Battery drain when ignition is off

Steve Kessler · #1 (**permalink**) 08-25-2002, 08:05 PM

I have been driving my Cobra for 1.5 years with no problem. Recently the battery went dead. I have discovered now when I turn off the ignition and park the car, there remains a current draw on the battery. Also at idle the alternator only puts out around 13 volts with the fan on and 14 volts with the fan off. At normal RPM, the alternator puts out the normal 14.2 volts.

I have checked the positive red cable from the battery to the starter solenoid and no shorts to ground. The problem seems to go past the starter solenoid to under the dash.

I am not too good at troubleshooting electrical things. Has any one ever had a similar problem?

Thanks,

Steve K

HighPlainsDrifter · #2 (**permalink**) 08-25-2002, 08:18 PM

Hi Steve,
You have a short somewhere, the way to isolate it to a single circuit is to start pulling fuses and checking if there still is a current draw with an ammeter. Look for obvious things like; an electric choke, a fan relay that has gone bad[happened to me 2 times] a short in the voltage regulator,etc
Good Luck,
Perry.

Candle · #3 (**permalink**) 08-25-2002, 09:00 PM

we had a similar problem to this on a 280zx that turned out to be the alternator having a short internally. this would still draw current with all the fuses pulled. but as the last post stated you will need an amp meter to chase this problem.
good luck

Bob Putnam · #4 (**permalink**) 08-26-2002, 05:51 AM

It's very likely to be a bad diode in the alternator. If you don't have an ammeter, disconnect the large lead to the alternator and use a test light between the lead and the alternator. It shouldn't light.

ron427 · #5 (**permalink**) 08-26-2002, 05:51 AM

Steve
You can test for a current draw with a standard 12v test light.
With everything turned off, disconnect either battery post and connect the test light in series with the disconnected cable and the battery post. A small draw will burn the lamp. Then disconnect wires, fuses, ect until the light goes out. You may want to start with the large postive wire on the alt. Using this method is easier than trying to detect a small movement on a gauge. If I can help, I am ron621@msn.com. Good luck.
RON

chipper · #6 (**permalink**) 08-26-2002, 10:40 AM

Consider a batter cut off as well, if don't have one. Won't get to the source, but is a good idea to help prevent fires and battery drain when the car is sitting. Had a stuck strater that started frying my wires-main cut off switch in the cockpit saved all. Can get them at a marine store-ignition protected ie sealed so the don't allow sparks. With a "key" or just a switch.Could be a problem with gas vapors in the trunk. cb

Cal Metal · #7 (**permalink**) 08-26-2002, 08:06 PM

My bet is on the alternator. If you don't have any test equipment, disconnect the alternator lead and recheck and see if you have a charged battery after a 24 hour interval. If you do, you know it is the alternator. If not, buy the test equipment and start the procedure.

swk550 · #8 (**permalink**) 08-27-2002, 09:27 AM

Thanks for all the help.

I have discovered when I unplug the wiring to the alternator built in voltage regulator, the current draw to the battery stops.

I don't know how much draw there is as the voltage test light does not light from battery red terminal to disconnected red positive cable nor does it light from disconnected red cable to ground. I do measure over 250ma with my voltmeter.

I bought a new voltage regulator assembly and grounded it to the engine. Same thing happens. So I reinstalled the old alternator and tested the engine. Now with lights and fan on, at 2500 RPM, the best the alternator can put out is 13.2 volts (14.2 volts is normal). This is the high output alternator Ford installed on the 91 Mustang 5.0 engines. Do I need to replace the alternator next?

I do plan to install a battery cut off switch. I have read various opinions on weather to install it on the positive battery cable or on the negative ground cable. Since my battery is in the trunk and my ground cable is in the rear of the car, I can install the switch in the trunk on either cable. If I were to install the switch in the transmission tunnel or under the dash, my easiest wiring modification would be to install the switch on the red positive battery cable. I welcome your input on this one.

Steve K

Steve Kessler · #9 (**permalink**) 08-27-2002, 07:20 PM

Thanks for the help.

You guys were spot on. It was a bad alternator. Replaced that puppy and all is normal.

Steve K