Parasitic draw??
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5th Wheeling
Joined: May 2021
Posts: 33
Likes: 8
From: Crofton, KY
Parasitic draw??
Got a 1989 f350 with the 7.3, an I’ve run through a whole slew of electrical gremlins in the 9 months I’ve owned it. Got majority of it figured out (for now at least) but batteries would still seem sluggish if it hadn’t started for a couple days. Disconnected batteries last week and let it sit, and reconnected them this morning and it fired right up. Batteries were bought from oreillys while out for Thanksgiving.
I’ve got my DVOM hooked inline with the positive cable and positive stud for one battery while the other battery is disconnected. It currently reads a max of 2.9 mA. (That was before I unplugged the charger in the 12v socket). After that, it reads a steady .07mA. That sounds pretty good, just wanted to be sure. What kind of range would I need to look for?
EDIT:
The mA readings I initially got were before I realized the meter was set for AC rather than DC. Changed the meter and rechecked. Holds a steady 2.54 mA with the charger unplugged and ~78mA when plugged in. It’s the style charger that has an LED ring around it to make it easy to see if it is getting power.
I’ve got my DVOM hooked inline with the positive cable and positive stud for one battery while the other battery is disconnected. It currently reads a max of 2.9 mA. (That was before I unplugged the charger in the 12v socket). After that, it reads a steady .07mA. That sounds pretty good, just wanted to be sure. What kind of range would I need to look for?
EDIT:
The mA readings I initially got were before I realized the meter was set for AC rather than DC. Changed the meter and rechecked. Holds a steady 2.54 mA with the charger unplugged and ~78mA when plugged in. It’s the style charger that has an LED ring around it to make it easy to see if it is getting power.
Last edited by RUSTEZE; Dec 12, 2025 at 01:04 PM.
Lead Driver
Joined: Apr 2016
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From: Bahstun
Mountain Pass
Joined: May 2020
Posts: 142
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The acceptable draw for a modern vehicle is 50mA. I would expect a lower reading on an older vehicle without modules. Either way, the batteries should be able to handle a draw smaller than that. The way I learned it was that the DMM should be placed in line between the NEGATIVE cable and terminal but that might not make a difference. Make sure you're not loosing continuity between the cable and post while you're disconnecting. Off the top of my head, I can not think of anything on these trucks that should be drawing power while off. But it sounds like your charger is what's causing the draw.
Mountain Pass
Joined: May 2020
Posts: 142
Likes: 9
One more thing... Ford recommends waiting 45 minutes after shutting off the vehicle and disconnecting the negative terminal (keep the multimeter in line and on) before you start testing. This is to let all the modules go to sleep. Obviously, the reasoning shouldn't apply to you but I would still wait to ensure proper readings.
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