To any and all first gen ls 400 owners wiring harness in trunk could possibly have exposed wire causing reverse light to come on which in turn confuses computer as to which gear to stay in so if your R light comes on when depressing the brake pedal you may want to have your wiring harness fixed and i tell you from actual experience this will take care of much much more than the light problem ie.... jolting when brake pedal pressed serious lagging hesitation under acceleration and your vehicle trying to switch into reverse while driving in D. Merry Xmas to all and to all a Good Night

 

Where in the trunk was your issue found?

 

In the left trunk lid hinge, of course - the most common electical problem on the gen 1 (90-94) LS400.

 

Just wanted to verify from the O.P. being he didn't clarify.

 

thanks but you're a few years late to the party
http://www.lexls.com/tutorials/lighting/bulbout.html

 

Hi, Everyone . New to the site. Lots of very helpful info!!. Drive and reverse light on @ the same time causing the Tranny to stay in 1st gear. Happens intermittently. I will check the harness ASAP.

Thanks

 

Hi from 2021! This happened to my daughter's 1991 LS400 and as her favorite Dad I was called to fix it.
A good vid on Youtube helped most, search

Ls400 dash light dim fix

Here are my pics, which I had posed them better. Apparently only the White/Black had an obvious weak point, so I soldered in a piece of 10 guage I had. This is a challenge to detect, sorry but I hope you will know it when you feel it (by flexing the conductors one at a time).



Squareish plastic strain relief cap removed (after TAPE cas snipped off to remove)

Best Wishes!

 

Thanks for those pics, they'll help many people in the future. Even though this thread is 10 years old, this problem will go on forever. Plus, all the times this problem has been covered and talked about I don't ever remember seeing any pics of the wires and hinge. I haven't had the problem yet on either of my LSs so I never knew how tight those wires actually are on that hinge. Wow. Thanks again for pics. Worth a thousand words, and more.

 

It's not about how many miles you have driven, it's about how often you opened the trunk!