Electrical Troubleshooting, Test, Don''t Guess

[buzz saw]

A recurring theme of a lot of posts is why wont my tractor, truck, car or whatever crank, or cranks slow with the starter? It has a new battery, etc., etc. Here in my opinion is how to test, dont guess.

Voltage drop testing with a voltmeterAll any voltmeter does is measure the difference in voltage potential between the two leads, so measure a fully charged 12 volt battery, 12.6 volts DIFFERENCE between the positive and negative terminals. So how can we use this to troubleshoot a no or slow crank situation? Start at the battery, do we have battery voltage at the terminals while attempting to crank, these tests must be done with the circuit under load in order to mean anything, static voltage tests while not under load mean nothing. If so leave the positive lead on the positive POST and place the negative lead on the positive cable TERMINAL, again load the circuit by cranking and read the meter. Ideally the meter should read 0 volts because the voltage potential is equal, but in reality we will probably see .1 or so volts. You just tested and verified a good connection at the battery post.Rule of thumb is no more than .2 voltage drop per connection.

Now place the negative lead of the meter at the starter solenoid lug, with the positive lead still on the positive terminal at the battery. Again load the circuit by cranking, we should have no more than .2 volts reading on the meter, if more than that, clean the terminals and lugs of the cable and test againstill to high reading, you just verified a defective cable. Remember we are checking voltage drop, under load, in actual conditions. It is absolutely necessary to load the circuit while testing, picture a battery cable with one individual strand of wire under the insulation, the rest are corroded and brokenif just testing static voltage with no load the meter will show no voltage drop, that is, the same voltage on both endsnow load the circuit and you will see a total voltage drop on the meter, battery voltage on one end and zero volts on the other, because the cable cant carry current, therefore the meter will read battery voltage.

The same can be done on the ground side, that is place the positive meter lead on the starter case and the negative on the battery ground terminal, load circuit and read meter. Same as before, less than .2 volts reading on the meter. Reading several volts?, Clean the ground cable ends and terminals, what about paint and grease at the starter mounting flange, check the actual ground cable from the battery to the grounding stud on the other end, using voltage drop testing. You get the idea.

Alternator testing, same thing. Use voltage drop testing. Test the positive cable from end to end, check the case ground back to the battery ground, you get the idea. Just remember, the circuit must be under load. Charging system testing, make the charging system work under load. A carbon pile battery load tester works well for this, but turning on every electrical accessory while testing works good too.

I hope I have explained this clearly, if not maybe John T or others can explain in better terminology. Trust me on this though, if you master and use this procedure, and get comfortable with it you can confidently troubleshoot these issues and not just guess and throw parts at it.

 

[buickanddeere]

Exactly . I have always wondered why so many mechanics have the time and money to throw random parts at a machine and hope they eventually solve the fault .

 

[504]

Now you just need to repost at least once a week.

 

[MarkB_MI]

Yep, that's pretty much it. I would add that any time you get a reading that doesn't make sense, back up and retest what you think is good. Oftentimes a bad meter lead or poor ground connection can send you on a wild goose chase.

If you think you have a bad starter cable connection, feel all the connections (carefully!) immediately after a start attempt. Bad connections will often get hot.

 

[Hobo,NC]

Ideally the meter should read 0 volts because the voltage potential is equal, but in reality we will probably see .1 or so volts. You just tested and verified a good connection at the battery post.Rule of thumb is no more than .2 voltage drop per connection.

To understand that...

Wires carrying current always have inherent resistance, or impedance, to current flow. Voltage drop is defined as the amount of voltage loss that occurs through all or part of a circuit due to impedance.

To put it to use.

The load should consume all the voltage that why we want to see

Rule of thumb is no more than .2 voltage drop per connection.

My starter is dragging lets test the positive side of the circuit... (The unknown the health of the battery lest say its good for now)

There are at least 5 connections on that circuit and possible 9 so the rule of thumb of .2 is a little out in left field : (

What gives folks a hard time with this is the word "voltage potential" remember the load should consume all the voltage and I think it will help you understand "voltage potential"

Voltage drop normally stands out like a sore thumb it will be a BIG number. Numbers at 1V are less are mute its normal at the end of those 5/9 connections if you have a bad connection you will see numbers like 5,6 9, 10 and 12V's.

 

[Hobo,NC]

I left the last part off...

Voltage drop normally stands out like a sore thumb it will be a BIG number. Numbers at 1V are less are mute its normal at the end of those 5/9 connections if you have a bad connection you will see numbers like 5,6 9, 10 and 12V's.

5,6,9 10 and 12V is the lost of the voltage potential the LOAD DID NOT CONSUME THE voltage potential.

 

[Russ from MN]

Lots of people are parts changers, not mechanics! So many time on here you read about someone that has changed a lot of parts, and their tractor still won't start.

 

[jimg.allentown]

The first thing to do when your tractor, truck, or car won't start is to go to your computer and post "my
tractor/truck/car won't start? What is wrong with it?"

