Slow battery drain...having problems troubleshooting.

[cpharmston]

Hey all,

I have a '98 Mercury Mystique that's giving me problems. The battery is draining while the car is off, and the speed of the drain is rapidly increasing. Last Wednesday it died over an 18 hour period, Friday it took 12 hours, and this morning it took less than 5. I've done some general troubleshooting, and I know that the problem is NOT:

• The battery. I had a new battery installed in January, so it was still under warranty. I brought it in Sunday, and it registered as "bad battery" so it was replaced. It did not fix the problem.
• The alternator. After I jump the car and drive it for 20 minutes, it will start up immdiately. It may be an alternator diode, but the alternator itself is still performing its job.
• A fuse (I think). To verify that a current was escaping the battery, I disconnected the positive lead from the battery, and joined them with a voltmeter. It registered a current, so I proceeded to take each fuse out one-by-one and checked the current after each, with the hypothesis that it would help me isolate the problem if I found a specific fuse that was causing it. The removal of no specific fuse or relay changed the current.

Other random oddities about my car that may or may not be important:

• Bad O2 sensor that I never bothered to replace.
• I bought the car at 40,000 miles and it now has about 160,000 on it. In that 120,000 mile stretch, the only non-routine repair that I had to make was a $50 replacement of the blower motor.
• The passenger-side speakers stopped working 3 years ago.
• Some of the dashboard lights randomly flicker on and off, and have been doing so for the last 2 years. Most notably the ABS light. There's an odd relationship between the vehicle's acceleration and the e-brake light, which also has been acting funny for a few years. While I'm driving, the light is on, but it flickers off when I use my regular brake.

I think that about covers it. Does anybody have any ideas? It may be a bad alternator diode, it may be a pinched wire somewhere...I'm pretty much at a loss. I'd very much appreciate any help.

Thanks everyone!

 

[amc49]

A bad diode is not an alternator doing its' job. Car if used during the day may start/run 2-3 days with no charging at all if battery new. 20 minutes is NO TEST of alternator good/bad. Throw a meter on battery while running and make sure you have voltage above the battery volts. Battery volts 12.8 max brand new, charging needs to be 13.5 to 14.5 volts. Anywhere in that range, MUST be slightly higher than battery volts. Also, same test you did by yanking fuses, do with the alternator wiring disconnected, see if leak goes away. Could be one way diode that stops alternator from draining battery while motor stopped. When I do fusepulling, I draw where fuses go or have fusebox diagram handy, I pull one by one AND LEAVE THEM ALL OUT to avoid confusion. Sooner or later EVERYTHING will be unplugged, narrows problem down some. I'd be suspicious of those dashlights, did you kill power to the instruments? Other things which draw power with ignition off, radio, clock or whatever it's in (trip computer), brakelights, courtesy system, seat belt interlocks, airbags, pull underhood and trunk lights. Lotta things pull power at all times now on modern car. You defeat enough things to narrow down what it is. Too easy to get confused one at a time and plug it back in. Be careful around the airbags, there are cautions about just powering them up, indeed, you should be afraid of an airbag module even uninstalled out of the car. I would pull any fuses to them first, leave out, and reinstall them last, both to ease your mind and then you don't think of them again until done with the repair. Spent 10 hours once on a car doing that same stuff, after got tired of battery running down. After like 5 miles of wiring harness out of car, found problem in solid state chip for seat belt interlock system. Problem can be anywhere, only good concerted logic will find it, but it IS DOABLE. Think of the $100/hr. you're saving by not getting Ford to do it, you'll never be paid better.

 

[Tony2005]

What engine and transmission?

Test the battery and alternator.

Check the engine wiring harness to make sure non are chaffed or exposed.

As a temporary solution, install one of those $5 switches which goes to the battery terminal (that you can turn on and off). I think most autoparts stores have them. This will ensure the battery doesn't go flat when you are not driving it.

 

[amc49]

Here's another thought. Unplug the entire alternator wiring FIRST and check for the leak. If still leaking then proceed to the rest of car, but leave alternator unplugged, both harness and main big wire. Have seen what appears to be a leak before through alternator that later proved to be "normal" as in nothing wrong. Thinking it had something to do with the small amount that went through meter did not allow for the voltage potential amount necessary to shut off a transistor or somesuch. Thought for awhile that alternator was the problem, it took me off where the real problem was.

 

[Linkman]

Hey all,

I have a '98 Mercury Mystique that's giving me problems. The battery is draining while the car is off, and the speed of the drain is rapidly increasing.

• A fuse (I think). To verify that a current was escaping the battery, I disconnected the positive lead from the battery, and joined them with a voltmeter. It registered a current, so I proceeded to take each fuse out one-by-one and checked the current after each, with the hypothesis that it would help me isolate the problem if I found a specific fuse that was causing it. The removal of no specific fuse or relay changed the current.

What value of current did you see? There is a normal current to support the alarm, PATS, keep alive memory, radio settings, etc. I don't recall it off the top of my head, but the Ford Service CD will give you a value - it's in the alternator/charging system diagnostic instructions.

 

[amc49]

Yup, there are lots of things pulling power with key off, you gotta weed all that out. Fun.