Need "constant power source" for aftermarket headunit - Mercedes Benz SLK Forum

SLK R170 General Discussion SLK 200, SLK 230K, SLK 320, SLK32 AMG

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#1 Old 06-11-2019
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Need "constant power source" for aftermarket headunit

I installed a pioneer radio in my 02 slk32. Everytime i start the car the radio resets to 12 and i have to go through the setup process again. I checked the constant and switched 12v power going to the radio. The switched works as it should, the constant drops from 12v to about 3v over the period of about a minute once the key has been removed from the ignition. If you open the driver door the constant goes to 12v again, close it and it drops to 3v. I did some research on this already some people say to swap the constant and switched power on the aftermarket radio harness but obviously that wouldnt work in this situation, some also say tap into the cigarette lighter for constant 12v but the cigrette lighter only gets power when the key is on. Is there something wrong or do i have to find another constant 12v source.
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#2 Old 06-12-2019
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Originally Posted by 02slk32amg View Post
I installed a pioneer radio in my 02 slk32. Everytime i start the car the radio resets to 12 and i have to go through the setup process again. I checked the constant and switched 12v power going to the radio. The switched works as it should, the constant drops from 12v to about 3v over the period of about a minute once the key has been removed from the ignition. If you open the driver door the constant goes to 12v again, close it and it drops to 3v. I did some research on this already some people say to swap the constant and switched power on the aftermarket radio harness but obviously that wouldnt work in this situation, some also say tap into the cigarette lighter for constant 12v but the cigrette lighter only gets power when the key is on. Is there something wrong or do i have to find another constant 12v source.

If the radio is going to dump every time the power drops out what other choices are there? Have you consulted a auto audio tech? I suppose you could come off the back of the starter motor.
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#3 Old 06-12-2019
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If theres no where to tap into near the radio i think the obvious choice would be to the positive terminal of the battery but i am really trying to avoid having to go through the firewall into the engine compartment with this.
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#4 Old 06-12-2019
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Originally Posted by 02slk32amg View Post
<snip> .. some also say tap into the cigarette lighter for constant 12v but the cigrette lighter only gets power when the key is on. Is there something wrong or do i have to find another constant 12v source.
I can't give you a definitive answer but the heater fan has a constant 12v ..
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#5 Old 06-12-2019
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Hi,

There is definitely a Constant Batt 12V + at the original H/U plug, and also an Accessories Position 12V + ..................

A small test lamp will prove, as I can't definitively remember the Colours right now but if test lamp says no, then check your Fuses, 'cos I know they are there on my 02 V6 .........................

IIRC on the Car is Yellow Red which I believe is Acc + , and I think from memory 12V Batt + is Red, Earth is Brown and do not forget to connect Blue /Black to the Pioneers Aerial wire (Blue at H/U ?) or your Radio Antenna amp will likely not work as it is "Wake Up Line"

Pioneers are noted for having those 2 connections back to front, (Red = Acc + and Yellow = Batt + on Pioneer Plug), Pioneer usually have 2 bullet connectors in their own wiring so you can swap the red to yellow in 5 seconds flat !!

That is what is wrong with your install

HTH, Cheers Dave
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#6 Old 06-13-2019
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Thanks for the information, however this was not the answer. The red switched wire and yellow memory wire were correct to begin with. The red switched wire (which was red on both the aftermarket harness and the pioneer harness) was getting power only when you turn the key to acc or on so that is correct. The yellow memory wire was also correctly matched up on the pioneer harness and aftermarket harness adaptor, however 12v power only stays on this wire (when car is off) with the drivers door open, as soon as you close it it slowly drops from 12v down to about 3v over the course of about a minute. It must be some kind of electrical gremlin. I did switch the wires around just to see and it didn't help.
I was able to figure it out however. I searched around for a constant 12v power source and i found one wire coming out of the hazard light switch (see pic) that had it, so i tapped into that and ran that to the yellow memory wire on the aftermarket radio harness and now everything works perfect.
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#7 Old 06-13-2019
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I forgot to mention, I did check every single fuse in all 3 fuse boxes and all are good.
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#8 Old 06-14-2019
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Hi,

I am glad you are happy with your fix

However you haven't really fixed the problem, only the symptom ...................

