4l80E Intermittent No 4th Gear

[JacobL]

I have a 1998 GMC 2500 pickup with 110k miles. The transmission worked fine until about a month ago.

80% of the time it shifts perfectly. But every once and a while it won't shift to 4th, the other gears shift hard, and the engine light comes on. Usually, if I shut it down for a few minutes and start it again, it will run OK again. The fluid is clean and smells fine.

I plan to get a code reader tomorrow and see what it says, but what's most likely to cause this? Speed sensors? Valve body? I'm hoping it's not the torque converter.

 

[NickTransmissions]

I have a 1998 GMC 2500 pickup with 110k miles. The transmission worked fine until about a month ago.

80% of the time it shifts perfectly. But every once and a while it won't shift to 4th, the other gears shift hard, and the engine light comes on. Usually, if I shut it down for a few minutes and start it again, it will run OK again. The fluid is clean and smells fine.

I plan to get a code reader tomorrow and see what it says, but what's most likely to cause this? Speed sensors? Valve body? I'm hoping it's not the torque converter.

Sounds like it's going into limp mode for some reason. When in limp mode, line pressure is maxed, all shift adapts are frozen and the trans defaults to second gear. You shutting it down is clearing whatever the condition is, a bit temporarily.

Post what codes you are able to read but note that budget code readers may or may not actually pull any codes beyond what sets for the emissions managment systems. Id get something capable of rendering live engine and transmission data to ensure you have the bases covered.

Id also test your TPS as well - this can cause issues similar to what you're describing.

 

[yevgenievich]

Often could be a failure of tcc apply circuit or if vss giving bad information. Ecu will see slip with tcc applied and keep increasing pressure, then it will keep 4th completely off until cycle restarts again

 

[Schurkey]

Post what codes you are able to read but note that budget code readers may or may not actually pull any codes beyond what sets for the emissions managment systems. Id get something capable of rendering live engine and transmission data to ensure you have the bases covered.

Or, to be more blunt...Code readers are consumer-grade junk. Almost useless.

You need a scan tool, or connector cable/dongle + software for a lap-top. (Scan tool simulator.) And bottom-feeder tools may be able to deal with engine codes and data, but may not be able to access transmission/instrument cluster/ABS/Air bags/etc.

 

[JacobL]

Sounds like it's going into limp mode for some reason. When in limp mode, line pressure is maxed, all shift adapts are frozen and the trans defaults to second gear. You shutting it down is clearing whatever the condition is, a bit temporarily.

Post what codes you are able to read but note that budget code readers may or may not actually pull any codes beyond what sets for the emissions managment systems. Id get something capable of rendering live engine and transmission data to ensure you have the bases covered.

Id also test your TPS as well - this can cause issues similar to what you're describing.

I plugged in a harbor fright code reader today and it gave me two codes that seem relevant.

P0122: Throttle Position Sensor 1 Circuit Low Voltage
P1122: Throttle Position Sensor Circuit Intermittent Low Voltage

 

[NickTransmissions]

I plugged in a harbor fright code reader today and it gave me two codes that seem relevant.

P0122: Throttle Position Sensor 1 Circuit Low Voltage
P1122: Throttle Position Sensor Circuit Intermittent Low Voltage

Yea figured the TPS may be involved...

@Schurkey has a post here where he explains how to test the TPS for voltage response with a scan tool. If your doesnt have the capabilities mentioned, there are several YT videos that walk you through the process using other things like multimeters, etc. I may even make one showing the procedure as it's a fairly common problem.

 

[JacobL]

Yea figured the TPS may be involved...

@Schurkey has a post here where he explains how to test the TPS for voltage response with a scan tool. If your doesnt have the capabilities mentioned, there are several YT videos that walk you through the process using other things like multimeters, etc. I may even make one showing the procedure as it's a fairly common problem.

Thanks for the help.

The TPS seems to be responding correctly at the moment, maybe an intermittent issue?

I think I'm going to replace the TPS and VSS for good measure.

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[NickTransmissions]

Thanks for the help.

The TPS seems to be responding correctly at the moment, maybe an intermittent issue?

I think I'm going to replace the TPS and VSS for good measure.

You must be registered for see images attach

Id keep that scan tool plugged in amd that view displayed at all times until you can definitively rule it in or out but those codes
wont set/store unless the PCM isnt seeing the correct voltage from the TPS at any given point (the exact criteria should be easy to locate via G-search).

You can replace the VSS if you want but it may be worth swapping the two of them and see what happens first, assuming you have a 2WD. If things change all of the sudden and you get ISS codes, you will have found another problem.

As an aside, I always keep extra parts like the TPS, VSS, MLPS and anything else in that fits in my pocket but would strand me if it goes bad, in my truck at all times.