Tech Dept.

Even though this is the Tech Dept., this is NOT a tech tip. This is what I did and I can not recommend it to anyone else. 

While working under the dash of a car. I was attempting to start a small nut which I could just barely reach with the tip of my fingers. In the process I snagged my index finger on something very sharp, slicing my finger. This netted a trip to the medicine cabinet for a band-aid. Now I noticed I couldn't feel the small nut through the band-aid and couldn't get my other hand in a position to do the same procedure. The cut was not bad enough to get stitches but too bad to attempt to put the nut on without the band-aid.  Back to the medicine cabinet and removed the band-aid and washed the cut with Hydrogen Peroxide then held the wound closed and put a thin line of super glue along the cut. Since super glue is hardened by temperature it set up right away. To my surprise, I could use the finger right away and could hardly tell it had been cut. In a couple of days the thin strip of glue came off and the cut was not completely healed so on with some medication and a band-aid. I am sure that how severe the cut is, helped determine the outcome and how clean I got the wound before sealing it up with glue.  

While working on another car, I got a steel splinter in my finger. Not uncommon for me. The usual magnifying glass and tweezers didn't do the job this time. The splinter was deep with none protruding. Oh well! grit teeth and start digging with a needle. Ouch!, Ouch!, Ouch!. Hmmmm! I heard of men in war digging shrapnel out of them selves with a bayonet. And I can't even dig this *%&$# splinter out of my finger. Back to the medicine cabinet. A tooth ache drop applied then I was digging for the splinter like it was someone else's finger. 

Tech Tip # 8

Problem- This is for any brand car. This problem will usually first be noted by inconsistent gauge readings. Like a fuel gauge that reads higher than it did earlier and you have not added any fuel, a temp gauge that moves when you turn some electrical component on. the tools needed to check this is a volt/ohm meter and a wiring diagram for the car.

First- Turn your volt meter to low volt DC scale. Connect the positive probe of the meter to the positive post on the battery. Be sure it actually contacts the battery post not the cable connection. Connect the negative probe to the hot post on the starter solenoid. Again the post not cable end. Now, turn the ignition key On , NOT  START. Turn all of the electrical components on. Head lights, heater fan, wipers etc and read the volt meter. If you have an analog meter (needle type) you may not see any voltage. On that type of meter turn to a millivolt scale if it has one. If you have a digital meter you should see a very low millivolt reading. You should not expect to see much of a voltage drop if any on such a heavy cable with no more amp use then what electrical components will use. If you do get a reading write it down. Now turn everything off including the ignition. Go to the fuse panel with your volt meter. Reset the meter to read battery voltage. Ground the negative lead and probe all of the fuses to find any that are hot with the key off. Their may be more than one. Pull that fuse and  probe each connector to find which side of the fuse was the power side. Now, connect the positive probe of your volt meter (set on a low volt scale) to the positive post of the battery and the negative probe to the hot side of the fuse you found to be hot with the key off. At this point you will probably read no voltage unless you have an electric clock the size of "Big Ben". Or 100 watt interior lights. Now, Turn on the ignition key. (not start) and note the meter reading. Then turn on each electrical component noting the voltage reading as you add a component.

For those of you who have never done this. It's called a "voltage drop test". It tells you something but it has limitations. It tells you how much voltage (the push) is lost to resistance when each component is turned on. The resistance being checked is the cables and connections between the positive post of the battery and the power at the fuse panel only. If you logged each reading you can see that each item made a different amount of drop. The limitations are that the amount of current draw of the component will affect the voltage drop so much that you may condemn a circuit when in fact the component may be drawing too much amperage. For example; a wiper motor can draw a high amperage running on a dry windshield. A heater fan motor that makes a lot of noise and and seems to blow only a small amount of air may be binding and drawing a high amperage. You should write all the readings down before you try to decide what to do. Think in general terms at this point unless you know exactly what the readings should be. Generally, you may see a low millivolt reading like below a half a volt. Which would normally tell you that you don't have a problem with that part of the circuit. But if you see a volt to a volt and a half or more you should do more tests on that circuit. Now you will need a wiring diagram for that car. Wire connectors are the most common cause of resistance. If your fuse panel is inside the car you may have a connector at the firewall. Remember that all voltage drop tests must be done with a load on the circuit being tested and it should be the normal load that circuit is used for.

If you don't find anything wrong at this point, don't panic. You have only tested a small part of the circuit. All circuits must complete a circle. Example; positive post of the battery to battery cable to starter solenoid to fuse panel to a switch to the component to a ground wire to the car body to a battery ground cable to the negative post of the battery. Plus all the connectors and even a possible relay in between. So the voltage drop test is used to isolate the resistance to a smaller and smaller area so it can be identified.  Move your + probe of the voltmeter to the power on the fuse panel and move the negative probe to the gauge or if this car has a printed circuit board you will need to refer to a wiring diagram and find the power to the instrument cluster. If you go directly to the gauge, keep in mind that most gauges operate on less than 12 volts. So, look at your diagram to determine where the "instrument stabilizer or instrument regulator" is positioned. It will either be on the instrument cluster or built into the gauge itself. Your diagram will usually show this. If it is on the cluster, it will usually be just two wire. Power (12 V) in and 6 to 8 V out.

Now you will not be checking from the gauge to the sending unit. You will be checking for voltage drops from ground of the instrument cluster to the negative post of the battery. Attach the neg. probe of the voltmeter to the neg. post of the battery (not the battery cable) and the positive probe to the ground terminal of the instrument cluster and as before start turning everything on noting changes in voltage. By using this voltage drop test you can check any circuit of any component. Just remember that the circuit must be "loaded" when you test and always keep the positive probe on the circuit toward the positive end of the section being tested and the negative probe toward the negative end of the circuit. A more common use of this test is on the starter motor circuit.          


 

Past Tech Articles

1. Polarize a Generator

2. Alternator / Generator Tests

3. Brake Problems after installing pads

4. Prolong Gearbox Life

5. Engine Oil

6. Ignition System

7. Cooling System

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