On board air - melting relays

[KSL22]

So I finally got my on board air installed and plumbed but I am having troubles with the relays overheating then failing. The most recent one failed while filling my third tire so I would guess after 10-11 minutes of constant current. What are the causes of over heated relays. What solutions do you have.

The relay is running a Smittybilt 2781 compressor. It is listed as having a 1/3hp motor but I do not know the actual amp draw. I have 10ga wire from the battery to the relay. From the relay I have the positive wire that came on the pump. The pressure switch is wired on the switch circuit of the relay. The compressor has a 40min duty cycle and has had not problem filling all four tires as it came from the vendor.

 

[Hopeless Diamond]

The 2781 shows as having a 45 amp draw. Your relay is probably undersized to begin with, then running it on #10 probably exacerbates the problem with undersized wiring adding voltage drop. Depending on how long of a run at least #8, if not going up to 6 or even 4. For the duty cycle, they say 40 minutes on, 20 minutes off in every hour. It's not meant to run for 40 minutes continuous, but have it run no more than ~66% of the time.

 

[proper4wd]

Yikes. Nothing like running wire and relays without knowing what the current in the circuit is. Out of curiosity, if you did not know how many amps the compressor pulls, how did you know what size fuse to run it off of?

(let me guess, there is no fuse.....)

 

[Robert Bills]

Try using a constant/continuous duty solenoid rather than a relay.

Something like this, available from NAPA or any quality auto supply store:

Wire it like this (you can use a 40 amp fuse rather than the 35 amp fuse shown since your compressor has a maximum 45 amp draw and substitute a check valve for the "unloader" valve):

 

[rayra]

Yikes indeed. Good thing the relay slagged before the puny wire did and set the vehicle on fire.
OP, you need at least a 6ga lead for that sort of sustained amp draw to have any safety margin. And fused and relay appropriate for the load.

 

[Pilat]

Yikes. Nothing like running wire and relays without knowing what the current in the circuit is. Out of curiosity, if you did not know how many amps the compressor pulls, how did you know what size fuse to run it off of?

(let me guess, there is no fuse.....)

Exactly!

 

[Robert Bills]

The 8 gauge wire illustrated in my post should be sufficient for OP because the 45 amp rating for his Smittybilt compressor is the "maximum amp draw" at start up/high pressures (150 psi) rather than his compressor's "running amps."

The wiring diagram I posted is from the Curry Enterprises onboard air system (NLA) that utilized a Thomas continuous duty milspec 12v compressor with a similar maximum amp draw. I had a nearly identical system mounted in my CJ-7 that I purchased from Brad Kilby/Kilby Enterprises/onboardair.com "back in the day." Even when the compressor was working hard for extended periods the 8 gauge wire never even got warm. In OP's situation, a 40 amp fuse would blow before the 8 gauge wire would fail.



http://www.smittybilt.com/docs/installation/2781.pdf

 

[rayra]

'should'.
Why not be sure, and bump it up a notch for high ambient temps too. Ensuring the wire can handle the load in any conditions. And then fuse it properly below the death of either the device or wire. 'Best practices'. 'should' = 'might'

 

[Robert Bills]

It's hard to argue against over-engineering an electrical circuit in an offroad vehicle, we all do it, but 6 gauge is overkill unless the wire run is longer than the typical underhood onboard air system.

Just for ******ts and giggles I emailed an acquaintance at Viair and asked what gauge wire Viair recommends for a compressor with a 45 amp maximum draw. The answer was 8 gauge for an underhood system and 6 gauge for wire runs 15' or longer.

Here is a chart that the OP can use to determine for himself the proper gauge wire for his system:

http://viaircorp.com/wiring.html

Note: the linked chart does not specify whether it is based upon the total length of the wire (positive + negative) or just the positive lead, but since most wire gauge charts specify total length that would be how I would read the Viair chart to ensure the proper safety margin. For example, using the Viair chart, one would use 6 gauge wire if the total length of the wire is 15' or longer for the positive and negative wire combined, not 15' of positive wire alone.

