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Radio power wiring in a Lexus RX-300 Was: Your thoughts?

troutscott

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Hi
I am installing a dual band radio in my Lexus RX-300. (.2m/70 cm transmit/receive)
I would really like to get a dedicated pair of 10 ga. wires from the battery directly to the radio but finding a way through the firewall is proving difficult.
Would you at all costs get direct power or simply find a 12v souce and keep the transmit watts down?
The radio is rated at 50 watt max output.
Thanks in advance for recommendations,
Scott
 

mmckenna

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I'd find a way to get to the battery.

Tapping into existing wiring is a great way to introduce RFI into your radio. The wiring for cigarette lighter plugs is often bundled in with everything else, including CAN BUS data, and all the other noisy things in your car. It can work, but it's often a source of headaches. Running dedicated power solves a lot of those issues and will let you run the radio at full power.

And you only need one wire. Run a #10 stranded wire from the battery positive terminal to a new fuse/breaker as close as you can reasonably get to the battery. Then run the #10 from the fuse/breaker to through the firewall and to your radio. Pull a ground from the chassis/body close to your radio. On many newer cars, there is a hall effect sensor on the negative battery lead, that's used to watch current consumption. Bypassing that by running a negative lead from your grounded (through the mount or through the antenna) and back to the battery can screw things up.

It can take some work to find a path. Look around existing wiring harnesses that pass through the firewall. Often they'll have large boots or grommets that you can —carefully— poke a hole through and run the new wire.

Back in my younger years, I'd tap into existing wiring for low current uses, like CB or scanner, and it almost always gave me issues.
 

K4EET

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Hi Scott,

First, let me ask what model antenna and where/how is that going to be mounted? 50 watts is a lot of RF energy and you want to use good quality coax, connector(s), antenna mount, etc. Don't skimp on the antenna system because RFI could wreak havoc with the electronics of the SUV. Make sure you have good RF grounding too. As for placement, keep the antenna as far as is possible from the cars electronics and signal cables.

As for the power, I would come off the battery with a fuse block. Like mmckenna said, you only need one wire since the other goes to frame ground. If you cannot get through the firewall and don't want to punch a new hole in the firewall or an existing feed-thru (wiring grommet, heater hose entry, etc.), see if you can find a path down the undercarriage, sometimes with cables inside the main structural frame on the side under the doors, leading to the rear storage area. Lots of times there will be plastic or rubber drainage plugs providing access to the rear interior. Once inside the vehicle in the rear compartment area, route the power cable under the carpet back up to where the radio is mounted. Be careful to not mash the cable or place the cable where weight will be subjected on top of it. You might have to go with a larger gauge wire but you can calculate that out based on resistance of the wire, maximum DC current draw and the maximum voltage drop that can be sustained before the DC Volts into the radio drops below specification.

Going through the firewall is best. For other routes, be creative. Finally, whatever you do, be forward-looking for things that would cause "rubbing" on the cable. Even though you will have it fused, it is obviously best to try to avoid anything that would be abrasive to the insulation causing a short to ground.

Let us know how you make out.
 

troutscott

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Thank you for the good information.
I will run a power lead direct from the battery.
I have a Comet lip mount on the back hatch paired with a Comet dual beam antenna.
Thanks again,
Scott
 

AK9R

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You've gotten good advice above and I won't contradict it. In my experience, there are often soft grommets in the firewall that you can usually get through. Of course, getting to those grommets in a cramped engine compartment or finding those grommets behind the dashboard can be a challenge.

All that said, I will offer anecdotal evidence that you might be able to "get by" with another approach. I've owned two Ford SUVs that had cigarette lighter sockets, aka auxiliary power sockets, somewhere in the rear of the center console.

In one of them, I simply plugged a mating plug into the socket and ran power from that plug to the radio. I do not recommend this approach and here's why. I was very careful to only run low or medium power on the radio so as to not draw too much current through that plug. I did run high power a few times. I later examined the plug and found some melted parts inside the plug. The heat generated across the contacts of a auxiliary power plug/socket while handling high current had melted the plastic in the plug. I'll say it again, I do not recommend this approach.

In the second vehicle, I got into the wiring inside the console and attached Anderson PowerPoles to the two wires going to the auxiliary power plug. I come off of those PowerPoles to my radio. This approach has been working just fine even at high power (drawing about 12 amps) for about 5 years now. Did I get lucky with this approach? Yeah, probably. @mmckenna is probably about to tell me that what I'm doing is wrong and I keep telling myself I'm going to install more permanent wiring, but I've never gotten around to it.
 

mmckenna

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Yeah, probably. @mmckenna is probably about to tell me that what I'm doing is wrong and I keep telling myself I'm going to install more permanent wiring, but I've never gotten around to it.
Hey, if it works, it works. Some cars seem to be a bit quieter than others.
 

chief21

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I would really like to get a dedicated pair of 10 ga. wires from the battery directly to the radio but finding a way through the firewall is proving difficult.
Not sure if the RX300 is the same, but I've wired up two RX350's by bringing the DC feed from the battery, across the vehicle to the right side, then through the firewall via a soft grommet used for the AC line in the general vicinity of the glove compartment. If you use a stiff wire or fish tape you should be able to pierce the grommet easily from the inside, then push the wire through until you can reach it from under the hood. Once you pull your wire inside the passenger area, it's an easy shot along the inside firewall from the passenger side to the console area or the drivers side dash.
 

n5ims

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Jul 25, 2004
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In one of them, I simply plugged a mating plug into the socket and ran power from that plug to the radio. I do not recommend this approach and here's why. I was very careful to only run low or medium power on the radio so as to not draw too much current through that plug. I did run high power a few times. I later examined the plug and found some melted parts inside the plug. The heat generated across the contacts of a auxiliary power plug/socket while handling high current had melted the plastic in the plug. I'll say it again, I do not recommend this approach.

In the second vehicle, I got into the wiring inside the console and attached Anderson PowerPoles to the two wires going to the auxiliary power plug. I come off of those PowerPoles to my radio. This approach has been working just fine even at high power (drawing about 12 amps) for about 5 years now. Did I get lucky with this approach? Yeah, probably. @mmckenna is probably about to tell me that what I'm doing is wrong and I keep telling myself I'm going to install more permanent wiring, but I've never gotten around to it.
One clean option for the lighter socket to Power Poles is this -->
CigBuddy, the portable cigarette lighter plug to Powerpole adapter <--. You can also find premade versions that have the plug with wires attached that have the Power Poles on them --> Amazon.com: Valley Enterprises Cigarette Lighter Plug with Anderson Powerpole Connectors, Length 6 Feet: Automotive <--. Since the price is about the same, it's up to which might work better in your situation. Just take the above warnings about current draw seriously when using the lighter socket to power a radio.
 
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