Small Engines (Lawn Mowers, etc.)/L120 charging system


QUESTION: Hey Eric, I've got a John Deere L120 that isn't charging the new battery. I put in 106777-0128-e1 into the B&S website to see about downloading,purchasing or asking for help but it says it doesn't have any results for that nmbr. The nmbr came off of the engine,more specifically the ohv cover. Do you know if their website is only operational between 9-5 Mon-Fri? Or why they don't recognize the engine model nmbr? Better yet, do you know of a link to a step by step process of checking the charging system? Here's where I'm at presently. Btty shows 12.5 v, voltage regulator shows about 29 VAC at yellow wires and 12.5 VDC at red wire at full throttle. I have jumped it off with my truck 3 or 4 times before I got the new btty. And I've seen where that is a no-no. So I guess it's quite possible that I shot myself in the foot and fried the regulator. Should I take a 60-80 dollar guess and get a regulator or take the time and try to troubleshoot it? Ain't nothing ever simple,is it? Sorry about the length of the question. I 'magine you're a fast reader. And I do appreciate wisdom from perfect strangers. Many thanks.....Man!!,65,000 - character limit. I've got room to tell you about my grandkids.

ANSWER: Try these URLs:

Double check the model number.  Based on the first 2 digits (10) it indicates it is a 10 cubic inch block which is a small, walk behind mower engine.

Let me if the URLs helps.

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QUESTION: Appears to be 406777-0128-e1. I'm trying to find a tach of some kind now to do the test on the URL you sent me. I guess it's best to check at 3600 rpms like they say? Do think jumping it off probably hurt the regulator or not always? Thanks for the speedy reply.

ANSWER: I wouldn't worry about a tach.  You AC voltage readings should be in the the alternator is bad your AC will be way off.

I have ruined regulators, diodes, switches, wiring...with using a car battery...too many amps.  There was usually other issues that contributed to the parts getting ruined.

The AC volts should increase with engine speed and decrease when the engine is slowed down.  

The regulator should have DC output if the AC input is good.

Make sure you check the wiring, connections...make sure everything is clean and tight as well as no shorts in the system.


---------- FOLLOW-UP ----------

QUESTION: Hey Eric, I had been checking DCV at the regulator with red wire plugs together and my test lead on the battery side of the connection. I disconnected the plug and checked red wire out of regulator and had 13.7. Had a bad connection at the plug. I appreciate your wisdom. I was leaning towards a new regulator. You did good. Many thanks. Enjoy your 30 minute weekend.

Make sure you keep you battery terminal clean...they might look clean, no corrosion, but is it a wise to disconnect the cables and clean the battery posts and cable ends with a wirebrush once in a while.  This will ensure a good connection and keep the resistance down.  Check out Ohm's law, increase resistance lead to more current...current, not voltage, leads to more heat which can cause components to overheat and it ensures your starter is getting max current to spin the engine.

Glad to hear it was just a connection.


Small Engines (Lawn Mowers, etc.)

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Eric A. Jones


Lawnmower Repair . Certified Master Service Technician from B&S. Have 23 years experience on B&S, Lawn Chief, Weed Eater, Echo, Peerless, Wheel Horse, Snapper, Atlas, MTD, McCulloch, Homelite and many other numerous brands. Specialize in electrical repair.


Born and raised in the midwest. Started tinkering with engines when I was about 14 on my Suzuki RM-80. I began lawn mower repair at a small hardware store. I knew absolutely nothing. I read lots of repair manuals and met an older fellow who taught me many lessons. I continued working on small engines through high school and paid my way through college working on mowers at the same hardware store. Decided to get away from the midwest and mower repair so I joined the Air Force. I repaired air traffic control electronic equipment and ended up in Hawaii where I got a part time job at Small Engine Clinic. I gained a lot of experience from the Small Engine Clinic and had a blast repairing small engines. I then took the Briggs and Stratton Master Service Technician test and earned my MST. I then traveled to Wisconsin where I attended the factory update training seminar and received formal training. Continued working on mowers part time as I completed 20 years of military. Retired from the military on a Friday and continued in the lawn and garden industry the next Monday.

MAS Aerospace Operations BA Mathematics AAS Electronic Communications AAS Electronic Technology

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