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Can hooking up jumper cables incorrectly cause one to have to replace the throttlebody?

I own a 2007 Toyota Camry XLe (6-cylinder) and accidentally hooked battery cables up to the wrong post. The car then, after fixing cables, started right up but would not go over 25 mph. My mechanic put it on the code tester and said it was my throttle body and it cost me $1258 to repair.

The Internet never mentioned computer being involved with this part but usually talked about a fuse. I can't imagine why Toyota would not have some kind of fuse or protection so that a simple mistake would not cost the consumer such a tremendous amount of money. I was bragging on the honesty of my mechanic but now I am beginning to wonder a bit.

Could this all be legit? Thank you for your time.

  • 2
    Beware the post hoc ergo propter hoc fallacy. Your problem arose after hooking up the battery incorrectly, so you assume the latter caused the former. But that might not be true. Consider WHY you had to connect the battery in the first place. It was disconnected for some reason. For example, if you were replacing a bad battery, it's possible an electrical problem caused both the battery failure and the TB problem. – barbecue Dec 30 '16 at 18:53
  • Have you called any other shops to see what they would charge for the same job? That will tell you if your mechanic is quoting the correct amount. Also, many mechanics will quote "worst case" cost so they know they can be less expensive in the end. Did you ask your mechanic why it was so expensive and give him a chance to explain? – CharlieRB Dec 30 '16 at 20:22
  • See my answer to this question mechanics.stackexchange.com/questions/39089/… – Fred Wilson Dec 31 '16 at 4:57
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There's a joke in electronic engineering about transistors being invented to protect fuses. Basically, it means that transistors blow faster, and so a fuse is practically worthless for that purpose. It still protects the wiring from catching fire, but not the circuit from functional damage.

To protect the circuit from functional damage, the designers therefore have to ignore the fuse and design the circuit itself to take whatever abuse that might be. That requires more parts and complexity that adds nothing to the functionality (and occasionally interferes with it), and so the less-likely scenarios from a normal-use perspective are often omitted, leaving the circuit vulnerable to them.

So basically anything beyond letting the car as an entire system do what it's supposed to all by itself carries a risk of damage if you do it wrong. Sometimes you're forced to interfere with it, like if you have a dead battery and need to charge it externally, but it still carries that risk and so you need to be careful about it.

I suspect what happened in your case is that when you hooked up a dead or very weak battery to a good one, backwards as you said, the good battery "won" and forced the bad battery to a negative voltage. Pretty much every silicon chip appears to be a short circuit past about 1.4V applied backwards, and so you got a bunch of current flowing backwards through a path that was never designed to do that.

If you're lucky, then the first device to start conducting effectively limited the reverse voltage to its own threshold, and everything else could still (barely) handle that. If you're unlucky (and more likely the case), then there were a bunch of devices that reached that point, and the relative current through each one depended on the relative wiring resistance that fed them. So now every electronic device in your car has that on its record as a possible latent failure mode.

  • In the case of a reversed power supply, the necessary protective circuits are VERY simple. A single diode will clamp the reverse voltage to 0.7 volts and keep it that low until the fuse blows. – Demi Jan 1 '17 at 2:08
  • @Demi Yes, but you still have to DO it. That's pretty rare in my experience. – AaronD Jan 1 '17 at 2:16
  • I guess you could put a big one yourself across the battery wires - just one will suffice for the entire system. Except it has to handle a kiloamp or so of current, otherwise when it blows, it then allows the same problem that it was supposed to prevent. Or you could use a small-ish one at the input of each device, along with a series resistor to limit the maximum current, but you can't get too aggressive there because that resistor also powers the device. – AaronD Jan 1 '17 at 2:26
  • You could (partially) solve that problem by putting another diode across the resistor, but then you might as well delete the resistor and the original diode altogether and just have the one diode that simply blocks reverse power. I have seen that, but it's still far from ubiquitous. – AaronD Jan 1 '17 at 2:26
  • Diodes generally fail shorted, rather than open. Note that it only needs to handle the current for long enough to blow the fuse, which ought to be an easier task. – Demi Jan 1 '17 at 3:14
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The problem with hooking up the jumper cables backward is that ANY electronic component in the vehicle is at risk. This could mean the throttle body (TB). It could be an alternator. It could be the computer. Anything can get fried as it looks like the TB was at the mercy. You are trying to force the electrons to go the wrong direction. To electronic components, this is what kills it. If your mechanic has been trustworthy to you thus far, there is no reason to doubt him this time. It appears putting your jumper cables on the wrong way has produced a costly mistake for you. I'd suggest you're lucky you only came away with frying the TB ... it could have been much worse.

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It could very much be possible that you messed up your throttle body, could have shorted something out if its electronic, which it most likely is, or worse you could have shorted something out in the cars ecu/computer. Best case scenario would be a fuse, but it doesn't sound like it. But A throttle body at most would be a couple hundred and it usually doesn't require a mass amount of work to be done, so that $1258 price quote does seem a little fishy, unless they're charging for something other than the throttle body part + labor. If I were you I'd take it to another shop just to get a second opinion and a price quote, cause i'm sure for just a throttle body replacement you could get a better price.

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    A quick search on google and you can find the OEM part for $600, which supposedly has a list price of over $900. AutoZone has an aftermarket for about $550. If it's a smaller shop, and they are paying close to list price for an OEM part, $1200 doesn't seem that outrageous once you add labor, gaskets, and tax. I would call a toyota dealership and just ask them to quote a replacement to gauge just how far over you are getting bent. – 8bitwide Dec 30 '16 at 18:38
  • You're right I think I was looking at a different part I think. Toyota dealership is definitely the way to go though – Iqbal Khan Jan 11 '17 at 16:37
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I find this puzzling. The most obvious way to hook up jumper cables incorrectly is to reverse the positive and negative cables at one of the batteries. That will produce a dead short across the good battery. The result should be quite dramatic with lots of arcing, noise, probably some smoke – but the voltage imposed on the rest of the car should be lower than normal, not higher. That said, arcs are weird things and something could have been damaged by arc related voltages.

Shorting the battery, and arcing in general, seems like the sort of thing that is both predictable and common enough that you might expect designers to try to protect against transients in a case like this. The protection would be designed into the inputs of sensitive devices. A fuse wouldn't help here. But, there is no easy way for us to know if such protection was provided, and if it was, if it was designed to handle a case like this.

So…, I think the questions you want to be asking are:

  • What is a reasonable price for troubleshooting the problem and replacing the, presumably failed, throttle body?
  • Does the diagnosis make sense? The throttle body itself probably isn't damaged, but some of the electronics associated with it presumably are. Would the failure of that component result in the symptoms you're seeing? Can those components be replaced individually?
  • What else could cause the problem you're seeing?

Ask your mechanic to talk you through the diagnosis. In the past you've trusted your mechanic, give him/her the chance to explain the logic of the diagnosis. It would also be a good idea to get a second opinion. If this all makes sense, then chalk it up to an expensive lesson and decide if the repair makes economic sense to you.

  • Why would it produce a dead short against the good battery? What component do you think would act like a short? – David Schwartz Dec 30 '16 at 21:50

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