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Troubleshooting the Briggs starter-generator system - Recovered Article


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By Kent

  • January 16

How do I troubleshoot my Briggs & Stratton starter/generator system?  My generator light stays on...  

Read the two-part response from Al Eden, a long-time Simplicity dealer.  I've posted it here for easy access and reference.


Subject: RE: Starter/Generator - Gen light on
Posted: 7/12/1999 11:49:31 PM

This starter-generator [SG] has 2 fields, one a very heavy series one for starting and another shunt wound one to charge.  The shunt one is connected to the armature, A terminal the other end is connected to the F terminal of the SG to charge the f terminal is grounded by the voltage regulator.  When the voltage at the reaches a level about 12.8 volts the cutout contacts in the regulator close and connect the A to the battery. As long as the voltage at the A terminal is higher than the battery current will flow to the battery, charging it.  The charge light is connected through the switch to the battery on one side and the A terminal on the other side.  With the key on and 12 volts on the battery side of the light and nothing on the A terminal side [generator not generating] the light gets its ground through the armature in the SG and is on. 

As the SG starts to turn, at idle speed the voltage at the A terminal approaches 12 volts the bulb has 12 volts on both leads and no differential between its 2 leads and is out.  As the voltage increases to about 12.8 volts the "cutout" terminals of the regulator close and effectively bypass the light leaving no voltage across it [out].  As the engine speed increases the SG output goes up and more current flows to the battery.  When the voltage reaches 14.2 volts OR the current reaches about 8 amps the contacts in the voltage regulator "VR" that ground the field term of SG open, internally there is a resistor across them.  This reduces the current that flows through the field, reducing the magnetic field in the pole pieces that provide the field for the armature, voltage at the A terminal of SG drops, this causes the contacts in the field circuit to close and this brings the output back up and then it goes too high and they open again and the cycle repeats. 

The voltage regulator contacts regulate the output by vibrating constantly. The output of the SG is controlled by the tension on the spring on these contacts. The regulator is calibrated by adjustment of this spring. the other contacts "[cutout contacts]" connect the A term to the battery when the voltage when the voltage at A is above about 12.8 volts and disconnect at about 11.8 to 12.2 volts this prevents the battery from discharging through the SG armature when its output is form 0 to 12 volts.  If the unit starts OK but don't charge, is the generator side field and or the voltage regulator.  Since these SGs are a combo and only have one generator field coil they only produce about 5 to 10 amps max.  If the regulator contacts stick and keep the F term grounded at all times [called full fielded] the SG overheats and the generator field burns out.  We replace these commonly. NOTE: If they are burned out most commonly the regulator caused it. HAVE IT CHECKED HOOKED UP TO THE STARTER GENERATOR!!! 

You can check the SG unit by hooking a voltmeter to the A terminal and grounding the F terminal.  The voltage should come up to 13 to 14 volts with the engine wide open.  If it goes to 15.5 or more the regulator is not connecting it to the battery as these units don't have the capability of going that high into a battery unless it is sulfonated and doesn't take a charge.  If the SG puts out OK, you probably have a regulator problem.  The A term of the starter generator goes to the arm or gen terminal of the regulator, the F term. to the F or fld. term of the reg. and the Batt or b term of the regulator goes to the Battery.  Do not get confused by the big solenoid lead to the Arm terminal of the SG. That puts 12v to the armature to start the unit only and is not involved when the unit is running and charging. If your red light is on, it either is not charging or the wire from the light is shorted to ground or not going to where it is supposed to go. 

Good luck and if you have any questions, ask and we'll try to explain them. 



Subject: RE: Starter/Generator - Gen light on More to look
Posted: 7/14/1999 1:26:23 AM

Roy, We see some things like that -- the starter-generator running too hot.  I'm glad that when we have to eat a bunch of hours on a weird problem, we aren't the only people that get frustrated. Now a little more about the SG.  One of the most difficult problems to find in a generator or alternator is the flying short.  Since the armature in a generator is belted up 3 to 4 times the crank speed, on one of these tractors this means it is turning between 10 and 15000 rpm. 243 brs are spec'd @ 4000.  Centripetal force is trying to make it fly apart, and the wire in each turn is pressing hard against the wire outside it in the coil. Varnish is the only insulation on the wire.  The whole coil is pushing against the fish paper and the retainer strips. Any of these areas is vulnerable to shorting.  When the unit is stopped and the armature is checked it will check OK both on a leakage test from the commutator to ground and an induced field test on a growler.  The short is gone and until it goes in orbit again will be fine. 

This is a not uncommon problem. You have checked the grounds, if you have any question run a wire from the ground strap on the reg. mount to the engine or SG. {for good measure].  Check that the regulator is somewhat "shock mounted". Next inside the SG check the rear bearing to see if it is loose, hopefully you are lucky enough to have one with a ball bearing in the back.  If there is excessive play in the rear bushing, this can cause the brushes to bounce. The commutator being out of round will cause this also. It should be turned in a lathe and the mica between the commutator bars undercut. Since you probably don't have access to an undercutter, you can use a hacksaw blade. Just break it in half and grind the sides down to take the set out of the teeth and make the cut narrower. .015 to .020 deep is good enough.  There can be no copper particles between the bars as it won't work if two are shorted together.  After you are done an starter/alt shop can "GROWL" it to check for these shorts. They can also do the turning and undercutting if you desire.  If the brushes have been bouncing look carefully at the flat side where they slide in the brush holder if they have a little indentation where the 2 ribs are in the brush holder they will very commonly "stick" bounce up and not slide back down to contact the commutator at higher speeds. Often they come down when the engine stops so the starter will work. 

There's basically 4 versions of SG:  short frame and the longer frame, clockwise and CCW.  CCW is found most commonly on the Cub Cadets. The short frames were used on engines up to 8 hp and don't have the torque to crank the larger engines.  In addition some were ball bearing on both ends and some have a bushing in the commutator end.  One of the reasons they run so hot is that they don't have openings in the end frames, and fans like generators do, so the heat stays inside. As for fields, the fields we use are wrapped in fiberglass tape, where the Delco fields are dipped in a soft plastic coating which we have seen "cold flow".  We see very few failures from them. Remember the CW units take different field coils from the CCW ones. The brushes are also different from regular generator brushes.  They have more copper in them to handle the starting current.  Not nearly as much as starter brushes have or it would tear up the commutator.  When it is not charging just ground the F terminal and see if it charges [running near full speed].  If it does the problem may be in the regulator. One division of our shop rebuilds starters, generators and alternators of all sizes and types big and small, we have one person dedicated to this area.  We often reset new regulators. If you trace it to the regulator, yell and I'll try to help you get it set. 

Good luck, 


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