1999 3.8L Electrical problem (short?)

TerpBE

New Member
Sep 14, 2010
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I have a 1999 3.8L Mustang that has been giving me problems lately. Last week the battery died so I jump started it. When I drove it around, I noticed that there was some radio interference, and at one point the electrical system seemed to cut out for a split second, causing several dashboard lights to come on for a couple of seconds. Then, when I turned off the car it wouldn't start again - the battery was practically dead. It seems like it might be a short somewhere. (I had a similar issue last year, but it turned out to be bad/corroded battery terminals, which I replaced).

I've jump started it a couple of times since then, but the battery won't hold a charge. To try to diagnose any potential shorts, I disconnected the negative wire, put an electrical tester between the wire and battery terminal, and started pulling fuses under the hood:

2000_fuse_1.gif


I noticed that the circuit connected to fuse "M" was drawing current, which isn't a surprise since it is the radio, and I assume uses power to maintain the clock and presets. But I also noticed that the tester remained lit up until I pulled fuse "F" - "Instrument Cluster / PCM".

Is there any reason that "F" should be drawing a current when the car is off, or does this indicate a short? If so, should a new instrument cluster fix the problem? For many years my odometer has only lit up intermittently, so I don't know if that might be a sign of a problem.

Does anybody have any suggestions? I think I can replace the instrument cluster relatively easily, but I want to make sure I'm headed down the right path before investing the time and money. Thanks.

-B.E.
 
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wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
IMO, the first thing you should do is drive to your local autoparts store and have the battery and alternator tested. Request an AC ripple check as well.

Double check the battery cables and grounds around the battery.

Its good maintenance practice to start with known good systems. Batteries don't really care how recently they were replaced.

How long does it take for the battery to run down? How much current is being drawn? An amp meter is needed here.

Note, the PCM does draw a fair amount of current which decreases as more systems are placed into sleep mode. Hence the reason we need to know the draw at say 5 minutes, 15 minutes, 30 minutes, and 1 hour.

If it were me, I would do a quick mini test by pulling all of the radio fuses and see if that makes an immediate difference.

Parasitic draws can be some of the biggest PIA to trouble shoot. Best to attack in a symptomatic fashion. Take notes. Follow where the clues lead.
 

TerpBE

New Member
Sep 14, 2010
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Thanks for the tips. My gut feeling is that it's not the battery, because I was getting radio interference and everything "cut out" for a second while I was driving, but it can't hurt to check it. The only potential problems would be that I'd have to jump start it once I turn it off, and I run the risk of it dying on the way there.

I don't have an amp meter right now - just a light-up tester, but maybe I'll pick one up. But knowing that the PCM still draws current when the car's off helps, because now I know that it's not necessarily an indication of a short.

The other thing that could be a problem is that I installed a FM modulator and some additional DC outlets a couple of years ago. There's probably a decent chance that I messed up something there that could be causing a short. I'm planning to try disconnecting them this weekend to see if it makes a difference. In the mean time I'll try running without the radio fuse(s) and checking the battery and alternator. Thanks again for the ideas.
 

A.Abercrombie

New Member
Sep 29, 2016
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My daughters 1999 GT is doing the exact same thing with the same fuse locations shorted to ground. What fixed the above issue? Any advice would be greatly appreciated.
 

Rusty67

15 Year Member
Dec 3, 2002
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Seattle area, WA
The GEM stays awake for about 30-45 minutes. That is likely why you are seeing a large draw from that ciruit. You absolutely need a multimeter if you are going to troubleshoot this yourself.
 

Moosee1955

Active Member
Sep 30, 2016
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Two things, buy yourself a small battery charger. Every garage needs one. Sears usually has a good selection. I like one that has a 6 amp to 10 amp setting. Good for a 5 to 8 hour recharge. Also disconnect all your aftermarket wiring. Stereo add ons, alarm, your d.c. Plugs. Nine times out of ten, electrical gremlins can be traced back to some add on item that needed to be disconnected from hot side or ground. Guys when you put any electrical items on, hook positive directley to battery with a fuse. Don't tap into any existing hots. They almost always cause a problem down the road. Same goes for grounds. A loose or dirty ground will make you pull out your hair. Keep us posted what you find.


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