1991 Pontiac Grand Prix Battery Drains Overnight

MIKLEMAK

MEMBER
1991 PONTIAC GRAND PRIX
6 CYL
FWD
AUTOMATIC
95,700 MILES

I seem to have a parasitic battery drain. A fully charged battery is dead in a day or two. I just bought a new battery and want to solve the problem before the new battery dies. I did a battery drain test on it and found nothing. Here's how I did it. I locked the doors and waited 15 minutes, then disconnected the negative battery terminal and attached an unpowered test light between the negative terminal and the battery and the light glowed brightly. While in the car with the doors closed and key out of the ignition, I removed all of the fuses, one at a time and the light continued to glow. Then, while under the hood with the doors closed and no key in the ignition, I pulled all of the fuses and relays under the hood. I disconnected the Aftermarket CD Changer and throughout all of this, the test light glowed. I disconnected the alternator and just about every electrical device under the hood and the light still glows. All of the accessories operate except for the power antennae and the left turn signal bulb is burned out. In the past 8 months I have replaced the: EGR Valve, IAC, TPS, spark plugs, all filters, brake pads, rotors, calipers, tires, etc, etc, etc.IÃ¢ï¿½ï¿½m stumped and am hoping you might be able to help; please.

2 Replies

MMPRINCE4000

MECHANIC
8,548 POSTS

Get yourself a DVOM and repeat the process, set to DC volts at the most sensitive setting (usually 100ths of a volt). Volts will drop until you pull the correct fuse, which will isolate the drain circuit.

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Saturday, November 21st, 2009 AT 8:59 AM

MATHIASO

MECHANIC
1,209 POSTS

Hello miklemak

On late model vehicles with computers and numerous electronic modules, the key-off drain on the battery can range from 20 to 50 milliamps, to as much as 300 to 400 milliamps. That is why the unpower test light will glow as long computers and modules memories are alive.

As a rule, the parasitic drain on most late model vehicles should be less than 50 milliamps one hour after the vehicle has been shut off and left undisturbed
Keep in mind, though, that opening a door, the trunk or turning anything on can wake up various modules and start the timer countdown all over again. So if you want to check the parasitic draw on the battery, leave the hood open.
Current drain can be measured with a digital multimeter (DMM) using the ammeter function.

1. Set the DMM to read milliamps (mA). If the meter is not auto-ranging, select the 100 or 1000 mA scale.
NOTE: the DMM should have a 10 amp fuse in it to protect the meter from possible damage if there is a large current drain on the battery.
2. Plug a battery backup into the cigarette lighter (or attach a backup battery or power supply to the battery cables) to protect KAM while disconnecting the battery cable
3. Disconnect one of the battery cables and connect the DMM test leads to the cable and battery post.
4. Unplug the battery backup and note the current reading on the DMM.
FINDING THE CAUSE OF THE CURRENT DRAIN

If the key-off current reading is above specifications (typically, more than 50 mA one hour after the vehicle has been shut off), the current drain is too high. It's time to start pulling fuses and relays to find the fault.
Pull the fuses and relays one at a time until the current reading drops
Avoid pulling the fuses for the PCM or other KAM-sensitive modules until you have checked all of the other circuits.
Once you've found the circuit that is causing the excessive current drain, check the relay, switch, module or other components in the circuit and replace as needed, or we can provide the wiring diagram for further help.

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Saturday, November 21st, 2009 AT 9:03 AM