Your Car / Stereo Wiring has an Electrical Issue, personally I would want to find and rectify that, because a decent Head Unit will be pulling a fair few amps on a circuit designed to run max 94w of yellow bulbs

A decent head unit with a real 4 x 50w will pull around 17 Amps flat out

HTH, Cheers Dave
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#9 Old 06-14-2019
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Originally Posted by Dave2302 View Post
Hi,

I am glad you are happy with your fix

However you haven't really fixed the problem, only the symptom ...................

Your Car / Stereo Wiring has an Electrical Issue, personally I would want to find and rectify that, because a decent Head Unit will be pulling a fair few amps on a circuit designed to run max 94w of yellow bulbs

A decent head unit with a real 4 x 50w will pull around 17 Amps flat out

HTH, Cheers Dave
If I'm understanding his post, the hazard wire is only powering the memory and the main power source is still switched so it should be enough to handle the hu. I do agree that it's an odd problem that should be invesitgated but it seems the solution will function.
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#10 Old 06-14-2019
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Hi,

I'm no ICE Expert, but I understood that the B+ was the one that had all the load including internal Amplifier, and the Switched Acc + is just a "Wake Up" line ..................

This is certainly the case with my SLK's recent Android install with Active Sub

I may be wrong, have been lately, probably because my head is else-where in V8 conversion mode right now

Cheers Dave
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#11 Old 06-14-2019
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Damn, i thought the yellow wire was just for memory and didn't pull much power. I guess you are right i double checked the connection at the back of the radio and the yellow wire is a bit thicker than the red one so it must be the main power source, However even the yellow wire at the back of the radio is not very thick at all it seems to be the same thickness as the hazard wire that i tapped into.
I tested the radio at max volume with the hazards lights on for several minutes and everything seemed fine.
I also tested the memory wire again from the factory harness (This time not connected to the radio) and it drops down to 0 volts (instead of 3) much quicker when it is not connected to the radio harness,
but again it only stays powered with the driver door open. I guess when the memory wire from the factory harness is connected to the radio, there is some kind of capacitor or something inside the radio that slowly looses power.
So what should i do put the harness back like it was, just deal with the clock resetting each time and wait till i find the proper fix or just leave it like it is.
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#12 Old 06-14-2019
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Also, i just looked up the wire harness i have (Metra 70-1786) and on metra's website it says its for either a slk230 or slk320, when i put in slk32 no harness comes up on the list, is it possible the slk32 factory harness functions differently somehow then the 230 or 320's
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Hi,

I'd leave it as is short term, bearing in mind your test you did

I'm gonna go out on a limb here .....................

Disclaimer :- I have never dabbled with a 32, and as said, I'm no ICE Expert .........................

but ...........................

Some, well, a lot of MB's I work on including my own S 55, my mates CL 600 which are similar vintage, plus Mels C Class which is newer, they have a wiring system that works over the Body CAN (no Acc wire) on their standard MB COMAND set up .........................

When you switch off ignition, the Stereo stays playing until one of these 3 scenarios .......................

1) Battery Voltage gets below a preset value

2) A time period, (half hour IIRC elapses)

3) You open the Drivers Door

....................... At which point the Unit powers down, switches off etc etc ....................

So perhaps the SLK 32 was Wired this way ????

If you give me your VIN I'll have a look at some point on MB WIS Wiring Diagrams to see if it is wired any different to my own 2002 SLK 320 V6 Car.

On that note, I was actually thinking of putting a 20 minute timer relay in the Acc + circuit on my SLK, because the Android has a long boot time, (that's their only disadvantage as far as I can see and we have 3 of them in our Cars, the C Class and S Class both work on Body CAN and boot up quicker, but the R170 has not got a suitable CAN, that's why I had to hard wire everything, but I simply used the Cars existing Batt+ and Acc + that the Car had at it's Plug) ....................

