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Rick, Factory Authorized Trainer
Category: Small Engine
Satisfied Customers: 8057
Experience:  Outdoor Power Equipment technical trainer since 1990, covering eight states.
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I have students at the University of TN Martin who are building

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I have students at the University of TN Martin who are building a 1/4 scale pulling tractor for entry into a competition in early June. This is sanctioned by the American Society of Agricultural and Biological Engineers (ASABE). We are using two 16 hp B&S Vanguard V twin engines Model # XXXXX type 0079-G1, code 08052111. The engines and two tires are provided by ASABE and cannot be altered or tinkered with other than to connect electrical power and adjust for proper governed speed. I say this to prefice my explanation of the problem. The problem is both engines have been started a few times and each has run for short periods of time but neither will now start. Yesterday, one engine would not start. Through trouble shooting we determined only one spark plug was firing. According to the manuals this indicates one of the diodes is bad so we ordered a new, 2-diode harness. This morning while starting and running the other engine, it developed the same problem, i.e. no spark at either plug. So it too will not start. We have ordered new diodes for it as well, but evidently something is causing this in terms of what we are doing. Or perhaps, we are just having bad luck. What we have done in making electrical connections is to connect power to the solenoid from a 12 volt battery and that is basically it.
Richard Shadden

Hello, and thank you for choosing JustAnswer. I'll be helping you today and am committed to providing clear and concise answers to your question.


Can you tell me what 2-diode harness you used and what manual said to do this? I'm a bit confused by that statement, since usually the diode is on the charging coil (to convert AC to DC for battery charging), not the ignition coil.


This sounds more like a grounding problem to me. There is a small ground wire that is connected to the base of the ignition module that has the purpose of grounding the coil and stopping the spark to kill the engine. If this wire is improperly grounded through damaged insulation, or any other contact to ground it will prevent spark from occuring.


Can you give me a bit more info please?

Customer: replied 5 years ago.

Thanks for your help. This 2 cylinder engine has a magneto ignition system with two coils (armatures), one for each cylinder. On page 58 of the BS Vanguard V-Twin OHV Repair Manual (No.272133-6/04), the two diodes are shown in the wiring diagram with one for each coil. The ignition armatures are electrically separated by two diodes installed in the wire harness that connects the ignition armatures to a single ignition switch. Information from a BS textbook (No. 2487 089484), page 200 states the diodes provide the independent ignition armatures a common path to ground so the engine can be killed with one kill switch to ground the primary circuit thus eliminating the spark to the engine. Without diodes each primary circuit of each coil would have a path to ground through the opposite coil. Our problem may not be in the diodes but may be something as simple as a ground, however, I believe we have a common probem with each engine since both have the same problem and the BS textbook as well as the repair manual indicates the problem is with the diodes. As I stated previously, we have ordered new diodes (come as two in a litttle wiring harness) and haven't received them. UTM is in a small town in NW Tennessee, i.e. out in the boondocks. We don't know if this will fix the problem. If it does, we hope to avoid a repeat, hence my reason for my inquiry to you. RS

Thank you. Now I understand. Have you tested for spark on each cylinder by completely disconnecting the the ground wire from each coil? That would eliminate the diode and ground wire and determine whether the problem was internal or external to the coil.


Also have you checked the diodes with a meter? Reading resistance you should have continuity in one direction, but no continuity if the leads are reversed.

Customer: replied 5 years ago.
On each engine we removed the wiring harness, with the two diodes, from the grounding contact on each of the two coils for the engine which then goes through the diodes to a common ground through the igintion (kill) switch. We did this to both engines and each starts and runs fine. The ignition switch even kills the engine without the diode harness which from my understanding is there to kill the engine by grounding out the primary circuit of the coil. So, how are we killing the engine with that wire totally detached? One thought I had is that the so called ignition swith is a multi-function switch with one duty being to energize the starter solednoid. I think it may also energize a fuel cutoff solenoid as well. My electrical schematics are not complete in that there is no schematic of the "modules" to which some wires connect. That may be because of some proprietary information BS does not want to let out. In any event the engine "ignition" switch allows the engine to start and shuts it off as well. We have received the two new wiring harnesses with the diodes for each engine but we are going not going to put them on the engine since we seem to be able to make it function with them off. I accept your work. Is that what I do to insure you are paid. I can't seem to find where to 'click accept'. Thank you for your help, I may be contacing you again later.

I don't supply an accept button until I'm reasonably sure we have solved the question, since I always prefer to earn my keep before getting compensated.


If you have a fuel shut-off solenoid on the carburetor, turning the ignition switch to the off position will starve it for fuel, so it sounds like that's exactly what is happening here. That will "kill" the engine as well as grounding out the ignition.


From your description of testing, it looks like you have certainly proven both ignition modules are good. The diode harness should resolve the shut-off issue based upon your findings.


Here's a rather generic diagram showing how those systems operate. Yours may be slightly different depending on the specific application.



I'm also posting a link to the book this diagram is taken from. There may be information in there you can use:


I'll go ahead and post an accept button with this response if you are satisfied with my assistance. Please let me know if I can help again. Thank you, XXXXX XXXXX of success!

Rick, Factory Authorized Trainer
Category: Small Engine
Satisfied Customers: 8057
Experience: Outdoor Power Equipment technical trainer since 1990, covering eight states.
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