Mechanics

BATTERY DRAIN?

1999 Ford Taurus • 143,000 miles

Battery full charged, key off, test light between unhooked neg cable and neg post the light is blinking bright and the theft light is blinking with the light. Pull what it says is the radio fuse #24 5A which in the manual it is the speedo, icp, rcc. The car is fine with that out. I have replaced the radio which had a tape get stuck and it hasn't made any diff, Don't know what would make tne light blink with that fuse in but can't figure out. Any ideas? Thanks Dale
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Punkoboy2
January 10, 2012.




You need to track the circuit -could be direct short to ground or a component problem
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Tiny
Rasmataz
Jan 10, 2012.
You can't use that common sense test light in series with a battery cable. There are computers trying to go to "sleep mode" which can take up to 20 minutes. Until then, it is normal to see a current drain of up to three amps. Your test light adds too much resistance and limits current flow so some computers won't be able to perform some self tests. They will reset and try again leading to a pulsing test light.

You can insert an ammeter between the battery post and cable but you must also bypass it with a jumper wire. If you start out with it on a 2 amp scale, you could blow the meter's internal 2 amp fuse. If you start out on the ten amp scale and wait for the computers to time out, that scale won't give you anywhere near the needed accuracy so you have to switch to a lower scale. That means removing a lead and placing it in a different jack, or it means turning the switch. Either action creates a momentary open circuit and wakes up the computers for up to another 20 minutes. To prevent that, you must use the jumper wire to bypass the meter any time you move a lead or turn the switch. Once you're on the new scale, remove the temporary jumper so all current goes through the meter.

Once the computers are timed out, unless the manufacturer tells you differently, 35 milliamps, (.035 amps) is the industry standard maximum allowable drain. Some manufacturers now specify up to 50 milliamps because there are so many unnecessary computers hung onto every part of the car. Each one has a memory circuit that constantly draws a little current. 50 milliamps won't even come close to lighting up a test light but it can be enough to drain a battery in a couple of weeks.

Caradiodoc
Jan 10, 2012.
I agree with caradiodoc. Good post. My 2003 Taurus has had some weak starts lately. Battery tested good, and it s a good one only 2 years old. Voltage at the battery about 14 when running with full light and ac load. No real volt drop from battery to alternator. No idiot lights on. Battery drain between 30 and 40 mA when car is asleep. Haven't figured out why the start gets labored. Do u think the starter is actually a possibility?

Tiny
Cjmj
Aug 6, 2012.
You're better off starting a new post. When you piggyback on this one only rasmataz and I will get automated e-mails directing us back here. None of the other experts will see your problem or have a chance to reply.

40 ma is not excessive and won't run the battery down for weeks. You can prove that by connecting a battery charger, then cranking the engine. If it's still too slow, suspect a dragging starter or an excessive "voltage drop". Voltage drops are easy to measure. First measure the voltage right at the starter terminal and engine block while a helper cranks the engine. If you find 9.6 volts or more during cranking, suspect the starter. If you find less than 9.6 volts, suspect a bad connection. This page will explain how to find the bad connection with voltage drops:

http://randysrepairshop.net/voltage-drop-tests-in-a-high-current-circuit.html

A common cause of slow or no cranking on Ford products is a corroded battery cable right at the starter. The strands of copper wire corrode away under the insulation where you can't see it. That will show up as a very high voltage drop.

Caradiodoc
Aug 6, 2012.

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