My 99 jimmy service engine light came on. The computer says component in transmission slipping. It shifts hard from 1st to2nd after the tranny warms up. What causes this

sounds like a transmission problem, does it slip or engine works to hard to move it? Check transmission strainer first

the modulator valve on the transmission may have gone bad, does it slip while driving or just simply won't shift easily? if you have to shift it manually from 1st to2nd to drive and it does fine it is a bad modulator valve on transmission or bad vacum to the modulator valve. hope this helps!!

Look for vacuum leaks. There are tons of hoses, replace dried and cracked ones....

The trans shift issue that you describe seems that the trans is being told to hold back the shift, there is a unit at the end of the vacuum tube, AT the trans, called a modulator. They have a rubber diaphram inside that fails and then trans fluid gets sucked into the engine, via this vac. hose. Pull it off and see if any trans fluid comes out...Should be ZERO.

Also, your vehicle must have regular trans service, especially you must change the FILTER. Disk material eventually clogs the filter, then trans fails really fast.

You may consider having a quick lube shop do the trans service, they use a machine that pumps new fluid in while unit is running, totally flushing converter, cooler,and lines...but DO replace the filter.

To be fair, the service suggestion has nothing to do with yoru complaint, necessarily...

Check and see if there is a check engine light code on dealing with the transmission. If not you can have the tranny checked with a computer to make sure everything is working right, but it sounds like the shift solenoid for it is going out.

whens the last time the tranny was serviced as far as filter and fluid the tranny may not be getting enough line pressure if the filter is dirty, or low on fluid, cuasing the slippage

Sounds like the trans. is shot, try a quick service (Fluid and screen) if that doesnt work try a used trans.

the tranny is slipping ,,in other words its starting to go.

Sounds like you need a solenoid pack to start, and possibly a service. If the tranny is not slipping, the solenoid pack is stuck and not tell it to shift up. Could possibly be a TCM, but not as likely.

At 65000km I had too rebuild my tranny cause it would not shift into od when it did it would become suggish found out it had a cracked drum that covers the od inside the tranny and certain temps affected it .rebuilt it and has been going stronge since have over 300000km on it now and its a 99 xlt 4.0L not the best on gas but reliable to get anywhere

SHift solenoid pressure switch