81 240D question about stereo power

[pj67coll]

Can anyone tell me if it's necessary for the constant 12V power to be hooked up to a stereo before it will get power? I assumed it would get power with just the keyed 12V hooked up when the key was in position 1. But I get nothing.

- Peter.

[Stretch]

Originally Posted by pj67coll

Can anyone tell me if it's necessary for the constant 12V power to be hooked up to a stereo before it will get power? I assumed it would get power with just the keyed 12V hooked up when the key was in position 1. But I get nothing.

- Peter.

It depends on the stereo. Some expect a constant supply and a connection via the ignition. Have a look on the back of the unit - sometimes there's a wiring diagram.

[pj67coll]

Originally Posted by Stretch

It depends on the stereo. Some expect a constant supply and a connection via the ignition. Have a look on the back of the unit - sometimes there's a wiring diagram.

I've already done that. I had hooked up the keyed 12V to the original radios 12V power supply, that's hot when the key is turned on. But I got no sign of life in the radio when I turned the key to position one. It has a yellow wire for 12V constant which is for the memory presets. I intend wiring that direct to the battery but not sure if it should be to the + or - terminal so have not done so yet. I assumed that I'd at least have power to the radio when the ignition is on as it's got 12V at the red cable, checked with the meter.

- Peter.

[jay_bob]

Most modern radios actually draw all their power from the "yellow" constant wire and only use the "red" switched wire as a power-up signal.

On the later 123s that had the option of the Becker 612 electronic radio, there is a 2-pin plug laying loose under the radio pocket. This connector has constant power and instrument illumination voltages on it.

Depending on how powerful the amp is in your radio, this constant lead may or may not be enough to run your radio. There is only 1 unswitched fuse in the 123, it is fuse 2, and it has the dome lights, clock, antenna motor, and this radio connector on it.

The standard 599 radio only needed switched power and ground. The bulb in the radio burned all the time that the radio was on. The 612 needed all 4 wires to operate.

[pj67coll]

Originally Posted by jay_bob

Most modern radios actually draw all their power from the "yellow" constant wire and only use the "red" switched wire as a power-up signal.

On the later 123s that had the option of the Becker 612 electronic radio, there is a 2-pin plug laying loose under the radio pocket. This connector has constant power and instrument illumination voltages on it.

Depending on how powerful the amp is in your radio, this constant lead may or may not be enough to run your radio. There is only 1 unswitched fuse in the 123, it is fuse 2, and it has the dome lights, clock, antenna motor, and this radio connector on it.

The standard 599 radio only needed switched power and ground. The bulb in the radio burned all the time that the radio was on. The 612 needed all 4 wires to operate.

Thank's for the input. My old radio was a 599. It had the switched power and ground but I didn't find any other leads lying loose anywhere.

I just got my new radio to work. Kenwood head unit. It holds the presets and functions fine. Just have one more question about this. How does the power antenna work? It has the antena up/down switch at the top of the center console. There was a black cable on a spade connector out the back of the old 599 that runs up to this switch. Is that how the hirschmann was powered? Thru the radio itself? Because right now it's not going up or down at all. I'm assuming I need to connect the power antenna cable from the back of the Kenwood to this cable?

- Peter.

[pj67coll]

It's all working. The Hirschmann is wonky and I'll have to work on that, but it's powering up and down now that I've connected the Pwr/Ant cable, pale blue/white on the Kenwood to the black wire going up to the antenna switch in the console. Time to rip it all apart and start re-assembling the interior.

- Peter.