1996 Ford Mustang Tracking down a short circuit

Tiny
THE MOO PROPHET
  • MEMBER
  • 1996 FORD MUSTANG
  • 6 CYL
  • 2WD
  • MANUAL
  • 120,000 MILES
I am working on a 1996 Ford Mustang, 3.8L V6, manual transmission. The power seats were non functional and the battery would be drained low enough to not start the engine after 24-48 hours. I found a bad power seat switch and replaced it, that problem is corrected.

I started checking the wiring out and this is where I am running into trouble. With the battery out, the ignition off, keys out, engine bay fuses oulled, and doors closed, I am seeing a few shorts to ground in the engine bay fuse box. The resistance between the fuses and ground is between 300 and 1500 ohms, and they are in the Interior Lamp fuse, the Audio fuse, and the Head Lamps fuse.

In the interior fuse panel I see similar shorts in fuses 6 (clock, anti-theft), 12 (Door locks), 14 (Power windows), 15 (Safety belt chime, gague cluster lighting), and 18 (Alternator warning light and EEC power relay coil).

I don't see any evidence of an accident, no pinched harnesses, no obviously burnt wiring. One headlamp socket was slightly melted around the ground wire on the lightbulb side.

Before I start tearing this thing completely apart to find a bad wire, I need to know if there are any common issues with the mustangs around this year, or if it is normal for these circuits to have some continuity with ground, or anything else to check.

One last thing, when I disconnect the main wiring harness in the driver's side wheel well through the firewall and a harness connector in the engine bay on the pass. Side, the shorts disappear in the engine bay fuse box, but, naturally, not the inside one.

I've got to sort out the electrical issues in this thing so I can drive it a couple of days and get all the other bugs worked out. I hate cars owned by teenagers and I HATE tracing wires.

So yeah, any particular wires/switched to check, etc etc whatever.
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Wednesday, November 10th, 2010 AT 7:56 PM

3 Replies

Tiny
CARADIODOC
  • EXPERT
Hi The Moo Prophet. Welcome to the forum. You're going about this in a way I've never heard of. Resistance readings are not a good way to troubleshoot these cars. 300 ohms is not a short, in fact, that would allow 40 milliamps of current to flow. Chrysler, (and other companies) say that at 35 ma, the engine will still start after sitting for three weeks. 35 ma. Is an industry-acceptable standard.

There is going to be a drain on the battery due to all of the insane computers' memory circuits. (That's how the radio maintains the radio presets and the clock, and the Engine Computer remembers its fuel trims).

It sounds to me like you already found the problem with the seat switch and your troubles are over. Are you searching for things you observed while looking for that original drain?

Caradiodoc
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Wednesday, November 10th, 2010 AT 8:10 PM
Tiny
THE MOO PROPHET
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Thanks, doc, I had forgotten about ohm's law. All I had around the shop was an ohmmeter. I'll check the actual amp draw tomorrow
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Wednesday, November 10th, 2010 AT 9:33 PM
Tiny
CARADIODOC
  • EXPERT
There's issues with that approach now too. Do you still think you have a problem?

Since the early to mid '90s many manufacturers are using various computers that have to go to "sleep" mode, typically 15 - 20 minutes after stopping the engine. During that time they can draw as much as 3 amps. That will pop the two amp fuses commonly used in a lot of digital DVOMs. If you start out on the 10 amp scale, then switch to the two amp scale later, the momentary break in the connection starts the 20 minute cycle all over again.

If this is what you run into, set your meter on the 2 amp scale, remove the battery cable, connect the battery and cable together with a clip lead, then connect the meter between the battery and cable. After about a half hour you can disconnect the clip lead. The computers should stay off and the meter will read the current drain. If you have to move the meter to a lower scale, connect the clip lead first so you never have an open circuit. That will prevent the computers from waking up and popping meter fuses.

The meter range switches are "break-before-make" meaning they break the connection from one scale before they make the connection to the next scale as you turn it. That is enough to wake up some computers.

Caradiodoc
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Thursday, November 11th, 2010 AT 12:38 AM

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