Battery drains overnight

Tiny
CORDELL6969
  • MEMBER
  • 1994 OLDSMOBILE 88
  • 3.8L
  • 6 CYL
  • RWD
  • AUTOMATIC
  • 75,000 MILES
I have a parasitic drain. I have tested using a test light and pulling fuse by fuse until I found the one that turned off the light. It is the 20 amp master fuse that powers PCM/VATS circuit. Question is: with the ignition off is there supposed to be any power going to that fuse? As soon as I plug it back in I can hear the throttle body recirculate.
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Monday, January 16th, 2017 AT 3:39 PM

21 Replies

Tiny
WRENCHTECH
  • EXPERT
That fuse only goes two places. The PCM and the Passkey decoder module. The problem is that the PCM in turn powers other things and there is no way to know the inside circuitry to isolate what it powers.

You have to try unplugging these things to figure out which one it is.
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Monday, January 16th, 2017 AT 4:05 PM
Tiny
WRENCHTECH
  • EXPERT
Here is the wiring diagram.
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Monday, January 16th, 2017 AT 4:09 PM
Tiny
CORDELL6969
  • MEMBER
Okay, I did not unplug the PCM (the three plugs) and no I have not unplugged the VATS. Is the VATS same thing as pass key?
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Monday, January 16th, 2017 AT 4:52 PM
Tiny
WRENCHTECH
  • EXPERT
Yes, it is. (Vehicle Anti Theft System)
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Monday, January 16th, 2017 AT 4:54 PM
Tiny
CORDELL6969
  • MEMBER
Okay, I suppose it is under the plastic at the column. I will try that next. I almost bought a new Ignition switch yesterday, but wanted to wait see if I found anything else before throwing more money at it. It is my son's car.
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Monday, January 16th, 2017 AT 4:57 PM
Tiny
WRENCHTECH
  • EXPERT
No, I would not go throwing parts at it. You are going to create more problems than you have now. That module is not in the steering column either.

https://www.2carpros.com/images/external/57435347_1.gif
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Monday, January 16th, 2017 AT 5:03 PM
Tiny
CORDELL6969
  • MEMBER
Thank you very much. I will let you know what I find.
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Monday, January 16th, 2017 AT 5:05 PM
Tiny
CORDELL6969
  • MEMBER
Unplugged VATS no go, must be PCM.
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Monday, January 16th, 2017 AT 6:47 PM
Tiny
CARADIODOC
  • EXPERT
Are you putting the test light in series between the battery cable and its post? That will not work on cars with computers. You can have a drain that is excessive, but not high enough to cause the light to glow. Also, you may have a computer that needs up to twenty minutes to go to "sleep" mode. Until that happens, it can draw as much as three amps. You need to use a digital amp meter to measure the amount of current flow, and there is a procedure you must follow with it to avoid waking up the computer and to prevent blowing the meter's internal fuse.
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Monday, January 16th, 2017 AT 7:20 PM
Tiny
WRENCHTECH
  • EXPERT
Yes, I probably should have talked about that Doc. The test light trick is just not doable any more. All cars have some degree of draw and the light just is not sensitive enough to read that.

Here are my prepared instructions on how to do this correctly and accurately.

You will need a digital ammeter and a jumper wire with clips on the ends to do this.
First rig any door switches so you can have a door open without triggering the interior lights and unplug the hood light. Remove one battery cable and attach the meter in series between the battery cable and battery post. Take the jumper wire and also attach it the same way. Leave the jumper wire on for at least ten minutes to expire all the automatic timers. Now remove the jumper wire and read the meter. Anything over 50ma is too much draw. The way you locate this is to start removing fuses one at a time until the meter drops to normal level. This will be the circuit with something staying on. Determine what components are part of that circuit and check them individually until the problem is isolated.
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Monday, January 16th, 2017 AT 7:25 PM
Tiny
CORDELL6969
  • MEMBER
Okay, yes the light was in series for quite a while I pulled each fuse and watched light. When I got to maxi fuse 20 amp under the hood that's when light went out. Yes I know I need an amp meter going to get one today, all I have is a bolt meter at this point. I did disconnect the VATS box no change so hooked back up. Note with that fuse pulled over night battery stayed at 12.1 volts. So it seems narrowed to that circuit. Before leaving that maxi in battery would had been 11.7 or less by morning.
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Tuesday, January 17th, 2017 AT 5:03 AM
Tiny
CARADIODOC
  • EXPERT
12.2 volts means the battery is discharged, so your 12.1 volts is not an acceptable reading. I suspect you're going to still have the same problem if you let the vehicle sit for twice as long.

Please allow me to add a few details. Wrenchtech is the first person I have ever run into who understands the need for the jumper wire to short out the amp meter. The reason is anything that creates an open circuit, (break in the circuit), can cause a computer to wake up and need another 10 - 20 minutes to time out again. I use 20 minutes in my stories because there are some '95 Chrysler products that need that long.

Most inexpensive digital amp meters have a common negative jack, a common positive jack for most things, and a separate "10-amp" jack for higher current readings. The highest current reading that can be taken through the common positive jack on most meters is 2 amps. Some computers can draw up to three amps for 20 minutes. There's two problems with using the meter's 2-amp jack. The first is you must disconnect the jumper wire so all current has to flow through the meter to take the reading, then the three amps will blow the meter's internal two-amp fuse. (&* $ ) Start all over! The second problem is if you were to wait for more than 20 minutes, then you remove the jumper, the desired current is too low to be measured accurately on such a high scale. You need to switch the meter to a lower range to get more accuracy. All meters use a "break-before-make" switch that breaks the connection to one of its ranges, THEN as you continue to turn it further, it makes the connection to the next range. That momentary break when turning the switch is enough to cause the computer to wake up again. Poof! Another blown internal fuse.

