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Premium Member
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Discussion Starter #1
I want to power a voltmeter, Fiamm horns, and an accessory plug (to run an air compressor) on the same relay.

1. Is it a good idea to run all three on the same relay?

2. If yes, what size fuse should be used?

3. If no, what needs to have a separate relay, and what size fuse should I use in the various relays?

TIA
 

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Sparky!!!
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8,697 Posts
you don't say what amperage relay you want to run, and how many amps the compressor runs, assuming you are running a standard 40 amp relay, you want the power from battery to the relay fused at 30 amps, and then your three other circuits would be fused at 10 amps. I personally would run the Compressor off of a separate relay, with nothing else on that circuit... 30 amp fuse in and out.
 

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Premium Member
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I have a 12 volt compressor (Slime brand) that is small enough to put in one of my saddlebags with plenty of room left over. It came with a cigarette lighter plug, but I cut it off, and installed battery charger plugs. If I need it out on the road, I will just clamp it directly to the battery. I have a small electric compressor to use at home. The Slime compressor has it's own internal fuse.

As for the horns, are you going to replace the stock horns with the Fiamm horns, or keep all of them? If you are replacing the stock horns, how much if any more current do the Fiamm horns draw? You might be able to use the stock relay. I replaced my stock horns with aftermarket horns of the same type, from JCWhitney, and connected the stock wires to them. They work fine.

There is no reason I can think of for using a relay for a voltmeter, you might want to use a small, 10 amp or so, fuse close to the battery to protect the wire in case it should short out against something, so it won't start a fire.

However, from what I understand, all of the stock relays, in the JB, are undersized and prone to failure. If it were me, I would replace the relays with larger ones, OUTSIDE the JB.


I used to do a lot of residential A/C work on the side, the old units had separate heavy duty relays for everything. They were easy and inexpensive to repair/replace, and didn't fail that often. Newer units have everything, including the relays for the indoor and outdoor fans, on a made in Mexico circuit board. They are seriously undersized, and fail on a regular basis. Due to the overall design, it would be extremely difficult to install proper relays. Fortunately, it is an easy fix, just replace the entire board, which is shared by many different units, for about $200, give the old one to the customer, explain what happened, and tell them that unfortunately, due to the lack of quality and size of the relays on the board, there really is no way to repair it properly, and they can expect another failure at some point in the future. I almost always tried to talk someone into repairing an old unit, but most of the time they wanted a new one, even after I explained to them that most new units were cheaply made junk. For some reason, most people seem to think that new means better, when in fact, it is almost always the exact opposite.

I have an old unit made in the early '80s, and it has no electronics on it. I have replaced both fans and a defrost timer, but never had a relay fail. I have installed both liquid line and suction line filters, so hopefully if the compressor had a hard failure, it won't contaminate the entire system, and can simply be replaced. I want to keep this old unit going as long as possible. Sorry for getting off topic. Jerry.
 

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Sparky!!!
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As for the horns, are you going to replace the stock horns with the Fiamm horns, or keep all of them? If you are replacing the stock horns, how much if any more current do the Fiamm horns draw? You might be able to use the stock relay. I replaced my stock horns with aftermarket horns of the same type, from JCWhitney, and connected the stock wires to them. They work fine.
there is no horn relay... the stock horns run off of the 30 amp main fuse... the only three relays we have inside the JB are the Starter, Fan, and Headlight. And yes they are all undersized, I have been rebuilding the JB's for several people adding in normal 30 amp automotive relays to the outside... and over sized diodes.
There is no reason I can think of for using a relay for a voltmeter, you might want to use a small, 10 amp or so, fuse close to the battery to protect the wire in case it should short out against something, so it won't start a fire.
the reason for adding a relay for the Volt Gauge is so that you can run a direct feed to the battery, but have the voltage shut off when the key is off.
 

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Old Truck Junkie
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4,133 Posts
More than likely the horn and air compressor will be used at different times, so one relay can do the job. And the volt meter will draw very little current. imo, one relay will work.
 

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The Professor
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3,147 Posts
I want to power a voltmeter, Fiamm horns, and an accessory plug (to run an air compressor) on the same relay.

1. Is it a good idea to run all three on the same relay?

2. If yes, what size fuse should be used?

3. If no, what needs to have a separate relay, and what size fuse should I use in the various relays?

TIA
The Fiamm horns will need a seperate relay since it will only be activated when you press the horn button, they will draw around 7 amps.

Use a mini fuse holder off the battery for acc. and air compressor, 10 amps will be fine.

Since the main reason for the voltmeter is to see when you are NOT charging, there is no reason to run it off a relay unless you want to, the volt variance is minimal. The easiest hookup is to use the unused ground in the HL bucket and get power from a running light.
 

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Old Truck Junkie
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4,133 Posts
If the comp has an on/off switch, you can run it right off of the batt. And disconnect when done with it. A quick connector like the battery tender has should work.
I was wrong if going the relay way at least two will be needed.
 

