Honda SOHC Charging System Troubleshooting
Contents
1. If an analog meter s needle swings backward that s good If forward reverse the connections On a digital meter you want a negative sign Start and rev the engine to 3 000 rpm The result should be at least one fourth the battery s amp hour rating In other words 3 amps in the case of a 12 A H battery When The Output is Low Step One Bypass the Regulator The first troubleshooting step is to bypass the regulator However before bypassing the regulator make sure the field coil isn t shorted because the bypass test might otherwise result in some rather dramatic electrical arcing Set your test meter to Ohms X 1 and measure the field coil s resistance Under 2 ohms is a short Over 7 Ohms is an open Both are bad If you replace the field coil skip ahead to Step Three to make sure the regulator isn t bad also Otherwise continue with the regulator bypass How the bypass is performed depends on whether your system has the rectifier and regulator as separate parts or together in one part All the Honda SOHC fours including the first year 1976 750 Automatic had a mechanical voltage regulator separate from the rectifier Starting with the 1977 model year the 750 Automatic got the more modern one piece regulator rectifier that would be introduced on the 1979 DOHC fours If your regulator is separate from the rectifier two piece system leave the alternator connector coupled and insert an unwound paper clip into the connector at the WH2. battery s positive terminal and the meter s black test lead on the field coil connector s power wire keep everything connected On everything but the 77 and 78 750 Auto the field coil power wire is the WHITE wire at the regulator On the Automatic having the later one piece regulator rectifier the field coil power wire is the BLACK wire at the regulator With the keyswitch on the reading should be 1 volt or less More than 1 volt points to a bad keyswitch or other switches or connectors which you ll need to check out If necessary repair or replace the indicated part and skip ahead to Step Three If on the other hand these double checks pass go on to Step Two Step Two Stator Test The resistance tests in the manual aren t conclusive tests for the SOHC stator The stator is best tested in a special way by stressing it Uncouple the alternator connector and remove the three yellow wires from the alternator side of the connector then re couple the connector Clip a 2 ohm 100 watt resistor 10 at industrial electronics supply houses across your meter s red and black test leads This is needed to properly load the stator Now start the engine With the meter set to AC Volts and the leads held to any two of the yellow stator output wires rev the engine to 5 000 rpm Repeat two more times to check all three leads The results must be the same for all three wires and at least 25 volts If less the stator is bad If on the other hand the re
3. Honda SOHC Charging System Troubleshooting Mike Nixon Here is a quick neat three step method of checking your bike s charging system However to use it you must understand the charging system For example note that there are two troubleshooting flow paths One for when the output is low and a different one for when it output is not low but the battery keeps going dead So first a few basics By the way a FAQ file covering this troubleshooting method is available here Note The CB650 SOHC model is excepted from this tutorial It has the same charging system as the later DOHC fours therefore go here to troubleshoot it Charging System Basics The Four Parts The SOHC s electromagnet charging system has just four major parts The two primary parts are the rotor and the stator The rotor is the rotating magnet and the stator is the stationary coil of wire The third charging system part is the rectifier As the alternator s magnet spins the induced electrical current changes direction in step with the magnetic poles Since alternating current won t charge the battery the rectifier rectifies the output to just one battery terminal The fourth charging system part is the regulator The regulator prevents the alternator from producing too much at high rpm because this would hurt the electrical components and the battery On the 77 and later 750 Automatic the rectifier and regulator are inside the same housing The Two Piece Rotor Hond
4. ITE wire location Run a jumper wire from the paper clip to the battery s POSITIVE terminal This bypasses the regulator If you have a 77 or 78 750 automatic that has the regulator and rectifier in the same housing one piece system put the paper clip and jumper in the same place but in this case connect the other end of the jumper wire to the battery s NEGATIVE terminal In either case with this temporary connection repeat the charge amps test but this time DO NOT EXCEED 2000 RPM to keep from overloading the system during the test If the alternator starts charging and it wasn t before the regulator is the problem Service if the regulator is mechanical or replace if the later electronic the regulator and skip ahead to Step Three If on the other hand the regulator bypass makes no difference in charge perform the following double checks First dangle that same paper clip from a pencil an inch from the alternator and turn the keyswitch on and off If the paper clip doesn t kiss the alternator each time the keyswitch is turned on there is no magnetism You ve already checked the field coil so all that s left is to examine the wiring leading into it including the connectors If on the other hand magnetism is present then there is probably some resistance in the circuit upstream Test for this resistance dynamically by doing a voltage drop test at the field coil With your test meter on DC Volts hold the meter s red test lead to the
