Drive it forever : Mystery problem a blown turn bulb

Dear Mike: The turn signals on my old Jeep flash really slowly when I’m signaling left turns, but work fine when I signal right. Even weirder is that if I have my motorcycle trailer hooked up, the left turn signal is fine, and the right side blinks really fast.

What’s going on?

A: One of your left-turn bulbs is burned out. The flasher relay on most cars uses a very simple thermal mechanism to flash the lamps. When the turn signals are turned on, the current to the two filaments (one front and one rear) passes through a resistance wire wrapped around a bimetal strip. When the wire heats up, it makes the strip bend, breaking the electrical connection at one end of the strip. The lamps go out, the bimetal strip cools off and unbends to restore the connection, and the entire process repeats.

When you hook up your trailer, your left turn signal circuit now has both the Jeep’s and the trailer’s signal lamps, so it operates at the correct frequency. The right turn circuit has all its lamps operating, and drawing current for all of them makes it blink faster.

To make the blinkers blink at the correct rate when the trailer is attached, switch to a heavy-duty flasher relay. This type uses a more sophisticated coil-and-capacitor LC circuit (or even a small circuit board with transistors) to operate at a fixed frequency regardless of load.

Warning: If you replace your flasher relay with a heavy-duty unit, the flash rate will remain constant when a bulb blows out, giving you no dashboard indication that a bulb has blown. If you don’t tow a trailer, you should use the simpler thermal variety, often called an SAE (Society of Automotive Engineers) relay.

Wait — there’s more. Aftermarket LED-style bulbs are becoming more common. But because they’re not compatible with an SAE-type relay, you’ll need to use the heavy-duty-style flasher if you replace your incandescent bulbs with LEDs.

Confounding this issue is the fact that some cars now use a computer-controlled flasher module that’s integrated into the lighting control module. It’s expensive to replace, too. A simple thermal relay costs about $2 at the auto parts store. A heavy-duty relay is a couple of bucks more. A new lighting control module can cost several hundred dollars.

Dear Mike: Why can’t I siphon gas from my 1996 Toyota Corolla?

A: Starting right after the 1973 oil crisis, most car manufacturers designed the filler necks of their new cars with a baffle a few inches down to prevent people from siphoning fuel. And just as well — the traditional method of starting a siphon by sucking on the end of the hose is a good way to aspirate gasoline and give yourself a case of chemical pneumonia.

Dear Mike: I have to keep my car parked outside at night. My son-in-law, who is a pretty good “Saturday mechanic” himself, yowls whenever I mention using a car cover. He says they scratch your car. I can’t see how they can do any more damage than bird droppings, tree sap and dust.

A: Using a car cover can be a good way to prevent many types of damage if you use one properly. If you must park in an area that leaves your car covered with cat-paw prints, bird poop and tree sap, a cover can help preserve your finish. It’s also possible to do serious damage with a car cover. A dirty cover, or a clean cover installed over a grungy car, can scratch the finish.

Use a high-quality fitted fabric cover, not a plastic tarp. Don’t cover a hot or wet car. Don’t leave the cover on during windy days; the flapping can chafe the finish. If it rains, remove the cover beforehand. If it’s too late, you can remove the cover after the rain stops, but don’t let the wet cover dry on the car or it will mark the paint. Launder the cover regularly to remove dirt or contaminants.

Install by placing the cover in the middle of the roof and unfolding it toward the corners, rather than laying it on one end of the car and pulling it over. Remove by folding the corners to the center and then lifting it off the roof.