I have 2003 Mercury Sable that I bought about a.

TERRY T

MEMBER
2003 MERCURY SABLE
74,000 MILES

I have 2003 Mercury Sable that I bought about a year ago. I've had dead battery issues with it the whole time. It seems that if I drive it every day its fine but if I go a couple of days it won't start without a jump, then its fine until I leave it for a day or more. The battery and alternator have been replaced and they both test ok, and there's no corrosion or other connection issues as far as I know. Until now I assumed its my fault for not driving it enough, but I've been doing some research online and now suspect I might have a parasitic drain. The interesting thing is that my previous car, a 2000 Grand Am, did the same thing and it also tested ok. I had a brainwave yesterday when trying to charge it (it wouldn't charge by the way but was fine when it was jumped). It occurred to me that I've always kept the auto headlights on, even when the car is off. I can't afford to go to a mechanic and have the car tested for a drain, so I want to see if I might be right with my deduction that leaving the headlight switch on auto all the time is draining the battery. By the way, these are the first cars I've owned that had the auto feature.

1 Reply

CARADIODOC

MECHANIC
33,729 POSTS

It's amazing what the engineers dream up when they assume we can't be bothered to turn a switch. Turning the feature off most likely will not solve the problem unless the computer is turning the head lights on at inappropriate times. If the computer is drawing excessive current, it will do that regardless of how the switch is set.

To not drain the battery enough to prevent starting for one day suggests a drain of less than half an amp. That would be less than the typical small glove box bulb draws. I'd pop that bulb out first just to be sure that isn't the culprit. Other than that, you need to perform a drain test at the battery. Unless specified differently by the manufacturer, the industry standard is 35 milliamps, (.035 amps), is the maximum allowable drain. At that rate a good battery will start the engine after sitting for three weeks.

You've also mentioned two car models known for developing generator problems. GM's are MUCH more common than Ford's, but if your generator has an intermittent problem, any testing has to be done while the problem is occurring. That means identifying that there is a problem first. Look for dimming head lights, a heater fan that slows down, or a dropping "Volts" gauge on the dash. If you see anything like that, the first step is to measure the battery voltage while the engine is still running. It must be between 13.75 and 14.75 volts. If it is low, Ford provides a convenient test point on the back of the generator for the next test.