No electrical power issue

Dremxx

New Member
Jun 13, 2019
3
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Ohio
I was driving down the highway in my 2002 mustang gt (138k miles) on my way to work when all of the sudden my car lost power. The car cycled on and off very quickly and i was able to drive home. I put the car neutral and let it sit for about an hour. When i tried to start the car again there was nothing. No headlights, no dash lights, no working key fob, and no radio. I have no idea what is wrong with it at this point as no fuses are blown. I replaced the starter as I thought that maybe starter solenoid had shorted out but this was not the case. There is power to the fuse box in the engine compartment and i can get the starter to crank when bypassing the relay and jumping it out in the fuse box. The car has a new ignition switch, starter, starter relay, and battery. I do have a car phone charger which has a led on it to indicate when it is getting power. The charger has power when the the car is off (the charger always has constant power) but as soon as you hit the brake pedal, turn the ignition or attempt to turn on the headlights the car looses all power. There is also a massive parasitic draw with the car as well that caused the battery to drain from 12.4v to 4.8v overnight.
Any advice is much appreciated. Thanks
 
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wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
I'm a big believer in Occam's Razor. A possible simple explanation would suggest that a weak ground is at the root of the problem. The starter relay is grounded at G102 which is located on the front left hand radiator core support.


IMO your problem is not a parasitic draw but instead it's a MASSIVE voltage drop problem. Here's some information on how to use the voltage drop test method to narrow down exactly WHERE the problem is.

Or you could use the ole "visual inspection" method. A good electrical connection is clean and tight. No loose wires. Bright shinny metal. No rust. No corrosion. Check all connections at the battery, battery terminals, and grounds around the radiator core support.

Howto perform charging system voltage drop test

The thing that throws many people is the part about testing is putting both VOM leads both on the same polarity (negative/negative positive/positive). Use the VOM to "frame" the current path to narrow down where the high resistance connection is.
 
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wmburns

SN Certified Technician
Aug 14, 2009
5,890
518
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Houston Texas
Turned out to be the body control module
It might help others if you included the "process" used to narrow down where the problem was and your experience with the repair.

I have to admit to being somewhat confused. Why? Because some of the systems mentioned (head lights for example) in the opening post do not go through the body control module (CCRM).

Is it possible the problem was in the battery junction box (BJB)? The difference matters because the 2002 model year does not use a PCM addressable active battery junction box (BJB).
 

BernS

New Member
Jul 20, 2019
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46
Connecticut
If you’re still working on this issue:
1. I’m having the same electrical issue in which the car kills all the circuits, except the anti-theft,...and cigarette lighter....
Searching has led me to believe it may be the ignition key transponder. Planning on having it serviced soon so I’ll update.
If it happens when attempting to start the car, try turning the steering wheel to lock and then turn the key.

2. The massive power drain may be your amps if you have the factory Mach system. Apparently something deteriorates inside the circuitry and it just drains. I can’t explain it, just learn to accept it. Used an inline fuse and lighted switch, run through the system’s fuse location, to power the amps only when the car is running and head unit is being used. I routed it this way so power is still being run through ground filter to avoid sound interference.

Hope this helps. GL