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Cobra 75WXST - Fuse tap amperage?

11K views 25 replies 9 participants last post by  Beastmaster 
#1 ·
I am installing a Cobra 75WXST behind the glove box in my 2014 JK. I plan to use a fuse tap, as documented here, but I don't know what amperage I need. Has anyone done this? What amperage fuse tap did you use?
 
#7 ·
That's why I'm aiming for an unused circuit. I agree, putting more power through a circuit that's already pulling load is a bad idea. Thank you for the rating and for your concerns!

Any reason you wouldn't wire to the cigar lighter circuit or straight to the battery? The Canbus system is notorious for issues if you add current draw to any circuit other than the cigar lighter. Also, most cb radios recommend wiring directly to the battery to avoid interference issues. The cigar lighter is a switched circuit, and it's not monitored for excessive current draw like the rest of the circuits are. IMO it's the best choice if you want an ignition switched circuit.
I think it not being monitored is actually a good reason to avoid it. I'm not as mechanically / electronically inclined as many of you folks, and if I break something I won't know how to fix it. I'd rather have the system shut my CB down than get cooked. As for wiring directly to the battery, I'd prefer a switched circuit.
 
#6 ·
Any reason you wouldn't wire to the cigar lighter circuit or straight to the battery? The Canbus system is notorious for issues if you add current draw to any circuit other than the cigar lighter. Also, most cb radios recommend wiring directly to the battery to avoid interference issues. The cigar lighter is a switched circuit, and it's not monitored for excessive current draw like the rest of the circuits are. IMO it's the best choice if you want an ignition switched circuit.
 
#8 ·
Always best to wire CBs directly to the battery to avoid introducing any noise in to the power feed. I would not personally wire my CB off of a circuit that is feeding something else.

If by switched you mean only on when the ignition is on then perhaps there is place under the hood to get power from an unused circuit (closer to the battery than your regular fuse box). Not sure, I know there is on my TJ.

With that said if you are using a fuse tap on an unused circuit I would just use a 10 amp and let the in line fuse that comes with the CB be the point of failure. It will be less than 10 amp.
 
#9 ·
Well, a discussion of the JK power system, and how the Canbus system works is beyond the scope of what I'm willing to post. But be aware that you may experience a CEL, or other malfunction indications depending on what circuit you add to. My understanding is that any circuit, other than the cigar lighter, that has additional load added can trigger a CEL, or other indications on the dash. Just trying to warn you and prevent disappointment or rework. I am actually wiring my cb right now. I took it out of my 2013, and it's going into my 2015. I will be wiring straight to the battery. I've left the CB on overnight a few times, and it didn't kill the battery, so I'm not too worried. In my 2010 I hooked it to the cigar lighter, but that dash was way easier to take apart.
 
#11 ·
I've been involved with 2-way radios since the 60's and the safest/best/most noise-free source of power a 2-way radio should be connected to is directly to the battery. There is just too much electronic noise on some circuits that would make it into the CB's receive and transmit signals, and there are other circuits that would disturbed (especially a JK) by wiring a CB into them.

Connected into the wrong circuit, a CB could cause anything from killing the engine to causing check engine lights. Especially on a JK that has more data links and networks than my first computer did.

If the JK has a power distribution center enclosure under the hood that has the bigger fuses and relays in it like a TJ has (it is mounted in front of the battery on a TJ), there is a single heavy-duty power wire going to a lug with a nut on it that is ideal for connecting a CB or ham radio to.

I would not simply tap into a wire to get your power from... such power connections are risky to say the least where a 2-way radio is concerned.
 
#12 ·
Thank you. I have been sufficiently chastised. You've all done well outlining the potential consequences of doing this, but I really don't understand what about this would trigger these issues. At first I wondered if it's because of tapping into a fuse that's already in use, but I'm talking about using an unused fuse so that doesn't really fit.

Then I thought maybe it's about excessive amperage coming through, but I would think a low amperage fuse could be put into the tap to protect from that. I also looked up the power draw; this unit has a 4 watt output and takes 12 volts (DC), so that's ~1/3 of an amp. In my head that doesn't seem like a lot, but I am obviously very far from an electrician.

