How to wire my stereo so it comes on when key is inserted.

[mazeone]

Hello---I want to wire my stereo so it will come on when I insert my key into the ignition. As it sits now you have to insert the key and turn to the on position to get power to stereo. There is times I would like to listen to my stereo without having to turn the key. I believe the yellow wire from stereo needs to go to the "s" plug wire coming from the ignition harness. Just not sure where all the wires need to go. Right now black goes to fuse box alone, yellow and red are spliced together and go to fuse box as well. I'm sure someone on here has wired there stereo this way. Any thoughts?

[Joel]

I have but you got the wires around the wrong way.
The black wire is a ground and should go to something metal not the fuse box.
The yellow wire needs to be live all the time to keep the head units memory.


What model bug?
If you have a 71 on that has the S terminal you can connect the red wire straight to the S terminal but make sure its fused somewhere.

But seems US cars with generators had an SU terminal instead which needs a relay as it;s a ground not a positive.

[mazeone]

It's a 1973. Can I take the red wire to the s plug wire and "t" it off to then go to the fuse box?

[Kevinx]

Buy a test light and find out what spades are poered on the back side of the fuse panel. On most stereos; the red is constant power, and the yellow is switched. There are a couple of extra spades that you can use
_________________
In life there is but one way out

[mondshine]

According to my Bentley manual, on the '73 models, terminal SU on the ignition switch provides a ground connection when the key is in place.

Look for a black/gray wire in the harness that plugs into the ignition switch.
In the trunk, on the driver's side (US) you will find the black/gray wire connected to a brown/blue wire which runs to the driver's door switch.
Once you've located this wire, check whether it provides ground or +12V with the key in place.

On my '71 car, the radio is wired like this:

The ignition must be switched on, then off (with the key left in place) for the radio to be powered. The diode might be unnecessary; I can't remember why I did that.

On late '73 (74 models & up), Terminal SU is +12V with the key in place.
The black/gray wire runs to terminal 85 of the seat belt warning relay.
I would still consider using a relay to power the radio; something like this:

Good luck, Mondshine

[badufay]

I like this idea... It is like the MkIII VW's. This will workout well since Im running a pertronix too!

Ben
_________________
Current: 2023 ID.4 Pro S, 2020 Golf TSi 1.4L, 2017 GTI S, 6 spd man, 76 Bus, 2.0L- 3 spd auto
Previous: 2015 Golf S TDI, 6 spd man; 2015 Jetta SE TDI, 6 spd man; 12 Jetta 2.5L, 04 Jetta 2.0L, 02 Golf 2.0L, 98 Jetta 2.0L, 98 Golf 2.0L, 98 Cabrio 2.0L, 95 Golf 2.0L, 87 golf GTI 16 valve 1.8L, 86 Cabriolet 1.6L, 80 Audi 4000 1.6L, 76 Audi Fox 1.6L, 76 VW Beetle 1.6L, 74 Audi Fox 1.5L, 69 VW Beetle 1.5L

[Joel]

[JerryMCarter1]

I wired mine so that when I flip the switch ( no key ) the radio goes on.
Like below -- the other two are for floor lights - thats a manual start on the left

_________________
Experience always triumphs over hearsay and You get to select which theory to believe

[mondshine]

Joel-
For 1974 models, my Robert Bentley manual refers to the actual contact in the ignition switch as H6, and the terminal as SU; Maybe US cars are unique in that respect.
I don't own a '74 model, but I did check the US wiring diagram.

You are probably correct about modern radios using the switched power wire as a trigger only; relay not necessary in that case.

[Ferguson]

[Joel]

x2

The old 80s and early 90s stuff from memory was like that and some of the cheap no name brands may have carried on that layout but all the bigger name brands were yellow memory and red switched ACC

You'll notice the yellow wire is much heavier gauge to power the internal amplifier and the red is quite small as its carrying no real load.