Then, out comes the parts cannon.

 

[John T]

Yo Buzz, you posted "maybe John T or others can explain in better terminology"

Hey you did good not much for me to add !!!!!!!!!!!!!!

I often use the terminology VOLTAGE DROP to impress upon non sparkies how much the voltage "drops" across a resistance such as a
loose/burned/carboned connection and remind them V = I x R. That's why they need to measure the voltage drop under current drawing LOAD
conditions IE with NO current the V = I x R voltage drop would be ZERO. A practical problem is keeping good solid volt meter connections
under cranking conditions and if its not good one gets erratic measurements. As you well know the Voltage across the battery posts as well
as the voltage that remains (after any bad cables or connection drops) on the starter post when cranking can help locate a problem. IE if
the battery voltage (+ to -) drops to say 11 when cranking and the voltage on the starter (starter to ground) is also the same 11 volts,
theres no drop across bad cables or connections or switches or solenoids etc. HOWEVER if battery voltage drops to 11 but theres only 10 on
the starter THERES A V = I X R VOLTAGE DROP ACROSS A RESISTIVE CONNECTION. I have seen MANY mechanical push starter switches and/or
electric solenoids develop resistance (causing voltage drop) due to carbon pitting and burning as well, of course, cable ends hidden UNDER
the outer visible crimp.....

Pleasure sparky chatting with you

Merry CHRISTmas

John T

 

[bc]

Thanks. Now you tell us buzz saw. All I need now is someone to do the cranking and also someone to crawl under the pickup in the mud to check the starter. Wife is always busy when those projects come up.

 

[Determined]

Overall I would say what you posted is very well written and easy to comprehend for individuals with reasonable mechanical/electrical knowledge.

To the guy who works in an office all day that just bought himself a 65 year old tractor for his 1.3 acre estate reading your post probably leaves him confused and set up to fail.

You have great advice but trying to teach out of a grade 12 math book to a kindergarten class does not work so good.

How often have you posted a bunch of well thought out checks for someone to try and they respond with a question like " So what does a distributor look like".

Sometimes you need to add little details like when you say put a meter lead on the solenoid lug it should be in BIG CAPITAL LETTERS with further elementary instructions hammering on things like the lug does not mean;

-the nut
-the washer
-the terminal

Add pictures where needed.

When it comes to batteries you can have top post terminals, side post terminals, screw terminals. They all have their own quirks when it comes to where to touch a meter lead to for an accurate test.

We often forget that half or more of what we know is not common knowledge and we need to step back and ask ourselves;

Would a 6 year old understand what I just wrote?

 

[buickanddeere]

Instead of Voltage Drop perhaps we should call it VD ?

 

[da.bees]

(quoted from post at 09:28:02 12/19/20) Overall I would say what you posted is very well written and easy to comprehend for individuals with reasonable mechanical/electrical knowledge.

To the guy who works in an office all day that just bought himself a 65 year old tractor for his 1.3 acre estate reading your post probably leaves him confused and set up to fail.

You have great advice but trying to teach out of a grade 12 math book to a kindergarten class does not work so good.

How often have you posted a bunch of well thought out checks for someone to try and they respond with a question like " So what does a distributor look like".

Sometimes you need to add little details like when you say put a meter lead on the solenoid lug it should be in BIG CAPITAL LETTERS with further elementary instructions hammering on things like the lug does not mean;

-the nut
-the washer
-the terminal

Add pictures where needed.

When it comes to batteries you can have top post terminals, side post terminals, screw terminals. They all have their own quirks when it comes to where to touch a meter lead to for an accurate test.

We often forget that half or more of what we know is not common knowledge and we need to step back and ask ourselves;

Would a 6 year old understand what I just wrote?

What you are saying determined might be true but if people insist on asking for help rather than turning it over to a garage,they need to begin some where and learn. Buzz did a god job putting in simple,easy to understand language. The birds that need to get with the program are those that strut and preen while spouting formulas that confuse and muddy the water for those trying to learn basics.

 

[Stephen Newell]

How would that test do if the makers of the tractor etc put a starter on that only had barely enough cranking torque to start it under great conditions? You can't blame the battery or the connections if the conditions are bad and the starter just needs more torque. I
have a tractor I replaced the starter, replaced the battery with as big a battery as will fit and cleaned the connections and run an additional grounding strap directly to the starter. When the temperature gets near freezing you can forget trying to start it, The
starter will just turn the engine maybe one revolution and stop.

 

[buickanddeere]

This is why it is so handy that starters are universal mount . With options on rotation , pinion gear pitch and number of pinion teeth .
Most engines have have the starter power doubled .

 

[Hobo,NC]

This test is not used to eliminate starter are battery issues its use to diagnose the circuit. You are looking to see if the load consumes the available voltage be it a low voltage circuit are a high voltage circuit.

The load should consume all the available voltage.