Car starter motor caught fire - how best to determine the root cause?

This past Sunday, something new happened to me me after almost 30 years of driving several different cars, all of them aged to 10+ years old and well into the “needing stuff fixed/replaced” part of their lives: my car caught on fire.

I was coming home from a road trip, having just driven 300+ miles from Annapolis, MD to Flushing, NY (in Queens, NYC), with a NJ Turnpike rest stop and refuel point along the way. I pulled into a supermarket about 2 miles from my home to buy supplies for dinner.

Upon getting back to the car after that 15 minute excursion, the car wouldn’t start.

And not in a way I was familiar with: the rr-rr-rr-rr of an engine that tried to start repeatedly with a weak battery, or the click-click-click and flickering dashboard of a battery too weak to engage the starter motor, or the total silence of a car that didn’t even attempt to start because the battery was 100% dead, or wasn’t entirely in “Park”, because of some immobilizer feature of the security system that was disengaging the starter, a non-functioning fuel injector, or (on a stick shift car) because I hadn’t put the clutch in (this car is an automatic, anyway).

I’ve experienced all of those, but this time it was different: the starter gave one “rr”, as if it was going to start the engine - but the engine didn’t start, and then it all stopped.

This is a 2008 Acura MDX, a ten year old car that I’ve owned for about 8 years (I got it “lightly used”).

I had just driven it 300+ miles from Annapolis, MD home to Flushing, NY, with a stop on the NJ Turnpike for food and fuel, with no problems, and had pulled in to a supermarket parking lot about 2 miles from my house; the car had been running fine just 15 minutes earlier.

The car has had a few aftermarket enhancements: a Viper car alarm with a remote start feature, and some aftermarket audio upgrades (amplifier, speakers, etc.), put in 8 years ago.

After 5 or so attempts at starting the car with the key, checking the usual reasons like “is the car in Park”, I gave the remote starter feature a try, which automatically tries 3 times before giving up. It had the same problem.

At this point, between my efforts by hand and the remote starter feature, I would guess I’d tried to start the car maybe 10 times total, when smoke began rising from under the closed hood. I popped the hood and I could see an open flame coming from the bottom of my car, below the the battery.

The FDNY arrived shortly afterward to deal with it, by which time the smoke was still everywhere, but the flame itself had fortuately died out. They removed two 80A fuses and said to try starting the car again, to no avail. They said “it’s because your aftermarket audio amplifier overloaded the starter.”

I eventually got the car towed to a service station, who replaced the starter, which had overheated and burned, which fire also melted some of the wiring, including the heavy gauge wires from the battery to the audio amplifier. They also said, “the aftermarket amplifier probably drew too much power,” pointing to the scorched wires that they’d removed from the battery that lead to the amp.

They also said my battery had failed cells and that they couldn’t start the car without jumping it every time, so I needed a new battery. That surprised me a bit since my car had not had any difficulty starting before the incident, and as I checked later, the battery was only about 2-1/2 years old (from Feb 2016). But OK.

I took the car to my audio and alarm installer, who I’ve trusted to do the installs on 5 of my cars over 25 years (including this one), after using several other shops I’ve felt were far less competent and knowledgeable. They said it was the usual case of blindly pointing the finger (as dealers and mechanics will do) at a non-OEM part as the culprit just because it’s easy - “this shouldn’t happen, that wasn’t there from the factory, so, there’s your problem!”

At the same time, my wife pointed out that the FDNY and car mechanic have professional POVs about “what started a fire” and “what parts are risky”, while the audio shop would obviously never admit to any kind of blame (which could result in liability) associated with an electrical fire.

After consideration, I’m inclined to believe the audio shop’s angle. Should I reconsider?

The starter caught fire, which is UNDERNEATH the battery, and the wires leading to the amp were burned. Fire goes up, not down. So the amplifier’s wiring was a victim, not the cause.

Electrical wiring doesn’t “short out”, electrical connections do, unless the cable had gotten chewed through by an animal or something, which it doesn’t look like it was.

In addition, as I’ve said, all my cars for the past 30 years have had aftermarket amplifiers, with no problems like this. It’s not like running an aftermarket amplifier is a high risk thing, I found that a suspiciously facile explanation. And I’m not running massive amps powering an array of subwoofers here, either - this is a family car, it’s just a single five-channel amplifier.

Besides which, the amplifier is drawing power constantly, while driving. The starter draws power only while starting, and this fire occured after repeated attempts to start the car. It’s the starter that caused the fire.

