Anyone understand 85 w123 wiring for radios? - PeachParts Mercedes-Benz Forum

JHZR2 · #1 09-08-2009, 11:18 PM

Hello,

Im working on an 85 300D, which came originally with the digital becker radio, not the old style analog, manual dial radio. The wiring is all hacked, and Im trying to get things worked out with a new radio.

In the older cars, the radio was powered by the harness that connected to the ashtray and gave it power. What Im finding is that the ashtray harness is now two hot leads, no neutral. I used an ashtray/radio harness from an older w123, and the wire I thought was neutral was hot - causing a short circuit and blowing a fuse.

What I don't get is how the ashtray is grounded. It appears (because I was blowing the #1 fuse when Id turn my lights on), that the ashtray has one hot that is switched, and one that is controlled by the lightswitch - an illumination hot lead. I dont think that the older cars had this.

Does anyone know about this? Did 85 cars share that same harness for hot wires? How does the ashtray get its ground, given that it has two leads and both are hot?

Other than me creating an artificial short, the car has no electrical problems, blows no fuses, and has all systems working. Any thoughts?

Thanks!

#2 09-08-2009, 11:40 PM

According to my w123 wiring diagram, there is red hot wire and a brown ground to the lighter and the ashtray light. These wires are also shown as feeding the radio. It does not appear that the brown wires should be hot.

ah-kay · #3 09-09-2009, 02:57 AM

My 84 190D radio wiring was butchered up also by the PO. Most old MBZ radio wiring and connector are pretty much the same. I measured them out and managed to put an after market radio in. I think brown wire is always chassis ground. Take a look at my post #6 below.

http://www.peachparts.com/shopforum/showthread.php?t=260074

A wire with a black sheathing is the antenna activation wire. It goes to the antenna switch then to the antenna itself. The ash tray is grounded by the metal chassis. IIRC, there is only 1 hot wire goes to the back of the ash tray female connector.

ImBroke · #4 09-09-2009, 07:31 AM

There is a switched 12v back there somewhere, if I was home I could tell you which one it is. Wont' be back for a week though. Good luck. Oh, I ran all new speaker wire though, no sense dealing with the fader mess.

JHZR2 · #5 09-10-2009, 09:36 AM

OK, got the problem...

It is actually correct that the ashtray has only a single 12V lead, even on an 85.

The problem was the other radio lead that the 85 cars have, which relates to the digital becker radio (which was not installed in the older cars). There is another harness, which uses a standard MB 2 round pin connector, which gives a constant 12V and an illumination 12V signal.

What I did wrong was that I tested these leads with the lights turned off. Turned off, I guess the lightswitch acts as a path to ground, so the red wire in the group is +12V, and the other lead acted as ground. When the lights are turned on, there is 0V between the two lines, as both are energized.

What happened was the radio worked perfect in all conditions, and the fuse #1 would blow when the lights went on. I thought the other lead in this 2-pin connector was ground, but it would energize with the lights on. The chassis of the radio is neutral, and I was attaching what I thought was a ground to it. Turn on the lights and there is a direct short from the lightswitch to the ground via the radio body.

Fixed that and all is well...