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What's draining my battery

I have a 98 Mercury Sable. If you start and drive the car at least once every 24 hours, no problems. However, if you leave it sit for extended periods, say 48 hours, the battery is totally dead and I need to jump the battery. Replaced the battery but this did not fix the problem. Took the Merc to a dealership service department who replaced the battery cables; did not fix the problem. They then replaced my new battery with a Ford battery and said that should do it. It didn’t.
Since it is over 14 years old, I didn’t want to spend a lot of money to diagnose the problem. The dealership said they wanted to find the problem and diagnosis would be no additional charge. They kept it for a week and finally threw up the white flag saying they couldn’t determine what was draining the battery after not running the car for 48 hours. Before I donate the beast, I was wondering if anyone else knows what is causing this.

The battery drain might be caused from a module that’s failing to go to sleep. Some modules can stay awake for up to an hour before they go to sleep.

If this is the case, you can’t use the old way of checking for a parasitic current draw by disconnecting the negative battery cable and installing a test light/meter between the battery and the cable. Because if there’s a module that’s failing to go to sleep, and you disconnect the battery, you force that module to go to sleep. Then when you go to test for the current draw and it’s not there.

There’s two ways to check for a current draw that might be caused from a module that’s failing to go to sleep without disconnecting the battery. One is to take a high resolution infra-red thermal gun and at point it at the fuses. If there’s a circuit drawing current that fuse will be warmer than the others. Or, you can use a volt/ohm meter and check for a voltage drop across the fuses. If there’s a fuse with a higher voltage drop across it than the others, there’s current passing thru that fuse heating up the fuse element causing the higher voltage drop.


Try to find a shop that specializes in electrical repairs and take the car to them. This problem shouldn’t be hard to find and fix and you don’t need the dealer service to fix it.

This is the proper way to check for parasitic draw:
Hook up an ampmeter in series between the negative battery cable and the negative battery terminal.
Lock the car and let everything go to sleep.
Check your reading in about 30 minutes.
50 - 60 milliamps would be okay. Less than 20 milliamps is preferable.
Make sure to set the scale to read in milliamps, NOT amps.
If it’s too high, start pulling fuses.
Occasionally, something unfused is the cause of the draw, but it’s uncommon.
I just diagnosed a 2007 Tahoe this way. The factory audio head unit was the problem.
The radio was turned off and the screen was dark, by the way. So just because something appears to be turned off is no guarantee that it isn’t causing a draw.
In my opinion, based on what you posted, the dealership replaced your new battery for no legitimate reason.
Here’s another opinion: somebody more experienced and with more patience should diagnose the cause of the draw.
Is the car alright otherwise? Unless it’s really beat up, it might be worth hanging onto it. After all, it is paid off, right?

That’s the old way to test for a current draw. Modern tech’s use this method for the reason I mentioned.


You will need a good, independent mechanic to find the parasitic draw. There are infinite things that can cause a battery to go dead over time. My son, when he was young, dropped a toy “dime” in my cigarette lighter. The dime was made of an aluminum alloy which was just conductive enough to run the battery down over a weekend. Since I’ve never smoked, the lighter was the last thing I checked.

Tester, I don’t want to say anyone’s right or wrong, but my way is still an accepted method of checking for a parasitic draw.
By the way, none of my colleagues check for a draw using the voltage drop method. And some of us aren’t even old.
Some of the old ways do still work, in my opinion.

Not anymore. If there’s a module that’s failing to go to sleep, and you disconnect the battery that forces the offending module to go to sleep. Then when you go to check for a current draw it’s not there.

I was taught this diagnostic procedure at one of the CarQuest Tech Training seminars I attended.


The question I usually ask is, is there something that is not working? For example, if you have a power window that doesn’t work, you might automatically think that it could not be the problem, but it could be.

