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Search Auto Parts Search 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm Sign up here! 1 2  View All Email This Post Auto Repair Home>> Automotive Repair>> Automotive Wiring and Electrical groundzer0336 User Mar 18, 2013, 7:11 PM Post #26 of 27 (1091 views) Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm Sign In just curious but, and not saying it would work, but for a situation like this where the ecm is seeing the pull of voltage, could a resistor be wired into the circuit and see that voltage drop. im not going to do it ha, seems like it might fry a computer but hypothetically. is that what is basically happening in the speedo itself. Discretesignals Ultimate Carjunky / Moderator Mar 18, 2013, 7:21 PM Post #27 of 27 (1087 views) Re: 97 lexus sc300, trying to test vss/bypass speedo and wire to ecm Sign In Actually no. A resistor won't work. The circuit from the ECM is actually pulled to ground by a transistor. The transistor is turned on and off by detection circuitry in the combo. The ECM probably sends out some type of voltage such as 5 or 12 volts. The ECM is monitoring the voltage on the circuit. So when the circuit isn't being pulled to ground by the transistor the ECM is going to see high volts. When the transistor pulls the circuit down the ECM is going to see near zero volts. We'll say for example the ECM is sending out 12 volts, so the ECM is going to see this 0-12-0-12-0-12 (off-on-off-on-off-on) signal being toggled by the transistor as the vehicle is moving. Hence your digital signal via a pull down circuit. The frequency of the on and off signal is how the ECM determines vehicle speed. Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved. (This post was edited by Discretesignals on Mar 18, 2013, 7:23 PM) 1 2  View All Email This Post   Automotive Repair     Auto Body Repair and Refinishing     Heating or AC Issues     Electrical and Wiring     Brake Systems     Car Stereo Repair Advice     General Discussions     Car Making Funny Noises     Engine Fault Codes     Engine Troubles     Exhaust Systems     Steering and Suspension     Transmission Troubles     Tricks of the Trade Automotive World     General Discussions     Antique Car Talk     Car Shows and Events     The Bargin Bin     Motor Vehicle Inspection     Car Care Tips     Traffic Tickets     Gasoline, Motor Oils, Concept Cars and Engines     Auto Insurance, Financing, Leases, Warranties Truck Repair     General Discussions     Truck Engine Repair     Truck Suspension Repair Motorcycle Repair     General Discussions     Motorcycle Engine Repair CarJunky     Open Posting     Board FAQ's     New Board Suggestions

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