davepl

Being spring and time to move stuff around, in the last several days I've had to jumpstart 5 vehicles. I did 4 of them correctly, then in the dark of a storage unit, hooked one end up BACKWARDS. Always wondered what that'd do. I'd make up some story about how it happened, but there isn't one. It was dark and I was in a rush, and it's that simple, my mistake.



A few sparks, some sizzling, and the faint smell of Chinese food for some reason. It was maybe a total of 4-5 seconds at most.

It was my LS3 '69 Camaro. The boosting vehicle appears fine, but the Camaro then wouldn't start because the fuel pump wouldn't run. I'm hoping the ECM is OK, but it has an unswitched battery connection too, so it's not a given.

The fuel pump is powered by an Aeromotive controller that has a direct circuit to the battery, so I'm assuming it's toast. It could be a fusible link or maybe an auto-reset breaker that has failed, if I'm lucky.

Now why reveal all this and put up with the inevitable abuse? Because I'm actually curious about what happens.

In fact, since the boosted vehicle's battery was apparently almost entirely stone dead, why didn't I wind up just setting up a negative charge environment in the car, whereas instead of having +12V, maybe for a few seconds it had -8V or so.

But why would a negative voltage roast things? Electrons would flow, but in the other direction, but why is that bad?

Put another way, why does hooking up cables backwards roast things and burn fusible links? It seems like it should just be current going the opposite direction on the same gauge wires, so I'm a little confused.

Can anyone explain this "hardware phenomena" to a software guy?

Zip Corvettes

That is the fastest way to take out the ECM.

davepl

Would not surprise me if the ECM is toast. Hoping it's not, but it's right there wired to the battery...

I didn't really get an answer here on this forum, but as near as I can figure, the reason things are so different when run backwards is due to circuits like diodes and regulators. A voltage regulator might limit forward current/voltage, but not backwards. The diode packs in the alternator don't block current going the other way, that sort of thing.

NSC5

It would be prudent for the manufacturers to provide reverse voltage protection for the ECM but they probably didn't.

You ended up with true reversed polarity because the vehicle battery is in parallel with the "jump" battery source and its ability to absorb power connected this way is far less than the source battery to provide power so you 12 volt bus rose immediately to a near 12 volt except with reversed polarity.

A lot of semiconductors, including those expensive dedicated processors, will be destroyed instantly (as in nano or picoseconds) while polarized capacitors will often outgas and spew their contents in milliseconds.

Just be glad it wasn't a new model because instead of the ECM and maybe but probably not an alternator you likely would have lost a boxful of dedicated processors.

Hopefully your radio was turned off; if not it probably isn't going to be happy. Or if it is one of the newer types transplanted into your '69 it won't matter whether it was turned off or not since most circuits are live in those whenever 12 volts is present.

davepl

Thankfully only the one ECM, not even a trans controller because it's a stick. Everything else was turned off except perhaps those things that get a direct connection:

- Fans, but those are on relays
- ECM
- Fuel pump controller
- Horn relay

That's all I can think of that has a direct battery connection.

NSC5

Out of that list I would expect only the ECM and maybe the fuel pump controller as having a problem.

Have you checked the ECM fuse(s)? A shunt type reverse polarity protection scheme is the most common and if the ECM has that protection built in the shunt provides a dead short to ground with reverse polarity which would have blown the fuse. It is a simple scheme to implement because all it needs is a high current diode from the + terminal to ground on the protected side of the ECM fuse; with proper polarity no current flows through the diode so it has no impact on normal circuit operation. The other scheme is the diode is placed in series with the 12 volt feed to the ECM and if the polarity is reversed no current flows; the downside of this method is around 0.7 volt is dropped across the diode in normal operation which further increases likelihood of "glitches" when the system voltage begins to drop due to a poor battery and heavy electrical load.

newskatercat

How are you making out?

davepl

Just working up the courage to go and start diagnosing things... Actually I was in Tucson for a week, just got back.

So I headed over there, confirmed the Aeromotive controller had power, but it was dead. So I jumpered to bypass it, power straight to the pump, and the pump (as expected) still runs. But it wouldn't fire, so I suspect the ECM is roast.

davepl

Weird... I started chasing voltages, checking fuses, all of that. I could not get it to fire, so as a last resort before replacing the ECM, I brought my laptop to see if I could communicate with the ECM at all... I was really hoping the ECM fuse block had a blown fuse as NSC5 speculated, but I didn't find any.

It answered right back, after which the car started. No idea what happened, I didn't "fix" anything, I just took things apart, looked, and put them back together. I did swap the IGN and FAN (unused) relays in the fusebox, but that's the only change. And I had tried starting it after that without success, so I don't think that was it.

So, long story short, it appears the only major damage was the fuel pump controller!

I have a fan relay setup and one of the relays appears not to be working, but I can't say for sure it's even related.

RedLS6

There are diodes which prevent reverse current flow, for various reasons ..... but there are also a lot of chips and circuits which are designed to operate on regulated voltages at something less than 12V. A reverse voltage will often defeat these regulators, and provide other sneak paths, which puts too much voltage stress in a spot that can't handle it. Sounds like the damage was limited here -