As for my choice of the word, "should," it was chosen because I hesitate to speak in terms of absolutes in an Internet post responding to a request for advice lest it be interpreted as a guarantee.

 

[4x4junkie]

Seems to me too much current going through a relay would burn/fry it's contacts. "Melting" says to me the plastic or potting around the coil is melting because it's an intermittent-duty relay being pushed into continuous-duty service, so the coil itself is what's overheating.

Perhaps I'm wrong, but that was the first thing that came to mind when I saw the word "melting" being used.

Whatever the case is, Robert Bills advice is sound. A cont. duty solenoid type relay is what's needed here (and some #8 or 6 wire and a 40-50A fuse!). Also if you were using typical spade type quick-connect terminals, that also is a no-no at this much current. I would suggest put the fuse between the battery & solenoid rather than between the solenoid & motor as shown on that diagram.

 

[dBeau]

I would suggest put the fuse between the battery & solenoid rather than between the solenoid & motor as shown on that diagram.

Good catch... I'd even go a bit further though and suggest that the fuse on the 14ga switch wire be moved closer to the hot side of the solenoid.

 

[Hopeless Diamond]

Always put the fuse as close to the power source (battery) as you can. The fuse protects the wire, not the device. If somethings starts chafing the wire and shorts out, it'll blow the fuse instead of turning the wire run into a fire starter.

 

[KSL22]

I admit I am not a electrical wizard. That is why I work in a field that has people to take care of that stuff. I came on here asking the question because I wanted to learn what was wrong. If I knew all the variables and had all the answers I wouldn't be here posting questions. Thank you to those that posted to help.

The fuse I choose was part of the relay kit and came with the relays and it is right at the battery. I choose the wire size based on what I read as what others had used in their setups using the same compressor.

The compressor also has a built in fuse / circuit breaker. I felt around the wires and such and the only thing hot at the time was the relay. I did not notice a lot of heat on any of the wires. I will look closer when I get home to see if I can tell which wire was getting warm.

I will look for a proper size feed wire to replace the 10ga. I will also look into the solenoid to replace the relay.

Will I need a relay for a switched air solenoid to air up my air bags off the tank, or can I get away with a wire from the switch that will be in the cab?

 

[KSL22]

Seems to me too much current going through a relay would burn/fry it's contacts. "Melting" says to me the plastic or potting around the coil is melting because it's an intermittent-duty relay being pushed into continuous-duty service, so the coil itself is what's overheating.

Perhaps I'm wrong, but that was the first thing that came to mind when I saw the word "melting" being used.

Whatever the case is, Robert Bills advice is sound. A cont. duty solenoid type relay is what's needed here (and some #8 or 6 wire and a 40-50A fuse!). Also if you were using typical spade type quick-connect terminals, that also is a no-no at this much current. I would suggest put the fuse between the battery & solenoid rather than between the solenoid & motor as shown on that diagram.

The relay itself was not melting. The wire plug adapter hooked to the relay was slightly disfigured around one of the connections. I did not get a close look at it a the time as I had just come down the Sherman Pass Jeep trail in a Ram Megacab. Lets just say I was a little bit mentally drained at the point I hit highway and tried to air back up. I will get a better look at it tonight and maybe even get a picture of the connector.

 

[rayra]

To split hairs, & showoff how brilliant & non-condescending jerk I am at the same time...

A fuse primary purpose protects wire and devices. ( a fuse is a device afterall...)
Device is any electrical thingamajig aside from a conductor/wire whats main purpose is to create a current path. Switches, relays, fuses & fuseholders, receptacles, etc. are devices.

You probably meant to write "Appliance".

I'm compelled to repeat a response I've made multiple times to these assertions. The fuse is to keep the vehicle / house from burning down. 'protecting' the wires and connected devices is a happy side effect.