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Automotive/testing charging system

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Expert: - 10/26/2007


Question
The battery on my John Deere LT155 lawn tractor died suddenly. The key was
not left in the ignition. The battery is so dead that it won't take a charge. I
want to figure out what happened before I put in a new battery.

I have an old analog multitester and thought I might be able to use it to check
out the charging system on the lawn tractor.

Can I get the engine running, then disconnect the battery, and put the tester
leads on the battery cables to see if there is juice flowing back to the battery?
If so, which DC settings should I use on the tester (DCV or DCA)?

Any other thoughts would be appreciated.
Thanks,
Barry Adamson

Answer
Your first order of business should be to check out the connections.  Batteries do not often "suddenly" fail so, unless the battery is very old, it is possible that the charging supply to it has simply gone open; maybe a connector stud has become loose ... ?  If there is no visible evidence of such a problem, your multitester can be used as a continuity tester to check for less-visible problems in connectors or wiring harnesses.  Use its low-ohms range and be systematic, working from the battery terminations back.

If that testing clears, proceed with voltage testing.  My assumption is that this equipment has a 12V system.  In that case, the DCV mode, 0 - 20V, would be suitable for determining if the charging system is providing the correct output voltage, likely in the range of 13.2 to 13.8 but perhaps as high as 14.2 V for this equipment.  However, it may be that a battery with some charge on it would be required to provide some field current to the generator or alternator to get it to begin inducing voltage; still, there often is enough residual magnetism in the iron core of the field winding to fulfill this task.  Therefore, try doing the measurement first without a battery present, and try again with a reasonably charged battery if that does not work.  An automotive battery would be suitable for this test (again, assuming the system is a 12V system) in the absence of the specified tractor battery.

Voltage is one thing - electromotive force (rise) or potential difference (drop), and current is another - flow of electrons.  If you want to verify that your charging system is providing current to the battery, you will need a DC ammeter that is capable of carrying whatever current your charging system can deliver.  Most modern digital multimeters can measure up to 10A (some to 20A) but generally for only a short time.  I doubt that your analog meter is set up to measure more than a few hundred milliamps.  In any event, even a garden tractor charging system may well exceed this if the battery it is supplying is low on charge, so you probably should see about borrowing a Hall-effect type clamp current meter, if you wish to check current flow.

For your purposes though, the presence of the correct voltage range should be sufficient for verifying the charging system.  If the system is good, then try a new battery.  Your old one likely has sulfated - the leading cause of failure in lead-type batteries - and has become more of a space heater than a battery!

Good luck ... EGK  

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Ernest (Ernie) Kenward

Expertise

The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!

Experience

A key skill in my work and hobby pursuits both is STRATEGIC TROUBLESHOOTING. I am a senior instructor in Electrical Engineering Technology at a leading Canadian polytechnic, my areas being Electrical Power and Industrial Control, electrical and electronics design and manufacturing, and AutoCAD and related CAD/CAE software - plus equipment problem-solving and new equipment design and prototyping. Hobby-wise, I have 30-plus years of experience in auto restoration, mostly in electrical and mechanical systems. Ongoing projects include a 1959 Edsel Corsair, my 1978 Ford E250 class-B motorhome conversion, and the care and upkeep of my Mercedes 300CD. My vehicles become engineering test beds for electrical and mechanical upgrades as ideas present themselves. This includes the design and production of circuit boards to restore or enhance features for which no OEM replacement parts are obtainable, or where better specifications or reliability can be had via newer concepts. Regarding the E250 RV conversion, I designed and continue to revise a custom power distribution system, managed by a Programmable Controller (PLC); this has made most revisions as easy as uploading new firmware as I develop it. The "mini" PLC is a powerful device for custom automotive control systems. One good example (there are many) would be the Moeller "Easy Relay"; these offer a wealth of control, monitoring and variable-and-status display options for such projects. A good example project which has worked well is that one for my RV noted above, which has been on the job - revised in firmware only - for a decade now. It is a load management and charging control system to avoid the sulfation-induced early failure that often befalls deep-cycle batteries used in RV power applications. The battery installed in 2003 lasted long enough to more tnan pay for the PLC that contributed to its longer life ... and the PLC will be there for the next battery as well!

Organizations
IEEE - senior member ... past WCC Student Activities; SME - senior member ... past chair, greater Vancouver chapter chair 318; Edsel Owners' Club - have served in various capacities on chapter executive during seventies; have been Power and Driveline resource on the Edsel Owners' Club "E-team" for more than a decade.

Education/Credentials
Graduate of UBC

Awards and Honors
Certificates of appreciation from IEEE and SME for work in student and chapter activities

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