So we all know that Yellow is constant 12v and red is switched 12v.

Which one is actually expected to supply power to the unit for the internal speaker amp?

• I have seen some radios claim that the primary power comes from the constant 12v, and the switched 12v is only used as a signal wire to tell the radio to turn on.   Much as the 12v illumination wire would signal dimming of the display.

• I have also seen some radios that claim that the constant 12v source is used for memory backup only, and the switched 12v carries the power to operate the radio.

I have installed the z130bt in my 2005 Mercedes C230 (W203 chassis).   This car has no switched 12v wire in the harness, forcing me to tap into the cigarette lighter for that switched power source.  The car uses a CAN-BUS digital communication protocol, which transmits, among other things, ignition status of the car.  So I have to believe that the car's stock radio expects it's power to come from the constant 12v source, and ignition status comes in through the digital bus.    The constant 12v source is rated 10A on this car.

Since the Avic needs a hardwire switched 12v, the common way to hook these up in the mercedes is to tap into the cigarette lighter power.  The cig lighter fuse is rated 15A, and since I don't use it as a lighter I have the full 15A available to power the radio if needed.

My son's Honda Accord has both constant and switched 12v in the car's harness,  but the fuse ratings are 10A for constant 12v and 15A for the switched 12v.   I believe the switched wire is rated higher because it is shared with the cig lighter ... but the side effect of that is that regardlesss of which wire an aftermarket radio expects to supply power, in the Honda it will get 10A minumum either way.

So, my question is, which wire is expected to power the Pioneer head units, and specifically their internal amps?   And, what is the required fuse rating?   Is it only 10A, which seems to be what cars have listed for their radio fuses?

Thanks.  John

Good question. I've wondered too. I'd like a definitive answer, not someone's conjecture. You'd think it would be in the installation manual. It was for the marine radio I installed in my boat.

I think the subject was broached a while back in discussions of how to avoid a reboot when restarting your engine. I don't recall if that was ever clearly resolved.

I guess a quick test might do it. Turn on the HU and let it boot up. Then remove the fuse feeding the yellow wire. It the HU stays powered-up, then the Red (switched) is the main line power. Otherwise, it must be the yellow.

The yellow 12v constant is the main supply wire.  The red wire is simply a turn on wire.  There are many things you can look at to arrive to this conclusion;

1. The fuse on the stereo itself(normally 10a and located on the back of the radio) is for the constant source.  If this 10amp fuse blows all memory settings are lost, proving that the main fuse on the unit comes from constant power.

2.  The constant wires on aftermarket decks are usually thicker(albeit not much) than the switched wire.  If the switched wire provided the power for the amp it would be larger gauge wire.

3.  Look at a PAC audio interface harness(for GM, Chrysler, etc.).  In the manual on a majority of these harnesses it calls out the 12volt switched is only 1amp.  These interface harnesses work well when wired up as stated, and only providing 1amp of current.

4.  Look at any amp, specifically the Alpine Power Pack.  It uses a 15 amp fuse to supply the power needed.  That fuse is located on the 12v constant wire, and usually is taking signal from the radio speaker output wires.

I'm actually asking this question in part because I may be trying an install of the Alpine Power Pack on my son's Honda Accord (Pioneer DEH6400).  The install instructions for this unit suggest you run the wire directly to the battery, or tap into a constant 12v power source that has 15A capacity.  But the Honda has only 10A constant, but 15A switched (due to the needs of the cig lighter).  So unless I can up the amperage on the radio fuse to 15A, then my only option is to wire the amp directly to the battery (one of their choices).   Or ... can I run the amp off the 15A switched line?

So this all brought about the question of which wire the radio is drawing it's power from.

Without a doubt B+ (the yellow wire) is the main current carrying wire, but this is only up to a point, as you have seen due to the radio fuse not being more than about 10A typically on vehicles. Acc/ignition is just a turn-on signal. While it can be used at a lower current application, it is not typically meant for higher loads.

If you need to pull more than this, like for an amp, etc, then you are correct. You will either need to tap into your fuse box to an empty/spare port and pull less than 20A, or wire something directly to battery. Regardless, it should be appropriately sized wiring and fuse for the application.

Okay, so we are clear that the Pioneer radios (or all radios?) use the constant (yellow) 12v as their main power source.

So the Alpine KPT-445/U needs 15A power, and the radio circuit has only 10A on many cars.  Has anyone hooked up the KPT-445/U (or any other low-power amp) to the car's switched circuit for the cig lighter, which is usually 15A?

Thanks.  John

Without a doubt B+ (the yellow wire) is the main current carrying wire, ... Acc/ignition is just a turn-on signal.

Please help me review this behavior in light of the issues I'm having with amp turnoff pop --

If the head unit uses the constant 12v as it's main power supply, then when you turn off the ignition in the car, the headunit should sense that a shutdown is in order and it should shut down gracefully, i.e. cut voltage to the accessory wire, wait a couple of seconds for accessories (especially speaker amps) to power down, then shut itself down.  I would also expect some sort of indication that the unit is shutting down, like the circular arrow progress indicator you see for 10-20 seconds upon startup.   I would also expect that the head unit would wait a couple of seconds to start the shutdown process to make sure the voltage interruption isn't you starting the car.

But instead, when I either turn off the car or start the car when the radio is already running under acc power, the head unit looks like it had the rug yanked out from under it ... screen black, no indication of anything still powered, and if starting the car, it reboots again from scratch after the car starts.

This also ties into my post in the troubleshooting forum, that after turning off the car I saw the amp stayed on for a couple more seconds before turning off with a loud "pop".  I've since moved the amp turnon to the 12v switched line (with a very small pop on startup now), but am wondering if the lingering 12v on the accessory wire was coming from voltage drain from the car's antenna amp, which is also connected to the same accessory wire.  The head unit wouldn't shut down instantly yet keep the accessory wire live for a couple seconds more, would it?   I mean, rule #1 in component configurations is "amp  = last on, first off".

Thanks.  John

** Update **

I did notice  that my USB drive power sensing (flashing amber) light stays flashing for a few seconds after I turn the radio off.  Since this is connected to the radio via the USB harness plug and not the remote accessory wire, I wonder if the head unit is actually leaving peripherals powered for a few seconds after it shuts off.   Again, this is not the behavior I would expect anyone to want.

** Update **

I received confirmation from Pioneer Tech Support of Pionara's statement of constant 12v (yellow) being the main current carrying wire.   The fact that the radio goes immediately blank when you cut power to the switched 12v is just the designed behavior of the radio ... it is not an indication that the switched 12v is supplying the power (although it sure leads you to believe that).

They also said that the accessory wire should cut voltage immediately when the radio is turned off.   The fact that mine retains voltage (thereby leaving the amp on for a couple of extra seconds, enough time to give a loud "pop") may be due to capacitance in the car's antenna amplifier, which I have also connected to the accessory wire.  I am going to try switching the antenna amp to the switched 12v instead, leaving only the speaker amp on the accessory wire, to see if it behaves as it should.

I have a Sony DAB radio (Model:DSX-A510BD) and I want to connect it to a England latest DAB + AM/FM car radio universal aerial amplified roof mount mast.The aerial has 3 leads (digital,AM/FM and a red power lead).Where do I connect the red power lead to on the wiring harness?Yellow - continuous power supply or red – switched power supply??