The question is, how does that happen and why? I’ve had problems starting a car before, and done the “many times trying to start the car” thing, and never smelled smoke or seen a live flame before.

Sometimes when a starter fails it can put an excessive load on the starter relay and melt the contacts together causing them to stay engaged. This will quickly result in a fire. The amps should not be pulling any juice until they are turned on. I wouldn’t want an 80 amp device on during the cranking cycle for sure.

Well, no, it doesn’t magically short out, but if you over heat a wire it’ll melt the insulation off and you can certainly end up with a hot wire shorting to ground. From there, any number of things can happen since that power had to find it’s way back to the battery.

I can’t speak to whether or not it caused the problem, but if your aftermarket amp is hot while the car is starting, that isn’t going to help anything. If that’s the case I’d see about wiring it into a circuit that shuts down while the car is cranking. You could also set up a dual battery system with an isolator. That allows you to run certain things (like an amp) off one battery while the other one does the normal car stuff, like starting. They’re both charged from the alternator.

It’s hard to say, but my guess is that a wire, over the years, was rubbing against something and this specific time it made it all the way through the insulation and shorted to ground or it just simply overheated. Once that happens, things go south in a hurry.

I am pretty sure the amp doesn’t engage immediately. I’ve noticed a 1-2 second delay for it to kick in after the engine starts, though if I turn the key one notch to enable electricity without starting the car, the amp also powers up.

Actually can’t understand why the audio installer used the starter as a power tap As stated above, a draw above the design specs can easily fail. Better to install a separate connection and fuse along with a high output alternator

I understand your trust for the installer, but this kind of sounds like a poorly though out installation for a high amp accessory.

Hmmm, yes, this is food for thought. Let me talk about this with them

I would think a delay would be good enough, maybe not?

I didn’t realize the amp and the starter used the same wires. I wouldn’t do it that way. For a high power device, I’d run new, adequately-sized and fuse-protected wires from the device to the battery, since the wiring to the power distribution center (fusebox) wasn’t designed for that load. It makes sense that auto audio installers would just run it to the starter, since it is a heavy wire and already connected directly to the battery. It does set the stage for something to fail, though.

When high current devices (such as a high-power amp or the starter) draw large amounts of electricity, they get the wires hot. For some things, like the starter, this is OK since it’s only used momentarily and then shut off, so the wires don’t get that hot and are allowed to cool off before they are needed again. When you start drawing nearly 2KW (2 x 80 amps x 12 volts = 1920) over that same wire, it might not cool down as quickly. As it gets hot, the connectors oxidize and the resistance of the wire goes up, both of which cause the device (either the amp or the starter) to draw more current, increasing the heat, and so on in a downward spiral that can end up causing a fire.

I have never had an aftermarket high-power amplifier in any vehicle (only aftermarket in-dash radios), and I have never had a starter motor start a fire. Yeah, with a sample size of one, you can’t really draw a conclusion, but I also have never heard of one catching fire prior to this post. OK, still a relatively small sample size, but such things are clearly not as common as, say, a battery exploding, a brand-new part being faulty, or a bunch of other unlikely things I’ve seen happen with automobiles. My point is that the system is designed specifically not fail in the manner yours has failed. It would seem to me that smart money would be riding on the modification that was not included in the original design as being responsible.

Amplifiers usually have a turn-on signal wire (blue or blue/white), and that wire is usually paired with a wire on the head unit. If the installer used that system then the amp wouldn’t draw much of anything until it got the “wake up!” signal from the head unit. The head unit is usually powered off when cranking, meaning it won’t send the wake up signal until after the engine has started or in the ACC position. An aftermarket installer might have done everything differently and overridden this system but that’s how I’d do it.

I’m not sure I’d blame the amplifier itself, but if the installer actually tapped into the high current lug on the starter for the amp’s power feed, that is about the most pants-on-head thing I’ve heard of in a while.

it’s possible the terminal had worked itself loose a bit, enough to cause a high-resistance connection at the starter high current lug, and it overheated while trying to start and caught the surrounding material on fire.

Fires started by overheated wiring tend to exhibit a phenomenon referred to by fire investigaors as “sleeving”.

If the insulation is bulged/expanded away from the wire to where it can slide on the wire, the heat came from the wire if it’s just melted but mostly snug to the wire the heat came from outside.

