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ECU shuts off when cranking, and stays off until reset

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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I have an 03 Mazda Miata I'm powering with an MS3pnp made for a 94. I've adapted the wiring to the best of my ability and all the tests check out in my Tuner Studio. I've had to manually switch on the main relay due to the 03 wiring relying on the factory ECU turning the relay on, which the MS3 doesn't do. Initially I had it connected to a random ground (ECU controlled the ground) but when I went to crank it, the ECU went offline, and stayed offline until I flipped the switch off and on. I relocated the ground to a more reliable source (chassis) but its still shutting off when I crank it.

I've already checked the ground source, swapped the battery to make sure its not a voltage issue, but I'm beginning to think for some reason its connected to a source that naturally shuts off during cranking. My question is, what could be my issue, AND if I was to wire in my own relay, how would I go about it? my understanding is that the ECU is powered off of the normal 12v circuitry of the harness.

It would seem the voltage across the ECU is dropping below the level where it shuts off.

To eliminate the possiblity of it being switched off (unpowered source) when cranking, disconnect the solinoid wire at the starter - this will leave the rest of the circuit intact but remove the cranking drop. If it shuts off you'll know it isn't the voltage drop (although this may be the case later).

If that made no difference and the ECU remained powered up, it would suggest a voltage issue. In no particualr order (and some probably missed), I'd check the voltage drop between the +ve battery teminal and the ECU +ve supply point with the engine running, and when cranking - this will indicate if the drop there may be a problem - as a general 'old school'guide we would try and keep the batter>starter drop less than 0.1 Vm but moder starters may accept more. Next I'd repeat it for the ground/earth points to battery. From those you can estimate the voltage drops across the ECU and where supply issues may be - I would suggest double checking that directly across the ECU connection points.

The ECU should have a 'Voltage' readout, if you compare that to what you're seeing across the power-ground, it will indicate the voltage drop in the wiring and whether it needs to be addressed.

If you post those voltage values, we may be able to go into it further.

Gord, because the ECU is shutting off, the car currently won't run. I'll do some multimeter tests and get back on here later. If it's any insight, the alternator isn't actually able to charge currently, so it's purely on battery power. Could that cause it possibly?

The NB2 uses an ECU side voltage regulator instead of an internal one. I don't have it hooked up as of now, but I was mostly going through my diagnostics before I got to the additional setup of the ECU.

For the voltage comparisons at the ECU, compared to at the battery, and the other points, the engine doesn't need to be running.

If it comes to that, you don't actually need to use the vehicle's battery to crank the engine - it's a bit of messing around, but if you disconnect the main vehicles cable from the starter - if you take the power to the ECU, etc, from that point, bolt the battery lead and the take-off together - you can take that out of the dropping voltage issue. Use an external battery only connected to a chassis or engine earth and the +ve side connected by cable to the starter terminal. This is different from a simple 'jump start' in that the two batteries are isolated from each other, rather than connected in parallel.

So I did some various testing, and ended up switching the circuit the ECU is powered on. Now it says on through cranking, but still no start.

I noticed when I turn the engine over my tuner studio doesn't read an RPM signal. So looking into that I'm going to first just try swapping my crank sensor.. any thoughts on this? I am going to try back probing the crank sensor to check for resistance