Voltage discrepancy on 12V+ to Megasquirt, is this a problem? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 11-23-2011, 11:56 AM   #1
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Default Voltage discrepancy on 12V+ to Megasquirt, is this a problem?

This probably sounds like a foolish question, but I figure it's better to ask and know than to guess and remain ignorant.

I recently added a voltage gauge to the dashboard on my TunerStudio (paranoia thanks to a defective alternator's untimely demise) and noticed that it reads about 0.5 - 0.75 volts lower than the system voltage when the car is running. Using my multimeter, I determined that the 12v power source (taken from the original power source for the OEM OBD2 ECU) itself is actually lower than the system voltage. I'm not sure if there is some sort of protection built-in to that circuit to prevent accidental voltage spikes from damaging the factory ECU, but my testing with the multimeter definitely shows it to have around 13.5 volts when the rest of the car is running 14.2 volts.

I assume that this discrepancy could throw off some of the sensor calibrations? A lot of the MegaSquirt's control is based on regulated 5v signal though, so I'm not sure if anything is critically affected by this.

I am inclined to find a new power source for the MegaSquirt, but I'm not sure how necessary this is, or even if the OEM power source might be safer somehow?

Other relevant information: 1997 Miata, stock engine, MS1 (pcb3), parallel install with dedicated CLT and IAT sensors and dedicated grounds, Innovate LC-1
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Old 11-23-2011, 12:18 PM   #2
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Measure the voltage at the injectors, the MS should match this. If not, calibrate as needed.
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Old 11-23-2011, 12:59 PM   #3
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OK, I think that answers my question then. The 12v power for the OBD2 (and MS at present) reads at ~13.6v with the engine running. The 12v power for the injectors reads at the same voltage. So my power supply is good. Thanks!

Out of curiosity though, is this normal for these cars? I've never heard anyone mention before that the power supply to the ECU is actually slightly reduced from that of the rest of the car? Is is supposed to be that way, or am I getting some unintended voltage drop at the fuse or relay?
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Old 11-23-2011, 01:26 PM   #4
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just a voltage drop, but you want the MS to read what the injectors see, since it does voltage based corrections to them.
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