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How to test a relay to determine if it is bad or the circuit board it is mounted on is bad

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auburnaviator

Well Known Member
This isn’t airplane related (except for the fact that it’s a relay on my shop A/C where I’m building my airplane) so if the thread isn’t appropriate please move/delete it. I went in to the shop this morning to start working on the plane and immediately knew something was wrong with the A/C. The fan was running but the compressor wasn’t so after some troubleshooting I narrowed it down to the compressor relay on the circuit board. If I jump the relay the compressor runs just fine.
So now I need to test the relay to see if it is bad or if something on the circuit board has gone bad. Looking at the back of the PCB it only appears that 1 of the four pins on the relay has a trace on it and the trace has good continuity with other pins that it runs to on the back of the board.
Just for reference the power wire ran to the COM terminal and the compressor feed line came off of the NO terminal and run to a solid state relay before continuing on to the compressor. The problem is I don’t know how to test the “bad” relay. Reference the attached relay picture. How do I test this with my multimeter?
 

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auburnaviator said:
This isn’t airplane related (except for the fact that it’s a relay on my shop A/C where I’m building my airplane) so if the thread isn’t appropriate please move/delete it. I went in to the shop this morning to start working on the plane and immediately knew something was wrong with the A/C. The fan was running but the compressor wasn’t so after some troubleshooting I narrowed it down to the compressor relay on the circuit board. If I jump the relay the compressor runs just fine.
So now I need to test the relay to see if it is bad or if something on the circuit board has gone bad. Looking at the back of the PCB it only appears that 1 of the four pins on the relay has a trace on it and the trace has good continuity with other pins that it runs to on the back of the board.
Just for reference the power wire ran to the COM terminal and the compressor feed line came off of the NO terminal and run to a solid state relay before continuing on to the compressor. The problem is I don’t know how to test the “bad” relay. Reference the attached relay picture. How do I test this with my multimeter?
Click to expand...
Appears to be a 12 volt DC coil (but coil MAY BE AC controlled) relay which controls a Single Pole Single Throw switch (inside relay) which is Normally "open" until the relay coil is energized, when energized the COM & NO contacts are "shorted". It's likely the two widely spaced bottom pins are in parallel to the slide-on contact tabs on the top of the relay (but may only be for mechanical package stability when soldering to a PWB).
Hope this helps.
 

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Here is a link to the datasheet.

Typically relays fail by the coils failing open or shorted. Rarer the arm fails, so you feel/hear the relay click, but the contacts cant close.

Ohm the coil, should typically be between 50-150 ohms. Can apply 12 volts and see if contacts close or not. With the relay out, do you have ground on the ground terminal of the relay socket and do you have 12v on hot side.
 
Diagram shown on relay. Apply 12-VDC to the coil and check relay contactor with an ohm meter (Open/Closed). Alligator clip-leads are helpful when doing this.
 
auburnaviator said:
Would this relay be an equivalent relay to replace the burned out one with, with maybe slightly better quality/specs? https://www.digikey.com/en/products/detail/american-zettler/AZ2100-1A-12DEF/12171683
I can get this one from DigiKey (a known reputable dealer and it ships from the US) later this week but if I have to order the Chinese original relay it takes forever to get here b/c it has to ship from China.
Click to expand...
Just comparing datasheets max specs, looks equivalent to me.
 
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