Blowing 30 amp fuse in box (Auto shutdown)

Tiny
MARILYN 94
  • MEMBER
  • 1994 DODGE DAKOTA
  • 3.9L
  • V6
  • 2WD
  • MANUAL
  • 200,000 MILES
After I replaced the fuse I was able to run it in the driveway for about a week. I started looking at the wiring. Found one wire under the fuse box under the hood that had burnt through the rubber coating. I cleaned it off, wires seemed fine so I wrapped it in electrical tape. Started it up and she blew the fuse immediately. At $4.00 a pop I'll start being more cautious. I'm still going through wiring. Checking grounds. Asking questions.
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Sunday, September 27th, 2020 AT 7:24 PM

9 Replies

Tiny
CARADIODOC
  • EXPERT
A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a 12 volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it's not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.

For intermittent problems like yours the bulb may be dim already. Watch what takes place when it gets bright. That's when the short is occurring. It could be due to the rocking of the engine when you shift between reverse and drive. It could be due to the body flexing when you drive over bumps in the road. The bulb limits current to a safe value when the short occurs, in this case about one amp. If the engine won't start with a brake light in the circuit, try a headlight bulb. The low beam filament will limit current to five amps, and the high beam filament will allow about six amps to flow.

You can do the same thing by using the light bulb in place of the automatic shutdown, (ASD) relay. That relay only turns on for one second when the ignition switch is turned on, then again any time the engine is rotating, (cranking or running). That means if you use the bulb in place of the fuse, you have to be cranking the engine to do the troubleshooting;... Not very practical. Instead, with the bulb in place of the ASD relay, you don't even have to turn the ignition switch on. The bulb will power up the circuit and limit current to a safe value. If you use the bulb in place of the relay, use terminals 30 and 87.

Items on that circuit include the injectors, ignition coil, alternator field, oxygen sensor heaters, and the fuel pump or pump relay. All of those items cause very little trouble. You're more likely to find a wire going to one of them is grounding out, similar to what you already found. Look for oxygen sensor wiring that fell down onto hot exhaust parts, and check the large bundle of wires that run along the left inner fender under the battery tray. That harness often slides back and forth as the engine rocks, leading to wires rubbed through and shorting to ground.
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Sunday, September 27th, 2020 AT 7:59 PM
Tiny
MARILYN 94
  • MEMBER
Thanks. I'm going to study what you've told me. Figure out what it means! It's rainy today. What do you mean by jumper wires? A piece of copper wire? And two spade terminals? You mean two 15 amp fuses. Is the negative battery terminal on or off the battery when I do this? I do have a circuit tester.
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Monday, September 28th, 2020 AT 6:39 AM
Tiny
CARADIODOC
  • EXPERT
I put these drawings together to see if they will help. This pertains to bypassing the blowing fuse. Figure 1 shows a typical Power Distribution Center, (under-hood fuse box). One fuse has been pulled out. It is shown expanded in figure 2. Figure 3 shows the items you'll need. At the top are the small jumper wires. You can find these at Harbor Freight Tools or just about any hardware store. They typically come in a package of about a dozen for around three bucks. The terminals on the right are generic crimp-type replacement terminals. These can be found at the hardware stores and auto parts stores, but you have to buy a box of as many as 50. Instead, ask any mechanic. They often have a bunch running around loose in their tool boxes. If necessary, you can tug off the insulator at the end so the clip can be attached. That would be the blue terminal on the top right.

At the bottom left I showed my favorite bulb, a common 3157 brake light bulb. It's easy to connect the jumper wires to, but be aware it is going to limit current to one amp. Once the short is removed, the engine likely will not run because one amp isn't enough to run everything on that circuit. It will run one the fuse is installed.

Connecting these parts takes all of a half minute. Plug the two spade terminals into the empty fuse socket as shown in the fourth drawing. Connect a clip lead, (jumper wire), to each one as shown, then the other ends to the light bulb.

If the short is constant, as soon as you connect the second jumper wire, the bulb will turn on full brightness and will get hot. Now you can move wire harness around and unplug things to see what makes the short go away. When the short is not in the circuit, the bulb will get dim or go out.

If the bulb is dim when you connect the last clip lead, the short is not there at that time. The same procedure applies. Move the wire harnesses around until the bulb flickers or flashes bright. When it does, whatever you're doing is related to the location of the short. No need to turn on the ignition switch, and no more wasted fuses.
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Monday, September 28th, 2020 AT 8:16 PM
Tiny
CARADIODOC
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Here's the procedure if you'd rather bypass the ASD relay. Either way the bulb will limit current to a safe value, one amp. The results will be the same but you must plug the bulb into the fuse socket or the relay socket, not both. If you plug it into the fuse socket, the rely must be in place. Or, if you plug the bulb into the relay socket, the fuse must be in its place.

The relay I drew is the common 1" cube relay used most often. Use terminals 30 and 87. If your ASD relay is the skinnier style, use the two larger terminals the arrows are pointing to in the fifth drawing.
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Monday, September 28th, 2020 AT 8:32 PM
Tiny
MARILYN 94
  • MEMBER
Did you get my reply from this morning? It's not showing up. New to site so not sure how site works. The relay has two terminals marked 87. The one in the middle is 87 also. Didn't realize I could send photos so easily. I'm a little tech challenged. The third photo of course is a long range look at my baby. Going for jumper wires. Thanks
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Tuesday, September 29th, 2020 AT 7:37 AM
Tiny
MARILYN 94
  • MEMBER
I just sent a reply and it not registering here. What am I doing wrong?
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Tuesday, September 29th, 2020 AT 11:28 AM
Tiny
CARADIODOC
  • EXPERT
I see you. That center terminal is "87A". It is not used in about 99 percent of applications. When the relay is not energized, as in turned off, terminals 87A and 30 are connected internally. When the relay is turned on, the internal movable contact flips to connect 87 and 30 together and send current to the circuit. At that time, the connection between 87A and 30 is broken. At the moment, I can't even think of a common circuit that uses terminal 87A. GM relays don't even have a corresponding terminal.

To insure I see your replies, whenever I post one, you'll get an automated e-mail message stating that, and it will have a link in it that brings you right back here. Click that link, then post your new reply. Don't try to start a new question. When we come back here, this has turned into a private conversation between the two of us. A new question can be read by all of the experts and some will try to help but they'll be confused from jumping into the middle of the conversation.
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Tuesday, September 29th, 2020 AT 4:10 PM
Tiny
MARILYN 94
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Ah. So email will be from 2 carpros. Com? And you are caradiodoc?
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Tuesday, September 29th, 2020 AT 6:35 PM
Tiny
CARADIODOC
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Yup. I'm one of about a dozen experts, plus the site owners, who are here to help. I have to travel to town to get on the internet, so you'll only hear from me in the evening. Don't panic if I don't get back to you right away. Also, I put most of my drawings together at home, then post them the next day.
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Tuesday, September 29th, 2020 AT 8:54 PM

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