To avoid all this heartache, remove the negative battery cable, install the jumper wire between the cable clamp and post, then you can connect the amp meter while waiting for the computers to go to sleep. Use the 10-amp range and the 10-amp positive jack on the meter. Disconnect the jumper wire. All current has to go through the meter now and will be measured. When it has dropped below 2 amps, reconnect the jumper wire, then it is safe to move the meter's positive lead to the common positive jack, and switch to the 2-amp range. Remove one end of the jumper wire, and take the current reading. If it is too low and you want more accuracy, reconnect the jumper, THEN switch the meter to a lower range.

Normally that was as far as I went with my wondrous story, but it occurred to me there's another potential pitfall after reading your discussion. When you pull a fuse to see if current drops a significant amount, if it does not, you risk waking up a computer when you put the fuse back in. That would blow the meter's internal fuse again. To avoid this, start over on the 10-amp range, then follow the same steps to get back to the lowest range. In the past we could have made some assumptions about the fuses for each circuit. Tail lights, for example, didn't involve a computer. Same with door locks and power windows. Even by '94, those days are long gone, and everything has a computer involved with its operation. The Engine Computer may need time to go to sleep mode because it is running some emissions-related tests. A Body Computer might stay on for a few minutes to watch for a theft system arming signal, or to turn off forgotten lights. Reinserting a fuse could mimic turning on a customer control, and that could cause a computer to wake up and blow the meter's fuse.

In the '90s, Chrysler said a good, fully-charged battery would have enough charge to crank the engine fast enough to start after sitting for three weeks, if the "ignition-off-draw" current was a maximum of 35 milliamps, (0.035 amps). That is the industry standard unless the manufacturer specifies otherwise. Cadillac used to allow up to 50 milliamps, and that is acceptable for many other brands now. If your battery is going dead overnight, you can expect to find around a half-amp drain or higher. That is equivalent to a # 194 glove box bulb. While that 35 or 50 milliamps is the standard, keep in mind you're looking for something considerably higher if it's killing the battery in a few days. Don't get excited over a drop of just a few milliamps when you remove a fuse.
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Tuesday, January 17th, 2017 AT 3:43 PM
Tiny
WRENCHTECH
  • EXPERT
Most of the DVOMs that I have used only have a self ranging 10amp scale and that has always worked fine.
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Tuesday, January 17th, 2017 AT 6:24 PM
Tiny
CARADIODOC
  • EXPERT
Yup. No switch contacts to worry about with those. I have over a dozen digital meters including one with "lab accuracy" from the mid '70s. That one is no more accurate than my 20 and 30-dollar meters, or the ten-dollar meters from Harbor Freight Tools. I have two problems though with auto-ranging meters that don't apply to most people. The first is I used to scrap out tvs and save a lot of the individual components. (Don't ever let your friends waste so much of their lives with this fruitless effort). I tested all the resistors to know which ones to save. With the auto-ranging meters, it is necessary to wait for them to find the right range. While that two or three seconds sounds trivial, it becomes real frustrating when you're testing 200 resistors. It was faster to sort the resistors by the ranges the meter used, then each test went real fast.

That time issue is of no concern for anyone else, but the other problem I ran into was failing to observe when the meter chose the millivolt range for me. For some reason the little "mv" is usually too tiny to get my attention. I've been fooled too many times and made the wrong decision when working my way through a problem, and I'm supposed to be the expert

I usually know what to expect when I select the range, so I can choose it accordingly, but I'm going to have to rethink owning an auto-ranging meter for exactly the test needed here. The extra cost for this feature is not much, unless you buy one from the guys who drive the tool trucks.
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Wednesday, January 18th, 2017 AT 6:25 PM
Tiny
WRENCHTECH
  • EXPERT
I recently bought a nice little meter made by Power Probe for $80

https://www.2carpros.com/images/external/pwpndmm101es.jpg
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Wednesday, January 18th, 2017 AT 6:33 PM
Tiny
CORDELL6969
  • MEMBER
Ok guys thanks I got a meter once I figure it out I'll test again I followed your advice on the jumper deal.
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Thursday, January 19th, 2017 AT 5:21 AM
Tiny
KEN
  • ADMIN
Thanks for using 2CarPros we are help to help, tell a friend.

Best, Ken
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Sunday, January 22nd, 2017 AT 3:01 AM
Tiny
CORDELL6969
  • MEMBER
Will do Ken thanks for the help.
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Sunday, January 22nd, 2017 AT 6:03 AM
Tiny
CORDELL6969
  • MEMBER
Well never found that drain exactly, however I did put a cut off switch on the negative side of battery. That stopped the drain but. So I have had battery load tested (bought in April 2016), Bought new ignition switch and lock cylinder has a new starter. When the car starts it runs like a champ.

So now here is the problem still with this car, it may start 20 times perfectly fine then will act like a dead battery when trying to start it, Battery is holding at 12.4-12.5 volts even when it just clicks and acts like it wants to go but wont, so I can throw a jump on it and it will start like new 15-20 more time or less then same thing. My son impatient as he is took the car to a mechanic, he pulled a couple codes that came back as the ignition module, but when it cranks over and starts it runs fine, so that has nothing to do with it cranking as far as I know.
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Tuesday, February 14th, 2017 AT 4:23 PM
Tiny
KEN
  • ADMIN
Hey,

This sounds like it could be the positive battery cable which can cause the intermittent problem you are describing. Look of bulging and soft spots at either cable end. This will be a sign the cable has failed.

Here is a guide on battery drains

https://www.2carpros.com/articles/car-battery-dead-overnight

Let me know what you find

Best, Ken

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Wednesday, February 15th, 2017 AT 9:27 AM

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