Tractor electrical testing update with pictures

Fri Jan 16, 2009 7:45 am

I have been reading of late several posts on starting problems.

First thing is having the proper tools in everybodys toolbox you NEED a DVOM (digitail volt ohm meter) a good one will set you back about less than $100.00 Craftsman has one in that range, I have a Fluke Meter MOST DIYers do not need this expensive of a meter. With a DVOM(digitail volt ohm meter) you can diagnose anything electrical on the tractor.

First start with the battery.Check the voltage at the top of the posts (DVOM set to DC voltage setting,all these tests will use this setting), a fully charged 6 volt should be about 6.5 to 6.6 volts, (12 volt battery needs to be 12.6-12.7 volts )if the battery does not pass this test it will have to be charged BEFORE you can continue.(a 6volt battery at 6 volts has less than a 10% charge in it).

With a fully charged battery (6.5 volts)in your cub, again set the DVOM to DC volts, observe meter polarity and measure from the top of the battery posts, disable the ignition and crank the engine over for 15 seconds(we are testing battery capacity or amperage draw) at the end of 15 seconds observe the voltage should be above 5.5 volts, the higher the better(means your battery has enough capacity to effectively operate the starter) remember on a distributor ignition you need voltage reserve to make the coil function as well as operate the starter. You need a minimum of 4.5 volts to have enough battery capacity to start the tractor if not the battery needs replaced.Always buy the largest battery that will physical fit in your application (see Bob's post in this section for his recommendations on batteries).

Next test the Ground and positive circuits. Remember that electricity will always take the path of least resistance and we will provide an alternate path and will measure how much electicity will take the easy route. Starting with the ground circuit put one on the top of the battery post end (observe meter polarity) put other end on the starter case, crank the engine(ignition disabled) and observe meter it should show no more than .5 volts (1/2 volt) the less the better on a 6 volt I like .2 volts or less. You just tested the complete ground circuit for total resistance. Now you can clean, replace and retest to correct the resistance in the groung circuit and you can test for your results.

Do the same thing with the positive circuit one end on the top of the battery post and the other end on the starter post and see what you have. Again you need to see less than .5 volts. Clean replace and retest as needed. Again less than .2 volts or less is desirable.

Add up your total resistance. For example if you have .5 volts on both the ground and positive circuits you have 1 VOLT resistance, couple that with a only 50% charged battery (a (reading of 6.3 volts on the battery voltage test)and you have a problem. You only have 5.3 volts (6.3 minus 1volt combined resistance) available to the starter BEFORE you even run the starter to crank the engine.

This test works for any vehicle 6 or 12 volt. 12 volt systems start out with more voltage (electrical Pressure) and can handle more resistance up to 3 volts and still operate where a 6 volt system is done at 1 volt total resistance and that is not much.

Correcting the resistance in the battery circuit will help your charging system, as it has to operate with the same resistance as your starter cicuit does. Measure charging voltage at the battery to get the true reading of the charging system.The automotive style battery needs 1.5 volts over fully charged battery voltage to fully charge the battery. A 6 volt battery needs a charging voltage of 7.8 to 8.1volts at the battery, and a 12 volt battery needs 13.8 to 14.2 volts at the battery to keep it charged.

Measuring voltage drop is not hard it just takes a little practice. Everybody on this forum knows the importance of clean connections and the right size battery cables. Taking the time to test and repair the resistance in your electrical system will solve many starting and charging problems

Last edited by cubguy's dad on Sat Jan 17, 2009 2:59 pm, edited 2 times in total.

Re: electrical problems & voltage drop test

Fri Jan 16, 2009 7:04 pm

Just a follow up on my earlier post. A DVOM is a good investment but shop around for a good price. Craftsman has a good unit, they are often on sale and they work well, Kyle paid $35.00 for his. You can spend more but that will be for features that you probably will not use.

With anything electrical start at the source, The battery is always the first thing to test.

We can test the battery voltage with our DVOM hooked up to the battery 6.5 volts means that the battery is fully charged and should have the capacity or power to crank the engine. Voltage less than 6.5 will mean that the baterry will need to be charged before you can test it.Test the battery capacity by using the tractor starter to pull amperage out of the battery while watching the voltage on the battery it should be a min.of 4.5 volts or more in order for the battery to have enough capacity to operate the starter and start the tractor.

To sum this test up 6.5 volts means the battery has a full charge ,at 6 volts it is less than a 10% charge in it. If the voltage drops below 4.5 volts it means that the battery does not have the capacity and is worn out and it will need to be replaced. I hope this clears up my earlier post a little (I tend to be a little on the long winded side)and I will continue to expand/edit the first post a little later on. Hope this helps someone.

