How to test a relay?

[collins_dan]

I suspect that the temperature sensor on my radiator has gone bad. It turns the fan on now at 140F, instead of 185F. It is not that old, maybe 3 years. Before I replace it, I'd like to test that the relay is working properly, as the original supplier said it needs a relay to operate, as it can't handle the load of the fan. I'm not really sure what I am testing for? I tested the voltage going to the sensor and it was 13 volts (alternator). This was before the fan had come on. I don't know if this is good or bad, the battery was receiving a similar charge at the time. Should I be testing for amps? Or is a relay either working, or not working? I would appreciate a little education on how a relay works, why a sensor would require a relay and how to test it. Thanks. Dan

[RotoFlexible]

Without knowing the details of your system, it's hard to say exactly what the problem is. But to answer your question: If the fan is turning off and on at all, the relay is working. If it wasn't working, the fan would be on all the time, or it would be off all the time.

Generally speaking, the "sensor" detects temperature. Something - possibly the sensor itself - has to convert the temperature into an "on or off" signal. That is, less than the threshold temperature, it's off; more than that temperature, it's on. "Off" is usually represented as an open circuit (no volts output) and "on" is a closed circuit (~13 volts output). There is no in between.

In theory, you can run the fan directly from this output. However, the sensor mechanism may not be able to handle the current requirements of the fan motor. That is where the relay comes in. A relay is simply a heavy-duty switch that is controlled by an electrical signal. If the signal voltage is 0v, the relay is open (no current goes through it). If the signal voltage is 13v, the relay is closed (current flows)*. It is the relay that powers the fan, so if the fan works at all, the relay is working.

What you need to do is trace all the wiring in the temperature and fan circuit, figure out which wires and terminals correspond to the sensor circuit (which controls the relay) and the fan circuit (which flows through the relay). Identify the components. Then you can make some more measurements and try to figure out what's going on.

* Yes, it can be the other way around, depending on how the relay is wired.

[collins_dan]

Thanks Andrew.

I'll check the relay, and see how it is wired. There's got to be about 5 of them located in the nose on the left side. I was helping a friend with his TR6 this weekend and all his fuses and relays are so much more easy to access, in the engine compartment next to the master cylinders.

Thanks, Dan