5. a s earliest electromagnet charging system as used on the SOHC has an extra part because it has a two piece rotor This permits the electromagnet to rotate and the wires not get tangled up Later versions as used on the DOHC engines used carbon brushes instead One half the field coil has the wires the other half the rotor is magnetized by the first half and rotates The Field Coil Regulator Relationship The relationship between the field coil and the regulator is the key to understanding this system The field coil s electromagnetism is controlled by the regulator which is wired between it and the battery When the battery reaches 14 5 volts the regulator interrupts the field coil s power feed which reduces its magnetism and alternator output diminishes Preparation for Troubleshooting Battery Load Test Charge the battery until a hydrometer shows that all cells are good then load test the battery You must start with a known good battery because the charging system is a two way system having both input and output Consequently battery faults dramatically affect charging system performance Don t skip this part Charge Amps Test Because the charging system is a two way street charge absolutely must be checked in AMPS not VOLTS Set up your multimeter for its highest current scale usually 10 amps Remove the bike s main fuse and clip your meter s leads in place of the fuse Check polarity by turning the keyswitch on
6. ading is 25 volts or more the rectifier is bad because the stator s output is stopping there Replace the indicated part and go on to Step Three Step Three Retest Step Three is simply perform the charge amps test again If you now get 3 amps or more you re home free Congratulations When the Output is Not Low Not all charging system problems are due to lack of charge If the battery is going dead yet it s in good condition and the system is charging then the battery is getting charge but is somehow losing it That is there is a leak in the system We must find the leak The same three step method is used as before with some variations Step One Test Breakeven Rom This test the breakeven test pinpoints what kind of leak we have Set up the multimeter for the charge amps test The meter will read backward that is discharge when the key is turned on Start the engine and from idle very slowly increase engine rpm Stop raising the rpm just when the meter crosses over from discharge to charge and reads 0 This is the breakeven point Note the rpm at which this happens It should be under 2000 rpm If over 2000 rpm you have a dynamic leak that is a leak with the keyswitch turned on On the other had if the breakeven is under 2000 rpm then the leak is a static one one that occurs when the keyswitch is turned off Remember which kind you have dynamic or static and go on to Step Two Step Two Find the Leak Find a dy
7. namic leak by unplugging suspect electrical accessories one at a time and don t forget to check for a stuck brake light switch Turn the bike off each time you unplug something and restart it and retest breakeven to see if there is a difference At some point the breakeven rpm will go down The last component unplugged at that point is causing the dynamic leak Repair or replace the component and go on to Step Three Find a static leak by doing a key off current check Set and jack your test meter for milliamps mA and connect it between the battery s NEGATIVE terminal and NEGATIVE cable DO NOT TURN THE KEYSWITCH ON that will ruin your meter Just read the meter as is Motorcycles with full time electronics radios alarms clocks etc will indicate between 10 mA and 20 mA Bikes without full time electronics should indicate 0 mA If the reading is high something is leaking If you can t find it it is probably the rectifier that is leaking current out of the battery Double check this by disconnecting the rectifier If the drain stops the rectifier is leaky Replace it or correct whatever component you find and go on to Step Three Step Three Retest If your leak was dynamic perform the breakeven rpm test again as in Step One If now the charging system s breakeven point is under 2000 rpm you ve got the problem licked Congratulations If on the other hand your leak was static perform the key off amps check again as in Step T
8. wo If the key off drain is 0 mA or if you have a radio or alarm under 20 mA you have fixed it Go for a ride to celebrate
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