Explanation for this resistance value in relay coil?


misterW

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I was testing my EEC power relay coil in my 1991 3.0 and noticed something strange. The resistance value of the control coil was 1 (infinite). This would seem to indicate that the circuit was open, which it shouldn't be. Yet, when I applied 12v to the two spades of the control coil, there was a click and the other circuit pathway closed like it should (based on resistance testing).

I tested a couple of other relays (a fuel pump relay from the same truck and an EEC power relay from a 94 ranger) and their control coil circuits showed resistance values of around 60 ohms.

So I figured the relay in my 1991, while apparently still functioning to a degree, had something wrong with it.

I just tested the resistance of the control coil circuit in the new relay that I bought and it was 1 also!

So what is going on here? Shouldn't the control coil circuit be closed? Wouldn't a value of 1 indicate a problem?
 


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lvwill

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I think in the case of an electromagnet coil what you read was correct. But how you you understood it may have been wrong. I may be wrong but the less resistance in a circuit is always better.
 

misterW

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Wouldn't a closed coil circuit have a resistance value? The other ones I tested did.
 

misterW

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Oh -- just in case this was causing confusion -- when I say the resistance was 1, I mean the multimeter said "1", or "infinity". Not 1 ohm.
 

8thTon

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It may not be a conventional relay, it could have a small circuit board in there with an electronic driver. In that case the “coil” is a high impedance input that you cannot measure in the normal way.
 

RobbieD

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Try reversing the meter leads. If it's a relay that has a diode across the coil to suppress inductive lockup, you'll get closed circuit one way (low ohms) and the correct coil resistance reading the other way. "Infinity" would be an open circuit.
 

lvwill

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Infinite resistance I do believe would be absolutely no continuity in the circuit. As in just hold the leads in the air. The coils in relays are really reliable it's usually the contacts that fail.
 

ericbphoto

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I think RobbieD got it right. Not sure what meter you're using. Most digital meters indicate an open circuit or infinity with "OL" for overload. If you got a reading of 1, that is close to no resistance and probably reading back through the flyback diode. Reversing the polarity of your leads should give the correct coil reading. I also agree that the coils rarely fail. It's usually dirty, burned or even welded shut contacts that cause a relay to malfunction.
 

misterW

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I'm just using a fairly cheap multimeter. As I said before, it gives a reading of "1" to indicate an open circuit. So, before I touch anything, it reads "1" to indicate that there is no continuity. And that is the reading that I was getting on the control coil circuit that puzzled me, because the relay seemed functional.

I tried reversing the polarity of the leads and it makes no difference.

If I was NEVER able to get a reading from the terminals of the coil circuit, I would assume there was something going on like 8thTon suggested (and maybe that is the case for this one), that prevents measurement in the normal way. But, I measured the fuel pump relay with no problems, and I measured the EEC power relay in my 1994 with no problems.
 

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HF meters display a '1' for an open (or above selected range) condition so it's not unheard of.
 


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