4l60e trans problem

I have a 99 suburban with a 100,000 miles I bought it with a bad trans. I pulled and rebuilt it(which i have never done before). The first short drive trans did great, shifted good through all gears and TC lock. After taking a longer drive down the highway the trans down shifts to 3rd and the cel (1870) comes on. If i come to a stop it starts in second and only shifts in to 3rd no 1st or 4th. If i stop and shutoff it off if shifts good till i run down the highway for 3-8 miles then the same thing happens. I've taken it to 2 tranny shops and both think its put together right. The slip rate is in the 200-250 even in lookup. I pulled valve body and had a trans shop install a sonnax tcc valve and A and B shift servo's . This did not help it. still does same thing. I did replace torque converter when i rebuilt it (advanced auto rebuild which worries me) Any advice?

If you don't have 1st or 4th, the common factor is the 1-2 or A Shift Solenoid. Since that was replaced, have you looked at electrical connector at the tranny or the wires from the connector to the solenoid.

Did any of the tranny shops use a scan tool to troubleshoot the shifting into 1st or 4th?

//2000CAyukon

Yes the trans shop hooked up a scan tool sad the slip nubers were to high thats why we put the sonnax upgrade. I don't think it is in a wire because it has 1st and 4th untill your at highway speed for 3-8 miles, it then down shifts and the CEL comes on (1870). Thats when i lose 1st and 4th. It will shift rite if i turn off and restart. It will shift fine all day if i drive around town. Just can't figure it out. :confused:

You need to do a full pressure test per shop manual for this condition, hot of course, if no manual I believe Sonnax has the diagnostic trouble tree.

I don't know what cel 1870 is, but if the ignition switch is beginning to malfunction,it can cause the tranny to do weird things.May be things that you are describing.When you turn your ignition ON,does the gear shift indicator illuminate?This was the tell tale sign for me when I changed my ignition switch.I hope this is helpfull.
chuck's