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Hi, I have a 93 SC400, and sometimes when I am driving, ( usually when I first start the car ) I notice the speedometer is still at zero!?!?!? :ph34r: but then when I take another look I notice it shoots up to the speed Iam traveling....this has happened to me twice. What could the problem be? And what can I do to fix it?

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  • 3 weeks later...

I just went through my instrument cluster, mainly to replace all the lamp bulbs. It requires removal of the first circuit board to get to the lamps. I noticed that the connections to the meter movements for all the gauges including the speedo and tach are made by screws which contact metal pads on the outer circuit board.

If someone has replaced bulbs before, they may have left these screws loose, or worse--stripped the threads. If you can get the instrument cluster out of the car (see a service manual for how to get it out) You can remove the rear protective cover from the back of it and see the screws. They are all silver in color.

The screws for the speedo would be left of center as you look at it from the back. Be sure not to tighten these very much, just enough to compress the split lock washer under the head. The washer supplies the contact pressure, not the torque. Just barely snug is the right way.

After lots of experience with electronics, I have to say this "screws carrying signals from circuit boards" design is really poor. It's given trouble in all kinds of equipment besides Lexuses (Lexii?) Anyway, it is one possible cause for the problem, probably the most likely. Hope this helps.

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:wacko: It's me again.. I need to add that if you aren't really sure of your skills with car electrical/electronics, refer this work to someone who is! It's so easy to screw up a car this complicated! I don't take any responsibility for what happens if you try this, it's just my personal observations on my car.

Good Luck ;)

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