Ok guys, over the next several weeks I will be posting different things about electrical to help you guys get the basic tools to diagnose an electrical circuit. This is my first posting, so here we go
Off the bat, remember that in all automobiles that we are dealing with a 12 volt system (car off). Also that the electrical in all cars is the same, they just use different ways of controlling the circuit but go about it in the same kind of way. Fo example, GM may use the multi-function switch and a relay to control the head lamps. While Ford may use a signal from the multi-function switch to the Body Control Module (BCM) which sends a signal to turn them on. The same basic idea, but different methods.
You must also keep in mind that we are dealing with Direct Current (DC). There are some Alternating Current (AC) systems in your car. Things like your alternator puts out a 3 Phase AC signal, but is translated into DC by sending voltage down 6 diodes on a rectifier bridge. (3 for positive 3 for negitive) Another example of this in your car is your car speakers.
Keep in mind that in any DC electrical circuit, that you need power, the load, and the ground. The positive on your battery is power and the negative is your ground. Another thing that you must remember is to NEVER EVER use an analog multi-meter to diagnose anything on a car. It does not have enough resistance to protect your circuit while testing. Make sure that you use a good Digital Multi Meter (DMM), this way your circuit is protected.
Your multi meter is your best tool for diagnosing an electrical circuit. A good DMM from Fluke is worth your money if you are diagnosing electrical problems on a regular basis. Crasftman DMM is alright, but is not fast enough to watch things like communication on a signal line. When you come down to something like that, you will kick yourself in the butt for not buying a Fluke, a Fluke 87V is fine. I bought mine from a pawn shop for $115, and I have gotten my money’s worth by a long shot.
Let’s talk a little bit about volts, amps, and resistance. Any electrical circuit has volts, amps, and some sort of resistance.
Volts is measurment of electicity. Think of a river and how it flows through the valley (wire).
Amps is the flow of electricity. Think of the river again and just see it as how hard the current is flowing.
Resistance is....well resistance blockage of the flow of electricity. This is measured in ohms on your DMM. Think of the river again and think of a tree falling into the river and blocking the flow of the water. It is restricting the water flow. This is usually something like a corroded electrical connector, or a module of some sort. There is always some sort of resistance in a circuit, but finding out what it should be is the trick. If you can get the grasp of how Ohms works, then you are golden.
Each circuit takes only a few things to really work. You need some kind of control (relay, computer, transducer, Etc…), circuit protection (fuse, circuit breaker), wire, power (battery positive). Ground (battery negative), and last but not least, THE LOAD (blower fan, coil pack, window motor, horn Etc....). If you do not have any kind of load, you will burn up the circuit. Let’s say you go test a headlight bulb in ohms (resistance). You will see that there is about 6 ohms of resistance in the bulb. Now how do you get the amps? If you take the voltage (12 volts) and divide it by the number of ohms (6) then you will get your amperage.
Example:
Ohms Law
v=AxR
12= Ax6
12 volts = 2amps x 60 ohms
In this case there are only 2 amps running through your light bulb. Now let’s say if you just have a wire, where there is only .02 ohms of resistance through the circuit.
12volts = Amps x .02 ohms
12=60 Amps x .02 Ohms
That is how fires start in cars, because people have no idea what they are doing, and wire up a circuit with no fuse inline. For an example a fuse that is rated for 10 amps will pop at 10 amps and no longer allows anything to be passed through the circuit.
A known good wire will have anywhere from 2-.002 ohms of resistance. This is the reason why wire burns because to many amps run through a wire that is only rated for a couple of amps.
So please comment on here and tell me what you think about this. If you guys want more, or is it a waste of time? Did this help you?
I will probably post one or two a week. Depending on how busy I get.
Thanks
Last edited by Snap on MOFO; 11-05-2009 at 09:01 PM.
Can we toss some electrical tech questions at ya as well?
I have a slight problem.
My car is 2001 Cobra wired, but I have a 1999 - early 2000 gauge cluster.
Problem is the rear defrost blows the fuse when I activate it. From what I found out, it is tied into my gauge cluster somehow. Why? I have no clue, but I suppose it retrieves a signal from the ECU to let it know when to shut off. Again, I have no idea.
Is it possible to modify the inline wire that travels up to the gauge gluster in order to prevent the fuse blowing problem?
