1995 c1500 4speed trans 4th gear

[Gilmore]

Well this may be a bit outside the forum year rules but I figured y’all might be able to help. My son has a 1995 chevy c1500 with a 4 speed trans 4L60 I believe. His transmission drives great 1-3 rd gear. When it shift to fourth it only momentarily shifts in to 4th gear and then immediately back into 3rd. Any thoughts?

 

[MarineOne]

First thing I'd recommend is pulling codes and freeze frame data.

2nd gear is good? 2/4 share a band, so if you manually shift the trans and it goes from 1st to 3rd (e.g. no RPM drop where you'd expect a shift into 3rd) you may have a worn/broken 2/4 band, a leaking 2/4 servo (they're separate but on the same pin), maybe a shift solenoid not fully stroking in the bore. There's a handful of things it could be, but with more throttle getting it to stay in 4th it would seem to indicate something line pressure related (since the PCM will command more pressure with higher engine load).

This could also be a torque converter lock up issue or a worn band and when trans senses slip it will pop back down to 3rd as a safety

 

[Gilmore]

Yes second gear works well. Pretty old truck and transmission so I wouldn’t rule out the converter lockup issues.