How do we test a Relay?
Thread Starter
Instructor
Joined: Jul 2006
Posts: 136
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How do we test a Relay?
I have searched for hours.
My cooling fans are in-op. I read all about how they work. I took a jumper wire to relay #45 and the low speed fans came on (both of them). So I know fans and wiring harness are good.
How do I test the relay to see if it is bad or not ?
My cooling fans are in-op. I read all about how they work. I took a jumper wire to relay #45 and the low speed fans came on (both of them). So I know fans and wiring harness are good.
How do I test the relay to see if it is bad or not ?
Instructor
Joined: Mar 2005
Posts: 147
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From: Lusby MD
I don't know
but I've always pulled a simular one that you know works from the fuse box and plugged it in the holes of the relay that was in question. If it worked, you know it was the relay.
It's always worked quick and easy for me. Hopes this helps!
but I've always pulled a simular one that you know works from the fuse box and plugged it in the holes of the relay that was in question. If it worked, you know it was the relay. It's always worked quick and easy for me. Hopes this helps!
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Relays consist of a small coil of wire wrapped around a metal core which acts like a magnet. Two of the terminals on the relay connect to that coil. If you measure that coil with an ohm meter it will read anywhere from 50 ohms to 1500 ohms depending on how the relay is designed and the purpose its used for.
The other contacts are connected to the SWITCHED terminals of the relay. Depending on the relay, there could be anywhere from 2 terminals to 3, 4, 5, ...........etc...
You will need the service manual schematic to figure out what the terminal # on the relay go to inside the relay.
Here is a sample of a schematic with a relay:
Say, if there were 3 terminals, one terminal would be connected to the power wire. The other terminals would be toggled by the coil. So, to properly test the relay, you would need to measure the switched contacts with an ohm meter. The switched contacts should read zero ohms when they are connected and infinity on the open contacts. To measure the resistance of both contacts, you would need to apply power to the coil and switch the relay. You would also want to check to see that there was INFINITE resistance on the open contacts when they are suppose to be open.
The contacts inside the relay can arc and carbon up which increases the resistance between the contacts. Once the resistance gets great enough the relay can still toggle properly but the relay contacts will not properly power the load. Ive seen relay that have 2-5 ohms of resistance and still work properly. Ive also seen relays with 5-10 ohms and cause problems in the circuit.
Bill
BC
The other contacts are connected to the SWITCHED terminals of the relay. Depending on the relay, there could be anywhere from 2 terminals to 3, 4, 5, ...........etc...
You will need the service manual schematic to figure out what the terminal # on the relay go to inside the relay.
Here is a sample of a schematic with a relay:
Say, if there were 3 terminals, one terminal would be connected to the power wire. The other terminals would be toggled by the coil. So, to properly test the relay, you would need to measure the switched contacts with an ohm meter. The switched contacts should read zero ohms when they are connected and infinity on the open contacts. To measure the resistance of both contacts, you would need to apply power to the coil and switch the relay. You would also want to check to see that there was INFINITE resistance on the open contacts when they are suppose to be open.
The contacts inside the relay can arc and carbon up which increases the resistance between the contacts. Once the resistance gets great enough the relay can still toggle properly but the relay contacts will not properly power the load. Ive seen relay that have 2-5 ohms of resistance and still work properly. Ive also seen relays with 5-10 ohms and cause problems in the circuit.
Bill
BC
Last edited by Bill Curlee; Oct 28, 2008 at 03:00 AM.
Thread Starter
Instructor
Joined: Jul 2006
Posts: 136
Likes: 6
Bill, your attention to detail and thoughness is just amazing. Thank you. I have no idea what all that means, but thank you for it. As always you put alot of appreciated effort into a answer. I see you showing me how and what to measure - and I think you are saying even though you measure it and it looks good - it still may be bad. So I guess I will swap one for another and watch the results. Again, Thank you.
