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Fusing Multiple Loads?

PDM Installation & Configuration

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This weekend I'm going to look at pinning my AIM PDM32 and getting some power to the car, but I'm not quite sure what is safe to do when grouping multiple Devices onto a fuse/channel.

Below I've attached a Fusebox Diagram and Google Docs Link with Grouped Circuit example and a Standalone circuit Example at the bottom. I'll be using some OEM systems such as power windows, door locks, and power-folding mirrors, which also come with their own control ECUs which draw way less power.

1. My question is, how does the OEM manufacturer group low current devices and higher current devices together surely this isn't safe? For example, the 30A 'POWER' fuse on the stock fusebox has both Window Motors and Window Switches, the switches are going to draw basically no current but the motors will draw a fair bit... Surely if the Switch encounters a problem that 30A fuse will not save it and cause a fire? I've heard that the manufacturer oversized cable gauge just like I have done to cope with this but then wouldn't the device be the next weak link in the circuit with the same fiery result?

2. If these lower current devices fail then surely the fuse wouldn't protect that low current device as it wouldn't peak past the 15A Fuse? I've been told the Manufacturers oversize the Wire gauge like I have done to account for this but surely then the device could be damaged and potentially cause a fire?

3. Or would I be completely wrong and these devices are all drawing very similar currents?

4. I'm leaning towards wiring as much as I can to singular outputs and then grouping Nonessential OEM systems like door locks and power windows together to fit everything on the PDM and retain proper logging of each channel, would this be the best method?

I am by no means a professional and this is my first project car ever, so any advice at all would help even the most basic of basic!

Google Docs Link - https://docs.google.com/spreadsheets/d/1vbO-NkdlVcojw_Vav-0xriVJ3L-2HuRunNHcezNSLDQ/edit?usp=sharing

Fusebox Diagram Link - https://knigaproavto.ru/shemy/en/toyota/mr2/379-1989-1999-toyota-mr2-w20-fuse-box-diagram.html

Thanks guys!

OEM's realize that some circuits are designed with safe current limits and don't need to be protected by a fuse. For example, your switch circuits directly shorts a wire to ground when active, but because of the current limiting resistor in the circuit, it draws an insignificant amount of current. So a short or open in the switch wiring just means the switch doesn't work. If you try to put that switch directly across the battery terminals, it will likely overheat and melt (or perhaps blow a 30 amp fuse first).

So, you can combine all these devices that don't need much power into a generic "dash power" or "Key-Switched power", and run with a current limit above whatever max current you can measure. I would save your controlled circuits for things you want to turn on/off programmatically, or need to measure current for diagnostic purposes (ie. fuel pump, water pump).

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