I’m replacing the original radio with a new one but I’m having some problems. 72’ 350sl (euro)

Car’s wiring is as follows (I think):
Brown - ground
Blue - ? (wiring diagram for 74’ 450 shows it going to the Auto Antenna, must have been an option but was plugged into the Becker even though the car had a manual antenna)
Green/Red – ACC? (diag shows going to fuse 6 which also services the Ignition Switch (ACC)
Orange (not part of the wiring harness) – goes to an ad on power antenna module, does not look to be OEM stuff

New radio is straightforward:
Yellow - battery or 12V constant
Red – ACC
Blue – power antanna
Black - Ground

My problem is that when I connect the obvious, that leaves me with the radios yellow and the blue from the harness but I don’t get any power to the radio. When I cross the yellow to the Red/Grn & Red ACC connection I get power (when key turned to ACC).
So really my question is…Is there supposed to be 12V constant from/with the wiring harness? The becker wound not turn on without ACC on.

Am I going to have to run a wire to the battery for the Yellow? That just doesn’t sound right.

Any ideas would be greatly appreciated.

BTW: Tonight I’ll put a multi-meter on them with and without ACC on.

MM results: red/grn - as presumed, hot when ACC on

I don't think I should run the Yellow straight to the battery, it just seems jerry rigged. Can someone recommend a better or preferred location to get constant power?

Check the fuse panel and see what circuits are hot without the key in the ignition (such as the clock). Your new radio only needs the constant hot lead to retain the memory functions, so it doesn't need to be a high-amperage circuit.

Run the yellow lead to the fuse panel if possible. Splicing into an already known hot lead can also work, but I would caution against this if it happens to be an alarm circuit...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

I'll by that, thanks

Note to self - when finding hot lead make sure door is not open!

If you are putting in a late model high powered radio then please check the fuse on the yellow wire before you just hook to an existing circuit, usually there is a 15 amp fuse on this line at the radio that is why I told you to run the wire directly to the power distribution block under the hood.(with a inline fuse at the block) This is really easy and it will save you the time tying to hook up to the fuse box. My 2 cents as I do this job every day!

Scott@lajollaaudio

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The Mercedes Specialists Since 1986

good point, that sounds even better, thanks

BTW: I did a quick search and found your post ref. the PDB up front. That was for someone else. Guess I should have done a better search initially, and I would have seen your post.

Ia m sorry as I thought it was you that I had helped before but I was mistaken.

Scott

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The Mercedes Specialists Since 1986