using relay to control air pump causes PDM 12V trigger switch stays on

Hi guys,

Due to large current the air pump draws, I am using a 4 pin relay to control the air pump. I connect the 86 pin to low side control of ECU, and 85 to 12V "ignition switch", which also connect with PDM and ECU's 12V trigger switch, and 12V + of keyboard and tail light as well. For 30 and 87, I connect 30 direct to battery 12V+ and 87 to air pump 12V+ , and pump - to battery -. It works fine, but when I turn off the ignition switch (12V disconnect with 85 and other trigger inputs), the PDM won't turn off, but keyboard will go dark. If I disconnect the relay, all works fine. I tried 3 new relays, they are all like this, I am very confused. I know there will be power surge when I turn off the relay due to coil in it that may still trigger the 12V trigger inputs, but I think there is a flyback diode inside relay to prevent this?

Please tell me where I am wrong and how to solve it, thanks.

Yuchen

Can you log the status of the PDM to determine what changes with the relay installed vs. not installed?

Does this only occur when the engine is running? If so it's might be the alternator feeding the 12V ignition signal through some unexpected path.

Yuchen, you are getting a 'back-feed' through the relay, through pins 85/86. In instances like this you need to choose to use different/separate connections of your 12V trigger to the ECU.

You could see what is causing the issue by disconnection one wire at a time on pin 85 or 86. The wire that turns off the PDM in the situation, will be the one causing the back-feed.

Hi David and Sergio, thank you for your reply. I have tried to see log from PDM, but I can't find any useful information. And this occurs at engine turned off situation, so must not caused by generator.

What I am understanding is this caused by back feed from coils in relay, I have tried 3 different relays, the problem stays, but when I disconnect the relay, problem goes away. So I am tring to connect a freewheel diode parallel to 85 and 86, see if that helps.

I want to know how does everyone else wire their circuit when they have my situation? I use PDM to power ecu, display, both of them and PDM has 12V switch pin, so I connect them together to a physical switch which get 12V from battery. Is this way right? After this I connect the relay pin 85 together with these 12V switch wires, then the problem happens. But I fear if I connect 85 to a power output from PDM, this high current or voltage back feed will damage my PDM or connected components, will it happen?

All of the PDM installations I've done, the PDM remains powered on all the time, or perhaps main 12V feed is disabled via a master switch.

PDMs are designed to be very low current in the standby mode (only monitoring a few switches). I would have the output that controls the power to the ECU also control the power to the Air Pump relay (that stops back-feeding power to the ECU). The PDM would then turn those off when the dash power switch is turned off. I always have a separate ignition switch (think of it as engine run/stop), so the engine can be cranked (and channels like oil pressure monitored) without the engine running.

Hi Divad, I have the same thought, but will the back feed voltage from relay harm the PDM's out channel? That's what I am afraid of.

If you have to use a relay, why don't you ground the trigger of the relay, and provide trigger power via your PDM output? That can't cause any backfeed.