Car Talk — Motorist who crossed terminals attempting to jump-start car lucky if only radio blew out

• Q. So, a co-worker needed a jump-start after leaving her lights on, and I thought I would be a good Samaritan and help her after work. Her car was parked on a busy road, and I had to pull up next to her in the opposing traffic lane to be able to reach her battery. In my haste, I put the positive and negative clamps on the wrong terminals on MY car (hers were correctly attached). When I got in my car to start it, I could see that the wires were smoking. I immediately got out and disconnected the cables. We ultimately needed to purchase new cables, and then we were able to start her car. The problem is that now her car works perfectly fine — except for the radio, which seems to be dead. When she brought it to the dealer, they said there is some kind of electrical system failure. Is that possible? Please help, as I may be responsible for money for repairs! •

RAY: I hope she’s cute. Then you can justify the expense you’re about to incur.

TOM: It’s not only possible that there was damage to the electrical system, it’s likely. In fact, if the radio is the only victim here, you would be not only a good Samaritan, but also a very lucky Samaritan.

RAY: Usually when someone crosses the terminals in a jump-start attempt, there’s a lot more electrical-system damage. One of the cars even can burst into flames. That’s usually our first hint that something’s wrong.

TOM: I think your mistake was jumping out right away and disconnecting the cables when you saw them smoking. If you had waited just a little longer, the radio would not be a concern right now.

RAY: What you did was send a whole lot of current through the ground wires, which are not supposed to be energized. So, lots of things can get fried, including in-car electronics, computers and the wiring itself. TOM: If you’re really lucky, you just blew a fuse or a fusible link. It’s unusual for a radio to be on its own fuse, but perhaps there are other problems with things on the same fuse as the radio that she just hasn’t discovered yet. Or perhaps it’s an aftermarket radio with its own in-line fuse. So start by asking someone to check that.

RAY: And get a little more information from the dealer about what he means by “electrical system failure.” He may just have seen some partially melted wires, and didn’t investigate further.

TOM: If it’s more than a fuse or a new radio that she needs, you should contact your insurance company. Better insurance companies should cover this type of accident under your comprehensive coverage. You may need to have your co-worker file a claim with HER insurance, which will then seek to collect from your insurance company.

RAY: We hope it’s just the radio. That would be a small price for this kind of mistake. And by the way, they now sell “idiot-proof” jumper cables, which won’t operate if you hook them up incorrectly. Go out and buy two pair.

• Q. I’ve had this 2001 Chevy Cavalier for almost five years. It’s been a fantastic car. I take care of it, and it takes care of me. Here’s the problem: Every year when the temperature starts to drop, my speedometer goes haywire. The outside temp only has to drop into the 40s for this to start. Morning, afternoon, evening — it doesn’t matter. One minute I’m right on the speed limit, the next it’s saying I’m doing 110 mph. I can be at a stoplight, and the speedometer still will read somewhere between 20 and 100 mph or higher. Doing the math on the interstate and side roads to calculate my true speed is really starting to irritate me. What could be causing this, and how can I fix it? •

TOM: You can fix it with a new instrument cluster.

RAY: This particular instrument cluster uses little stepper motors for the speedometer and tachometer. And because those two gauges get the most use, they tend to fail first.

TOM: There’s no speedometer cable in this car. Information about the car’s speed is sent to the speedometer by the vehicle speed sensor, or VSS. But it’s very unlikely that a bad VSS would cause these particular symptoms.

RAY: I’m guessing you’re going to have to get your instrument cluster repaired or replaced.

TOM: Getting a new one from your Chevy dealer will cost you hundreds of dollars. But you don’t need to buy a new one — after all, the rest of the car is more than a decade old.

RAY: I’d look for a mechanic who’s willing to source one for you from a junkyard. Of course, the danger here is that you’ll get a used one with the same problem, since it’s a fairly common complaint.

TOM: But your mechanic can at least get a warranty on the part so that if it turns out to be faulty, he can go back and get you another one. You’d still owe him for some additional labor, but not too much, since he’d be able to test the part before putting the car back together.

RAY: And given the age of the car, a used instrument cluster will take this car either into deep old age or until something much more serious breaks. Good luck.

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