensor with the takeout part. There was also a rear sensor in the takeout transmission, and the connector was all munged-up, but it also measured about 1390 ohms.

There is no rear sensor in my transmission; the hole is filled by a dummy plug, and the output shaft sensor is on the tail end of the transfer case (already changed that). Explains why the connector was all munged-up on the takeout rear sensor: there was no cable plugged into it, and the socket cavity was open to road grime.

Well, got the 'Burb off the hoist, and went for a short spin on the grid road. Tranny is shifting as it should, shifts are properly timed, and smooth, no harsh shifts at all. Did about six miles, with several stops and starts, all good.

So, was the input speed sensor bad, or was it just that unplugging and replugging the connector (which was apparently real clean), cleared a flaky connection? I also handled the plug on the pass-through connector, which connects the internal electronic components to the exterior wiring harness. Maybe something came good there? I hate not having a definitive answer, but I guess a modest road trip is in order, to see if the tranny continues to behave properly. SES lamp is out, too.

 

[zora]

You never had a problem with the speedometer not working?
It could have been an electrical contact as you already suspect.
Would have been interesting to see what a transmission scan tool would have shown as a code.
Good luck and hope you solved it.

 

[gordr]

Thank you, zora.

Curiously enough, the speedo on this truck has been off since I owned it. Seems to be a baseline calibration error. It's a metric speedo, being a Canadian truck, and it seems to be always 20 km/hr out. If it indicates 70, I am really doing fifty, and if it indicates 120, I am really doing 100. I got it in 2007 as government surplus with only about 100,000 km on the odometer (which is accurate). Showing 235,000 km now. I sure don't think the speedo error is related to the transmission problem. And the speedo worked all the time it was in limp-in mode, too.

 

[zora]

the tires are the size that match the tire decal on the drivers door frame?

 

[gordr]

Yes, correct size tires. And wrong size tires would affect both odometer and speedometer, as it would change drive shaft revs per mile. My speedo issue has to be in the speedo head circuit itself; the odometer is dead-nuts on. Checked it again yesterday.

And the truck did a 160-mile round trip to Calgary yesterday, plus a bunch of urban driving in the city, and the transmission worked perfectly, so I am going to assume the input shaft speed sensor was indeed faulty.