850 Transmission Intermittently not engaging into 1st gear

[tryingbe]

I wasn't aware there is a "thicker" ATF.

[polskamafia mjl]

I don't want to throw parts at it if thicker fluid is all I need.

It's not. Get the codes read and then you you can make an educated decision on how to proceed.

[95turbowagon]

One option is running a bit of Lucas transmission fix my issue occurs less often. I seen a thread post were a guy installed a trans oil cooler and said he resolved his issue. The guy said he had six months with zero issues. I feel like the solenoid replacement would be a proper fix. I replaced a similar solenoid on a Lexus suv of mine. They seem to get worn from oil sludge dirt etc. they begin to clog and intermittently stick in a an open or closed position.

[abscate]

My 850 will use 1st/2nd gear fine when it's cold, but will not shift into low gear once the tranny is warm. It does show the S1 solenoid code, but why would the electrical solenoid work cold and not warm? I don't want to throw parts at it if thicker fluid is all I need.

When warm, electrical resistance goes up, current goes down, magnetic field goes down, solenoid fails.