Switched 12v to Injectors

[Stingy49]

Hey all,
I'm building up my ms3 with an ms3x for my stock ka24e. I was wondering why the injectors need switched 12v when they are being controlled by the ms3x? The fets used on the ms3x have a 70v drain-source clamp which is well above 12v so it doesn't seem like an issue to me. Is there something I am missing? I would prefer to leave the injectors hooked up to constant 12v so I don't have to modify the stock wiring harness. Thanks for any input y'all have.

[grom_e30]

if its powered of a constant feed you may find the 12v is back fed through the ecu preventing it from powering down when you turn the key off.

this goes for all things the ecu is controlling switched 12v is the way to go

[DaveEFI]

Hey all,
I'm building up my ms3 with an ms3x for my stock ka24e. I was wondering why the injectors need switched 12v when they are being controlled by the ms3x? The fets used on the ms3x have a 70v drain-source clamp which is well above 12v so it doesn't seem like an issue to me. Is there something I am missing? I would prefer to leave the injectors hooked up to constant 12v so I don't have to modify the stock wiring harness. Thanks for any input y'all have.

Your stock harness has 12v to the injectors at all times? Never seen that. They are usually switched via the ignition switch (and maybe a relay etc), same as other engine electrics.

[kjones6039]

I'm with Dave on this!! I've never heard of such a thing.

What make and year? I'd like to look at the schematics.

Ken

[Stingy49]

Thanks for all the replies!

The car is a 1989 Nissan 240sx with the ka24e motor. You can see how it is wired in the snip from the FSM I've included. Sorry about the low quality image, the pdf it's from isn't the best quality. http://i.imgur.com/xcbhziR.png. It's definitely an uncommon setup from what I can tell.

Also, I asked one of the EE's at work and they were explaining to me that the mosfets used to control the injectors can be turned on by the fluctuations in the 12v line while the motor is cranking. The source and drain pins are capacitively coupled which means that those fluctuations can bring the gate voltage high enough to turn on the fet.

I was hoping to avoid cutting up the stock harness but it looks like that is unavoidable . Thanks again for the advice.

[DaveEFI]

Interesting - never seen that arrangement before.

I'd do all the power side by the wiring diagrams given in the manuals linked to from this site. No point in guessing things might be OK. I do often start off with a stock harness as a basis - but they always needs quite a bit of modification. Perhaps get one used - if you want the ability to swap back to the original injection easily.

[billr]

Have you verified that schematic? All manuals have some errors in them...

[Stingy49]

Have you verified that schematic? All manuals have some errors in them...

Checking that tonight

[Six_Shooter]

I've never seen a Nissan that DIDN'T have the 12V switched to the injectors.

[Stingy49]

I checked today after work and the injectors have 12v relative to the neg terminal even while there is no key in the ignition.

[Six_Shooter]

That's just a bad idea overall.

Even though you don't want to change it, it'll be safer in the end to change it.

[Matt Cramer]

I checked today after work and the injectors have 12v relative to the neg terminal even while there is no key in the ignition.

Yep. You can get away with this with the MS3X injector drivers, but not with the main board drivers. Why Nissan ever did this is beyond me.