View Full Version : Radio ACC to Aux Battery

04-07-2008, 02:03 AM
Note: This mod only applies if you rewired your aux battery to your cargo lights, radio, and cigarette lighter. The diodes are used to isolate the two batteries.

When I rewired the cargo lamps/radio/cigarette lighter to the aux battery (http://sprinter-source.com/forum/showthread.php?t=2360), the only thing left to do was what to do with the ACC line on radio. I like the fact that I can turn the radio on/off using the ignition key, but I didn't like the fact that I had to leave the key in the ignition to use the radio.

I came up with an extra switch when I was working on my rear fog lamp project. I thought it would make a great switch for the Radio ACC. The only problem was it was just SPST (Single-Pole, Single-Throw) switch. A SPDT (Single-Pole, Double-Throw) switch would have been better, but I wanted to use the Fog Lamp switch because it was available. I started to noodle around the electronic possibilities so that I could use the switch. I was about to use a relay which would make the wiring job more of a hassle, but I'm glad I found a simpler way.

A SPDT switch (left circuit) would have been the ideal switch to use in a two battery setup. Toggle the switch to select which power source to use for the Radio ACC. Only problem was the OEM Fog Lamp switch was a SPST switch.
By using a couple of diodes (right circuit), I can now use the OEM Fog Lamp switch for my purpose.

If I didn't use the diodes and I had both (ignition and fog lamp) switches closed, the batteries would start charging/discharging each other. Not a good thing long term if one battery becomes weak. The diodes act like an OR gate, providing power to the Radio ACC either from the Main 12V or the Aux 12V.

It's definitely not like a SPDT switch where I can decide where I would like to source all my current from. If two switches were closed in the diode scenario and the batteries were equally charged, an equal amount of current (about half from each battery) would be sourced. Likewise, more current would be sourced from the more powerful battery.

Fog Lamp Switch on the left.

New Front/Rear Fog Lamp Switch in place of the old Front Only Fog Lamp Switch.

04-07-2008, 02:03 AM
Reusing the the old connector, since I'll be using a new connector for the New Front/Rear Fog Lamp Switch.

In the picture below, Red/Yellow is power for the Radio. Normally from the Main 12V, but after doing this mod (http://sprinter-source.com/forum/showthread.php?t=2360) power is now coming from the Aux Battery. The Black/Red wire provides 12V to the Radio ACC from the ignition key.

The bundle of wires behind the gauge cluster. Gauge cluster was removed for this mod.
Black/Red wire was cut to solder a diode in series. The other diode branches off (green wire) to the output of the Fog Lamp Switch. The other green wire soldered to the Red/Yellow line (at the connector) supplies Aux 12V to the Fog Lamp Switch.

Heat shrunk some tubing and tie wrapped the wires neatly.
Neatly stuff the wires back into the dash and reinstalled the gauge cluster.

The Fog Lamp Switch and its connector is relocated to the right side of the steering column.
Jumpered and soldered a couple wires to the dash illumination wires on the rear defroster switch connector.

Here's what I have now:

1. Fog Lamp Switch OPEN, ignition ACC Closed; Radio ACC current sourced from Main 12V battery.
2. Fog Lamp Switch CLOSED, ignition ACC Open; Radio ACC current sourced from Aux 12V battery.
3. Fog Lamp Switch CLOSED, ignition ACC Closed; Radio ACC current sourced from both the Main and Aux batteries.

I may swap switch positions though.

I wished I had done it sooner. Now I can use my ignition to turn on/off the radio or I can use the radio without leaving the key in the ignition.

03-04-2017, 12:59 AM
This is a great idea. Does anyone know of a SPDT switch that matches the factory look?

03-04-2017, 05:53 PM
This is a great idea. Does anyone know of a SPDT switch that matches the factory look?
You can always buy them from MB: 83623