MS3X Pin 29 (N2O 12v input) is actually outputting 10v?

[gerzand]

Hello,

I have just completed wiring my nitrous system, utlizing my MS3X as the nitrous controller. I am, however, having an issue where the MS3X board pin 29 (nitrous input 12v trigger) is actually OUTPUTTING 10v. Is my unit defective or can anyone explain this? Could it be software (tune) related?

Thanks, Andy

[grom_e30]

http://msextra.com/doc/ms3/hardware.html possibly jp8 ms3x 'Nitrous in' pull up?

[SymTech Laboratories]

http://msextra.com/doc/ms3/hardware.html possibly jp8 ms3x 'Nitrous in' pull up?

That's exactly right, it's normal to measure ~12V on the NITROUS_IN wire as long as JS8 is set.

[gerzand]

http://msextra.com/doc/ms3/hardware.html possibly jp8 ms3x 'Nitrous in' pull up?

That's exactly right, it's normal to measure ~12V on the NITROUS_IN wire as long as JS8 is set. Just ground the wire to activate the circuit.

So I took the tip from the previous poster and disassembled my unit, and indeed found that JS8 was set (jumped). I removed the jumper, and also checked to make sure that SC12 to JS9 wire was also soldered. It was.

I took the car on a test drive, with all of the same settings (Nitrous ON, min RPM, Min Throttle position, etc..) and it was a no go

I can confirm with my voltmeter that my "enable" switch is working, as MS3X pin 29 is receiving 12v from the switch. Also I can also confirm that removing the JS8 jumper did fix the previous issue. But now im stumped why nitrous is no longer activating.

Im on firmware v1.1.0 official, and Tunerstudio v2.08

NITROUS.msq

Thanks for your insight guys

[dontz125]

Read Symtech's previous message again - you are SUPPOSED to see 12-ish volts. That's what a pull-up resistor does! The CPU activates the nitrous system when the signal goes LOW. Since removing the JS8 also removed the +12v (which wasn't a problem), AND stopped the nitrous circuits from operating, I would suggest replacing the JS8 jumper.

[gerzand]

Read Symtech's previous message again - you are SUPPOSED to see 12-ish volts. That's what a pull-up resistor does! The CPU activates the nitrous system when the signal goes LOW. Since removing the JS8 also removed the +12v (which wasn't a problem), AND stopped the nitrous circuits from operating, I would suggest replacing the JS8 jumper.

From http://msextra.com/doc/ms3/hardware.html

"JP8 is jumper for a pullup on the "Nitrous in" input. Normally the nitrous system expects a 12V input here and the jumper should be omitted. If re-using this input for a ground-swiching input then install the jumper."

Does the manual contain false info....or can I just not read?

Am I not just recreating the "pull-up" by supplying my own 12v?

My circuit is set to provide 12v to the input, so i'd like to use it that way....Unless there is some reason it wont work that way.

[dontz125]

Sorry I missed your comment about the +12v FROM the switch.

I've spent the past few minutes going over the MS3X schematic for the nitrous system, and I find it ... puzzling. With nothing more than a few resistors, the CPU will accept a going high OR a grounding signal, with no software setting that I can see. I'm also seeing BAT54 shunt diodes dumping +12v into the Vcc bus, which strikes me as a fast way to fry expensive parts.

I'm baffled, so I'm going to sit down and shut up on this. I'm dying to read what a dev has to say on this ...

[elaw]

Looking at the schematic, I can see where the confusion comes from!

Keep in mind that all the CPU cares about is whether it's seeing a logic "high" (greater than about 1.5 volts) or "low" (less than about a volt) at its input.

The most straightforward setup is with JP8 installed, and an external switch that closes to ground. So when the switch is open, R18 sends +12 volts (from JP8) to the processor via R19. D9 "clamps" the input to make sure the processor doesn't see > 5 volts to avoid damaging it. That is, technically, feeding +12 volts into the +5 supply, but R19 limits the current to an insignificant amount. When the switch is closed, the R18/R19 junction is shorted to ground, so the processor sees 0 volts and all is well.

If you have a switch that closes to +12 instead of ground, things get more confusing. You definitely don't want JP8 installed, since that will provide +12 just like the switch, so the CPU will always see a "high" input level.

The problem comes when the switch *opens*... what's really needed is something to pull the input down to ground so the CPU can see a "low" input level. In TunerStudio, if you go to the "CAN-bus/testmodes" menu and select "Special options", there is a setting for "Nitrous-in pulldown". Setting that "on" will enable a function inside the CPU that pulls the input to a low level except when it's pulled high by an external voltage source... that should do what you want.

The other option would be to install a small resistor, say 2.2K, from the switch to ground so when the switch is open, the input on the Megasquirt is pulled down to 0 volts.

[gerzand]

Eric thanks for your response but the canbus testmodes setting for nitrous in pulldown was already set to ON. Now im really scratching my head.

[elaw]

Can you check the voltage on MS3X pin 29 with the switch *open* (and engine running or at least the ignition on)? I think you've already verified it's around +12 with the switch closed, right?

The "pulldown" in the processor does not supply very much current... it might be that it's just not able to pull the voltage close enough to zero with the switch open. The external 2.2K resistor I mentioned above should fix that problem.

[gerzand]

Solved! The pulldown setting needed to be set to off. Strange!

[jsmcortina]

http://msextra.com/doc/ms3/hardware.html possibly jp8 ms3x 'Nitrous in' pull up?

That's exactly right, it's normal to measure ~12V on the NITROUS_IN wire as long as JS8 is set. Just ground the wire to activate the circuit.

That won't work.
The JP8 jumper should pretty much never be installed. The firmware treats the NITROUS_IN input as active high (as it says in the docs.)

James