ACC bypassing crank.

[Saut de Chat]

So You are sitting in the car, listening to a great song while waiting for someone. They show up. As You start the car to leave, everything resets. -_-'
I was driving a commodore recently in which this is not a factor.
The stereo stays on ceaselessly while starting the car and also does not turn off until You either remove the key, or open the door, even in the 'off' position.
How exactly is this achieved? I am soon to install a 'carputer' in my two and would very much like not to have to boot twice.
What is the reason for everything being turned off when You start the car?

[Grrrrrrr!]

What is the reason for everything being turned off when You start the car?

couple of reasons,

1) turning off electrical loads gives more juice to the systems that need it while cranking.. starter motor and ignition/ecu & fuel pump.
2) starter motor draws a lot of current, when you let go of the key and the contacts open it can generate some pretty harsh spikes, these can kill cheap electronics.

How exactly is this achieved? I am soon to install a 'carputer' in my two and would very much like not to have to boot twice.

Its just a matter of wiring a relay up in such a way that it takes it feed from a source that isn't interrupted by the key being turned to the cranking position. Most ignition barrels disconnect the ACC relay line when the key is in the cranking postion from what i recall.
Even this may not be enough, that big load of the starter motor cranking brings the battery voltage down, sometimes to < 8V (any combination of old battery, crappy wiring, cold morning & high compression engine) so your carputer might brown out and reset anyway.

[Benckj]

You might be having a battery or ground wiring problem as with the MR2's they don't disconnect the AC when starting. I'd look at why your electrical system can't cope and possibly how you have the stereo wired into the circuit.

[Saut de Chat]

Hmmm, I had not looked into the wiring, just assumed it was the same as most.
My radar detector also resets on start, and is just plugged into the cigarette lighter.

[Malcolm]

You might be having a battery or ground wiring problem as with the MR2's they don't disconnect the AC when starting. I'd look at why your electrical system can't cope and possibly how you have the stereo wired into the circuit.

I think the OP was referring to the ACC (accessory power), and you may be referring to the AC (air conditioning). ACC definitely shuts off when cranking in all Toyotas I've come across.

I'd suggest the best way to achieve what you want is to use a feed from ACC and a feed from AM1 or AM2 (two circuits switched when the key is "ON"), place a diode in each line (to prevent one circuit powering the other) and use those to trigger a relay which powers your stereo. Then it'll be on whether the key is in ACC, ON or cranking. You can also get clever with electronics and use a latching relay so once it's on, it stays on until you open a door (which now you mention it, our work Commy is as you describe and I'd quite like that in my Camry ). If you need more details/a diagram of how to do either of the above, I can probably do that

[Saut de Chat]

That is exactly what I needed, thanks.

[Malcolm]

This circuit will do what you want. Some notes:
Relays shown by the boxes are standard automotive "horn" SPDT relays, pins labeled with the standard numbering convention.
The momentary switch, and the driver's door switch, need to be normally open switches

The fundamentals of the operation are: current from ACC turns on the relay on the right, whose output is routed back to the ACC line so it latches itself on. This "feedback loop" is connected through the normally closed terminals of the left-hand relay. In order to power down the right hand relay, you must energise the coil of the left hand one to open the contacts - this is done by either switching the door switch or the momentary push button (my plan for the camry is to install a smash button on the dash somewhere so I can turn the stereo off without opening the door if needed). Opening the door or pushing the button while there is power coming from ACC will NOT turn off the headunit.

[Malcolm]

Just a further note, you need to add a diode to the ACC supply otherwise this circuit will power everything on the ACC circuit, but that may not be an issue unless you have another high current draw device on ACC that you don't want powered up the same as the headunit. I'm gonna make up a small one of these circuits using veroboard and PCB mount relays to try out in the Camry sometime this week

[Malcolm]

I received the parts I need to make the above circuit on veroboard, will construct it in the next couple of days and post up some pics.

[Malcolm]

So, it made it up yesterday. I made a few changes from the diagram above to make it more compact and so I could do it on veroboard. For the high current relay on the right I used a 16A SPST relay, and for the one on the left which only carries a small current, I used a sub-miniature type DPST. I also added a diode to the ACC signal to stop the low current relay powering the vehicle's ACC circuit when it is switched off, and another before the door switch so it wont have any funny interactions with circuits in the car that also use the door switch.

Finished:




And I found a little earphones case I had which was the perfect size for an enclosure


I haven't actually tested it functions properly yet, but given the simplicity I'm pretty confident it'll be fine.

[Saut de Chat]

Thanks for all this information. It will help me heaps. Nice, tidy setup too.

[Malcolm]

Let me know if you feel like making it on veroboard with PCB mount components, I'll put up a veroboard diagram and part numbers from RS (parts cost about $20)