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general guide for chasing shorts?


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vancouverbc 
Copper - Posts: 72
Copper spacespace
Joined: September 27, 2004
Location: Canada
Posted: March 01, 2007 at 7:40 PM / IP Logged  

Electronic shorts in a car. I'm curious where the shorts most commonly occur. For example, I had

an internal short in my ignition coil that was blowing fuses. Also, instructions on how to test wires , connectors, etc..to chase down the short.  Voltage drop vs resistance? Can you stab the multimeter probes through the

plastic insulation of wire?

1991 240sx
alarm=model 2620.web central door locking system
Hymer 
Silver - Posts: 695
Silver spacespace
Joined: November 20, 2004
Location: United States
Posted: March 05, 2007 at 9:51 AM / IP Logged  
1st off a short is kind of sa pain to trace, but easy to diagnose. it is a direct short to ground somewhere in the circuit. Trying to find it is another story. Basically you need to eliminate components. IE aftermarket heated seats are blowing fuses. This is pretty common, due to a poorly planned install. 1st, pull the fuse from the +12v side of the ckt. next ground the black probe of your meter, and put the red probe on the FEED side of the fuse holder. [you should get 12v on 1 side and 0v on the other. This tells you the short is farther down the line than the fuse is. Next, disconnect the seat heater. replace the blown fuse with a smaller 3 amp or so, if the fuse blows right away then there is s short between the fuse holder and the wire running to the seat heater plug. [remember the heayter is not connected.] so now ya go through and check the wires between the fuse holder and the heater connector. Case 2 if the fuse does NOT blow, then check with your meter at the heater plug, you should get 12v. This will tell you that the heater or the switch or thermostat controler is causing the short... It's just basically block troubleshooting. In your case with the bad coil, this method probably would have worked, just unplug each coil pack until the fuse stopped blowing. Unfortunatly you'll go through a lot of fuses and such this way. you can also do it by comparing resistance measurments. 1 wire on each of three coil packs shows OL with reference to ground, and the 4th coilpacks wire shows 0 ohms indicating a shorted coil pack. As for testing wires, there are a bunch of different methods. I like the "bed of nails" aligator clips you pinch the wire between this thing and all the little pokers make contact with he wire without stripping the insulation away, also most tool trucks have a set of adapter pins to plug onto single wires or multiple ones just for this purpose.. but stripping insulation away and re tapping it or shrink tubing it works ok too...
Tire Proz Stillwater Mn
High End Restyling and Comlete Repair
master5 
Silver - Posts: 1,123
Silver spacespace
Joined: October 10, 2006
Location: United States
Posted: March 05, 2007 at 3:02 PM / IP Logged  

I'll give my take as well since I basically spend my entire life chasing down shorts.

First off there really is no particular place where shorts occur most..basically they can occur anywhere..any time..for many reasons. In my industry it is common to see wires pinched under seats..or wires hanging against exhaust manifolds. Sometimes a screw is put through a wire as well.

Some devices can internally short...motors..speakers and a host of other components. By simply disconnecting the suspect component..then checking the wires for a short with a meter can quickly tell you which one is the culprit.

As far as voltage drop vs.resistance.. Voltage drop is a sympton of resistance...that about sums up the relationship for this topic. A dead short to ground (zero resistance)causes blown fuses..and in the worst case burned up stuff and/ or wires.

Everything posted by Hymer is great advice so I see no need to add anything else. If you have any other questions feel free to ask away.

KPierson 
Platinum - Posts: 3,527
Platinum spaceThis member consistently provides reliable informationspace
Joined: April 14, 2005
Location: Ohio, United States
Posted: March 05, 2007 at 3:36 PM / IP Logged  

Divide and conquer.

Thats really all you can do.

It's also important to identify exactly when the problem is occuring, especially if there are several components on one fuse.

For instance, if you keep blowing a headlight fuse when you turn your headlights on check to see if the fuse blows when you only turn the parking lights on (if they are on the same fuse, which most are not, but its the only example I could think of).

Also, as said above, unplug as many things as possible and keep testing with a meter (I don't recomend using small fuses to test as they are somewhat costly, especially if you use 25 of them!)  If you know you have a short to ground you can measure the fuse holder output side to ground and determine when the short has been fixed.

Kevin Pierson
master5 
Silver - Posts: 1,123
Silver spacespace
Joined: October 10, 2006
Location: United States
Posted: March 05, 2007 at 5:05 PM / IP Logged  

While on the topic...I have seen a few times a problem with a short come down to this.......a dime fell into the cigarette lighter socket. On some American vehicles you will lose the interior lights...radio..clock..and perhaps more. Also have seen it cause in a few caddilacs all kinds of crazy feedback with gauges and lights doing weird stuff.

In many imports you lose the radio and sometimes other accesories as well .

By knowing all the circuits on the blown fuse (read the owners manuals) I did a visual inspection of each device until I saw the coin. Now I know by experience to check in the lighter socket first.

Another example of many in the world of 12v automotive electronics. Just thought I'd share.

dualsport 
Silver - Posts: 983
Silver spacespace
Joined: September 27, 2005
Location: United States
Posted: March 18, 2007 at 11:56 PM / IP Logged  
I'd suggest saving the fuses while diagnosing, and just get a spare bulb, and connecting it in place of the fuse that's blowing. You can use the blown fuse to provide the contacts in the fuse block; soldering wires to each side.
A solid short to ground should net you a bright light, and you can go through your process of removing connectors, wiggling wires, etc, and watch for the light to go dim, at which point you probably identified the problem.
There are some hokey "short finders" sold at low cost, which I was disappointed to find were simply flasher units that intermittently opened and closed the shorted connection, and a galvanometer was used to pick up the current pulse. Too crude- guess I was expecting RF signal injection or something nifty like that for my $9.99 general guide for chasing shorts? -- posted image.

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