So for my R170 example if I re fuel, I switch off Engine, Android powers down, come out of paying for fuel and drive off, it takes a period to all boot up again, about twice sa long as the other 2 Cars, so a timer relay would leave it running, and I could just hit the pause or mute as I got outta the Car .......................

Now I confess, I'm going to check all the many settings Menus, because there may be a "Delay Shut Down" option or "Sleep Mode" rather than "Power Down" in one of the many many settings I've not played with yet ......................

Yeah, it would be really hilarious if I wired up and installed a Timer Relay etc only to find the H/U will do that anyway So best I have a good look first eh

HTH, Cheers Dave
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#14 Old 06-14-2019
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Quote:
Originally Posted by Dave2302 View Post
Hi,

I'm no ICE Expert, but I understood that the B+ was the one that had all the load including internal Amplifier, and the Switched Acc + is just a "Wake Up" line ..................

This is certainly the case with my SLK's recent Android install with Active Sub

I may be wrong, have been lately, probably because my head is else-where in V8 conversion mode right now

Cheers Dave
It's possible I need to 'stand corrected'. Used an assumption over a fact. The good news is I learned something and my ignorance didn't result in a social media battle that seems quite the fashion in this day and age. Thanks.
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#15 Old 06-15-2019
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Hi @Mn car guy ,

Thanks for that, I actually don't think either of us were wrong the more I think about it ......................

Older Single Din style head units did just power the memory from B + (again IIRC)

The newer stuff with processors etc actually work the other way round, the B + is actually it's main power, and the Acc + , (or even no Acc + so CAN Bus connections on really later units like my Androids I play with these days), is what actually just gives it a wake up call

No one will ever (well very rarely) get a battle from me, life's too short, I also have to think of others on forums, and a lot of times guys are just doing their best to help someone, as we both are here

I can at times sound a little blunt or abrupt, I don't mean anything by it, it's just the way I have always been............. Say it how I see it
Trouble is with a keyboard sometimes the words do not look on screen like they would in a conversational banter going on between 2 petrol head guys in a workshop

Often times I'm busy and just rattling out a message to try and give someone an answer, and yes even though I have been in the motor trade for 42 years + and am an MB Indie self employed Garage Owner I do not know everything, far from it and like a lot of us I learn something new every day ....................

Anyway, cheers,

Dave
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#16 Old 06-15-2019
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For a R170 I show Fuse 5 - PK/YL/DG to be hot in ACC and run and Fuse 8 RD/DG to be hot all of the time.
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Well I decided to return the wire like it was originally in the harness, I don't want to risk something melting/starting a fire by hooking up the constant power wire somewhere else. Its not worth it to have the time not reset on the radio when i can just use the clock that is on the tachometer anyway. I made a video of what is happening, if someone can figure out why this is happening and how to fix it then great if not ill have to live with it.

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#18 Old 06-15-2019
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This is the schematic for the Bose system. Note the amp has only Left and Right inputs, no front/rear.
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#19 Old 06-15-2019
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Yes, you can see on the schematic RD/DG which i believe is Red and Dark green this is what i have my voltmeter plugged into on the video, so it should be hot at all times but instead i'm getting all these weird voltages that eventually drop to near 0 after the car is off.
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#20 Old 06-15-2019
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Hi,

I believe what you are seeing is feedbacks as various Modules go to sleep, a multimeter places no load, on a power wire you should really use an old fashioned 5w Sidelight bulb as a test lamp, that is a far more accurate test as it should light brightly. On more hefty stuff like pre fuses I use a 21w Indicator Bulb

Look at where the Wire goes, see if it goes to a Fuse, via a connector maybe etc, check any connectors fuses etc, agin do not use multimeter, physically remove fuses and make sure connections are good

I'm gonna get told off, Mel want's her Chinese, but looking at that Schematic it runs from H/U direct to Fuse Box under hood

HTH, Cheers Dave
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