The load from the amp is trivial compared to the power draw of starting the engine. The “horses not zebras” answer would be to suspect that the starter somehow stayed engaged and overheated, they are only designed to run for a few seconds at a time. It would take a pretty catastrophic failure of an 200w amp to overload the same wiring that can handle the starting load of as much as 2-3 kilowatts.

if it stayed engaged but wasn’t audibly cranking the engine, then the armature would have to have been stalled (locked) too. Which is within the realm of possibility; a locked armature condition is pretty much the maximum current you’ll pass through a brushed DC motor.

There IS a separate set of fuse protected wires to the amplifiers, hooked up through a small box to the battery - those are the two fuses that the FDNY pulled and said to try again, which did nothing.

It is surprising that they were two 80A fuses, though, that seems very high (too high). Though my memory as to their amperage could be faulty because I don’t have them in front of me, they’re in the car, that my wife drove off today; I just remember it was a surprisingly big number.

Checking the manual for what I believe is the amp - and apparently it’s not a a single 5 channel amp in this car as I had thought, but two amps, an Alpine PDX-F6 (4 channels for cabin speakers) and F4 mono (for the subwoofer).

The online manual for these amps state that they should be wired with two 30A fuses; perhaps they installed it with one 80A fuse to each amp? I’ll have to check with them when I bring the car back to them this weekend (after my wife returns with the car).

Still, I am not sure the fuses were actually blown; when I can, I’ll test them with a multimeter. Of course, if they’re supposed to be installed with 30A x2 per amp and instead were wired with one 80A fuse per amp, that would allow much more power through than was supposed to go, though I would think the main risk there was of frying the amp, not of overloading the battery as it powered the starter.

No, there is a separate set of lines going to the battery from the amp(s) and through a fuse box and attached to the battery terminals (on top).

So if that were the source of the fire, how to explain the starter getting burned (which is below the battery)? And whatever power draw the amplifiers represent, it shouldn’t be “spiky” on startup, a really high load would be constantly at that level while in use even while driving, draining my battery and killing my alternator, which it hasn’t been.

See, this is what I initially thought, that the remote starter (the last thing I tried to do, perhaps foolishly) somehow over-engaged.

According to the installer, though, the remote start feature should be no different than my manually turning the key in the ignition - so if that were the case, the fire would have triggered just from me turning the key 10 times in 2 minutes, which shouldn’t start a fire, either, right?

You are not going to overheat he starter to the point of Fire unless it somehow STAYS engaged. Like the engine starts but the starter fails to disengage.

Dozens of normal starting attempts will most likely discharge your battery long before they would overheat the starter to the point of fire.

The brushes are part of an armature assembly and are spring loaded. The brushes are actually solid carbon/graphite and will wear down after time. Power is supplied via a metal/brass clip with wires that lead to the lugs.

It’s possible the brushes wore down to the metal clip, and said clip got wedged against/between a commutator pad (I think that’s the name) and then proceeded to short out. There does exist a very real possibility that a relay can become and remain in a latched state given certain circumstances. We’d have to examine the schematics for the starter and associated circuitry. But, there’s also a solenoid within the starter and that is also susceptibile to being in a latched state given specific circumstances. A nice thing to have would be the starter itself for examination.

If the relay is ok, my guess is that the failure is a combination of brush failure and over saturation of the starter solenoid causing a runaway condition that caused excessive current draw. The amp wires were probably affected due to heat dissapation for the positive and if there’s a ground somewhere where the amp is located (or the amp is grounded) then that negative wire served as an alternate return path for the current.

My money is on the starter.

most starters don’t use a separate relay any more, the solenoid performs two functions: 1) moving the pinion into engagement with the ring gear, and 2) bridging the battery contact to the starter motor feed contact. and yes, I have seen cases where the contact plate “stuck” itself to the contacts and wouldn’t disengage.

They pulled them not because they were bad but because they wanted you to try starting the car without a large load already on the system, drawing power away from the starter.
If they had blown, they could have left them in if they wanted to.

Also, if the installer used 80a fuses instead of 30a fuses, it would only allow more power through in the even of some type of fault. The fuses don’t limit amperage, they simply stop the flow of electricity if it goes above a certain amount (and/or for a certain length of time).

OK that makes sense then. And confirms that I doubt the amplifier’s load was a factor in the fire.

The larger 80A versus 30-40a fuses just means that if there were a power surge, it’s the amps that might get fried, not the other way around (that some kind of “surge” from the amps would cause a problem to the battery and other connected electrical components), because the amps are not a source of power but a draw.

Looking online for starter related fires, they do occur, for all makes. Even boats as well.

Without the starter to look at, it’s all a guess really.