Last edited by cubguy's dad on Fri Jan 16, 2009 11:25 pm, edited 2 times in total.

Re: electrical problems & voltage drop test

Sat Jan 17, 2009 5:57 am


I think this is an excellent post. I work with electronics every day and if you follow this explanation of “how to trouble shoot starting problems” you can resolve any electrical no start / hard starting issues and in fact any electrical circuit problems.

The only thing that’s a little confusing to me is when you talk about voltage drop and use the term resistance. Your explanation shows there would be resistance (high / low resistance) in the circuit when performing the voltage drop as you explained. When I see the term resistance I think of disabling the circuit and performing an ohms test.

In my opinion, doing a resistance check with an ohms meter is ok, but, if you make a circuit work and use the voltage drop method like you explained you will find problems a lot quicker.

When testing low voltage / amperage circuits (milli volts/amps) sometimes you have no choice but to disable the circuit and check with an ohms meter. On occasions I still disable the circuit and run battery plus voltage to locate hard to find “opens” or high resistance connections using the voltage drop method and putting a headlamp bulb on the end of the circuit.

Like Eugene stated, if you can post pictures on some simple checks, I think it would really help with a lot of electrical issues.


Last edited by Jack fowler on Sat Jan 17, 2009 10:22 am, edited 1 time in total.

Re: electrical problems & voltage drop test

Sat Jan 17, 2009 10:41 am

Since I do not yet own a meter, can you explain the "observe meter polarity" a little bit.
Some volt/ohm meters are sensitive to polarity. Meaning that when you check/test a direct current circuit you must have the negative meter lead connected to the negative side of the circuit and positive meter lead to the positive side of the circuit. This is particularly true with the analog type meters (meters with a needle).

Purchasing volt/ohm meters.
Some of the inexpensive meters are very good. You can find meters in the $10 to $15 range that will work quite well for all of the electrical work you will be doing on a tractor.

The only major problem with inexpensive meters, that I have found, is that the fuse will burn out when I screw up. It's cheaper to throw the meter away than to buy and install a replacement fuse.

First time meter purchasers:
Buy one of the inexpensive meters first. Above all, look over the instructions before using.

Caution. For all meters. Remove one of the leads from a circuit being tested before changing positions on the meter's dial. Changing the meter's dial position under load will burn out the fuse.

Re: electrical problems & voltage drop test

Sat Jan 17, 2009 2:56 pm

An update on this topic with pictures. We used Kyle's 1947 Cub as the test tractor. His tractor has been sitting in the shop since just before Thanksgiving. The shop temp. is at a constant 40 degree 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 (Digitail 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 everthing eles 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 inacurate.

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 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 pics 1 and 2. In picture 3 I am holding both leads while Kyle is getting ready to crank the tractor (ignition off). The 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 curcuit.

Next we will test the positive curcuit. 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 you Kyle had .5 volts on the positive and negitive 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 one 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 Kyles charging system to see if it is up to specs.

To next test is for 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 test). Observe meter the voltage should fall between 7.8 volts to 8.1 volts. If it does your charging system is producing enough powerto maintain the battery. In the picture Kyle's 47 shows 7.88 volts at half throttle. The tractor ampmeter 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 ampmeter that 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 ampmeter 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. ... 0_0141.jpg

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.



Mon Jan 19, 2009 9:13 pm

An amazing amount of great information and the organization is first-class.
If I could only think as well as you write...

One thing I noticed is regarding polarity. Every digital meter I've had does not require attention to polarity when measuring voltage. I realize the negative sign (-) that's automatically displayed when the polarity is 'wrong' might confuse novices, but if someone understands about why it's there, not having to be concerned with polarity makes these meters easier to use.

Of course if you have an analog meter, you have to watch the polarity.

Like you said, I don't know how people can fix electrical problems without one.
Your post is going to help a lot of people (including me) make troubleshooting a more rewarding experience.
Thank you for taking the time to write it and for taking all the pictures for us.

Re: Tractor electrical testing update with pictures

Thu Jan 22, 2009 7:25 pm

Thank you so much for doing the "electrical teseting for dummies" segment. You do't know how many times I did the "Tim the Tool Man-Uhhhh!" when I read posts involving electrical testing/maintenance. Now, if I can go through this 6 or 7 times I may figure it out.


Re: Tractor electrical testing update with pictures

Thu Jan 22, 2009 7:45 pm

Outstanding, this is something that even I can understand :D :D