Master solenoid - a weird question

[Delta Whisky]

So - I'm mocking up my instrument panel on the workbench and fired up the EFIS and EIS for the first time. By accident I touched the master relay and discovered it was pretty warm, borderline hot. So I pulled out an IR thermometer and read 143 F. That seems strange to me but it might be normal. I wouldn't expect measuring the temperature of a master relay to be a normal thing to do, but just in case others have some sort of experience in the matter I thought I'd post the question. What's normal??

[Shadowrider]

It’s normal. On both of our planes they get hot.

[jrevens]

I agree with Dustin... they get very hot. I think the ones commonly used in aircraft, manufactured by White-Rogers, are usually rated to somewhere around 105 deg. C (221 deg. F), so the normal operating temperature can be pretty darn hot without damaging them. I've found that they usually get hot enough to hurt if touched. If my memory serves me correctly, I think the coils draw somewhere around 1 amp... approximately 14 watts @ 14v. It's a shame to use that much power, but they are reliable and relatively inexpensive - the alternative would be some kind of much more expensive solid-state relay. Any electromagnetic coil is going to produce heat. I've noticed that solenoid valves commonly used for water can get similarly very hot to the touch.

[Kitfox Guy]

Interesting discussion. I noticed how “warm” the master relay gets last year when I had to replace mine. No complaints though as the unit was over 20 years old. According to their website they have a projected life expectancy of 100K to 250K operations. With that being said they are built to withstand the heat.

[Shadowrider]

Interested when it failed what happened? Hopefully it failed on the ground? I ask because I have a lot riding on that relay.

[Delta Whisky]

Thanks for your responses. I'll go back to work now!!:)

[Kitfox Guy]

Interested when it failed what happened? Hopefully it failed on the ground? I ask because I have a lot riding on that relay.

It failed on the ground, after I landed at an airport with the intention checking out the little airport and putting a few gallons of petrol in the tanks. All of my electrical went dead, and I initially thought the battery was dead. I opened the cowl and with limited tools on hand determined the battery had power. I didn’t have a voltmeter, but I suspected it was the master relay as I couldn’t hear the normal solenoid “click” when the master switch and ignition were turned on.

Unfortunately, the airport was a quiet small remote airport about 75 nm from my home. To make matters worse it was a Sunday afternoon and there was absolutely no one at the airport. Lucky for me about an hour after I landed a Mooney owner showed up at the airport and gave me a ride back to my home base. I came back two days later, replaced the master cylinder and all went well. I think we all have a lot riding on the master relay. They are a very reliable piece of equipment and my master cylinder was 20+ years old when it failed.