Where are the N2O electronics experts? Need help.

[TT5.9mag]

Gentleman, I installed an NOS 2 stage window switch (only using one stage) on the D100 over the weekend and got it all wired up. The instructions with regards to rpm signal were vague and the unit I am using is primarily designed for efi stuff so typically you tap in to a single coil on a COP setup. The instructions say “coil trigger” wire. Now, I am using it on a carbureted application with a TPS (already had it for my trans) and a single MSD coil and mopar electronic distributor fired by a 4 pin HEI module. I have the rpm signal wired to the neg side of my coil with my tach and the “c” terminal of the hei. The RPM on the unit at idle is correct and tracks exactly with my tach. As I rev the engine slowly the displayed rpm goes erratic and jumps all over the place (while the tach continues to be accurate) rendering the unit basically useless as the function of a window switch is to trigger the nitrous at a preset RPM. I’ve read about maybe needing a resistor inline (what value?) or maybe needing an MSD tach adapter to clean up the coil signal. Anyone have any words of wisdom? Thanks in advance.

This is the window switch
https://www.holley.com/products/nitrous/controllers_and_accessories/controllers/parts/15982NOS
These are the instructions
https://documents.holley.com/d0323b6734a254e5eb0279950d473fc3af8651e0.pdf

 

[MadScientistMat]

I'd first try grounding the unit directly to the battery. Also, where's it getting 12v power? These tend to be very common noise sources.

If that doesn't work, I would only then try filtering the input signal. You can try a 1k, 1/2 watt resistor inline first, they're pretty cheap
If that doesn't work, this module might. I designed that one when I worked there, but I no longer have any affiliation with them other than knowing what's inside the box.

 

[TT5.9mag]

I'd first try grounding the unit directly to the battery. Also, where's it getting 12v power? These tend to be very common noise sources.

If that doesn't work, I would only then try filtering the input signal. You can try a 1k, 1/2 watt resistor inline first, they're pretty cheap
If that doesn't work, this module might. I designed that one when I worked there, but I no longer have any affiliation with them other than knowing what's inside the box.

Thank you. I was hoping you’d chime in and I’m glad you did. I am powering the unit from an unused key on fuse location on the original fuse block of the truck. The ground is currently to bare metal on the firewall shared by 3 other grounds. I will move that directly to battery neg and see if there is any change. Also, Holley tech said 500-600 ohm resistor in line “should” work. I’m more inclined to listen to you. I’ll try the 1kohm 1/2 watt first. And if that doesn’t work I’ll try the isolator module you built.

 

[TT5.9mag]

Ordered these.
McMaster-Carr
I’ll have them later today and I’ll give your suggestions a try tonight and report back.

 

[MadScientistMat]

The 1k value was more of a starting point without full info on what's in the box. If you lose the signal, try a lower valve.

But I'd move the ground first. The power point sounds OK - typical trouble spots are things like the coil positive wire.

 

[TT5.9mag]

Have to apologize, worked late last night and didn’t mess with the truck. Hopefully today I’ll get a little time.