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Old 04-09-2019, 02:33 AM   #1 (permalink)
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Mitsubishi Reversed battery, now not turning over

TL;DR - installed battery backwards, replaced alternator and starter before realising my mistake. Alternator fuse was blown, replaced that, engine cranks but doesn't run. ECU or fuel pump? Or something else?

So dummy me went and bought a 1993 3000GT SL from a dude on craigslist for $548, who got it from police auction for $350, and it's been nothing but problems since. This is expected; I've rebuilt a VR-4 before.

The main issue up front was the battery wasn't charging, as I learned driving the car home and having it die 5 miles later on the freeway. Alternator was covered in old grimy oil and not charging. Replaced that, replaced the 5-year-old battery it came with, and in an amazing lack of thought managed to swap battery terminals (new battery looked the same as the old battery but had swapped terminals and I didn't double check).

I tried turning it over, not knowing the battery was backwards, and noted the battery voltmeter was pegged at zero, and the flashers stayed on and didn't flash when prompted. Should have stopped there. Starting did nothing but spin the starter backwards and not let it come out. I briefly smelled fuel after this.

I thought (somehow) my starter suddenly went bad, so I replaced that and after replacing that I discovered the battery was backwards. I swapped it around, tried again, and the engine spun about no problem, but found the alternator fuse was blown and it didn't start. Replaced the fuse, and it still doesn't start.

Has plenty of fuel (came with a full tank), battery is good (new), spark is (probably?) good, intake is good, compression is good. Only element missing is fuel. Either the fuel pump is dead, which is fine, or the ECU is fried, which is less fine. Could also be an issue with the MPI circuit or ignition circuit, I haven't been able to test for spark or fuel in the cylinders yet, nor check diodes of fuel pump check connector. Fuses are all visually OK but haven't tested them individually. What's most likely wrong?

Oh and to add to this, the car is parked in a no-parking zone and it's racking up parking tickets. So uh, sooner I get this fixed, the better XD
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Old 04-09-2019, 07:56 AM   #2 (permalink)
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I'm surprised you got it as far as cranking with the reversed battery. On the other hand after blowing fuses aside the main 120a nothing except the starter and some unnecessary systems are left so you won't draw over 120a and can proceed to crank with reversed polarity. Keep in mind just cranking does not require a ecu.

So if some fuses did blow and you put the battery correctly in now. It can still crank.

The ignition switch (keylock) and neither the main power relays in the engine bay care about polarity. The draw is fused and if you would exceed that, the 120a and 40a big fuse for battery power would have blown. All smaller parts like ecu and so on would not get power at all because of how the circuit is wired. And if, - the *sub distribution fuses would blow first. Since you replaced the alt fuse, and can crank, that would likely rule out the 40a battery and 30a ign switch fuse to.

There is most likely nothing vital burned up there. The fuses however for MFI or other systems likely burned out. Please get a multimeter and check them, especially fuse 1(gold square mark) in the chart and just to be sure 4 and 6(you have to unbolt those) number 6 supplies all the fuses with battery power in the small box in the car (red) and fuse 4 supplies the ign switch and so all ign sources there (Blue).

The 1 fuse (gold square mark) supplies the MFI relay with battery power so it can run the fuel injectors and other sensors. The fuel pump gets power from the ign switch. (fuse 4) which supplies fuse15 (in car) and then sparks /Coil ptu and maf get power from fuse12 in the car which again gets power from fuse 6in the engine bay.

Also make sure to check the ecu related fuses in the car, number esp. 19 and 15)

In short. Get a multimeter and test the fuses. It's likely nothing except some fuses fried.
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File Type: jpg mfi n ecu power chart_1554814590043.jpg (900.0 KB, 9 views)

Last edited by enkaio; 04-09-2019 at 08:13 AM.
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Old 04-09-2019, 11:44 AM   #3 (permalink)
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Default Re: Reversed battery, now not turning over

Thank you! I'll look into this deeper later today and report back with my findings.
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