BACKYARD MECHANIC: Parasitic drain suspect with battery problem

Hey there, time traveller!
This article was published 08/04/2011 (5336 days ago), so information in it may no longer be current.

QUESTION: My daughter has a ’98 Taurus and has had a problem for the last five years. It appears that the battery will not hold a charge. She has replaced the battery numerous times and has the same problem with the new ones. She has replaced the alternator once. From Internet research, I found there are hundreds if not thousands in North America with the same problem. Suggested solutions always refer to the alternator or battery. I would be interested in your opinion.

ANSWER: From your description, where the battery and alternator have already been replaced, I would think the problem is more likely a parasitic drain on the electrical system that is discharging the battery. Fifteen years ago, this type of problem would arise only occasionally and then typically on luxury vehicles equipped with lots of electronic features. Today, this type of problem is fairly common, as even lower-priced cars and trucks are equipped with many computers and electrical options.

On your Taurus, the two main computers are the engine computer and the GEM (generic electronic module) module. Engine computers can cause a parasitic drain but it happens rarely. The GEM module operates body electrical systems such as lights, locks and alarms and because of the many inputs and outputs, it is a much more likely suspect. The GEM module is mounted in the left rear quarter of the car behind the trunk upholstery and has many wiring connections to it.

When you unlock the car, operate a switch for a controlled system, open a door or turn the ignition on, the GEM module “wakes up” and uses electrical power. After a time period without any inputs, the GEM module will turn off the controlled system and go back to sleep. If the module stays awake, then it will drain the battery, even though it may not appear that anything is operating.

A battery drain can be tested by installing a digital amp-meter in series with one battery cable. With the car in a sleep mode, the drain should be less than 25 milliamps. This test takes some time because you have to wait for the computer to go to sleep, which could take an hour. Also, if you operate any high-current circuits such as ignition or interior lights while the amp-meter is in place, it will overload the amp-meter and blow a fuse in the meter, so know what you are doing before connecting the meter.

A simple way to test to see if the GEM module is going to sleep is to leave a door open so the interior lights are on. Don’t touch anything on the car and in about an hour, the GEM module should turn the lights off and go to sleep. If the lights don’t go off, the GEM module is staying awake. The problem could be an input to the GEM module keeping it awake, but more commonly it is a problem in the GEM module itself and it needs to be replaced.

 

QUESTION: I got my first oil change done today on my 2010 Civic EX-L and had them put in Mobil 1. Can I expect better fuel economy with synthetic oil? I know for sure that Mobil 1 made my last car, a ’96 Saturn SL2, start noticeably easier in winter but I don’t know if it improved the mileage.

ANSWER: Synthetic oils flow easier at colder temperatures while still protecting engine parts when engine temperatures get hot. Their wide operating range of protection is a primary benefit. Vehicles that are operated for short-trip driving especially in cold weather will probably see a benefit in better fuel economy compared to a conventional oil. I say probably, because you could install a light-grade conventional oil that would start easy too, but it wouldn’t provide high heat protection like the synthetics.

Will your Civic get better economy from the synthetic oil? I am guessing not, because the factory oil fill for many manufacturers is already synthetic oil. You will likely experience better economy, however, because moving drivetrain parts are becoming more polished with use. Keep the synthetic oil. It will provide better fuel economy than the same grade conventional oil in most driving situations.

 

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net