Blowing Fuses When Activating Power Running Boards With Alarm

I want to use a negative output from my alarm system to operate the +12v power retracting running boards on both sides of the vehicle.

Alarm: Prestige APS997Z

Vehicle: 2008 Ford Expedition (limited)



The diagram shows what I am trying to do. I use the roof rack a lot, and I cannot access the roof without running boards in the deployed position.

Normal running board operation: When a door opens, the running board on that side of the vehicle deploys. Otherwise, the running board is retracted.

My initial testing:
I tried wiring directly into the running board motors, but quickly learned that this is a very complex system involving multiple sensors. Instead, I tapped into the “door open” sensor wires. These wires read 0v when the door is closed and +12 when door is open. Using a simple on/off switch I successfully controlled the position of one of the running boards. I want my alarm fob button to activate a +12v on/off latch to each of the front door sensor wires, making the door appear open, thus deploying the running boards on both sides.

The problem:
I keep blowing fuses! I have blown the 15a in-line fuse, a 20v ignition fuse, and destroyed a 1a diode (I’m using 3a diodes now). The running boards will deploy for a short time before a fuse blows.

The relay that converts negative output to positive output seems to work fine; it delivers +12v volts output.

Yes, I realize that my approach has shortcomings...like the fact that the siren will sound if I pseudo-open the door when armed. Yes, I realize that I can deploy the running boards using the car’s menu, but that’s a very clunky process.

What am I doing wrong that's causing fuses to blow?

The alarm output is only rated for 200mA-250mA and it needs to be wired to the relay coil (pin 85 based on your diagram).
You need to switch the wires going to pin 85 and 30.

The alarm output will trigger the relay coil and 12V will be fed out of pin 30 to the vehicles SJB.

Thank you audiokid1.

I must have an additional problem because I'm still blowing fuses even after making that adjustment. In an effort to troubleshoot, I created a less-complex setup (diodes removed; only one door sensor wire). Even with this simpler setup it took about 1 second to blow a 15a in-line fuse going from the alarm to the +12v ignition wire.



Suggestions for troubleshooting? Thanks!

More troubleshooting results:

I used a simple on/off switch to control the automatic running board on one side of the car. I grabbed +12v directly from the car's battery.
-- If I do this with no fuse in the circuit, it operates fine.
-- If I have a 15-amp in-line fuse in the circuit, that fuse blows after about one second.

Is the solution to simply use a more robust fuse?
Am I correct that I will have to isolate the "door open" circuit from my alarm system (relay?) to prevent the 15a alarm fuses from blowing?

Thanks!

What happens if you just apply chassis ground to pin 85 on your relay instead of using the alarms aux output?

If you have any wiring details for the running boards that you can share it may help with diagnosing the issue.

[solved]

Evidently the door ajar system in the vehicle works like this:
- When the door is closed the door ajar circuit is complete/closed
- When the door is open the door ajar circuit is broken/open

Manipulating this circuit required severing the door ajar wire and using relays as shown in the attached diagram. By default, relay pin 30 and 87a are connected, resulting in a completed circuit. In other words, the door ajar system works as normal based on the actual status of the door. But when the alarm remote triggers the alarm's negative output, relay pin 30 and 87 are connected, and because 87 is connected to nothing, the door ajar circuit is broken. This effectively "overrides" the actual status of the door, telling the vehicle that the door is open even though it is not.

It works like I wanted. I can click a button on the remote and the running boards will deploy and stay deployed until I click the button again. As I mentioned previously, this is not a perfect solution: if the alarm is armed and I deploy the running boards then the siren will sound because the alarm thinks the door is open.

Factory running boards can be put and kept in the deployed position through the settings menu of the instrument panel or infotainment system.