ē 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 ó 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! ē

RAY: I hope sheís cute. Then you can justify the expense youíre about to incur.

TOM: Itís not only possible that there was damage to the electrical system, itís 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, thereís a lot more electrical-system damage. One of the cars even can burst into flames. Thatís usually our first hint that somethingís 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 youíre really lucky, you just blew a fuse or a fusible link. Itís 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 hasnít discovered yet. Or perhaps itís 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 ďelectrical system failure.Ē He may just have seen some partially melted wires, and didnít investigate further.

TOM: If itís 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 itís just the radio. That would be a small price for this kind of mistake. And by the way, they now sell ďidiot-proofĒ jumper cables, which wonít operate if you hook them up incorrectly. Go out and buy two pair.

ē Q. Iíve had this 2001 Chevy Cavalier for almost five years. Itís been a fantastic car. I take care of it, and it takes care of me. Hereís 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 ó it doesnít matter. One minute Iím right on the speed limit, the next itís saying Iím 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? ē

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

RAY: Iím guessing youíre 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 donít need to buy a new one ó after all, the rest of the car is more than a decade old.

RAY: Iíd look for a mechanic whoís willing to source one for you from a junkyard. Of course, the danger here is that youíll get a used one with the same problem, since itís 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. Youíd still owe him for some additional labor, but not too much, since heíd 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.

Got a question about cars? Write to Click and Clack in care of this newspaper, or email them by visiting the Car Talk website at www.cartalk.com.