I have a '99 GMC Sierra 5.3 Last spring I noticed when it was cold in the morning around 50*F and below, my tranny would not shift until high RPM'a around 3500. Some times it wouldn't even shift. If I let off the gas pedal it idles down and seem to disengage but if I give it gas again it idles up to the RPM range for that gear weather its 1500 or 3500 and engages. It usually shifts out of 1st gear 2500-3000RPM and 2nd gear around 3500. I do not have any tranny slippage in gear or shifting out of gears.

After I drive a little bit down the road it improves and when I get into 3rd gear it will shift into overdrive with no problems. Once it has warmed up for the day it works just fine. I forgot all about this problem until this week when it got cold out last night. Did the same thing as last spring, all summer long I have not had 1 problem with this transmission until now, and its cold in the morning. I know that Transmission problems do no just fix themselves (I have replaced enough in my time to know). So I am hoping that this might be an easy fix. Like when I had to replace the nylon gear on my governor for my 700R4 back in the day, or a sticking valve body or just another fluid change.
This transmission was rebuilt 15,000 miles ago.
Thanks,
Jason

put one tablespoon of brake fluid in the filler tube. DO NOT put more than that. What you have is your lip seals are getting hard and the brake fluid will soften them but too much will disolve them. The cause of this problem is too much heat in the trans or the person that rebuilt your trans did not use a quality rebuliding kit. The brake fluid is only a temporary fix. Heat is the worst enemy of an automatic trans. If you decide to keep the truck than make sure you flush out your trans cooler located at the bottom of the radiator.

Take it back to the rebuilder.

I'm not exactly sure what you have going on BUT I get phone calls (at my transmission shop) every fall and winter for similar issues,on gm vehicles (and usually fords/dodges too) their is a transmission fluid temp sender built into the pressure manifold on the valve body. On 4l60e/4l65e transmissions (like your truck has) the temp sender tells the ecu how warm the transmission is,to help it warm up faster and avoid chuugging and drivability issues the ecu delays the transmission from going into lockup until the fluid reaches 77 degrees f,you will notice this the most when it is real cold outside but can notice it any time its below 77 deg f. Heres what happens-you jump in the truck in the morning,take off right away,transmission shifts 3 times,to 2nd,3rd then overdrive BUT it does not go into lockup and you feel it flaring/stalling in overdrive. Once you get a few miles down the road the trans reaches temperature and ecu tells it to go into lock-up which most people mistake for a shift into overdrive (its already in overdrive). Then it doesn't do it again until it sits overnight and fully cools down. If you add a big aftermarket cooler the "problem" (its not really a problem) is more pronounced as it takes even longer to warm up. Now the fancier transmissions like the allison (and maybe even your 4l60e-but I haven't seen this) also have shift inhibit which keeps transmission from shifting to overdrive/5th or 6th gear when it gets even colder like zero-allison won''t shift to 6 th gear,-20 it won't shift to 5th gear but obviously its not that cold anywhere yet. In your case if transmission is turning 3500 rpm's and not shifting at all you might have other problems like a bad pcs solenoid (screen becomes plugged and it won't flow enough fluid until transmission warms up/fluid thins out). If you think its more then just delayed lock-up pm me with your phone number and I'll give you a call and go over the possibilities with you,Smitty

There was a wiring harness upgrade on those late-90's trucks with the 4L60. I had similar problems with mine including what you describe and occasional/frequent inability to lock the convertor. I don't know if the upgrade extended that far out (mine was '96) but it's worth a look.

The 96 had a harness upgrade,it was a grey or white ground wire gm part#12167310. The upgrade only applied to the 96's though,it connected two ports of the computer that had a intermittent ground problem. I have probably rebuilt 75-100 96 chevy 1/2 ton transmissions and we checked everyone for the harness update,there has only been 2 that we did that didn't have it done already by the dealer,Smitty

I thought we had some 97's and 98's that had the problem... maybe not- long time ago.

We bought 6 new trucks in 96- all with 4L60's I bet I bought two dozen transmissions before I found out about the fix.

Thanks for the responses guys Smitty and Chris.

Smitty, I finally got around to driving the truck again yesterday. I started down the road and it worked like there was never a problem. Shifted through all the gears, no slippage, all RPMs matched usual speeds. Drove it about 5 miles and I thought it fixed itself. (again) I was heading back, going slow through the subdivision and I heard a different pitch in the drive train. It sounded like the transmission was slipping again. I stopped and put the truck in reverse, it revved up and slowly engaged into reverse. I backed up put it into D and headed down the street again. It slowly slipped through gear 1 and then 2 and 3. It never felt like it engaged into any of the gears. Kind of felt like a hydrostatic tranny that doesn't have individual gears. (like a big stall converter was on it)

I took it to my brothers shop and he is going to change the fluid and filter. Is there any senders or solenoids that can be replaced when he is in there?

Thanks once again,
Jason

If I was guessing I would say the one way roller is failing in your stator in the torque convertor unless your filter is severely plugged which rarely happens on 4l60e's. Its slipping in too many different gears to be one particular clutch pack,sprag or gear failing. If there was one solenoid that I would at least pull,clean and inspect it would be the pcs solenoid. You need to pull the 1-2 acumulator housing to get it out,the screen could be plugged on it but if it is its because something else is starting to fail. You could also have a pressure regulator valve hanging up giving the trans virtually no pressure but again it usually leads back to something failing in the trans and polluting everything ,causing valves to hang up. A line pressure test off the main pressure port could confirm this but in any case it sounds like the best thing to do would be to pull it out,dissassemble it and inspect everything. Are you getting a service engine light? The 99's have a fairly advanced computer and it will tell you if see's a component slipping or solenoid shorting out,not responding etc. If you pull the pan and find excessive debris or metal shavings again its time for it to come out,no solenoid will fix that,Smitty

OK heres the scoop. The tranny works great for about 5 mins regardless of temp. The it starts slipping, I do not believe this to be clutches slipping. If I pull over to the side of the road and turn the truck off. Pull the key, out and then immediately start it again, its back to locked up and running fine, for about 5 mins. I did pull P01870 transmission code.
I am at the shop pulling the pan off as I write this.
Thanks,
Jason