Ongoing Speedo Problems With '99 Ls - Need In-Depth Technical Advi

[dthouston]

My '99 LS400 had the "doesn't work intermittently but banging on the dash sometimes fixed it" problem for some time.

I was able to fix it by 1) buying the Lexus service manual and 2) very carefully removing and disassembling the cluster and 3) "hot wiring" the four pins from the speedo directly to the Combination Meter ECU pins F1-4, thus bypassing the press-fit edge connectors (which are a source of problems just about anytime they're used - shame on Toyota/Nippon Denso for designing it that way!).

I don't recommend this procedure to anyone unless you can read a schematic, are VERY competent with precision soldering, and have a spare $1000 if you slip and destroy the circuit board.

BUT THIS DID FIX THE PROBLEM... for about 2 years.

But now I have a different problem (or at least the symptoms have changed): rather than being completely dead, the speedo now reads about 60mph too slow. That is, until you get to 60+ mph, it reads zero. But it then rises to 15-20mph (ie, the needle isn't completely "stuck" at zero).

I re-checked my "bypass surgery", and those connections still seem to be OK. And the needles don't seem to be "stuck"... they swing easily off their stops. And the two SIN and COS cores in the air-core speedo still read ~200 ohms.

• Does anyone have any suggestions as to what to check next?
  
• Does this year model use a sensor on the transmission or is the speedo driven off the ABS front/rear speed sensors? They're "only" $200 each (not installed).
  
• How can I "drive" the ECU electronics (or at least the speedo part) when the cluster is on the bench?
  
• Is there a more complete schematic available for the Combination Meter ECU than the one in the service manual?
  
• Anyone know how to test an air-core speedo beyond simply checking its coils for continuity?
  

Thanks!

DT