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Voltage Drain

  #1  
Old 01-12-2019, 11:35 PM
rjp1965
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Default Voltage Drain

I am trying to figure out what may be causing my battery to drain when my 1998 Ram 1500 5.9l is parked.

Here's what I've done thus far:


1) Followed the Ignition Off Draw Test in the service manual.
  • The voltage drain is ~ .4 amps.
  • I have pulled each fuse and each relay to try to isolate which circuit the draw might be happening in. None of the "normal" fuses resulted in any change / reduction in the draw.
  • Pulling the 50 Amp fuse labeled "Battery" in the under hood box and pulling the IOD fuse from the fuse panel on drivers side end of the dash were the only two that reduced / eliminated the draw.
2) I didn't have the problem prior to doing the following work all at the same time:
  • Disconnected / reconnected steering column connections while replacing the heater core and evap core. Note: I pulled the computer off of the firewall, but did not disconnect it.
  • Disconnected / reconnected the air bag while pulling and replacing cruise control set switch.
  • Installed new brake light switch on brake pedal.
  • Removed and reinstalled the stereo to get a stuck tape out.
  • Installed a new power lock mechanism on passenger door.
3) Does the fact that the draw doesn't appear to be in any of the circuits, does that indicate that it's a problem with the PCM/ECM (whatever the correct term is for the computer mounted on the firewall)?

Any advice or help would be appreciated.

Thanks.
 
  #2  
Old 01-13-2019, 02:44 AM
DerTruck
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According to the wiring diagram the 50A fuse only supplies power to the dash fuse panel and downstream equipment, the PCM is not connected that way but via fuse 3 in the PDC. 0.4 amps x 12.something Volts is around 5W or about a small light bulb. The IOD fuse should be fuse 12, that one has a number of lamps on it: Glove box, dome lamp, cargo lamp, underhood lamp, vanity and courtesy lamps. Plus some other things like the radio memory power, see page 8W-12-12 in the service manual. You probably need to go around everything and disconnect it unless there's an obvious light stuck on.
 
  #3  
Old 01-16-2019, 10:09 AM
99DR
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Have you cleaned your battery terminals and ground? If there is resistance in the battery cables, there will be a small draw across them/it, and your battery will drain very slowly. A .4 amp draw is very small. I'd look at the battery cabling before messing with anything else.
 
  #4  
Old 01-16-2019, 11:34 AM
wrnchbndr
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You're troubleshooting seems to be on track. Have you searched your truck in the dark looking for light? Do you have lit vanity mirrors? Sorry about mentioning the obvious but I've had my own share of brainfarts. Radio auto-antenna circuit? USB accessory plug/adapter? Defective radio clock circuit? Bent or pushed pin in one of the connectors that you disconnected doing maintenance? I think you're safe disconnecting the battery and then carefully disconnecting the PCM and doing a current draw measurement but someone else please comment if its not. I know that on my truck, the alternator field control is in the PCM. Bad diode in the alternator? I think the current draw of 0.4A rules out a defective battery and general electricity principles don't support dirty battery connections but weird stuff happens. I got something weird happening to my '97 1500 that I'm gonna post right now.
 
  #5  
Old 01-16-2019, 11:48 AM
99DR
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Originally Posted by wrnchbndr View Post
You're troubleshooting seems to be on track. Have you searched your truck in the dark looking for light? Do you have lit vanity mirrors? Sorry about mentioning the obvious but I've had my own share of brainfarts. Radio auto-antenna circuit? USB accessory plug/adapter? Defective radio clock circuit? Bent or pushed pin in one of the connectors that you disconnected doing maintenance? I think you're safe disconnecting the battery and then carefully disconnecting the PCM and doing a current draw measurement but someone else please comment if its not. I know that on my truck, the alternator field control is in the PCM. Bad diode in the alternator? I think the current draw of 0.4A rules out a defective battery and general electricity principles don't support dirty battery connections but weird stuff happens. I got something weird happening to my '97 1500 that I'm gonna post right now.
There's nothing really weird about it. When a cable has resistance, it is a resistor. Resistors draw amperage. I had a similar issue once that drove me nuts. Then I found a ground cable that was swollen in the middle. Replaced it, problem solved. When I cut the insulation from the cable, it had a ton of corrosion separating the copper wires.
 
  #6  
Old 01-16-2019, 12:20 PM
wrnchbndr
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Not exactly. But I totally have seen circuits act in apparent disregard to the rules so I'm not saying you are wrong about the corrosion. I'll also add that inspecting and addressing the battery cables is always a good idea. Corrosion between two differences of potential can certainly cause current flow but adding resistance to a series path should generally decrease current flow. I=V/R. I've seen a lot of stuff that makes no sense but keep one foot in the door while the other contemplates Karma and the existence of a giant mutant space goat on its way to eat our planet.
 
  #7  
Old 01-16-2019, 12:24 PM
99DR
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Hey, if you happen to see that goat, let me know. He jumped out of the back of my Ram when I was driving home to my farm! On the voltage drop, imagine replacing the corrosion with a small light bulb. they are both resistors, and both have a small draw.
 
  #8  
Old 01-18-2019, 12:08 AM
rjp1965
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Originally Posted by 99DR View Post
Have you cleaned your battery terminals and ground? If there is resistance in the battery cables, there will be a small draw across them/it, and your battery will drain very slowly. A .4 amp draw is very small. I'd look at the battery cabling before messing with anything else.
Thanks for the question. Yes, I actually installed new terminals and a new battery after this started. I will check the rest of the cabling now that you've mentioned it.


 
  #9  
Old 01-18-2019, 12:11 AM
rjp1965
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Originally Posted by wrnchbndr View Post
You're troubleshooting seems to be on track. Have you searched your truck in the dark looking for light? Do you have lit vanity mirrors? Sorry about mentioning the obvious but I've had my own share of brainfarts. Radio auto-antenna circuit? USB accessory plug/adapter? Defective radio clock circuit? Bent or pushed pin in one of the connectors that you disconnected doing maintenance? I think you're safe disconnecting the battery and then carefully disconnecting the PCM and doing a current draw measurement but someone else please comment if its not. I know that on my truck, the alternator field control is in the PCM. Bad diode in the alternator? I think the current draw of 0.4A rules out a defective battery and general electricity principles don't support dirty battery connections but weird stuff happens. I got something weird happening to my '97 1500 that I'm gonna post right now.
Thank you for the suggestions. I'll work through them and see what I can figure out.
 

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