'99 Ls400 Speedo Needle Light Is Out

[dthouston]

The speedo was intermittently not working on my ’99 Lexus LS400. Banging on the dash would (usually) make it work again, so I pulled the cluster and direct-wired the four pins protruding from the back of the speedo to the #1 ECU circuit board… bypassing the flaky edge connectors at both the speedo guage and the “F” connector's ribbon cable (pins 1-4).

Note that this is NOT a job I would recommend for the faint-hearted, unless you're really good at precision soldering under a magnifying glass (I have the factory service manual and a background in electronics repair, but even so, this job took the better part of a weekend).

But the speedo now seems to work great - at least it shows the speed reliably.

HOWEVER, the speedo needle LIGHT is now not working! (arrrgh) I guess I broke it, although I was incredibly careful when I pulled the speedo.

Does anyone know how the speedo needle light is driven? Does the '99 LS use LED's or cold-cathode? And if the latter, where's the step-up inverter?

There are only four pins on the speedo guage (SIN +/- and COS +/- since it's an air-core guage), so the needle light must get its supply from them. And since the speedo is working, I assume the “break” is somewhere internal in the guage… between the two coils and the needle, or in the needle itself.

Any hands-on electronics advice would be greatly appreciated!

DT in Houston

[dthouston]

I think I may have just found what my problem is, thanks to an annotation made by Jim Walker to a diagram in his article at: http://www.lexusownersclub.com/Lexus_Circu...ard_Removal.pdf

Jim said: "These 6 holes carry the high voltage from the circuit boards through the special long screws to .. the guage needles"

So that's how the CCFL voltage gets to the needles! Those engineers at Denso aren't dummies!

And sure enough, I used one of those "special long screws" as a convenient place to hold down a plastic cable clamp I added to restrain the wires I used to bypass the edge connectors.

Well, duh!

DT

[90LS400Lexus]

Sorry to hear of your misfortune. I remember someone else not too long ago having an instrument cluster issue and banging on the instrument panel quickly led to an array of other issues. Hopefully it did not cause more damage.

There was another thread on this not too long ago, about the problem you have... I will see if I can find it.