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'88 Silverado...Multimeter Test

A

Anonymous-0

Well-known Member
I'm pretty close to zero in understanding how to operate a multimeter in testing for a possible battery drain on my '88 Silverado. I am getting a reading of 12.91 volts and 1 amp. Does this sound right/normal?

I hope to get serious with some of these ptojects around here as this long winter is s-l-o-w-l-y losing its grip. My nice '88 Silverado has sat long enough.

How long should this vehicle be able to sit without the battery running down?

Thank you,
Glenn F.
 
Testing for battery drain with a volt meter is not going to give much usable information. The semi conductor electronics will leak enough voltage, and memory circuits will draw enough to show voltage when the amp draw is extremely low.

A test light will give a more useful result.

I like to look everything over, doors shut, everything off, hood up (check for hood light), give everything time to "time out" as in automatic lights, etc.

Using the test light, clip one end to a good engine ground, stick the other into the negative post of the battery, then carefully remove the cable from the battery WITHOUT breaking the circuit. This keeps any timers from resetting, or confusing any electronics, etc.

The test light may faintly glow, but not burn full brightness.

If it passes this test, a healthy battery should hold it's charge at least 4 to 6 months, probably longer.
 

A multimeter is an excellent tool for all kinds of electrical problems. Once you switch the leads to test for amperage, disconnect batt cable and hook one lead to cable and other to batt terminal, then make sure you dont overload the meter since the meter itself will be carrying all electrical loads for the truck, most meters are rated /fused at 10amps. You dont want to hit the starter, ignition sw, power windows or something like that.

A general rule of thumb on excessive current draw is 50 milliamps, 50mA or 0.05A. Any more than that you need to start unplugging stuff to find the draw.
 
I had one of those pickups. The battery drain is significant it will discharge in a few weeks under the best of circumstances. But one amp seems excessive.

Back in the old days, you could check for battery drain by hooking a voltmeter in series with the battery. If you saw battery voltage, then something was drawing current. Two things have happened since them: modern vehicles draw quite a bit of current when turned off, operating clocks and keeping the memory alive in the various computers (your '88 has several microcontrollers). The other thing is we no longer use analog meters. Digital multimeters have much higher input impedance than the old VOMs. The result is you really can't use the voltage test any more.

OK, so that leaves current. So you have your meter in series with the battery and it shows a one amp discharge. Just start pulling fuses until you find the one that drops the current to a more reasonable level and go from there. I'd start by disconnecting the alternator.
 
You need a multimeter with a amp scale and/or a milliamp scale. The ones from Harbor Freight for $4 have both. Put it first on the 10 amp scale and hook in-line with a battery cable and the battery post. Negative or positive. Nobody knows which way electricity flows anyway. If you get a zero reading (make sure you don't have a hood light that's on) - switch to the milliamp scale. Up to 50 milliamps is normal and allowable. 100 is somewhat problematic. Anything more will give you issues. A typical car battery can last around 800 hours with a 50 miliamp draw. I.e. one month. Many newer cars draw 30 to 50 milliamps all the time. Also - all batteries self-discharge over time even when hooked to nothing. 2-10% per month.
 
Thank you for the exceellent feedback. I'm going to be looking at it again today.

A related queston: Could excessive amp draw cause intermit engine stall at low rpm?

Thank you,
Glenn F.
 
(quoted from post at 09:19:29 03/16/14) Thank you for the exceellent feedback. I'm going to be looking at it again today.

A related queston: Could excessive amp draw cause intermit engine stall at low rpm?

Thank you,
Glenn F.
Click to expand...
I don't think so unless the amp draw is really excessive. LIke maybe 40 amp or more.
 
My truck had intermittent stalling caused by a bad pickup coil in the distributor. One of the leads was broken. It was not easy to diagnose; I had to leave it with a shop and they ran it for several hours hooked up to their scope before it quit.
 
System voltage to the ignition would need to get below 6-7 volts to make it quit. You'd need one heck of amp draw on a system that is normally at 14.2 volts to get down that low, that quick.
 
Does it restart after it stalls? Could be a fuel issue. Hal
PS: Our 1981 Mercury Cougar was doing that when my wife drove the car which were usually short runs. I drive it about once a month and run it about 65 miles. The speed limit is 55mph, but everyone runs at least 60. It doesn't stall now and I'm carrying about 600 pounds in the trunk on the return trip. Hal
 
Yes, restarts perfectly. It's like you simply turned the key off. Idling in the driveway, at the mailbox, or at a traffic light.

Previous owner had replaced everything imaginable. Just a nusiance.


Glenn F.
 
Something ought to check on an 88. The nut on the starter solenoid where the positive battery cable goes. The main feed to the truck's electrical system is also on the same post. Often that nut is not tightened properly since it's hard to get too. Many after a while cannot be tightened since it's a metal bolt in a plastic cap. Plastic cracks and then you cannot tighten. I've seen many with chronic problems after a starter-motor change.
 
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