How To Perform a Tractor Electrics and Voltage Drop Test

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How To Perform a Tractor Electrics and Voltage Drop Test

Postby cubguy's dad » Sun Jan 04, 2009 6:44 pm

We used Kyle's 1947 Cub as the test tractor. His tractor has been sitting in the shop since just before Thanksgiving. The shop temperature is at a constant 40 degrees (F) unless we turn up the heat. We did turn up the heat but the battery was still cold. All events are real, so here we go. Let's see if Kyle's Cub measures up.

First picture is of two DVOM's (Digital Volt Ohm Meter). The yellow one is a parts store special at about $25.00, while the Craftsman is Kyle's and was about $35.00 on sale. These are a step up from entry level but both have rubber cases that will stand up to some abuse and hard knocks. Read the instructions carefully. When you have mastered this instrument you will wonder how you did without it.

The next picture shows a Open Circuit Battery Voltage Test. Hook up the meter as shown observing the meter polarity. By this I mean Red lead on the Positive post and Black lead on the Negative post. With the engine and ever thing else off measure the Battery Voltage. We are looking to measure the state of charge that the battery currently has. A fully charged 6 volt battery will have 6.5 volts. A fully discharged battery will have 6 volts, anything in between will be various states of charge. For example 6.3 volts in a battery will equal 50% charge. Anything less than 6.5 volts and you will have to charge the battery to continue or the next series of tests will be inaccurate.

The next picture is the Battery Capacity Test. Can the battery produce enough amperage to crank our tractor? Hook up the meter the same way as the last test. Crank the engine for 15 seconds and observe the voltage at the end of 15 seconds. A good fully charged battery will have 5.5 volts at the end of this test. For me I would like to see 5 volts as the lowest acceptable level that will leave a little margin left for extreme conditions. At 4.5 volts the battery needs to be replaced before you can continue with the next tests. As you can see ,Kyle's battery has 5.49 volts and is in good condition. We can know proceed with the next test.

The next series of pictures shows the Voltage Drop Test. We are measuring how much resistance this Cub has in the Ground Circuit. We go from the battery post to the starter case and we will measure how much electricity takes the easier path through the meter. Hook up your meter leads as in pictures 1 and 2.


In this picture I am holding both leads while Kyle is getting ready to crank the tractor (ignition off).

This last picture is our measured resistance in the Ground Circuit. The resistance needs to be less than .5 volts (1/2 Volt). Kyle's resistance is .2 volts. That is good. I like to see .2 volts (2 tenths of a volt ) or less.

So far the 47 has passed let's see what happens on the positive circuit.

Next we will test the Positive Circuit. Perform the test the same as the Ground Circuit only this time we will test the Positive Circuit for Resistance. We are looking for .5 volts or less. Anything more than .5 volts and you will need to clean, check and tighten then retest. The Positive Circuit is easer as it has a direct path to the starter.


In the last picture, Kyle's positive circuit is showing .19 volts and that is great.

Once again the 47 has passed.

Now is the time to do some math and see how much total resistance we have in the Battery Circuit. Add .2 plus .19 and we get .39 volts of total resistance which is really good. Here is what this all means. As good as Kyle's tractor is, he is still losing almost .4 volts in his Battery Circuit. While he is cranking his tractor with a fully charged battery his battery fell to 5.49 volts. However, subtract the .39 volts total Resistance and the starter only sees 5.10 volts. Imagine if Kyle had .5 volts on the Positive and Negative circuit for a combined resistance of ONE volt. Even with his good battery, the starter would only see 4.5 volts and this is the lowest acceptable voltage for the starter.

Now imagine the Charging System. The generator out-put is 8 volts, but because of the high resistance in our battery circuit, 1 volt is consumed by our high Resistance and the battery sees only 7 volts. Not enough voltage to maintain the battery to full charge let alone operate the lights. Next we will check Kyle's Charging System to see if it is up to specs.

The next test is the Charging System No Load Test. Start the tractor and run for 5 minutes to stabilize the regulator. At part engine throttle, measure voltage at the battery just like you did in the first test (Open Circuit Battery Voltage Test). Observe that on the meter, the voltage should fall between 7.8 volts to 8.1 volts. If it does your Charging System is producing enough power to maintain the battery. In the picture Kyle's 47 shows 7.88 volts at half throttle. The tractor amp meter shows about 8 amps charge.

Again the cub passed.

The next test is the Charging System Load Test. Can the generator produce enough power to operate the lights and maintain the battery? At part engine throttle turn on the lights to the highest setting and observe the meter. Kyle's cub showed 7.25 volts at the battery and 1 amp on the tractor amp meter which is enough to run the lights and keep the battery up. You need to have 7 volts minimum in this test.

Kyle has a Farmall spotlight on his tractor and we wanted to test the Charging System to see if it produced enough power to operate that as well. Same test as above but we had the spotlight on as well. The voltage at the battery showed 7.16 volts and the amp meter showed 0 amps.

Charging System passed.

The last picture is of the battery in the Cub. Always install the largest battery that will physically fit in you application. This one is from our local Auto value parts store and has 650 amps. This is the correct size battery for your Cub.

Kyle was pleased with the results as we had not tested his tractor before. All the tests took less than a half hour to do. We had fun doing this and hope this helps someone out.

Experiance is knowing what NOT to do the next time.......
1937 John Deere A (Big John)
1953 Farmall Cub (LiL Red)
Lots of projects.

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