MS3 PRO fan on,system off

[PeteJ]

I'm doing the final wiring on my Pro and ran into a strange one. My fan is wired using PWM 3. with power on at the ecu the fan works as it should, when i shut the ignition off the fan goes on regardless of engine temp.

[dontz125]

When the processor is off, the driver transistors are uncontrolled. If the fan is powered when the box is not, the behaviour is unpredictable.

[PeteJ]

The ECU is controlling the ground to the relay. the only way the fan can go on is if the ECU completes the circuit.

I will check for a short to ground between the relay and the ECU.

[winstonusmc]

a pull up resistor would stop that. 10k ohm to 12v should do the trick. Make sure its at least 1/4 watt.

[PeteJ]

A resistor from where to where? I just checked and all the PWM outputs go to ground when off. If that is by design that seems wrong to me, why would you have a circuit "activate" when power goes off.

The issue is that i have the fan relay wired to constant power so i can run them using an override switch while in the staging lanes. Are there any outputs that stay off when you shut power to the ECU off?

So I found this in the manual and now I understand what it means...☺

Any device connected to these outputs must be set up so that it is powered off when the ECU is shut down. Relays or solenoids connected to these outputs can back feed power into the MS3-Pro if the relay coil or solenoid winding receives power with the key on. Wire any device connected to
these outputs to switched 12 volt power.

I will try using one of the high current outputs tomorrow.

[winstonusmc]

You would place the resistor between the wire that goes from the relay to the Megasquirt and 12v. You could put the resistor right at the relay. Back when my MS3 controlled the fan directly, I ran it from "idle" on the MS3X and didnt have any issues with it staying on. Now it is run over CAN Bus to my fuse box.

[PeteJ]

Thanks Winston, i didn't have this problem with the standard MS3, the PRO is working differently.

[jsmcortina]

Make sure that the 12V feed to the relay coil is only on when the MS3-Pro is on. That's the key.

James

[PeteJ]

James, if that's the case then I have a problem with the MS3Pro cirucit. Any design should always fail/go off to a "safe" condition. Having the system go off and have the relay energized is what I'm trying to avoid. Do all the outputs on the MS3-Pro work this way? I'm open to using the spare injector and/or high current outputs.

The net is that there are many times where I need to run the water pump and the fan between rounds to cool the car down and I don't want to have the entire system powered up. Need be I can use a double pole/double throw switch to toggle the power to the relay coils but I'd like to avoid the extra wiring if at all possible.

[dontz125]

If you look at the standard wiring diagrams, you'll note that the Main and Fuel Pump Relay contacts are powered directly from the battery, but the coils are powered through the key switch. Do the same with the fan and water pump relays. The coils and injectors (and anyother systems you want to only be active while the engine is ON) should be powered through the FPR (or a parallel relay also triggered by the FPR driver); when the engine is off, even with the key on, the fuel and spark systems are OFF.

[ashford]

its not a fault with the ms3. the v3 board has the same characteristic on the injector drivers. it is the fet driver power supply, it has a capacitor on it after a diode so until the capacitor drains the fet is on, for me this lasted for 15-45 seconds depending on temperature. what i did was put a 10k across the capacitor and that cured that issue for me

[jsmcortina]

James, if that's the case then I have a problem with the MS3Pro cirucit. Any design should always fail/go off to a "safe" condition. Having the system go off and have the relay energized is what I'm trying to avoid. Do all the outputs on the MS3-Pro work this way? I'm open to using the spare injector and/or high current outputs.

Yes, they all work that way. Fix your relay coil wiring and the problem will go away.

EDIT: This is covered in the MS3-Pro manual, section 4.11.

James

[PeteJ]

James, I'm looking at the manual and there is no section 4.1.1, only a 4.1 and 4.2. Do you mean 4.11?

There is something strange going on here. I moved the fan to High Current 3 and the problem went away. I'll call DIYAutotune in the morning, something isn't right.

[Matt Cramer]

The PWM circuits are optimized to drive solenoids that are parked in the middle of their travel. This means clamping the flyback voltage to the MS3-Pro supply voltage to slow down the solenoid closing action as much as possible. It is possible to clamp the flyback at a higher voltage using a zener diode, but this needs to be higher than the battery voltage can ever go or the clamping circuit can trigger the relay when it should not be triggered. We did this on the high current and injector outputs, which are clamped at 36 volts. However, this would not work on the PWM circuits as this makes the valve close too fast, resulting in poor control with IAC or VVT solenoids.

So the failsafe needs to be put into the system as a whole as it wasn't practical to put the failsafe entirely inside the MS3-Pro. In this case, the failsafe mechanism is to not have anything wired to the PWM outputs connected to constant 12V power, ensuring they will turn off with the ECU.

[PeteJ]

Thanks Matt, that makes sense. Using high current 3 solved my problems. I need 1 and 2 for boost control and the Trans brake relays.