Turns out swapping a '99 Cobra cluster into a '00+ car is NOT a direct swap (for the most part). You can use the gauges, but you need to swap out the large circuit board on the back.
For some reason there is some wiring that goes through the cluster for the rear defrost. On the cars with the defrost button down by the shifter it's the yellow wire with black stripe that goes from the defrost switch into the cluster. Out of the cluster is a black with yellow stripe that goes to the defrost relay.
On the '99 Cobra cluster I had these two wires were unused and connected to ground on the PCB causing the rear defrost to stay on all the time (even with the key off). It also caused the rear defrost fuse to blow under the dash.
The post '99 clusters have some sort of logic circuit in the cluster board that controls the rear defrost. I don't know why though. Maybe the defrost timer is integrated into the cluster on the newer cars. I compared both large circuit boards side-by-side and there are significant differences for these two rear-defrost wires. It's not a simple wiring change.
So, if you want to use a '99 Cobra cluster in a car with the defrost button by the shifter ('00+ I think) you can either:
1) Swap the odometer board AND main circuit board from your original cluster to the Cobra one (ideal)
or
2) Pull the wires at position 9 and 10 (yellow/black and black/yellow) out of the left harness and this will disable the rear defrost.
Riley
I figure the proper fix would be to get a 2001 Cobra gauge cluster, but I'd prefer to keep the current one since it's pretty current with my mileage and I don't think it would be easy to find one with matching mileage anyways.
So, lemme know if you can tackle this twist.
Ken
____________________________________ Mods *BRAND NEW 2000 4.6L DOHC COBRA MOTOR w/ T45* 294.18 rwhp and 295.96 rwtq
*MAC catback*H&R Springs*Front Bilsteins, w/ Tokico rear illuminas*8.8 w/ 3.55's*MM CC plates*Subframes*Cobra F&R Brakes*
Can we toss some electrical tech questions at ya as well?
I have a slight problem.
My car is 2001 Cobra wired, but I have a 1999 - early 2000 gauge cluster.
Problem is the rear defrost blows the fuse when I activate it. From what I found out, it is tied into my gauge cluster somehow. Why? I have no clue, but I suppose it retrieves a signal from the ECU to let it know when to shut off. Again, I have no idea.
Is it possible to modify the inline wire that travels up to the gauge gluster in order to prevent the fuse blowing problem?
I figure the proper fix would be to get a 2001 Cobra gauge cluster, but I'd prefer to keep the current one since it's pretty current with my mileage and I don't think it would be easy to find one with matching mileage anyways.
So, lemme know if you can tackle this twist.
Ken
Most of the time when you are talking abou blown fuses, you are talking about a short to ground or a short in your load. Lets go ahead and practice ohms law for your case. If you have a 30 amp fuse for your rear defroster, then your resistence between your power side of your circuit of your defroster, to your ground side of your defroster should be at least 1 ohm or so. But in most fords, fuses power more then what they actually say in the fuse block.
12=ohmsx30amps
you get about .4 ohms before it blows. if you go to the power side wherever the connector is of your rear defroster and then go to a known good ground in your car with your multi meter, you should a big amount of resistence becasue your rear defroster is nothing but a huge resistor. if you see something like 1 ohm, then you have a short to ground somewhere in your circuit. Make sure that your car is off and no power is running through this circuit. Your multi meter sends its own voltage through the circuit and it comes back and makes an math equation out of it and determines the resistance. If you have your car on, it will see that voltage and give you erratic readings. Also make sure your switch for your defroger is closed (pressed).
Whats your mileage. I have an 01 gauge cluster i'm trying to get rid of. Has around 32-33K miles on it. I can't remember i took it out so long ago. My defrost doesn't work either, but my car is also parked for the winter and stays in the garage so it doesn't need to work.
____________________________________
Performance Red Coupe #1594 6/18/2001
Whats your mileage. I have an 01 gauge cluster i'm trying to get rid of. Has around 32-33K miles on it. I can't remember i took it out so long ago. My defrost doesn't work either, but my car is also parked for the winter and stays in the garage so it doesn't need to work.
Hey guys. For one of my new articles, I will be showing you how to diagnose a circuit. But in order for me to show you, I will be needing to put up pictures. How do you do that? LOL. Thats right, good with electrical but not with pc's
Linear Mode
