mfitzpatrick2
New User
I thought I would share a story that proves proper electrical testing saves time and money.
After the fact, I got involved in an issue with a farm semi truck that was having problems starting, charging, and dead batteries.
The farmer began having trouble with low batteries and staring issues, so he took it to a local truck repair shop for repairs.
After a day or so he was told the batteries were bad.
All new batteries installed.
A week or so later "more problems".
Took back to shop and they checked the batteries again and found low voltage from alternator so they replaced the generator. Volt meter on dash
showed 14.5v but the tech was checking at batteries. Interesting?
New alternator installed.
Again more problems starting!
Took back to shop
Tech said "it must be the starter because the alternator is new and the batteries are new". In dash volt meter showed 14.5v and load testing of
starter at batteries showed starter draw problems?
Starter replaced.
Again more starting problems.
The starting problem seemed to be inconsistent?
This guy was frustrated!
I looked it over and everything looked good
New batteries, cable ends were clean and tight, alternator tight and showed 14.8v at unit.
Starter looked good, tight, cables tight and clean.
Now for some testing
Load test of batteries showed low but good after a charge?
Alternator voltage test at batteries showed 12.5v while dash showed 14.5v?
Checking battery voltage while cranking showed low draw but slow cranking?
Time to do some voltage drop tests!
From post on back of alternator to battery showed 2v drop? This matched the dash voltage compared to Alt voltage difference?
From starter solenoid post to battery post 2v drop while running and 4v while cranking?
Voltage drop test from battery post to cable and solenoid post to cable. Probe through the insulation so I can test the cable to end connection.
All good, so cable ends are good but the entire cable test 4v drop?
This cable is 4/0 50 inch long with a 24 inch section inside a plastic cable protector near the bell housing. This was the only area of the cable
that could not be seen.
I cut the protector off and found a 4 inch long section wrapped in electrical tape?
Cut the tape off and Problem Found!!!!
Wire insulation must have been damaged and was wrapped with tape but this allowed water/salt to penetrate. Wire was green and rotten.
Ran these same tests on the ground side and all tested good.
The alternator power wire went to the starter solenoid where it connected to the battery, so power must go through the bad cable to charge the
batteries. Had it went direct to batteries the charging would not have been an issue.
At this point we have no way of knowing if the original alternator, starter, or batteries were actually bad?
Rule of thumb is max .1v drop per connection. So reducing the number of connections should be considered whenever possible.
Ground connections are a good example, battery to cable, cable to frame, frame to engine block, engine block to starter, etc.
If you can, run direct from battery to starter. I have seen 7 or more connections with grounds, each having the potential for voltage drop.
You need to count all connections, post to cable end, cable end to cable, cable end to solenoid stud, etc.
5 connections is equal to .5 or 1/2 volt and still be considered acceptable.
Consider all connections on positive and negative side and they add up fast.
Big engine in cold weather needs all the power it can get!
Moral of the story? All possible testing must be done to properly diagnose before just throwing parts at problems.
Time and money wasted!
After the fact, I got involved in an issue with a farm semi truck that was having problems starting, charging, and dead batteries.
The farmer began having trouble with low batteries and staring issues, so he took it to a local truck repair shop for repairs.
After a day or so he was told the batteries were bad.
All new batteries installed.
A week or so later "more problems".
Took back to shop and they checked the batteries again and found low voltage from alternator so they replaced the generator. Volt meter on dash
showed 14.5v but the tech was checking at batteries. Interesting?
New alternator installed.
Again more problems starting!
Took back to shop
Tech said "it must be the starter because the alternator is new and the batteries are new". In dash volt meter showed 14.5v and load testing of
starter at batteries showed starter draw problems?
Starter replaced.
Again more starting problems.
The starting problem seemed to be inconsistent?
This guy was frustrated!
I looked it over and everything looked good
New batteries, cable ends were clean and tight, alternator tight and showed 14.8v at unit.
Starter looked good, tight, cables tight and clean.
Now for some testing
Load test of batteries showed low but good after a charge?
Alternator voltage test at batteries showed 12.5v while dash showed 14.5v?
Checking battery voltage while cranking showed low draw but slow cranking?
Time to do some voltage drop tests!
From post on back of alternator to battery showed 2v drop? This matched the dash voltage compared to Alt voltage difference?
From starter solenoid post to battery post 2v drop while running and 4v while cranking?
Voltage drop test from battery post to cable and solenoid post to cable. Probe through the insulation so I can test the cable to end connection.
All good, so cable ends are good but the entire cable test 4v drop?
This cable is 4/0 50 inch long with a 24 inch section inside a plastic cable protector near the bell housing. This was the only area of the cable
that could not be seen.
I cut the protector off and found a 4 inch long section wrapped in electrical tape?
Cut the tape off and Problem Found!!!!
Wire insulation must have been damaged and was wrapped with tape but this allowed water/salt to penetrate. Wire was green and rotten.
Ran these same tests on the ground side and all tested good.
The alternator power wire went to the starter solenoid where it connected to the battery, so power must go through the bad cable to charge the
batteries. Had it went direct to batteries the charging would not have been an issue.
At this point we have no way of knowing if the original alternator, starter, or batteries were actually bad?
Rule of thumb is max .1v drop per connection. So reducing the number of connections should be considered whenever possible.
Ground connections are a good example, battery to cable, cable to frame, frame to engine block, engine block to starter, etc.
If you can, run direct from battery to starter. I have seen 7 or more connections with grounds, each having the potential for voltage drop.
You need to count all connections, post to cable end, cable end to cable, cable end to solenoid stud, etc.
5 connections is equal to .5 or 1/2 volt and still be considered acceptable.
Consider all connections on positive and negative side and they add up fast.
Big engine in cold weather needs all the power it can get!
Moral of the story? All possible testing must be done to properly diagnose before just throwing parts at problems.
Time and money wasted!