If you just want to try something, I’ve used the old method described with a test light. Started pulling fuses to see when the light dimmed. One turned out to be an intermittant problem with the rear load leveler module. It was intermittant so after messing around pulling fuses for an hour, all of a sudden I got a brighter light. Another time it turned out to be a door handle turning on the interior lights with no one around and draining the battery. The usual suspects are trunk lights, glove box lights, lighters and so on. If you got spare time and you’re lucky, you might find it. If not, you’ll have to go the new tech route or take it to an electrical shop.

Thanks to all who responded. I am not mechanically inclined but its good to know the vehicle can be fixed reasonably easily. Yes, the car is paid for and in good shape otherwise.

So if you’re not mechanically inclined, it might be worth spending a few bucks to have a pro take a crack at diagnosing your problem. Maybe you’ll get lucky and he’ll find your problem relatively quickly.

Tester, us do-it-yourselfers can learn a lot just by reading your answers. I used to use a test light between the cable and post but was aware it didn’t work with modern cars.
I did have a car with a battery going dead at randomly overnight. Some mornungs it would be fine, others dead. One night I went out to the van 5 hours after I parked it and heard the cooling fans start up. It was a 2002 Mopar minivan and it had a bad fan control module,It was a bear to find,it was underneath the bumper cover and bolted to the top of the steerl bumper.
Don’t think any diagnostic method would have helped with that.

That’s why I’m on the board. To try and help the D-I-Y’er. And help them understand how a modern vehicle works.


Rereading your replies, it appears your procedure is the only way that will work if the problem is a module that fails to go to sleep. I didn’t know that and I learned something - as I often do from your posts.

How often is the cause a failed module vs the many ways parasitic drains have been showing up for years? My sense is it is low enough such that the age-old lo-tech process described by db4690 is still valuable to know.

Another option is a micro-amp clamp on DC ammeter. They use a hall-effect sensor to measure the magnetic field produced and have resolution down to 1 ma. Due to the detection mechanism, they can also show current flow direction in the wire (if that’s important to you). One of these babies can be had for around $380. If you routinely do this kind of work, might be a helpful addition to the arsenal…

There’s a way to test for a module that fails to go to sleep using the old method. But it requires that a battery disconnect switch be installed. Such as this or this

Install the switch prior to testing. Then when you go to test for the parasitic current draw, connect your meter to each side of the switch. Now when you open the switch the circuit is still complete thru the meter. So the battery still hasn’t been disconnected. So now you can pull fuses/relays to determine what circuit is drawing the current.


This is a nifty tool so see what kind of current goes through a particular fuse.
One would imagine that, once you suspect one particular part of the circuit, you could leave it in line for a while to see what the current does.

It probably isn’t something you’d want to leave inline all the time. I wouldn’t trust this thing to not catch fire if there’s 20 Amps going through it. I also wouldn’t trust it while driving but it would be a good way to see parasitic current draw once you park the car, them being usually small currents.

Sable/Taurus have issues with sticky door switches that would keep the the dome light on after the doors are closed. Usually squirting WD40 inside the door latch mechanisms fixed the problem. Although the first thing a competent dealer mechanic would have done is with diagnostic software identify if any of the door switches is bad.
Its possible if you are not the original owner that the previous owner pulled the dome light bulb, but there could still be another light associated with door switch such a glove box light that could still be draining the battery.
So the big question is does the dome light work as it should with the doors opened and closed?

I like americar’s suggestion. My dad once bought a 1973 Cutlass S as a used car. The car looked and ran great, but if it sat for more than a day, the battery would be dead. His mechanic replaced the battery, but that didn’t solve the problem. I went down one evening and disconnected the negative cable. I then scratched the cable clamp against the post and there was a pretty good spark. I reconnected the cable and thought I would pull fuses one at a time to see if I could find the problem. I had my head under the dashboard and was ready to pull a fuse when I happened to glance at the console between the seats. There was a little bead of light. Inside the console was a bulb that would light when the console lid was opened, but wouldn’t go off when the console lid was closed. I removed the bulb and the problem was solved.
My suggestion is to go out at night and check the car thoroughly. There may be a light in the glovebox, trunk, under the hood, etc. which has a sticking switch. You can do this check yourself for free.