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gymguy

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  • First Name
    Bill

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  • Lexus Model
    '93 LS400 & '94 LS400
  • Lexus Year
    1994
  • Location
    North Carolina (NC)

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  1. As others have said...fix the ECU first. All your symptoms are usually present in most unrepaired ECU's in '93 and '94 LS400s.
  2. Just a wild guess. Two years ago I fixed the ECU (new caps) and did a general timing belt, water pump replacement, plugs, etc on my '94 LS400. My wife limped home one day in our '94 recently.. It was running very rough (sounded like only four cylinders) and smell of gas let me know that this probably wasn't ECU or plugs. Did some diagnostics and found I could fire plugs on one bank of 4 cylinders but not the other bank. I tested primary and secondary resistances on the ignition coil on the bad side. The coil was open. I was able to get a new one (in stock) from Advance Auto or AutoZone...can't remember which. Replacing it was not fun but it has been running for 2 months now...no problems. Check your coils.
  3. Had a 93 with same problem. After much work I discovered that the ECM (computer) had bad capacitors that eventually start failing. There's a topic somewhere on this forum that details the fix which involves repaired computer that will work but you have to either find one in a junk yard that has already been fixed or find a company that does the repair. I was lucky to find a wrecked 94 that had been repaired and had a warranty on it. I am an electrical engineer so I can repair the old computer with new caps (cost about $10 for the parts). That's why I'm here to find the info on parts location so I can fix it and use it as a spare. The repair companies that repair these things charge about $900 for one with a warranty.
  4. Indicated Speed at 65 MPH is actually 75 according to my GPS. At 55 it is about 64. My tire size has not changed for 5 years. Six months ago the reading was accurate. I have another LS400 (1994) with readings that agree with the GPS.
  5. Fixed most of my LS400 instrument issues (lights, gas gauge, sticking needles) but the speedometer is linearly low on actual speed (error increases as speed increases). I know this because of my GPS. I suspect that the electrolytic caps on the meter board (C2, C3, C4) attached to the needle movement may be the culprit since they are the same age as those that have failed on the lights and gas gauge circuits. I believe this speedometer design uses a hall effect sensor on the transmission that sends pulses to the meter board where a microprocessor counts the pulses and drives a stepper motor attached to the needle. This seems to be the typical design used for analog meters today. Does anyone have a schematic of the meter board or have experience with this problem?
  6. Your solution suggests to me a possible permanent fix. While fixing my flickering dash light problem I noticed that the meters attach to the printed circuit board (PCB) with screws and flat washers. As an electrical engineer I recognize this as a very poor way to make a connection. The problem is fourfold: 1)the PCB has a different coefficient of expansion than the screw. 2) the PCB is somewhat plastic and when compressed exhibits a property called "cold flow"; that is the board flattens under pressure. 3) Thermal cycling will cause the connection between the PCB and screw to loosen due to the other two factors. 4) A mechanical connection between 2 conducting materials can and will probably corrode over time unless noble materials are used such as gold. One solution to this problem is to add some spring to the system such as a lockwasher. The lockwasher allows for a connection to be maintained even during thermal cycling and cold flow. A lockwasher, one that has teeth on it, will bite into the PBC and screw head and form a gas tight connection that won't corrode under normal conditions. If I start having this problem I plan to remove the instrument cluster, remove the screws and replace the flat washers with either a standard lockwasher of a star washer.
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