- Joined
- May 28, 2001
Saw this on the Bullet. Thot it may be of use.
You will need a volt meter, prefferable a digital one. The help of a friend will make testing a lot easier as well. And possibly a set of booster cables if the circuit you are going to test is really long.
A few things that you should know first. When you use a volt meter it displays the difference in voltage from one lead to another. So when you place it across the battery on the + and - posts, and it reads 12.79volts, it means that the difference in voltage between the two posts is 12.79volts.
All the circuits in your car complete themsleves at the battery, specifically between the + and - plates inside the battery. So any extra resistance outside the battery, any where in the circuit is a bad thing.
Industry stanard for automotive production cars is a .5 total voltage drop in any system. Except maybe your starting system, as the old style starters just draw a ton of amperage. The .5 is divided up as .3 positive and .2 negative. So when we are testing the positive leg of the circuit we will want to see less than .3 of a drop, and less than .2 on the negative side. This is while the device is in use. i.e. if you are checking your lights, they need to be on while doing the check.
So here is how to do the test. Put one lead on one point of the circuit, and the other lead at another point. Say at both ends of a wire. The meter should read zero. Now turn on the device. The reading on the meter is how much the voltage drops between the two leads. If it is on both ends of a wire, then it is how much the voltage drop down that wire. If it is over .3 + or .2 - it needs to be replaced.
Here is an example testing the starter system on an older Ford. It is slow cranking, with a new starter and battery, so we will check for voltage drop issues. First thing we do is put one lead on the positive terminal of the battery, and the other lead on main post of the starter. Try starting it and we get a reading of 1.5volts. So we know there is a 1.5 volt drop on the positive side of the circuit.
So leaving the lead on the battery, we move the other lead up to where the main battery cable from the starter goes to the fenderwall solenoid. We repeat the start, and still get a reading of 1.5volts. So we know that cable is not the problem. (Could have also put the leads on each end of the wire to determine this, but takes longer to do the test).
Next you put the lead on the solenoid's post, and try the test. Still 1.5volts, so we know the connection between the post and the starter cable) is good. So we move the lead to the other post of the fenderwall solenoid, and repeat the test. Now we get a reading of .5 volts on the meter, instead of the 1.5volts. So it would appear that we are losing 1 volt through the solenoid. To confirm that we can put a lead on each post of the solenoid, and try starting it. In this case it does confirm that we dropping 1 volt through the solenoid.
But we still have .5 volts drop. So now we check the cable end at the solenoid, to the battery post. It reads .5volts still. So we then go to the battery post to the cable end at the battery itself. We get a reading still of .5 . So that would mean that between the battery post and the cable end we are losing .5volts, in other words, we have a bad connection.
So you replace the solenoid, clean the battery cable end at the battery. Also preventive clean the solenoid cable ends too. Test the complete system down to the starter again, and get a reading of .2 volts. So we are happy. Moving over ot the negative side. You move one lead to negative post of the battery, and another lead to the body of the starter.
You start the motor and getting a reading of .7 on volt meter. Now you just follow the ground path back every where there is a connection. So first you would go from the negative post to the bell housing, then block, then the ground strap clamp at the block, then to the frame rail, etc, etc. Where ever the drop goes away, then the what ever is between your last point and the new point is likely where the drop is. Always double check it by actually putting the leads on both ends of the item being tested (both ends of a wire, between a clamp and bolt, two posts on a solenoid or relay, two posts on a kill switch, etc).
It should take less than a couple of minutes to check a complete circuit. If the circuit is really long, say with the battery in the trunk. You can clamp a booster cable to the battery, and then to the one lead of the meter, thus extending the lead's length. You can use a small wire even, just booster cables are readily available.
You should do it on any circuit you install, and it does not hurt to go through your car, daily driver included every now and then.
If people did they would have found that the GM hot start in the 70's was mostly because the 4ga starter wire needed to be 1ga starter wire.
You will need a volt meter, prefferable a digital one. The help of a friend will make testing a lot easier as well. And possibly a set of booster cables if the circuit you are going to test is really long.
A few things that you should know first. When you use a volt meter it displays the difference in voltage from one lead to another. So when you place it across the battery on the + and - posts, and it reads 12.79volts, it means that the difference in voltage between the two posts is 12.79volts.
All the circuits in your car complete themsleves at the battery, specifically between the + and - plates inside the battery. So any extra resistance outside the battery, any where in the circuit is a bad thing.
Industry stanard for automotive production cars is a .5 total voltage drop in any system. Except maybe your starting system, as the old style starters just draw a ton of amperage. The .5 is divided up as .3 positive and .2 negative. So when we are testing the positive leg of the circuit we will want to see less than .3 of a drop, and less than .2 on the negative side. This is while the device is in use. i.e. if you are checking your lights, they need to be on while doing the check.
So here is how to do the test. Put one lead on one point of the circuit, and the other lead at another point. Say at both ends of a wire. The meter should read zero. Now turn on the device. The reading on the meter is how much the voltage drops between the two leads. If it is on both ends of a wire, then it is how much the voltage drop down that wire. If it is over .3 + or .2 - it needs to be replaced.
Here is an example testing the starter system on an older Ford. It is slow cranking, with a new starter and battery, so we will check for voltage drop issues. First thing we do is put one lead on the positive terminal of the battery, and the other lead on main post of the starter. Try starting it and we get a reading of 1.5volts. So we know there is a 1.5 volt drop on the positive side of the circuit.
So leaving the lead on the battery, we move the other lead up to where the main battery cable from the starter goes to the fenderwall solenoid. We repeat the start, and still get a reading of 1.5volts. So we know that cable is not the problem. (Could have also put the leads on each end of the wire to determine this, but takes longer to do the test).
Next you put the lead on the solenoid's post, and try the test. Still 1.5volts, so we know the connection between the post and the starter cable) is good. So we move the lead to the other post of the fenderwall solenoid, and repeat the test. Now we get a reading of .5 volts on the meter, instead of the 1.5volts. So it would appear that we are losing 1 volt through the solenoid. To confirm that we can put a lead on each post of the solenoid, and try starting it. In this case it does confirm that we dropping 1 volt through the solenoid.
But we still have .5 volts drop. So now we check the cable end at the solenoid, to the battery post. It reads .5volts still. So we then go to the battery post to the cable end at the battery itself. We get a reading still of .5 . So that would mean that between the battery post and the cable end we are losing .5volts, in other words, we have a bad connection.
So you replace the solenoid, clean the battery cable end at the battery. Also preventive clean the solenoid cable ends too. Test the complete system down to the starter again, and get a reading of .2 volts. So we are happy. Moving over ot the negative side. You move one lead to negative post of the battery, and another lead to the body of the starter.
You start the motor and getting a reading of .7 on volt meter. Now you just follow the ground path back every where there is a connection. So first you would go from the negative post to the bell housing, then block, then the ground strap clamp at the block, then to the frame rail, etc, etc. Where ever the drop goes away, then the what ever is between your last point and the new point is likely where the drop is. Always double check it by actually putting the leads on both ends of the item being tested (both ends of a wire, between a clamp and bolt, two posts on a solenoid or relay, two posts on a kill switch, etc).
It should take less than a couple of minutes to check a complete circuit. If the circuit is really long, say with the battery in the trunk. You can clamp a booster cable to the battery, and then to the one lead of the meter, thus extending the lead's length. You can use a small wire even, just booster cables are readily available.
You should do it on any circuit you install, and it does not hurt to go through your car, daily driver included every now and then.
If people did they would have found that the GM hot start in the 70's was mostly because the 4ga starter wire needed to be 1ga starter wire.
