Questions and Answers

Wrangler Jumped Backwards Q. Hello, My question concerns what could happen if, in the process of attempting to jump-start a dead battery, the cables are connected backwards (i.e. positive to negative, negative to positive) and left that way for about 30 seconds. This happened to me and subsequently I had to replace my alternator, my stereo was blown, a 50-amp fuse was blown.

I also had to replace the battery, but I think this was the initial problem. My mechanic said that the problems with my car, other than the battery, are due to crossing the cables. In fact, before I had told him that the cables were connected improperly he asked me if that was what happened.

He noticed that the alternator was charging at 38 - 39 volts. However, other mechanics have said that the damage to my car could not have been the result of connecting the jumper cables improperly and that there must have been other problems to start with. What do you think?

My car is a 1993 Jeep Wrangler with a 2.5 liter 4 cylinder engine and manual transmission. It has 106,000 miles. Thanks for any help you can provide.

Kevin...

A. Well Kevin, I would say you are lucky that's all that happened. The devices in a car are designed to run on DC current. That is to say there is a definite positive and negative. when you reverse that current, as in hooking up jumper cables backwards, current flows in the wrong direction and will damage some electronic components. Radios, amplifiers and CD players are the most vulnerable. It is also possible the diodes in the alternator could be damaged.

Most EFI systems have a Safety Relay installed to protect the main Powertrain Control Module (PCM), such as the one shown here that Nissan uses to protect there computers. This relay is wired backwards so that if someone hooks up jumper cables backwards, the relay opens and cuts the PCM off preventing any damage.

Let's put it this way, if everything was fine before you hooked up the jumper cables, and these problems started after hooking up the jumper cables, then it is safe to assume they were caused by hooking up the jumper cables backwards.