1987 740 Auto Trans wont shift till warmed up

The Check Engine Light is on? Did you read the DTCs.

The second link has the answer, the first link says it is a known issue. It is times like this you need the Dealership or a good auto electrician.

Transmission won't shift to third until temp rise

ClubLexus https://www.clublexus.com > forums > 713188-transm...
Apr 1, 2017 - Transmission won't shift to third until temp rise ... The solution is to drive slow and keep rpms at 2.5k and 3k at max till the engine and transmission warm up.
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Owner of Lexus RX300 with 182000 miles. Recently the ...

JustAnswer https://www.justanswer.com > lexus > 3wsxt-lexus-rx3...
Aug 22, 2010 - Recently the transmission won't go into overdrive. Stays in 3rd gear like when you press the o/d button to do so. When car is cold (live in ...
1 answer · Top answer: Hello and thank you for using just answer! The check engine light is on correct?

https://www.google.com/search?q=Lexus+RX300+will+shift+into+overdrive+once%2C+and+after+slowing+down+it+won%27t+go+past+3rd+gear

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Im assuming it is a 4 speed transmission and if you are using the "tap shift" which is for performance situations and you wouldnt typically need 4th gear. What speeds are you trying to go into 4th? If you are going too slow it wont upshot because you rpms might drop too low. Will it not go into 4th at highway speeds?

change the coded solenoid and the reset the ECM and after a few days read the fault codes again

shift solenoid for third gear maybe bad up inside the transmission pan

too heavy a motor oil can cause hard shifting also,but when its cold out let your car warm up longer before you take off,it might help

your shifter cable is going bad.

When you first start driving it usually takes atleast 4 miles to warm the transmission to operating temperature. The temp of the motor may say it is warmed up, But, the trans fluid and differential fluid only begins to warm as you drive and the parts are actually working. THe computer has sensors that keep the trans from shifting to OD until the temp is up as not to add stress to the motor until warmed properly. Hope this helps and have a good day.

There are to many things that can cause this problem, best thing to is go get the ECM scanned for DTC codes at a transmission shop or Auto zone or Parts Source since they scan for free.
The ECM should have DTC code in it's memory on how the transmission is Performing and have the Transmission line tested for Fluid pressure.
Good luck and hope this helps. Oh and also ways check with more then on transmission shop if there tell you that you need a new Transmission,
It could be a simple as the transmission filter falling off causing the lost of transmission fluid pressure. Check the transmission fluid too.

the neutral safty switch tells the comp. what gear is in. when its not working the light wont move and the car will not go threw the gears prop. the switch is nown to go out. it is located on the tranny or down under the shifter

The transmission/computer is designed not to go into overdrive until it reaches a certain temp and I don't remember what the temp is. Mine does same thing and it has 250,000 miles on it. When you start your truck up do you start it and warm up in neutral? The transmission fluid won't flow unless it is. There is know way to force it to shift into overdrive until it warms to the set temp.

Hope this helps
Howard