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Finding a Short


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Zilverado 
Copper - Posts: 92
Copper spacespace
Joined: October 03, 2004
Posted: October 14, 2006 at 8:07 PM / IP Logged  

geepherder wrote:
With a blown fuse, you should read 12 volts across it (when power is applied).
If it's not blown, you will not have a reading.
That's how those smart glow fuses work. The small bulb is connected to both terminals of the fuse, or in parallel. We know that electricity follows the path of least resistance right? Well under normal circumstances (fuse is good), the fuse has minimal resistance (basically zero). The bulb, in comparison, has a much higher resistance. It's much easier for current to flow through the fuse than the bulb, so that's what happens. When the fuse blows, the path of least resistance is through the bulb.

So, if I follow what you're saying (and I think I do), my system is functioning the way it should and I do not have a short under this symptoms???

Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder
geepherder 
Platinum - Posts: 3,668
Platinum spaceThis member consistently provides reliable informationspace
Joined: October 27, 2003
Posted: October 15, 2006 at 6:19 AM / IP Logged  
Yes, that's right. It sounds like you had a problem with either the trailer wiring, or had a bulb that was shorted across.
My ex once told me I have a perfect face for radio.
Zilverado 
Copper - Posts: 92
Copper spacespace
Joined: October 03, 2004
Posted: October 16, 2006 at 3:52 PM / IP Logged  

geepherder wrote:
Yes, that's right. It sounds like you had a problem with either the trailer wiring, or had a bulb that was shorted across.

All is still well, so I hope I got it. But just one little further clarification if you don't mind...

While I understand the "path of least resistance through the fuse" thing, I am a bit confused why I get a 12.2v reading when I measure across a broken fuse, with no bulbs in the sockets? To me, the lack of bulbs would mean the circuit was incomplete, and I should get 0.0v - - but I get 12.2v. What am I missing here.

Either way, like I say, things appear to be working fine, so I'm glad about that. Just trying to learn as much as I can here. Thanks.

Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder
Hymer 
Silver - Posts: 695
Silver spacespace
Joined: November 20, 2004
Location: United States
Posted: October 16, 2006 at 4:00 PM / IP Logged  
That would be because there is a ground element on the [load] side of the fuse...   [sometimes with the bulbs in you'll get this], sometimes not... with the bulbs out, you should get no path to ground at all.... and just get a 0v reading... but on the feed side of the fuse you should always get 12v while grounding your black lead of your meter... Glad ya got it fixed.... good luck!
Tire Proz Stillwater Mn
High End Restyling and Comlete Repair
Zilverado 
Copper - Posts: 92
Copper spacespace
Joined: October 03, 2004
Posted: October 16, 2006 at 4:14 PM / IP Logged  

Wow, thanks for the quick response.

So now I'm super-confused ;-)) By what you're saying, I should get no voltage with the bulbs out, but I did - - at least I think I did. That's what kinda confused me. I guess I'll take another look and see, maybe I was imagining it...

Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder
Hymer 
Silver - Posts: 695
Silver spacespace
Joined: November 20, 2004
Location: United States
Posted: October 16, 2006 at 4:18 PM / IP Logged  
just think of it this way... when you pull or blow the fuse, it's just like cutting the 12v feed wire... one side should have voltage and one side won't... pretty simple.... don't let the glow fuse thing confuse ya... just try to keep it basic....
Tire Proz Stillwater Mn
High End Restyling and Comlete Repair
geepherder 
Platinum - Posts: 3,668
Platinum spaceThis member consistently provides reliable informationspace
Joined: October 27, 2003
Posted: October 16, 2006 at 7:17 PM / IP Logged  
Sorry for the confusion, I misunderstood you. I missed the part about the bulbs being out. However, since you have a remote start, the wire connected to the brake wire may have continuity to ground through the unit. That's probably why you got the 12 volt reading.
My ex once told me I have a perfect face for radio.
Zilverado 
Copper - Posts: 92
Copper spacespace
Joined: October 03, 2004
Posted: October 16, 2006 at 11:17 PM / IP Logged  

geepherder wrote:
Sorry for the confusion, I misunderstood you. I missed the part about the bulbs being out. However, since you have a remote start, the wire connected to the brake wire may have continuity to ground through the unit. That's probably why you got the 12 volt reading.

Actually, the RS should be out of the equation right now, because I disconnected the brake pedal lead to the RS to help with the trouble-shooting.

But now that you mention it, there is one other possibility. The brake pedal is also interlocked with the safety switch that releases the console shifter (to take it out of PARK). I'm guessing that's where the ground is. Not sure why it took me this long to figure that out - - it seems so obvious. I guess it's that "can't see the forest for the trees" thing.

Thanks for ALL the help - - this place is a HUGE help!!!!

Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder
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