2015 Mazda6 won't start (but has power) after new battery - Mazda 6 Forums : Mazda 6 Forum / Mazda Atenza Forum
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post #1 of 9 (permalink) Old 11-20-2018, 02:56 PM Thread Starter
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Question 2015 Mazda6 won't start (but has power) after new battery

The car: a 2015 Mazda6. I canít turn up anything specifying GT, et cetera (without a VIN search, I guess), but sheís a sedan and seems to have the default engine specsÖ?

The problem: She wonít start; just clicks (once) as everything lights up.

The time frame: Since I dropped a new battery in a few days ago, stone-ignorant of the need to drain the ECU. I also tried, initially, to put the battery in backwards, frying fuses (which Iíve since replaced).

Yes, in hindsight, I KNOW that was stupid! Iíd assumed that Western batteries and Japanese would have terminals on the same side. Go ahead and laugh.

The location: Arkansas (the mid South).

The notes: I donít begrudge dealership employees a living (at all), but taking her in now could take a painful bite out of the family Christmas. Thatóand conviction that I made a mess of my wifeís car, so I need to clean it upóis why Iím desperately hoping to fix it myself.

A check-power-steering dash icon lights up at start now, too (and wasnít before). Not certain why itís appearing, but I suspect that itís a clue.

Research suggests possible ECU damage, but I donít see anything that clearly looks like an OEM ECU under the hood. I havenít found much thatís helpful on the Web, either (after multiple attempts). Is it in the interior, maybe on the driverís side? How much would I have to take apart to reach it?

I did read where someone pulled an ECU fuse and replaced it, but I canít find one labeled ďECUĒ in the hood or interior fuse boxesí lid diagramsÖ?

Any/all constructive suggestions are welcome. Thanks for reading this far!
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post #2 of 9 (permalink) Old 11-20-2018, 06:41 PM
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Photo shows where ECU is. Plastic cover protecting it.



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post #3 of 9 (permalink) Old 11-20-2018, 07:41 PM
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If you connected the battery backwards (positive and negative reversed) odds are you've destroyed module(s) in the vehicle. Maybe all of them. Check all fuses; you might have gotten VERY lucky but I wouldn't take a wager on it and you don't want to know exactly how bad this could be in terms of dollars.
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post #4 of 9 (permalink) Old 11-20-2018, 08:03 PM
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Your battery supplies 12.5-14 volts to the system. The computers on your car use anywhere from 6 volts to 6 millivolts. They do this by using voltage converters, but when you hook the battery up backwards you bypass those converters. It can fry your computers very quickly.

Get a scan tool and see what the check engine light is. Maybe all that power just blew a small component somewhere. Or, you may need to replace an entire ECU unit. Or, worst case scenario, you damaged one or several of the dozens and dozens of sensor relays throughout the vehicle, which would practically total the car.

Scan tool first, then take it to the dealership.
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post #5 of 9 (permalink) Old 11-21-2018, 01:21 PM Thread Starter
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I'd have replied sooner, but I can't get this forum to load in the Web browser on my phone. Had to wait and use my wife's desktop: 'preciate it, MadStyle!
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post #6 of 9 (permalink) Old 11-21-2018, 01:23 PM Thread Starter
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Quote:
Originally Posted by tickerguy View Post
If you connected the battery backwards (positive and negative reversed) odds are you've destroyed module(s) in the vehicle. Maybe all of them. Check all fuses; you might have gotten VERY lucky but I wouldn't take a wager on it and you don't want to know exactly how bad this could be in terms of dollars.
OUCH--hard to hear... But I value truth over cushy speech, any day. Thanks for your reply, TickerGuy!
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post #7 of 9 (permalink) Old 11-21-2018, 01:27 PM Thread Starter
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Quote:
Originally Posted by Iamadoorknob View Post
Your battery supplies 12.5-14 volts to the system. The computers on your car use anywhere from 6 volts to 6 millivolts. They do this by using voltage converters, but when you hook the battery up backwards you bypass those converters. It can fry your computers very quickly...
Thank you for the informative post, Iamadoorknob! I'm trying to keep up the hopes... but I'm hearing you.
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post #8 of 9 (permalink) Old 11-21-2018, 01:30 PM Thread Starter
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I'm HOPING to at least reduce the cost of a dealer visit, if it can't be avoided--and I've pulled all the smaller fuses. Do I need to replace the fatter ones, too?

Any idea which module(s) would affect power steering? Where would they be?

I'm guessing that I'd be out over a grand if I'd taken her in already...?
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post #9 of 9 (permalink) Old 11-21-2018, 02:06 PM
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Quote:
Originally Posted by Juan14Six View Post
OUCH--hard to hear... But I value truth over cushy speech, any day. Thanks for your reply, TickerGuy!
Yeah, being a designer of electronic "things" by both amateur practice and (as part of my writing code years ago) by trade, I know how the usual design practices work in "hostile" (lots of noise and spikes expected, etc) environments work.

There's usually a zener on both power and signal inputs designed to shunt overvoltage spikes. The problem is that a zener has a "reverse breakdown" that is used to shunt off the spikes; over "X" voltage and it conducts (to ground in this case.) So long as that spike is of relatively low-current (on a signal line a series resistor is used to limit the current it can carry) this is very effective at preventing overvoltage spikes from getting into the circuit beyond that point and destroying what lays beyond.

Unfortunately most semiconductors don't behave well at all when an *opposite* polarity current appears. A zener will immediately conduct in the "forward" mode (typically at 0.3 or 0.7v, depending on whether it's germanium or silicon) and present FULL reverse voltage shorted to ground. Whether the resistor in-series is enough in that situation to prevent the diode (or the resistor, especially if it's not a noise spike but a persistent voltage) from being destroyed, or whether what lays beyond it will survive the reverse voltage is unknown. Ditto for linear and switching-mode regulators used to drop the +12V rail to the CPU and external circuit voltage requirement. If the reverse voltage gets through *them* the odds of those external circuits and CPUs surviving is near zero.

It's possible to design devices to be tolerant of this situation (the use of a standard forward-biased diode on the feed to the regulator will do so) but you have to design for the intentional 0.7v voltage drop that produces and deal with the heat dissipation that comes with it. I have no idea if that is being done in common automotive applications; the heat dissipation issue, especially for high-load electronic circuits, is where the problem can become quite serious.

The other common reverse-voltage protection scheme used with electronics is to intentionally reverse bias a diode across the DC input terminals with enough current-carrying capacity that the fuse will blow before the diode is destroyed if power is connected backwards (and an appropriate PIV rating for reverse breakdown.)

This is why I suggested checking ALL the fuses; if you got lucky you will find one or more of them burnt and in that case that protection MIGHT have been effective.

I just don't know what, if any, protection Mazda (or their suppliers, such as Mitsubishi in their case of the ECU) employed with their modules as I've never torn one of them apart to look.
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