I've got a 2005 80e that I slapped a hd2 kit in and put behind my 6.0 with a Jake's converter and stand alone micro squirt TCM. I don't have a first gear once the trans warms up. I can feel it when I first start it up after a few blocks it will kick me out of it. It also lags shifting into second whether it's from 3rd to 2nd or 1st to second. I've replaces solenoids and made sure my work was properly cleaned and done right. Would like to avoid a tranny shop if possible. Any advice would be greatly appreciated.

 

So do you have 2nd when it's warm?

the forward clutch is the only one engaged during 1st gear.
all subsequent gears up after that rely on the forward clutch...so if there is an actual problem with the clutch...you will see it in all gears.

If you have 2nd when warm...I would suspect low AFL pressure...likely needs the valve body reamed/fitted with larger valve...should vacuum test the rest of the valve body while it's out...pay attention to the manual valve

Last note...do a pressure test. Checking line pressure is easy and if you don't have a gauge...they're like 35 bucks on amazon

 

Yes I have second when warm. It's only first gear that disappears when warm. Thanks for the info!