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Forcing Accessory-Mode in Vehicle after stopping the engine

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Sebastian S.:
Hello everyone,

I'm a long time watcher of the EEV-Blog and do have some interest to electronics because I'm a car mechanic and profess in diagnostics of electronic functions. Unfortunately we don't go as deep as opening ECUs and figuring out what's wrong within them as the manufacturers don't provide data to do so. I am able to read schematics, take measurements and identify basic parts thanks to the blog.
What I'm not able to is figuring out what parts I need to make my wish a reality. I want to have the Accessory-Mode in my car active after I turned off the engine. Since this is a modern car with a push button instead of the conventional ignition switch this has proven rather hard. I am now at the point where I have sketched down how I think it could work and hope you can help me pick the correct parts.

So basic information first:

* The Modes Accessory (further refered to as [ACC]) and Ignition (further refered to as [IG]) are controlled by 2 independent relays
* Said relays are switched on and off by the Start Stop Unit (further refered to as [SSU])
* Upon pushing the Start button once, [ACC] is turned on. Pushing a second time, [IG] is turned on. If the clutch (MT) or Brake pedal (AT) is depressed the engine will start in any mode
* While the engine is running and the Button is pressed everything will be turned off
* Software configuration is no option according to the manufacturer, the ECU is secured with Anti-Theft-Bolts and getting it out to check for internals takes a long time
My requirements for the component to add

* Receive both 12V and Ground from the vehicle to operate. Voltage source can go up to 16V
* Receive 12V from [IG] as signal so turning off the vehicle can be detected. Again spikes up to 16V should be covered
* Receive the Signal from the drivers side door so opening the door can be detected (Switch is connected to ground)
* Send 12V from point 1 out to [ACC]-relay
* Dark current should be as low as possible and not exceed 5mA when Vehicle is locked
* An adjustable or fixed timer should be available and start as soon as [IG] is turned off
* Function: Upon Turning on [IG] the module also starts sending power to the [ACC]-Relay. Upon switching [IG] off a timer starts (e.g. 40 seconds) and keeps the Output to [ACC] alive. If either the timer reches its end or the drivers side door opens the power output to the [ACC]-relay is cut and normally turned off.
I will attach a prototype schematic of how I want to wire this up to aid you in my probably confusing description.

Mechatrommer:
1) you should not tap directly from battery (topmost line), you should tap after 200A fuse, or better yet, put your own 5-20A fuse to the battery tap.
2) i guess the line that goes to SSU (bottom ACC) relay control is to bypass SSU control? i wouldnt do that. i will put another relay parallel to ACC relay and do independent control from there. so i will not mess with SSU module's output/power pin.

what part needed? it depends on your preferences. if me i will go mcu way much simpler and less component. if you go analog way i'm guessing you will need more part for timer (555 and shift registers), reset or latch switch everything, much harder if me doing it. ymmv.

cs.dk:
Which car? There may all ready be a solution out there for you..

Sebastian S.:

--- Quote from: Mechatrommer on January 18, 2019, 03:27:29 pm ---1) you should not tap directly from battery (topmost line), you should tap after 200A fuse, or better yet, put your own 5-20A fuse to the battery tap.
2) i guess the line that goes to SSU (bottom ACC) relay control is to bypass SSU control? i wouldnt do that. i will put another relay parallel to ACC relay and do independent control from there. so i will not mess with SSU module's output/power pin.

what part needed? it depends on your preferences. if me i will go mcu way much simpler and less component. if you go analog way i'm guessing you will need more part for timer (555 and shift registers), reset or latch switch everything, much harder if me doing it. ymmv.

--- End quote ---

1) Of yourse I won't be tapping into that wire, I realized that mistake right after I posted this here. There'll be a seperate fuse for that later on.
2) Fair enough although I don't think the relais itself is capable to be diagnosed. I will change that however to do as you said if it's possible, since the ACC-Relays is in the engine compartment while everything else is in the cabin. I'll think about what to do here.

I thought about going for a MCU myself, but lacking the knowledge I don't know which to pick for my specific use-case nor do I really know how to look for one as there are a lot of them. I stumpbled upon a PIC16F18323 but am not sure if it will do the trick.

@cs.dk It's a 2016 Mazda Miata (ND) and unfortunately noone in our german community has found a solution to that yet.

Mechatrommer:

--- Quote from: Sebastian S. on January 18, 2019, 03:46:04 pm ---I stumpbled upon a PIC16F18323 but am not sure if it will do the trick.

--- End quote ---
any mcu will do the trick including that you mentioned. you just need to learn how to program it and put decent 5V voltage regulator and spike protection to it.


--- Quote from: Sebastian S. on January 18, 2019, 03:46:04 pm ---@cs.dk It's a 2016 Mazda Miata (ND) and unfortunately noone in our german community has found a solution to that yet.

--- End quote ---
this is very easy circuit any german guru can make it in 5 minutes. lack of solution may hint something, safety reason maybe. to not risk flattening the battery or burning a car. or maybe simply no one has the need yet except you the first one.

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