Batttey Drain Testing

[vesnyder]

I am testing my electrical system for what appears to be a parasitic draw on the battery. I disconnected the negative terminal and used an amp meter to measure the amperage draw by connecting the gauge to the batter terminal and cable. I noted when I am measuring the draw that the trunk light does not come on. It is only showing .33 mAmps but the trunk light does not go on. Is there something I need to do to ensure I am getting an accurate reading? I get a sense that the meter is not closing the circuit entirely and I am not getting an accurate amp draw reading. What should I be doing that I am not?

[dad911]

Check the fuse in the ammeter.

I prefer to use a 'clip on' DC ammeter for these tests, its easy to pop the fuse.

[timmy2]

The trunk light not coming on may be a clue.
Does it come on when the negative cable is connected without the meter in the circuit?
If so you may have a bad fuse in the meter or you are on the wrong scale.
You need to be on the 10 amp scale, with the lead plugged into the correct amp socket on the meter.

Type of meter? Picture of connection?

[911pcars]

Did the trunk light work before testing?

The connection of the meter between the – (neg.) battery post and the disconnected ground cable is correct. With the meter in series, all system current is being measured. Suspect the trunk lid switch is still open, thus no current flow to the trunk light. Switch a dome light ON, meter should indicate current flow of a few amps.

BTW:
.33 milliiamps = 0.00033 amps. You sure? Check the decimal place.

Convert milliamps to amps - Conversion of Measurement Units

Sherwood

[vesnyder]

Thanks - I did have the setting on the meter set incorrectly. Now that it is working and the trunk dome light goes the draw is at .833 amps with the dome light on and about .183 amps with it off. Does that sound about right? What should be the threshold draw?

[timmy2]

Pull the clock fuse and see if it goes to zero.

[911pcars]

I think this video pretty much covers it:



Sherwood

[rick-l]

My 88 draws about 100 mA.

The first owner installed a state of the art alarm system (for 1988) including lo-jack. I can't find the lo-jack and every time I go a little further removing all the alarm systems the car won't start until I figure it out and bypass something.

[Algernon]

Originally Posted by vesnyder

... about .183 amps with it off. Does that sound about right? What should be the threshold draw?

OOh! Still sounds a bit high! 40AH battery with 0.2A drain wont last more than a week!

[911pcars]

It's very probable that vehicles with smart keys, alarms and audio systems suck up additional electrons as we sleep. While overnight current drain isn't good for the life of a battery, a daily use vehicle shouldn't feel any short-term effects. However, for garage queens, a battery maintainer is essential equipment to keep the battery as fresh as possible.

Sherwood

[darrin]

Do you have power windows? If so, does your dome light work when you open a door (especially your driver's door)? If not, can you still roll up/down you windows after you open your driver's door?

This is a pretty common source of parasitic drain (the window circuit remains active after turning off the car and is deactivated when the door switch signals that the door's been opened -- if the door switch has failed, the circuit will remain open indefinitely and will slowly drain the battery). Easy to repair/replace.

[vesnyder]

Fuse #5 on the '81 SC Cab shows as windshield heater? That is the fuse that is causing most of the draw. Need to do some checking to see if I wired the amp of CD player to it but when I connect it I hear a click coming from under the dash. Need to do some more research but could this be the clock' fuse?

[Bob Kontak]

Clock fuse is on the circuit that reads OXY. Close to the driver on the row of fuses. Same circuit as the glove box light and the under front trunk light.

I am pretty sure. Not able to double check my car and too lazy to look at the electrical diagrams.

[DaveMcKenz]

The easiest way to check for a current flowing in a fused circuit, is to measure voltage drop across the fuse. If there is no current flow, the value will be 0 milivolts. If there is current present the voltage drop across the fuse will be 10-30 milivolts. You don't pull any fuses, just use your digital voltmeter to measure DC voltage across the fuse terminals. You can check 20 circuits in one minute.
Good luck,
Dave

[vesnyder]

I was counting from the wrong end and it is fuse #17. According to the manual it covers the defrost , wipers and emergency flashers. The wipers still work without the fuse and I hear a clicking under the dash when the fuse block is powered. Any idea what could it be that is making the noise? The only thing that does not work that I have found is the emergency flasher.

[A horse with no name]

Hi, try checking for the leak with your multimeter's 'Resistance' position as you may have a short in one of the harness's, including trying the 'Continuity' position. Check by using both of these multimeter positions on all circuits, such as the rear brake and park lights etc.

Often it is a ground problem but you have already checked all of the body grounds.
Small electrical leaks can often be very hard to find. It's a slow operation, but by doing one at a time, you have to unfortunately check all of the cars electrical components and as you know, there are just sooo many of them

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