So, honestly, can someone please either explain this to me, link me to some kind of documentation, or at least just give me some key words to search with?
 
#13 ·
Another question. The CB I've purchased, a Cobra 75WXST, is a handheld model that plugs into a remote control box. The connection from the fuse would be to the remote control box, not to the radio itself. The reading I've done suggests that connecting a power relay to a fuse and then an accessory to that power relay is safe. Would the control box count as a power relay and thus make this safe?

I apologize for not mentioning this before. I didn't really think it was important until now.
 
#14 ·
The Cobra 75 is like any other cb, in that it has a power lead and ground lead. It's design is such that some of the circuits are in the "box" which has no controls on it. This allows the "box" to be tucked away behind the dash, or anywhere the user wants. All the controls are integrated into the handset. Other than this, it's operation is the same as any other compact cb.

A relay is used when a device draws too much current for a switch to handle. Compact automotive switches generally can only handle a few amps, whereas compact relays can easily handle 30 amps or more.

A relay isn't really needed to install a cb. When I was talking about a "switched" circuit earlier, what I was referring to was a relay switched circuit. While a relay itself won't induce noise into a circuit (if it is working correctly) you are rolling the dice if you do anything but wire directly to the power source.

I think you will understand a lot more when you read the instructions that came with your cb. Also, I hope you will read the tech section on the firestik website, it is excellent and answers a lot of cb related questions.

Tech-Docs Index
 
#15 ·
The Cobra recommended fuse size is 2 amps but a 3 to 5 amp fuse is fine too. The transmitter's 4 watt output doesn't mean a 1/3 amp power draw at 12 volts like Ohm's Law may seem to indicate.

No need to connect it to a switched source of 12 volts in fear of the CB draining the battery between uses if accidentally left on. It would take weeks for an idle turned on but unused CB like the Cobra to drain the battery of an undriven Jeep. Forever if it was driven every so often. :)
 
#16 ·
I took some time and read through the installation manual (link) and the Firestik technical directions.

From page 8 of the install manual:

5. Connect the red wire marked “BATT” (+) directly to the positive side of the battery or to a connection on the fuse box that is always on.
From the Firestik "Things Every CB'er Should Know" document (link):

17. If you are hearing whining noises from your radio while your vehicle is running, it is probably due to "dirty power" being supplied to the radio. Under dash power may be more convenient, but the "cleanest" power will be found by running the radio's power leads straight to the battery.
What I'm getting from the Firestik document is that using a fuse will probably create some audible noise, but there's no mention of electrical damage. The installation manual from Cobra seems to indicate that using power from the fuse box is safe, but also suggests the use of an always-on fuse location. Neither of these seem to indicate that there's anything unsafe about using a connection to the fuse box. Is an always-on fuse location a more direct link to the battery? Otherwise, why might Cobra advocate the use of that type of connection?

I'm sorry, guys. I am trying to understand this stuff and I know you are trying to explain it to me. Maybe I'm just hearing what I want to hear, but I'm just not getting it.
 
#17 · (Edited)
No, Firestik wasn't saying a fuse causes noise, they don't... there should always be a fuse in the power circuit. They were simply saying if you are getting noise from your current source of power, connect directly to the battery as was already recommended above.

Why is it so difficult to just power your new CB directly from the battery which is the only guaranteed source of clean noise-free and problem-free source of power? That is why experienced 2-way radio installers always recommend some form of direct connection to the battery.
 
#18 ·
No, Firestik wasn't saying a fuse causes noise, they don't... there should always be a fuse in the power circuit. They were simply saying if you are getting noise from your current source of power, connect directly to the battery as was already recommended above.
Okay, understood. I'm just still trying to see where the potential for damage to the vehicle's electrical system comes from. So far I've found where damage to the radio can come from (e.g. failure to tune the antenna), but nothing that indicates some of the consequences listed above.
 
