complex! And probably so have I!!!!

Willy asked for a "light" to come on. He said "Ideally, I'd like them to go on when the system is calling for the fans to come on but the fans don't come on ... ie.. fans are faulty.. get it?

I believe the fans are switched on by supplying "ground" through a relay. If that's all he wanted, you could use an LED with the common side supplied by the ground action of the relay. The LED would LIGHT!

But the second part of the statement (in RED), is the kicker! The fans DON'T come on. To do that you NEED to measure CURRENT. That's the only way to know; VOLTAGE is ALWAYS present but current only flows if the fans 'work' and the relay engages.

Measuring VOLTAGE does nothing to help you learn about the fan's operation or non-operation.

Tkamiya ideas:
1) Voltage tells you nothing!
2) Coolant temp sensor measurement means you have to "tap" into that ckt which MAY load it and change the trip points if YOU ARE NOT REALLY CAREFUL. As you know these 'trip points' are not that well defined meaning that your ckt would have to be adjustable to 'fit' it to the car. Now having this information still doesn't do anything toward knowing what the fans are doing.

However, this is not to say that I don't see your point: You want to AVOID a disaster such as an overheated engine. So maybe this would be a 'last ditch' warning system, probably audible, to tell you that something is about to burn up!!!

That type of system, upon second thought, could be built relatively easily. Very similar to the concept of my Cool Harness except that it has an audible and/or visual alarm. Humm. . . . . have to think about that.

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Regards . . . . JimF
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'94 S500 Cpe

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