Dear Tom and Ray:

My wife heard a report on a local radio station warning people not to use jumper cables on modern cars. The guy said they could cause "unstable voltage" and damage on-board computer operations. I own a 2005 Subaru and a 2007 Ford. When I contacted both vehicles' dealerships, each told me it is OK to use jumper cables. Additionally, each told me that the devices they use in their repair departments to start cars with dead or low batteries are just like jumper cables. So? What's the truth? Are my dealership's service representatives ignorant? Please advise! Thanks! -- Rob

RAY: It sounds like you're describing my brother, Rob. Unstable voltage and on-board computer problems.

TOM: Actually, your dealership's service representatives are exactly right. We jump-start cars all the time at the shop, and have never had any car suffer from "unstable voltage" or computer damage when it was jump-started correctly.

RAY: Now, there are lots of problems that can be created if you hook up the jumper cables BACKWARD.

TOM: Right. That can cause unstable voltage, unstable angina, fried computers, angry customers and lots of explainin'.

RAY: So, when you DO jump-start a car, you want to take great care to make sure you're doing it correctly. If you're not sure how to jump-start a car, we have a diagram and instructions on our Web site at www.cartalk.com. Print 'em out and stick 'em in your glove compartment.