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need help building a low voltage switch

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deathlucky

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I wired up my car thermo fans yesterday with 2 fans with 3 relays so there is a high and low speed. they are hooked from the ignition to the relay coils to the ecu as the wiring diagram shows in the workshop manual. but for some reason the ecu is not triggering the relays. i have tried earthing the wires that go to the ecu to the chassis and the fans work fine.

the plan is to run an engine temp sensor that changes resistance around 3k at operating temp and around 22k at room temp and have 2 switches hooked up 1 to turn on at about 2.9k for low speed and 2.7k for high speed has anyone got any ideas on how to make these switches

thanks
 
i think the margins between 2.9k and 2.7k are so nerrow. n this case you may need a very good sensing method, i hope wheston bridge will may work.

just post the temperature cherecteristic of the RTD and the temperatures you like to switch them. you said its range is 3k-22k for 25-120 deg?? i doubt you made a mistake in calculation.
 
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