Power constant not hot enough

i'm re-installing a head unit in my '83 Mercedes 240D. someone ripped my other out, so i figured another install shouldn't be so difficult, beings the wires are all right there freshly disconnected.   ;(

i hooked it all up, seemed to go well, then i realized i didn't hook up the power constant correctly...i lose my settings/clock each time i turn the car off.

so i go to find my power constant with a voltmeter, and basically i find only ONE wire that and it seems to be too weak for the head unit to run on. i compared it to the ignition power and it's hardly 1/3 the power outtage.

i'm searching like mad for another cable to use for constant, and i can't find anything else!!

anyone ever see a power constant go weak? is it a problem with my electrical system somehow? i'm totally lost and very bummed i can't use my new head unit.

That might be the dimmer wire.  Make sure a fuse isn't blown from when they cut your wires.

Is your car in the vehicle wiring section of this site (in the left column)?

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That's my opinion. Take it, leave it, or correct me.

thanks guys.
i checked the fuses, it's these mega old school type and they all look OK to me. worth a double check.

my car is on the wiring section, but it doesn't seem to tell me much, other than there is no dimmer, but you're probably right that the "constant" power pumping only about 3-4 volts probably isn't the right constant anyway.

https://www.the12volt.com/installbay/detail.asp?info=alarm&year=1973-83&make=Mercedes&model=240/280%20Series&ID=18336&type=Stereo

i just can't see why i'm unable to find any other wire. unless it was one of the wires i've found and somehow is dead now they've ripped it out...

assuming you found the correct wire, a bad connection or bad wire for that matter will give a low voltage reading.

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Prove your connections, use a meter!
I promise, I'll behave!

damn, i guess my only options now really are to either take it to a shop and pay someone else to deal with it, or try to run my own power constant.

you can do that, right? just run a wire from the battery direct?

problem is its REALLY difficult to get through my firewall without drilling, and it's a really tough wall, i don't have a drill for it in either case ;(

Yeah, you might just want to take it into a shop.

trace the suspect wire to the fuse box and measure the voltage there. if you see 12 volts there go ahead and replace the wire.

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Prove your connections, use a meter!
I promise, I'll behave!

electrostatic wrote:

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trace the suspect wire to the fuse box and measure the voltage there. if you see 12 volts there go ahead and replace the wire.

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that's a great idea, but too bad i would have to tear the crap out of the whole dashboard to follow that wire. also, this super old school fusebox is might confusing as is.

i just called a shop...i got a feeling it's going to be a tough fix.

Try just getting your constant 12volt from a lead you use for an amp.

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you cannot get taller by hammering on someone elses head.

arniek wrote:

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Try just getting your constant 12volt from a lead you use for an amp.

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you mean the blue/white cable that comes from my sub's amp?
i thought that was getting power from the head unit?

or is that cable itself giving power to the head unit?

in either case, i'll try. that may be how i did it before. something just isn't right here.

lastly, i was thinking to go to the fusebox, try to measure volts of each fuse, find one with a similar weak 3-4V strength, remove it and make sure the cable also lacks power, then replace that fuse.

these old school fuses are weird. sometimes they look fine, that is, no break, but they run so damn thin, it looks suspicious to me.

What he means is if you have an aftermarket amp installed, you already have constant power running to it.  You can fuse there, and run a lead up front to the radio.  You can also test the wires at the ignition harness and see which is constant.  Either way you won't have to run all the way to the battery.  Be sure to fuse close to where you connect for power wherever you do.  Just use some 16 or 18 gauge wire and a 10 or 15 amp fuse will do fine.

Also, any wires you don't use behind the deck need to be insulated (use tape, crimp caps, etc.).

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My ex once told me I have a perfect face for radio.

I have sometimes been in the habit of soldering or wrapping the constant and the battery lead together so the unit can work without the key in the ignition, not sure if you want to do it that way.  If you are going to cowboy it and run all new wire, be sure to have fuse along the line(s) of current... if not going through the fuse box.

Mercedes are a huge hassle, just to even get to the damn dashboard speakers it is tough.  First installation I failed.  I let my friend take it to the shop cause I did not want to mess it up any further.  German engineering is too advanced for it's own good.

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I have not failed. I've just found 10,000 ways that won't work.

When you do not know what you are doing and what you are doing is the best -- that is inspiration.

His problem is not with the switched wire, though, so connecting the two together would not allow him to turn it on with the key off.

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My ex once told me I have a perfect face for radio.

well, thanks everyone here, this forum is awesome, but my PROBLEM IS SOLVED!
long story short, i think i've got some minor electrical problems. pulling the fuse connected to that power constant out and back in a few times and it seems to hold a steady 12volts finally.

the fuse is fine, it just needed to be taken out a few times, and seemingly random, now it pumps out a good 12v.

i have had some funky electrical issues lately, so it all makes sense. anyway, it may not work perfectly for long, but it does now and at least i know where the problem is.

thanks all here. this forum is great. i'm lovin my system once again.

now i just gotta remember to keep taking out the faceplate from my car, or someone will jack my doodie again!