Battery clamp errors can spell disaster

Dear Mike: A friend of mine told me that I should always disconnect the positive terminal of the battery first to keep the electricity from spilling out of the battery. I can’t see how this would matter. I just unhook whichever terminal is closest first. I’ve never noticed any spilled electricity, and I think he’s just pulling my leg.

A: Let me relate something that happened to a friend of mine, a dentist who likes to occasionally work on his sports car. In preparation for some other work, he elected to remove the battery from the car, using a box wrench to loosen the battery clamps. The wrench made contact with his wedding ring (he’s left-handed), and the ring in turn made contact with the battery holddown. A massive short circuit welded his ring to the holddown and to the wrench, which in turn was welded to the positive terminal.

Car batteries store an enormous amount of energy, and they’re optimized to deliver it in a very short period of time. A shorted car battery can easily deliver several hundred amps — more than an arc-welding machine.

Within a second, my friend’s wedding ring was almost red-hot, and only a fast reaction with his other hand to break the connection by hitting the wrench prevented it from remaining connected long enough to melt. The ring had to be sawed off his finger at the emergency room, and it was more than a month before the burn specialist was sure he wouldn’t lose the finger altogether.

Two lessons here.

First, remove all jewelry when working with tools, even something as simple as a box wrench. That’s fairly obvious, eh?

Second, always remove the battery’s ground clamp before loosening the positive. If you remove the negative clamp and inadvertently complete a circuit to ground, there will be no current flow because the ground clamp is already grounded. Subsequent shorting of the positive terminal to ground will then produce no current flow because the current has no return path to the negative post.

And, of course, always reconnect the ground last.

Dear Mike: My dad was a mechanic and instilled in me that I should never start and then turn off an engine without getting the temperature gauge to at least register some heat. This is to keep totally cold oil from clogging up the pistons. However, my service manager says this doesn’t apply to newer engines. What say you?

A: Unheated oil? Clogging up the pistons? Where do these things get started, anyway?

From the top: Shutting down an engine that hasn’t reached operating temp will have zero short-term effect. As far as warming up an engine before driving away, that old nugget no longer applies because modern engines have better lubrication, tighter clearances and, in general, are more resistant to sludge formation. Start the motor, buckle your seatbelt, adjust the mirrors and drive off at normal speeds.

I suspect your father was over-generalizing from another reason to warm up an engine thoroughly: Low-temperature running is rich running, and will cause unburned fuel, water and acids to collect in the crankcase. A few weeks of being driven only to the corner for groceries or to the train station for commuting to work, and these contaminants will build up, ultimately turning to sludge.

So every engine needs to be driven for a half-hour or so after it has warmed up to boil off the gruel and water. But doing that once or twice a month is plenty. It’s not necessary to come to full operating temp every single time you drive. Starting it for a few seconds and then shutting it off is probably less harmful than starting it and letting it partly warm up, as your dad suggests.

Dear Mike: I have a problem with the ABS in my 1992 Buick Park Avenue. One day as I was driving, the car just started to slow down all by itself. After a lot of looking and trial and error, I unplugged the rear-wheel sensor and the problem stopped. My question is, since I unplugged the sensor do I still have all-wheel braking?

A: If your ABS (or at least the system on your Buick) packs up, you’ll still have brakes on all four wheels, but without the benefit of the anti-lock feature. On most vehicles, this is not an issue, but on some — notably pickup trucks — this can lead to lack of stability under heavy braking as the rear brakes lock up early.

If unplugging the sensor makes your problem go away, don’t be fooled. You’ve simply disabled the entire ABS, masking the symptoms. The sensor can be checked for proper function without replacing it.

I suggest you get a mechanic who has a scan tool to check for trouble codes and a good multimeter to check that sensor, because it doesn’t sound like you’re ready to tackle this on your own. Something in the ABS is making that wheel brake by itself, so get it fixed soon.