jaydmdc2

hey i went to turn my headlights on when i left work today and went to turn my headlight on and my passenger lowbeam light wasn't on so i was assuming the buld just blew. so i havei tried anothr buld and still nothing. I then check the fuse and it's blown, put a new one in and it blows as soon as i turn the headlight's on everytime now i know that usually means a short in the wiring i check the red and black wire that runs from the socket back to where the loom starts and there's no cuts or anything in either wires. so then i wire up the headlight socket directly to the battery red wire to battery and black to a ground with a inline fuse on the red wire and try to turn the lights on and it blew the inline fuse and still no light. does this mean that the actual socket is bad?

EGAS

You cant run the lights directly off the battery even if you were to relay a fuse.. You would need to control the current flow and its def. not ment to have 13-14.2V going to it..

I would check with a multimeter what your getting going into the fuse as well the other outlets.. this is something in the relay broke. your running to much current and your fuses are saving your car. If it blows twice theres a problem.

Your best bet is to rear resistance from one end to another for a break in a wire somewhere..

jaydmdc2

can you explain to me exactly what i have to do to test it with a mutimeter and what am i testing volts,amp,ohms or what and you said there is a relay to run each light right? if the relay goes bad would that cause no power to the lights at all. i tried hooking up my driverside lowbeam socket up to the pasenger side wired it in and still nothing so i know it's not the socket and for some reason now it's no blowing fuses anymore just no power at all to the passenger lowbeam all other lights work low and high's. any help anyone with testing with a mutimeter

MajorsEF

you mess with any wires lately? something might be grounding out or some wire might be touching

Fabrik8

Why can't you run lights right off of the battery?
You definitely can; that would be the ideal way to run a light bulb. The fuse and relay are just there for protection and for power switching respectively. Power comes from the battery, goes through the fuse (sometimes more than one by the time it gets there) and then the relay, and through the headlight. It's always connected directly to the battery.

Lights aren't current limited (except ultimately by the fuse), the resistance of the bulb filament (when hot) sets the current that will pass through the bulb.

Ok, so here's what to check and what not to check.

First, I wouldn't check the current draw of the bulb with the multimeter. The current measurement sections of a multimeter are fused, and the fuses aren't that easy to find locally. Because all of the current passes through the multimeter to do a current measurement, chances are that if you're blowing the headlight fuse then you'll likely blow the multimeter fuse. Plus, you already know that something about that headlight circuit is drawing too much current, so measuring that current wouldn't tell you anything you didn't already know.

So what you do need to do is measure the resistance between the two wires to that headlight bulb. If you are blowing fuses when the circuit is active, there is a short somewhere. You'll likely have a rather low resistance between the wires, and it would have to be somewhere on the headlight side of the relay if it is only blowing fuses when the lights are switched on. That is, unless the relay itself is the problem, in which case it would be a good idea to remove the relay.. This makes sure that you're looking at the wiring itself when you're checking resistance, and not getting fooled by a shorted-out relay or the bulb filament.

Anyway, check resistance between the two wires, and then check the resistance of each wire to ground. One of them is grounded already obviously. With the relay and the bulbs removed, you should see a fairly high resistance or none at all. Use the functional headlight circuit on the other side of the car to verify whether what you are seeing with the multimeter is correct or not. If you do see a pretty low resistance short to ground (which will probably also appear between the two wires because one is already grounded), there are different ways to track it down. Visual inspection is the first thing to do obviously, and you may have to untape and unloom as much wiring harness as you need to get to everything.

If everything looks good with the wiring, you can plug the relay back in (not the bulb) and test that with the fuse removed. Do the same resistance checks as before using the headlight wires, with the headlight switch turned on and off (you want to see if the relay is shorting out when it is switched on). Do not check the power side of the fuseholder, it will likely still be connected to power and you don't want to potentially damage your multimeter by doing a resistance reading on a live circuit. You can use the DC voltage setting on the multimeter (measure between the fuse terminal and ground) to find out which side of the fuseholder is power and which is connected to the relay.

If that looks good, you should check the input side of the relay. This is easily done by testing resistance between the relay side of the fuseholder (again, no fuse!) and ground, with the switch on and off. This is also testing whether there is a short inside the relay (or in the wiring between the fuseholder and the relay) that is only there when the relay is switched on. The reason you need to test on both the input and output sides is that if the relay has failed, you can't really predict what is connecting to what inside the relay, so you could miss something by only testing the input or output.

I think I'm done typing now. I'm already one beer into this post.

EGAS

I didnt read everything in your post haha yet..

Iam saying you can not run directly from the battery to a source DIRECT.. without anything.. I know my electronics yet iam not great at explaining it. Just as the same i can get to my house but cant give directions.. haha

The bettery is putting out 12v your alt is going to raise the output the battery will put out. so what you were asking of 12v prior your getting 13.8-14.2V... so if you know what your doing you can use the battery direct as a power source yet you need the proper resistance.

EDIT: do what you must. Yet i wouldnt run it directly.

Fabrik8

No.

Resistance has nothing to do with any of this. If you raise the voltage on a light bulb, it will just get brighter.

The battery and the alternator are directly connected. Whatever voltage level the alternator is putting out is whatever voltage is going to the battery also, so everything connected to battery power is also connected to alternator power, and everything is at the same voltage. Your headlights are connected directly to the main power circuit in the car, which is directly connected to the alternator and directly connected to the battery. So when the engine is running, the headlights are getting voltage the alternator is putting out, and when the engine is not running (and therefore the alternator is not moving), the headlights are getting voltage at whatever voltage the battery is charged to. This is why your headlights are brighter with the engine running, and stuff like your window motors move slower with the engine off. If everything was connected to the battery only, nothing would change whether the engine was running or not, because everything would always have the same voltage (and the current is already set by something else; see below).

This is the same with a fuel pump, amplifier, fan, whatever load is connected to the main power circuit in the car is directly connected to both the alternator and the battery at all times.

I think the breakdown here is that you are thinking about loads which aren't current limited. Anything that has resistance (the windings of an electric motor, the filament of a light bulb, etc) already limits its own current, according to V=IR (voltage = current x resistance). So in the case of a light bulb, or a motor, the current it will draw is the voltage divided by the resistance. That is the max amount. One of those things has to change for the current to increase. Either the resistance has to somehow become lower, or the voltage has to get higher.