Fuse(s) question
 

This is either going to show my brilliance or my ignorance with respect to electrical issues and probably the later but here goes...

Do fuses in the fuse boxes have a "flow" to them? In other words does the electricity flow in on one side (+) and out the other (-)?

If so does that mean that if I test the voltage at a fuse that should be live and there is no juice, is the problem "upstream" on the + side? OR could it be caused by a bad ground (or something) anywhere it the circuit?

Electrical 101.........
 

TWJ,

Fuses don't have control on the direction of the flow of current (electron) in the electrical circuit. They are simply a device to prevent over-loads. If you want to determine the flow or direction of the electrical current, do this simple test:
1). Locate a fuse terminal that has power. Use a test light instead of a multi-tester.
2). After you have located an active circuit, remove the fuse. Test both ends of the fuse holder to determine which one is LIVE. The terminal that will test positive for voltage is the INPUT side or source of power.
3). The current flow (electron or electricity) is from INPUT to output (load side). The standard arrangement for a fuse block is one side is the INPUT and the other side is the load side (output).

Now that you have found the INPUT side, you could also determine if the input side is hot all the time or controlled by a switch. Remember that not all INPUT terminals have power all the time.

If the INPUT terminal is tested positive for power, the other side should also have power unless:
a). Fuse is blown.
b). Poor or bad contact/s.

So in summary, you could check the presence of voltage (power) down the line. An open line (cut) will cause interruption in the current flow, so with a poor connection or bad contact. HTH.

Tony

Thanks much Tony! Exactly what I was looking for.

Next question(s): Could a bad ground "down the line" (on the load side) from the fuse terminal cause the entire circuit to loose power? That is both the input and the load side. Or if there is no input does it (only) mean a problem "up the line" from the input fuse terminal?

If you do not have a complete circuit (i.e. a continuous connection) from the positive battery terminal to the negative batery terminal, you will not have electrical flow (current). Each circuit in your car has this.

When you use a tester like a test light or voltmeter, you usually ground one tester lead (which is connected to the negative battery terminal) and touch the other lead to where you expect to see power. If the circuit (path) from the battery positive terminal to the point where you touched the lead is continuous, current will flow and will be evidenced by the lamp lighting or the meter showing voltage. You can move along the circuit to various points to determine if the circuit is intact or not. If the circuit goes dead at some point, the fault is between the last good point and the dead point.

no.
if you remove ground from say a light bulb, there is still 12v at the fuse. that is called an open. a lot of people say they have a short, but generally a short blows fuses or melts wires due to a direct path to ground resulting in high current. with an open, usually it just does not work.

i just went through something interesting with someone. they said they have 12v on BOTH sides of one of their dash lights, even with the bulb removeed he had 12v on both pins of the socket. so he thought he had 12v shorted to ground. actually it was quite the opposite. he had an open ground. (the dimmer in the light switch provided ground and was not working)
what was happening was, the 12v went through the other bulb and since it could not return back to the battery, it was available at the ground side of both light bulbs.
you see, you cant measure voltage across something that does not have resistance. for all practical purposes, the wire on the ground side of a bulb has no resistance, but when that wire is removed, now you have invinite resistance and you measure 12v. in the electronics world, the ground side of a bulb is considered to be electrically the same point as the neg post on the battery.

i hope i have not confused you or given you more info than you need, but knowing what you see on a MM or test light means more than the test equipment itself.
personally, i prefer the test light. they are cheaper and easier to use. when you are upside down under a dash, twisted like a pretzel, it is much easier to see the light rather than trying to hold a MM and read a number. plus, dont get caught up in the numbers. 11.98v, or 12.1 or 12.3, its all good.

usually, you can think of electricity like a water hose

voltage is like (~~) pressure

current flow (amps) ~~ flow rate

and resistance is present also - it cuts down on the flow (current) downstream

if there are 2 paths, then the electricity will flow down both of them ("parallel") and the amt. of current in each path will be set by the resistance

current will flow thru each thing on the same path (series)

first exception to the analogy is that the "water" has to have a ground to get "back" - the entire chassis (body) of your car is the ground (gnd); it looks like a bunch of little crosshatches on a vertical line in the schematic