Electrical problem. Need some help.

[jbwrangler]

I am trying to figure out why the headlights on my 2000 JD 5105 are not working. I have a short somewhere. The lights would blink on an off as you hit bumps, but now they don't work at all.

Now for the part that is odd to me (as an electrical novice). When I remove the bulb and put my meter on the connectors that the bulb plugs into, I get a 12v reading, but as soon as I plug in the bulb, the reading goes to 0v. The bulb is good. I verified by connecting to a 12v battery with some wire.

The ground wire appears to be the culprit, but not sure how to find where it is shorted. Any tips or tricks from the pros?

Thanks in advance.

 

[JimRB]

I do not think you can have a short in a ground wire. The definition of a short is a fault to earth or ground. A short should blow fuses or pop circuit breakers. You may have a crummy connection. Crummy defined as corroded or broken. Use the ohm setting of your multi-meter to check the resistance of the ground wire. Probably black in color.

 

[BobRip]

With the lights on wiggle the connections of the wire between lights and the battery. You have a loose connection somewhere. Not a short. Look for the flickering during the this process

 

[ecbaatz]

I agree, loose connection or a bad ground. It could be the switch itself, but unlikely. I would look at the wiring first.

 

[jbwrangler]

I agree, loose connection or a bad ground. It could be the switch itself, but unlikely. I would look at the wiring first.

Why would connecting the bulb cause the voltage to go from 12v to 0v on my meter?

I connected a wire from the Positive wire on the bulb socket to the positive terminal on the bulb and another wire from the frame to the neg terminal on the bulb and the bulb lights up fine.

 

[aczlan]

Why would connecting the bulb cause the voltage to go from 12v to 0v on my meter?

A bad ground. It has just enough of a connection to make your meter read the voltage, but not enough to power the bulb.

Aaron Z

 

[Samuellm]

Think of a bad ground or lose connection as a kink in a pipe.

If you hold you finger over the end it will slowly build pressure (your meter) but if you try to water your garden (your bulb) you have no flow.

 

[boggen]

looking at tractor data, to double check size of machine...
TractorData.com John Deere 5105 tractor information

ya tractor most likely has 1 or 2 relays between the key/on switch on tractor, fuse box, and the lights.

you also most likely have 2 to 3 wiring harnesses plugs that the power feeds through for the lights, between the light and the switch, and then maybe another to the fuse box (pending on were it is located). there may be a "engine wiring harness plug" before wires go up and into cab/front dash (were steering wheel is), might be located under the floor board / hard to reach area possibly, and then another wiring harness inside the cab/front dash, needing to remove sides or cover on dash to get to it.
--these wiring harness plugs can and do come apart / break. check them. also if you take them apart, you can individual test each wire (if you know the color coding), though i would first check the connections / corrosion / broken pins, pulled out wires, or like.

most wiring harnesses have a ground wire in them. but when ya split out from the wiring harness, it gets a little iffy if a ground also comes out of the wiring harness. for lights, there could be a chance a ground wire comes out of wiring harness and also connects some place on the lights. other times ground wires goto only a couple specific spots on the tractor.

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a shop / service manual for your tractor would have a detailed diagram showing all the wire connections. along with color coding, and were your wire harness plugs are.
you might try contacting your local dealers, and see if they would email / fax you a copy of the wiring diagram for the shop/service manual. so you have something to work from.

 

[jix]

Think of a bad ground or lose connection as a kink in a pipe.

If you hold you finger over the end it will slowly build pressure (your meter) but if you try to water your garden (your bulb) you have no flow.

This is the correct answer: Ohm's Law..Voltage = current multiplied by resistance. If there is no current(because of high resistance) there is no energy to light the bulb, but there is high voltage because of the high resistance, until you attach the load. Look for a poor connection (hint: use an ohm meter, not a voltmeter) What is happening is that All of the applied voltage is dropped over the bad connection and there is no EMF electro-motive force left to push the necessary current through the bulb Voltmeters do not load the circuit, so they need very very little current to register voltage. In a hgh resistance fault, voltmeters can be misleading. You need to measure resistance, not voltage.
You would be surprised to know how many automotive mechanics never understand this, A 12V test lamp probe measuring from each suspected connection to ground will reveal where the fault is, in a series circuit. Not necessarily so in a parallel circuit.

 

[skyhook]

You have an open ground somewhere,
when you test with your meter, you are effectively creating a ground with it when you ground the test lead giving the voltage a path to ground through the meter.
Remove the meter and you have now, removed the ground.
A test light will give you the same results.