mith612
mith612 Reader
9/30/16 11:10 a.m.

About this time last year, I bought a 1991 Miata with a supercharged 1.6 running Megasquirt. Included with the sale was a race shop built 1.6 that needed assembly, a log manifold, and used Nissan T25. Two weeks ago the build of that engine was finished and it was installed in the car. Now that I am tuning the car for the different engine (larger injectors, etc) I've noticed that the fuel pump runs continuously when the key is on. I hadn't noticed it before because I had always just started it and let TunerStudio live tune the car as I drove.

So I took the computer out of the car, and it is a PlugNPlay unit for 90-93 Miatas from Stratified Auto. Tracing the wiring, the violet wire (connector 1C) for the Circuit Opening Relay is connected to the +12V pad along with the white/red wire (connector 1B) that is +12V switched. The car is using a GM-style IAT sensor that is directly wired into the computer, and there is no jumper in the AFM connector. There is also no jumper in the Diagnostics connector and the ST SIGN fuse was already removed.

Reading more about MS makes me believe that the violet wire should be connected to the FP (fuel pump?) pad. It's my understanding that circuit provides +12V for 2 seconds if there is no RPM input; with RPMs it provides continuous power. That way the fuel pump would only prime briefly instead of running continuously when the key is on, engine off. Can anyone confirm this?

Nothing connected to FP:

TLDR: Should the violet wire (1C - Circuit opening relay) on a 1991 Miata be connected to the Fuel Pump circuit in MS?

MadScientistMatt
MadScientistMatt PowerDork
9/30/16 11:22 a.m.

This is a MicroSquirt Module on a proprietary board. There isn't much documentation out about Stratified's units; you will need to contact them directly for support.

kb58
kb58 Dork
9/30/16 12:12 p.m.

Nah, just try it. For safety, put a light bulb between that fuel pump connection point and 12V. Turn on the ignition and see what it does, then start the car and see what it does. Done.

Keep in mind that the FP point is probably designed to only drive a relay, not a fuel pump directly.

Also, the output may go high OR low depending how it's designed. Easy to fix with the relay.

mith612
mith612 Reader
9/30/16 12:29 p.m.

According to the documentation, the jumper pads are labeled per B&G Microsquirt naming convention. So it makes sense for the fuel pump wire to be connected to FP so it would work as designed. It's just weird to me that they would build it to go straight to switched 12V.

mith612
mith612 Reader
9/30/16 1:48 p.m.

Tested the FP connection as kb58 suggested: Lightbulb wired between FP pad and 12V. Key off, light is on. Key on, light turns off and stays off (does not change state after 2 seconds). Voltage from FP to 12V when the key is on drops to about 8 volts. Something seems wonky with this circuit.

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