#19 · (Edited)
Ok, I'm not going to defend my and the other recommendations any further. I'm just saying that modern vehicles like JKs have more data busses and circuits with have high impedance devices that are more susceptible to damage from RF, spurious voltages, etc. You can accept the recommendations or not. I'm done here... sheesh. :)
 
#20 ·
You came to this forum with a question and are getting advice and answers to questions that you didn't ask.

That is because it comes from many hundreds of years of collective experience.

Power distribution in late model Jeep vehicles is all controlled by a computer. Most if not all of the controls (light, heat, AC, Radio, wipers, engine, etc) are also controlled by the computer.

The computer is susceptible to stray radio waves from devices like the CB you are trying to install.

One of the tried and true ways of minimizing the potential of damage to the electronic systems in your Jeep is to wire the CB directly to the Battery.

A second way is to make sure your antenna is tuned for the lowest SWR you can achieve.

You will never completely remove the potential for damage since the function of the CB is to put out radio waves in order for you to communicate.

You can ignore Jerry's advice and come back here with questions like: why do my lights flicker or the wipers come on when I key the mic. You still may run into those problems even if you power the CB directly from the battery.

But the first question you will get is: "How did you power the CB"?

Do it right the first time.
 
#21 ·
I apologize to any who have become frustrated with my questions, in particular those who feel that I have attacked their suggestions. That was not my intent, nor was it my intention to imply that the things you know to be fact are wrong. It is in my nature to not simply accept that something is, but to instead try to understand why something is. When I'm told the reason why is a set of potential consequences, I want to understand how those consequences can come to pass so I can work to avoid them.

The most specific answer I've seen so far is RFI. RFI, as I understand it, is a subset of EMI. Taking a leap from here, I'm guessing the concern is that "dirty power" can cause an EMP. Anyone who's seen a sci-fi movie in the last ten years knows EMPs are bad for electronics. So, if I'm picking this up correctly, that fairly strongly instills fear in me. It makes me wonder how the vehicle is protected from such things from other accessories, like my FM radio.

Hearing that these problems can occur even after connecting the CB directly to the battery and in all other ways Doing The Right Thing™ translates in my head to CB's being unsafe for my vehicle. That makes me want to trash the entire project. That's an unexpected conclusion to this thread for me, but life is full of surprises.

Thank you all sincerely for your advice, warnings, and explanations. It is by far better in my mind to avoid destroying my Jeep than to get answers I didn't want to hear.
 
#23 ·
The most specific answer I've seen so far is RFI. RFI, as I understand it, is a subset of EMI. Taking a leap from here, I'm guessing the concern is that "dirty power" can cause an EMP. Anyone who's seen a sci-fi movie in the last ten years knows EMPs are bad for electronics. So, if I'm picking this up correctly, that fairly strongly instills fear in me. It makes me wonder how the vehicle is protected from such things from other accessories, like my FM radio.
Radio frequency interference (RFI) is the radiation or conduction of radio frequency energy (or electronic noise produced by electrical and electronic devices at levels that interfere with the operation of adjacent equipment.

The key part is "at levels that interfere with the operation of adjacent equipment."

While your cell phone, FM radio, GPS receiver, or intermittent wipers and even the computer that controls your Jeep, all emit radio frequency energy, it is at a very low level, especially compared to a 5 watt output CB transmitter.

If you think that we are alarmists, please scroll down through the posts in this forum and read the posts about CB installations and and the issues.

We are just trying to give you the benefit of our experiences.

BTW I have successfully operated 100 watt Ham Radio transmitters in vehicles for many years without destroying any of the vehicle electronics (or myself).
 
#22 ·
Damn dude, come down from the ledge. :) It's not that hard. Installing a CB is a great beginner project if you want to learn how electronics work in a vehicle. I see no reason to give up before you even start. Chances of you "destroying" your vehicle are nil. Take a deep breath.

My 2010 wrangler had every possible issue and problem you can imagine with the cb. My troubleshooting caused my to check, double check, and triple check every possible part of the system, and in the process I learned a lot. I never even came remotely close to destroying anything though. Just my sanity. ;-)
 
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