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Old 07-21-2008, 11:37 PM   #1
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Default MS Ground Wire Question

The ECU Wiring diagram in the DIY MS thread for the 94 shows the ground wires being attached to two pins one in from the end, but the two end pins on the plug are heavy black wires; clearly ground wires. Is it actually correct to plug these into the colored ones next to the grounds?

I assumed that if it was incorrect there would have been corrections or questions, but I also figured other people may have noticed the same thing I did but I didn't find any questions about it in the searching.

If I do splice into the colored wires, why are they colored? Where do they go?

Thanks, sorry it's a stupid question I should know the answer to.
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Old 07-22-2008, 02:07 AM   #2
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The four wires at the end are all grounds.

2A and 2B, the black wires at the edge, would actually be good ones to use as primary ground. They go to two different ground points, and are fairly heavy.

2C (black/green) isn't really critical. It's connected to ground at chassis, and carries the shield for the O2 sensor.

2D (black/green) is an interesting wire. It's a ground, however it does not go to ground in the harness. That wire, in stock trim goes to ground through the ECU, and provides the return for the coolant temp sensor, the EGR sensor, the TPS, and the O2 sensor. In particular, the TPS and CLT sensor won't work if it's left floating. So that one needs to be connected to the MS ground, with the understanding that it's not providing a ground for the MS, the MS is providing a ground for it.

Ideally, if I were you I'd connect all four wires to the MS ground plane. That'd be pins 1-2, 7-19. You don't have to hit every single pin at the MS, and it does not matter which wire goes to which pin. Use four heavy wires and hit two pins with each, and you'll be golden.
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Old 07-25-2008, 09:13 AM   #3
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Ok. now that I have an answer to that, how about my other wires?

I have started wiring into the stock harness, but there are a number of wires I don't know what to do with.

For Instance: There are two blue and white wires, and two green and white wires. Only one of each shows up on the ECU pinouts. Does the other just get tied in and not connected?
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Old 07-25-2008, 11:21 AM   #4
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It all depends on how your DB-37 is wired, dude. Specifically, those four striped wires you have are designated IAC1A, IAC1B, IAC2A, and IAC2B. But Miatas don't have stepper IACs.

In reality though, most of us tend to re-purpose those pins for other uses. In my car, the fan control, table switching, and EBC go through those lines. I just keep track of which function was on which pin when I built the MS, and then wired accordingly.

If by "Only one of each shows up on the ECU pinouts." you mean that there's only one of each pictured in the "How to make and install your own Diy MS" thread, you're right. Brainey has re-purposed those two wires to do other functions that the MS "bundle-o-wires" does not come with. I hope you're not just blindly hooking up wires from a pre-built DB-37 to pigtail harness. You need to make sure that the grn/wht wire you use is the one from pin 31, not the one from pin 29, and that the blue/white wire you use is the one from pin 25, not pin 27.

Why is this important? Because in the following picture, you built the CMP input to come in on the "IAC1A" pin, and the SparkB output to go out on the "IAC2B" pin. Meanwhile, the other two IAC pins are getting used for other functions later on.

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Old 07-25-2008, 11:45 AM   #5
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I'm not blindly following, but to continue the metaphor, my eyesight isn't very good either. I built the pigtail myself, per the DIY instructions in Brainy's thread.

I'm doing my best to understand what I'm doing but I have trouble finding the answers through searching. When I ask, you and brainy answer and you do a great job of explaining. I just feel like a newb asking all these questions I know are answered out there somewhere, I just can't find them.

While I'm being a newb, can you or somebody explain what the idle valve is/does? I know all about how engines work, and I'm excited to start tuning because I'll at least know what I'm changing, but I have no idea what this PWM thing is.

I assume it is a small valve that controls the amount of air allowed into the IM when the butterfly is closed, but I always thought it was just a small hole with an adjustment screw.
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Old 07-25-2008, 01:25 PM   #6
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Quote:
Originally Posted by skidude108 View Post
... I have no idea what this PWM thing is.
PWM = pulse width modulation, which describes how the valve is controlled. An electrical engineer could give you a more technical answer. But you've got the right idea of what the valve does.

I also have a '94, and if you followed Brain's writeup you'll end up with four unused wires from DIY's bundle coming off the DB37. At first it made me wonder what I screwed up but not to worry, your car will run just fine if you're doing a parallel install. Just tape off the ends and tuck them out of the way for later.
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Old 07-25-2008, 01:52 PM   #7
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I'm not doing a parallel install I don't think. Besides the added tuning for idle, fan control (which I already did the mod on the board for) and the AC which I will tackle later, I don't really know what the advantage is to parallel.

I don't mind the idle tuning so long as it's possible to get it correct eventually. I'm using 460cc injectors so I don't think the stock ECU will even idle them anyway.
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