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Old 11-18-2004, 11:45 PM
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tracking down a short - continuity

I'm trying to track down a short or open circuit in my Toyota pickup. I have narrowed the search down to the "dome light" circuit which, while not blowing the fuse, does show a drain of power when I put an amp meter on it.

This circuit has one hot wire coming off a 10 amp fuse which apparently branches off at one or more points further along in the circuit (somewhere buried up under the dash). I have located a few of the wires on this circuit at or near their end points. I have done a continuity test from the single wire at the head of this circuit to these end point wires and I show 0.00 resistance in each of them. I also show 0.00 resistance when I test these various end point wires to one another.

My question is, can there be yet another wire branching off this circuit that is shorting or open even though I am getting continuity from the main wire to all of these end point wires?

(Hope this makes sense to somebody - I'm a little dazed and confused after spending a day trying to track this down)

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Old 11-19-2004, 08:23 AM
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Cboy,

Couple of thoughts.

Is the concern with the vehicle battery is draining down and going dead?

Are you putting the amp meter in series in the circuit?

How many amps is the circuit pulling under what should be a no load condition?

I don't believe you have a short to ground or the fuse would be blowing. Nor is there an open or there would be no amperage.

Most likely one of the circuits is feeding a component (cigarette lighter, radio, something like that as a guess.) When faced with a condition like I think you have (draining battery) I begin eliminating components by disconnecting until the amperage draw goes away.
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Old 11-19-2004, 10:30 AM
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Quote:
Originally posted by advanced design

Is the concern with the vehicle battery is draining down and going dead?

Are you putting the amp meter in series in the circuit?

How many amps is the circuit pulling under what should be a no load condition?

I don't believe you have a short to ground or the fuse would be blowing. Nor is there an open or there would be no amperage.

Most likely one of the circuits is feeding a component (cigarette lighter, radio, something like that as a guess.) When faced with a condition like I think you have (draining battery) I begin eliminating components by disconnecting until the amperage draw goes away.
1) Yes - the concern is battery drain. If I leave the fuse in for the "dome light" circuit, it will run down the battery over night. As long as I don't install that fuse, there is no battery drain. BTW, this fuse is located in the fuse box under the hood - not inside the cab.

2) I am testing with the amp meter by simply testing across the two poles where the fuse has been pulled. I place the red probe on the side of the fuse socket coming from the battery and the black probe to the side of the fuse socket going to the circuit. I don't know if this is called "in series" or not.

3) When I test (as described in #2) I get a reading of .94 when the scale is set at 20m and a reading of 1.0 when the scale is set at 200m.

4) I get the above reading when I have disconnected all of the "loads" which my Toyota manual says are on that circuit (it lists the following: interior lights, door courtesy lights, clock, and ignition switch light) I have pulled all the bulbs in the dome lights and disconnected the harness plugs to those lights. I have disconnected the wiring plugs to both doors (which include the door courtesy lights), and I have disconnected the wiring plug to the ignition switch (which includes the ignition switch light). I have also disconnected EVERY wiring harness plug from the back of the main fuse box in the cab and removed that fuse box from the vehicle. And I have the radio, cigarette lighter, cigarette lighter courtesy light, the ash tray courtesy light, the glove box and the glove box courtesy light all pulled out of the vehicle and laying on the floor.

I'm pretty much at a loss as to what possible load there could be on this circuit other than some sort of short which is big enough to draw amps but not big enough to blow a fuse. And more importantly I am now at a loss as to how to troubleshoot this problem.
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Old 11-19-2004, 10:54 AM
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Hi Cboy,

Electrical problems are some of the most challenging and frustrating. Sounds like you are really covering the bases on this one.

Yes, you have your amp meter in series on the circuit. I am not sure on the scaling of your meter wether it is showing 1 amp, 0.01 amp or 0.001 amp.

Suppose it is possible the wire is grounding through a resistance to ground. Draws a little amperage but not enough to open the fuse. Make sure there is no voltage on the circuit and check for continuity to ground.

If you show continuity to ground the sorce is most likely the cause of the draw (obviously). Wish I knew then of an easy way (other than systematically tracing the wires) to find the ground but I don't.
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Old 11-19-2004, 01:29 PM
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YEEEHAAAAAA,

After many frustrating hours, a totally torn apart interior, and way too many cuss words, I finally tracked down the cause of my electrical problem. The relay for the power antenna --- which isn't even listed in the manual as being on the "dome light" circuit, which is the one that was giving me fits. The actual problem could still be beyond the relay itself (possibly in the antenna motor) but at least I have it zeroed in and I can just pull the relay and drive the truck like normal.

Thank you advance design as well as the folks who responded a few weeks back on an earlier thread I had regarding this problem.

Now it's off to many more hours putting everything back together.
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Old 11-19-2004, 01:58 PM
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Hey Cboy,

Yep, that is a great feeling. Glad you found it!
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