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Premium Member
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7 amps for horns? Those must be some horns. What do the stock horns draw? As for the battery clips on the compressor, I did it that way so I could run the compressor off any battery, without the need for an accessory plug. I also have a larger, Black&Decker Airstation (really crappy brand I know) 12 volt compressor set up the same way, with a spliced on cord long enough to reach from the battery to all 4 wheels on my car and truck, which I take along if I am going on any kind of trip. Again, it has it's own internal fuse.

And while you should have no problem connecting a voltmeter to any hot wire on the bike that is hot whenever the bike is running, I would still conect it directly the the battery.


On my truck, I ripped out all the electrical, installed a large fuse box under the hood next to the battery, and connected it directly to the battery. I ripped out all the switches and wiring inside the cab, and built a custom switch panel with all new toggle switches, and all new wiring, with a separate circuit for everything. I used a pushbutton for the horn. I have a row of 12 toggle switches, one for everything on the truck, and a few more for some extra stuff I added. And every single circuit is fused right at the battery. I haven't had any yet, but if I ever do, there won't be any issues tracing down electrical problems. Jerry.
 

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Sparky!!!
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8,697 Posts
7 amps for horns? Those must be some horns. What do the stock horns draw? As for the battery clips on the compressor, I did it that way so I could run the compressor off any battery, without the need for an accessory plug. I also have a larger, Black&Decker Airstation (really crappy brand I know) 12 volt compressor set up the same way, with a spliced on cord long enough to reach from the battery to all 4 wheels on my car and truck, which I take along if I am going on any kind of trip. Again, it has it's own internal fuse.

And while you should have no problem connecting a voltmeter to any hot wire on the bike that is hot whenever the bike is running, I would still conect it directly the the battery.


On my truck, I ripped out all the electrical, installed a large fuse box under the hood next to the battery, and connected it directly to the battery. I ripped out all the switches and wiring inside the cab, and built a custom switch panel with all new toggle switches, and all new wiring, with a separate circuit for everything. I used a pushbutton for the horn. I have a row of 12 toggle switches, one for everything on the truck, and a few more for some extra stuff I added. And every single circuit is fused right at the battery. I haven't had any yet, but if I ever do, there won't be any issues tracing down electrical problems. Jerry.
Wonder how hard this would be to do to our bike
 

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Premium Member
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Discussion Starter #10
My apologies for not providing more information in my initial post. My electrical knowledge wouldn't fill a thimble. The compressor is an 8A 96W designed to plug into a standard "cigarette lighter" outlet. It has an internal 8A fuse. The PO left the battery tender leads on the bike and from what has been posted, it appears that it may be easier to change the compressor plug to an SAE 2 pin that will mate with the tender leads. Therefore, no need to include it on the relay. Anyone see any problems doing it this way? I have no idea which of the two compressor wires would be + or -, and would it matter if I get them switched?

The relay I have is 40 amp. If I need a different one, I will get it.

Couldn't find any info on how many amps the Fiamm horns draw. I wanted to run them direct from the battery because I remember reading on the forum that you wouldn't get the full sound using the stock wires. I plan to remove the stock horns.

Seems I read somewhere that running the voltmeter off the battery gave a better reading, but I could be wrong on that, and it might not amount to a hill of beans difference.

So, if I am interpreting things correctly:

1. Run the compressor directly off the battery, no need for a relay.

2. Use the relay only for the horns.

3. Wire the voltmeter as lance suggested.
 

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Sparky!!!
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you need to figure out wich wire is positive and which one is negative on the compressor, to do this take apart the cig adapter plug on the compressor and the wire that runs to the outside two prongs is negative, the wire that runs to the spring loaded tip is positive.

the relay you have is fine. and you can hook up the voltmeter to both leads inside the HL bucket. the White w/ Blue stripe is + and the Black w/ Yellow stripe is -
 

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Discussion Starter #12
Thanks to one and all! Looks like rain, so I may get started on this today.
 

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and the Adventure Cycle
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Something else you could do for the compressor is to get an extension cig lighter plug like this...


Remove the male end and attach a two pin plug for the tender plug.
That way, you could also use it for a cell phone charger/GPS/MP3/whatever else uses a cig lighter plug
 

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Premium Member
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Discussion Starter #14
Something else you could do for the compressor is to get an extension cig lighter plug like this...


Remove the male end and attach a two pin plug for the tender plug.
That way, you could also use it for a cell phone charger/GPS/MP3/whatever else uses a cig lighter plug
Yes, I'd forgotten about the need to charge my cell phone. Keep it in my riding jacket and only turn it on when I need to make a call. Good suggestion!
 

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Sparky!!!
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8,697 Posts
I added a cig light adapter behind the right cover, wired directly to the battery... I just have to make sure to unplug my cell when the bike is off... but usually take the phone with me any ways
 
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