Speedo Operation

zeus87gn · October 13, 2008

Well, I found out this weekend that the speedo in my SC400 is reading SLOW. LIKE 20MPH SLOW.

When it reads 40mph, I am going 55. When it reads 45, I am running over 60.

Thank the good Lord above, I didn't get a ticket going to my sisters for a visit.

But I did have a lot of fun on those back roads!! :D

First thought was the gear on the cable was wrong, but there is no need for that as the car is (supposed to be) all original. So yesterday, after I figure out new and interesting ways to lift this thing in the air to change more of the trani fluid, I find there is no speedo cable. Ok, it's got to be an electronic speedo. But an electronic speedo shouldn't bounce at slow speeds like this one does(????). I'm lost here.

I did find 2 sensors at the end of the trani that oppose each other, in the tail end where a cable would typically be.

They look like small TPS's.

Would these be the speed sensors?

Is it possible for one to be bad and the computer is not interpreting the signals correctly because of that?

Forgive me for asking such a NOOB type question, but I haven't weeded through the 3300 pages of the manual quite yet.

I don't want to throw parts at it and not fix the problem.

Hopefully, someone has run into this.

All comments/suggestions/opinions/whatever are greatly appreciated.

Z

zeus87gn · October 21, 2008

Bump to the top.

I hope someone has had this problem.

UCF3 · October 24, 2008

It's a little unusual for your car to be running 10mph slower than what a radar picks up. The two sensors on the tranmission housing are the #1 and #2 speed sensors, but my experience is you should have the check-engine light on if either of them are malfunctioning. it almost sounds like your car needs to be recalibrated.

Have you by any chance changed your wheels?

zeus87gn · October 24, 2008

Thanks for the reply!

Nope, as far as I can tell the wheels are original (16" Lexus rims) and the tire size matches the glove box sticker.

I don't have any lights on the dash showing for service, but I also havne't hooked up my scantool to it yet either. From some of the other threads I have read, a generic scantool won't work on these cars. You have to go to Lexus.

I have weeded through the wiring diagram and it appears the speedo gets it's signal thru a few places from sensor #2. The ECM, ABS & another that I don't remember right off. I can't tell what the #1 sensor is for. Is it a backup?

UCF3 · October 24, 2008

this will help, w/o a scan tool.

http://www.lexls.com/tutorials/intro/codereading.html

I think you've reversed which sensor does what, but i might be wrong, but one of the them is your wheel sensor. The Trac light should have been on, but i guess it could malfunction without indication.

zeus87gn · October 24, 2008

I can get things mixed up easy enough, no doubt there.

With that link you posted for me, I should be able to find out something.

I don't believe I have the TRAC system.

I haven't seen any lights in the dash, but I don't drive it regularly.

It's for the Mrs. right now. But I still have to fix it.

Thanks for the link, I'll let you know what I find out.

zeus87gn · October 24, 2008

Just for curiousity, I called my local stealership to ask around about this. They said they have run across 2 other SC's that have had this problem. One had a bad wire and the other had to have a new inst cluster.....to the tune of $2k. :huh: