How Hooking Up Jumper Cables Backwards Instantly Caused $200 In Damage To My Car

The red clamp goes on positive, the black clamp goes on negative

Illustration for article titled How Hooking Up Jumper Cables Backwards Instantly Caused $200 In Damage To My Car
Image: David Tracy

I’m trying hard to fix and sell some of my cars, but they are literally breaking faster than I can mend them. Yesterday, I went to move one of my Jeeps so I could take photos for prospective buyers. The battery was flat, so I went to jump the Cherokee with my Jeep J10 pickup. Then I made a mistake that cost me big.

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I’m in the process of selling my 1991 Jeep Cherokee five-speed. Keen to get as much scratch as possible, I bought two new bumpers and a junkyard fender/fender flare, and had them all painted at a local body shop. I also replaced the front seats, installed new stock-size tires, and fixed the broken driver’s side vent window. Now the vehicle looks damn good and is ready to be sold:

Illustration for article titled How Hooking Up Jumper Cables Backwards Instantly Caused $200 In Damage To My Car
Image: David Tracy
Illustration for article titled How Hooking Up Jumper Cables Backwards Instantly Caused $200 In Damage To My Car
Image: David Tracy
Illustration for article titled How Hooking Up Jumper Cables Backwards Instantly Caused $200 In Damage To My Car
Image: David Tracy

Yesterday, after cleaning out the interior, I went to move the XJ so I could take some better photos for my Facebook Marketplace listing. The Jeep had a dead battery, so I pulled my Jeep J10 pickup right up close to the XJ’s nose, and broke out the jumper cables.

I hooked them up, and in short order, began seeing smoke coming off one of the J10's battery posts. Shortly thereafter, smoke billowed from my Jeep Cherokee’s Powertrain Control Module, or PCM.

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Illustration for article titled How Hooking Up Jumper Cables Backwards Instantly Caused $200 In Damage To My Car
Image: David Tracy

I corrected my jumper cables, and went to start the Jeep. It cranked and cranked and cranked, but it did not fire. The Jeep previously almost always fired right up, even after sitting with a dead battery for a few months. Something was broken; clearly it was my PCM. As I am a cheap bastard, my first instinct was to try to fix this computer myself, so I took the PCM apart and plugged it in so I could better identify where the problem was. I found a short and a huge charred section of the computer board:

Here are some close-up pictures of the charred board after I removed three offending resistors and one capacitor:

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Illustration for article titled How Hooking Up Jumper Cables Backwards Instantly Caused $200 In Damage To My Car
Image: David Tracy
Illustration for article titled How Hooking Up Jumper Cables Backwards Instantly Caused $200 In Damage To My Car
Image: David Tracy
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This cannot be fixed. At least, not easily. And unfortunately, manual transmission Jeep Cherokees are hard to find in junkyards. The ones that I do find are rarely 1991 or 1992 models, and since those two are the only years that will work for my Jeep, I had to resort to buying a used computer from eBay. This was not cheap:

Illustration for article titled How Hooking Up Jumper Cables Backwards Instantly Caused $200 In Damage To My Car
Image: David Tracy
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After some haggling and after tax, I ended up dropping $212 — all because of a silly, easy-to-make mistake like switching my jumper cable clamps. If someone pays the $7,500 I’m asking for this little machine, then I’ll probably forget all about this unfortunate incident.

Still, it’s just another example of how an experienced wrencher like myself will probably never stop making foolish wrenching errors. Just a month ago, I wasted 12 hours because I lost a valve keeper inside of my engine, knocked over freshly-lapped valves, and used the wrong brake flaring tool fitting. I also nearly burned down a garage by using a torch to try to get a carb cleaner-soaked jet out of a carburetor. Then there was that time I actually used a rented thread-chaser as a nut and had to tear back into my wheel hub to recover it. Oh, and I once seized my engine because I used the wrong main bearing cap.

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But making mistakes is all part of wrenching. Just try to make sure that the mistakes you make don’t get you killed, and you’ll grow into an excellent wrencher.

Sr. Tech Editor, Jalopnik. Owner of far too many Jeeps (Including a Jeep Comanche). Follow my instagram (@davidntracy). Always interested in hearing from engineers—email me.

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