bumperm
Veteran Member
What ritcheyvs said.
Also with a voltmeter you can easily isolate your problem in just a few minutes.
With a load (current draw) on the battery (turn lights on or if you have a helper, glow plugs or similar load), measure voltage at battery terminal posts, not on the connectors, measure on the center of the terminal posts. If above 12.5 or so, good, proceed. Move the positive voltmeter lead to the wire terminal - should read the same as before. Move the negative probe to the wire terminal, should be the same. If not you can measure "voltage drop" (or loss caused by a bad connection) by measuring across that connector or length of wire. If it were the positive battery terminal bad, you would put the meter positive on the center of the battery terminal, and the meter negative on the connector - if you get a voltage reading, that is the "loss" across that connection.
This works as long as there's current flow through a circuit, you can measure from point A to point B with your meter and see what the voltage loss is through a wire or component. In a high current circuit, say a starter motor running and pulling 100 amps or more, you expect to see a loss of 1 volt or a bit more. With a smaller load, say headlights on, the loss across the wiring (if you measure from the bat positive to the positive terminal of the lamp) should be just a fraction of a volt, (e.g. 0.13 volts).
By checking this way, you won't be replacing perfectly good parts. Note that a wire seldom almost never goes bad unless it is physically damaged. More often the problem would be a poorly crimped, corroded, or loose connection. If wire corrosion itself is suspected (again rare) copper corrosion is green in color. Also a lead acid battery can have what is called a "surface charge" - wherein it looks good on a voltmeter (12.5 to 12.8 volts) but the battery has little to no capacity - if a load is applied, as ritcheyvs said, the voltage drops precipitously or "collapses". Measure across the battery with your meter will show this, as a good battery will show a small voltage drop as a load is applied, then will typically rebound a little as internal chemistry and temperature rise a bit.
good luck,
bumper
Also with a voltmeter you can easily isolate your problem in just a few minutes.
With a load (current draw) on the battery (turn lights on or if you have a helper, glow plugs or similar load), measure voltage at battery terminal posts, not on the connectors, measure on the center of the terminal posts. If above 12.5 or so, good, proceed. Move the positive voltmeter lead to the wire terminal - should read the same as before. Move the negative probe to the wire terminal, should be the same. If not you can measure "voltage drop" (or loss caused by a bad connection) by measuring across that connector or length of wire. If it were the positive battery terminal bad, you would put the meter positive on the center of the battery terminal, and the meter negative on the connector - if you get a voltage reading, that is the "loss" across that connection.
This works as long as there's current flow through a circuit, you can measure from point A to point B with your meter and see what the voltage loss is through a wire or component. In a high current circuit, say a starter motor running and pulling 100 amps or more, you expect to see a loss of 1 volt or a bit more. With a smaller load, say headlights on, the loss across the wiring (if you measure from the bat positive to the positive terminal of the lamp) should be just a fraction of a volt, (e.g. 0.13 volts).
By checking this way, you won't be replacing perfectly good parts. Note that a wire seldom almost never goes bad unless it is physically damaged. More often the problem would be a poorly crimped, corroded, or loose connection. If wire corrosion itself is suspected (again rare) copper corrosion is green in color. Also a lead acid battery can have what is called a "surface charge" - wherein it looks good on a voltmeter (12.5 to 12.8 volts) but the battery has little to no capacity - if a load is applied, as ritcheyvs said, the voltage drops precipitously or "collapses". Measure across the battery with your meter will show this, as a good battery will show a small voltage drop as a load is applied, then will typically rebound a little as internal chemistry and temperature rise a bit.
good luck,
bumper
