So I'm trying to install a new stereo HU in my car and have run across a problem. I need a good place to tie/splice into an always live 12v line, preferably as close to the battery as possible without actually having to run a line through the firewall.

I tried tying something into the fusebox/terminals, and gave up after a couple hours of being upside-down. I can't use anything from terminal 2 because there is a short somewhere in that line. Any ideas?


I drive an 84 300D

There should be a spade terminal on the back of the tach/clock marked "+12".
This is the always on +12V that runs the clock. At least I can say it's always
energized, even with the key off.

__________________
Bob
'82 300D Petrol B-G Metallic

Try browsing in the Media section, plenty of info there.

Or, assuming an unmolested factory harness still in the car, it ought to look like this one:

http://www.tehnomagazin.com/Auto-radio-car-connector/Becker-Car-Radio-Wiring-Connector.htm

That wire draws power from fuse 2, so your short may actually be in the old harness or in the console.

A bit more info helps for advice/troubleshooting: how much power are planning on drawing through it? You've got a switched line selected already? Are you bypassing the fader?

You can unscrew the fusebox and work from above, too.

I keep the presets on my stereo by running the memory wire to a splitter off of the clock.

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'81 300SD

There are both 12V+ constant "hot" and 12V+ switched "hot" terminals on the headlight switch also. The nice thing is that the connectors are captured Philips head screws so you know you're getting a soild electrical connection all you need to do is use a ring terminal on the end of your power wires.

thats the route i am going to take whenever i find the time to install my HU

I can't ... or won't use anything on terminal 2 because finding the short is out of the question at the moment. I tried unscrewing it from above, which resulted in me spending an hour trying to put it back because I still couldn't reach anything. For the selective live, I plan on just using the one from the harness, as I don't plan on pulling anything from it.

Well, the head unit says 50Wx4, so I thought it was going to pull 200Watts at max (not that I would do that), or at least make provisions so that if I ever DID want to, I could, but the HU has a built in 10amp fuse... well, 12V at 10 amps, that's only 120 Watts, so how in the world can it outpud 200W? I don't know.

I by every means plan on bypassing the fader, but that isn't really the issue here.

The problem, if it could be called that, with my radio, is that the always live line is the thick wire, and it has the inline fuse. The selective live, or ignition live or whatever the crap it is called, is a tiny little wire. I am afraid of pulling too much juice through an existing wire for say, the clock, and having my car burst into flames like somebody else's did recently.

If ye wise ones believe that running these lines from the headlight switch is fine for the load, then that would save me many headaches, I just wanted to know if it was safe before I turn on my headlights, and then all of a sudden my radio, and lights, go out (eats the fuse).

On many cars, I've run a fused power cable off the + battery terminal to a power distribution block. Then, run wires to power amps which normally fit under the seats or in the trunk and to the antenna if your stock antenna system needs to be replaced. See if the cigarette lighter is constant or switched. I usually make a wire with terminals that plugs into the stock harness so I don't have to cut the harness. It makes removal easy in case there is ever a short and I want to eliminate aftermarket items from the system.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

You should have a constant +12Volts in your original stereo harness.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles

If you're willing to consider running a line through the firewall without drilling, I'd look at the A/C grommet under the passenger side footwell. It's a large oval grommet with 3 hoses running through it. It exits the firewall immediately under the battery tray. And if yours is in a comparable condition to mine, it ain't in great shape anyway and is pretty ragged in the engine bay.

I think you can see it from below the car, or you can pull the battery and the battery tray and it's right there. It's clearly visible from the pax footwell after you pull the plastic trim.

You should be able to push a relatively stiff good gauge wire from the inside of the car to the engine bay, run it straight to the battery and put an inline fuse right there. You'll need to find some connector/adapter for the battery terminal/post; but I think those are readily available at parts stores.

I think the inboard-most hose is just the drain tube, so there shouldn't be any temperature or shortcircuit concerns. You could fill any gap formed by the extra wire through the grommet with shoe-goo or some RTV sealant.

Voila - a fused constant power line, no drilling needed, and it's on the 'clean' side of the dash.

I'd be a bit gun shy of adding extra demand onto the light switch.

I'm still waiting for the Crutchfield adapter for my harness.

Agreed. This was the case with both of my 126 300SD's. There most definitely should be a 12v constant in the harness. As well as a switched 12v and ground. All the wires should be there to use.

I dont have a FSM for a 123 to pull pin information though. You should be able to just probe each pin on the harness and check voltage to ground.

Furthermore: to answer your question about the 50x4, 120watts item. The 50x4 is not a rating that you should be really paying attention to the real rating you should be looking at is RMS value. This value is typically 16-22 watts rms per channel, largely depending on what caliber of head unit you get. These are the values that your speakers will see more times than not. Unless you get an external, trunk mounted-style, amplifier you wont see anywhere near 50 watts at each speaker

22 x 4= 88 watts.

This 50x4 and other such ratings are all over the place on car stereo equipment. Just look at rms ratings. Match those.

Also, your factory wiring should be sufficient to run a "50x4" hu. I had a "60x4" Pioneer for years in my 300SD and never gave me a problem. I ultimately installed a Kenwood amplifier in the trunk too.


For what its worth, installing an external 4 channel amp makes a world of difference in terms of sound quality, provided of course you're using good speakers.

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

There is most definitely a constant 12v+ on the harness, as well as a switched 12v+.My problem is that the constant 12v+ is goes to terminal 2 of the fuse box, which for me at the moment is not working, which is kind of why I am asking. Also, if I add in a large gauge wire, with a good source, I can run my radio, as well as amp, and other things off of that one.

I learned of a way to narrow down where the short is. Apparently on fuse 2, there are three clusters of wires which get attached via screw. I can just touch a cluster to the terminal until the fuse blows, which will tell me which cluster has the short. This is where it gets tricky though, how can I find out which of the wires on the bad cluster is the culprit without cutting into them? Trying to strip, twist/solder, and heat shrink wires in that tiny little area.... I fail to see that ever happening.

Hmm.. What all is on the terminal 2 fuse? It should say on the fuse box, I believe.

__________________
Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
Check here for compatibility, diagnostics, and availability!

(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

According to my recently acquired 123 fuse box cover ... terminal 2 is an 8 amp (white) fuse.
Terminal 2 provides protection for :
Clock, trunk light, diagnostic, roof light front and rear, hazard warn. system, warning monitor term 5, (electronic radio, autom. antenna)

Id be looking at the trunk light wiring first.

This type of meter is invaluable to us for several reasons.
Measures ohms
Measures volts
Measures amps
Extremely short learning curve as we have more important
things to do than getting a phd in meters.

It is reasonably accurate and mine have taken a beating and still work.

Lastly they are cheeeep and very available.

Attached Thumbnails

 

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

What the fuse box cover and the service manual say, as well as what reality seems to say, are all in contradiction about terminal 2. I don't know who to believe anymore.... I'll go out and see what I can do. The MB CD's say that the automatic antenna is NOT part of circuit 2.
I'll go see if I can restore functionality to part of circuit 2. Hopefully I can get the 12v constant to the HU, and I will put in a whole new line, on though the passenger's side straight to the battery for the amp, as was said earlier.