Trouble Shooting Field Excited Charging Systems

With today’s economy, may people are bring out their older machines and they need to bring them back to life.  The older Honda CB750/900s are notorious for problems with their charging systems.  The Yamaha JX650/750s also have these same issues, to a lesser degree.  And these problems often go misdiagnosed, resulting in extra money spent for no reason.  Either the shop or the customer absorbs this cost, but either way, no one is happy.  Rick’s Motorsport Electrics, Inc. has been testing, manufacturing, and rebuilding these charging components for over 30 years.  “Our rebuilt Honda rotors are the company’s biggest seller,” says owner Rick Shaw.  Rick offers the following information to aid with a proper diagnosis the first time so everyone is happy.

“When a charging failure occurs, it is most likely the alternator rotor,” says Shaw.  “Most people replace the rectifier regulator because it’s the least expensive piece and hope the problem is solved.  Unfortunately they call back in a couple weeks when the problem resurfaces and we find out their bad rotor took out the replacement rectifier/regulator.”  What happens? Heat and vibration break down the insulation on the copper wire inside the rotor, resulting in a “short” of the current from one coil to the next.  This prevents the electro-magnetic rotor from providing sufficient magnetic strength to produce a strong charge in the stator.

A failed rotor can also result in a secondary problem.  When the internal “short” occurs, the resistance in the rotor goes down and current flow increases.  This puts a heavy load on the rectifier/regulator and may burn out the regulator unit.  Simply replacing the rectifier/regulator because it looks bad will yield the same results – the bad rotor will continue to burn out new rectifier/regulators.  When the rotor fails, always replace the rectifier/regulator to prevent future problems.

Follow these test procedures from Rick’s Motorsport Electrics and you’ll be on your way to a proper diagnosis.

To test a rotor:

• Clean a spot on each of the 2 slip rings
• Set a digital OHMs meter to 0-20 OHMs
• Put one probe on one ring, and one on the other
• Record the OHM reading

(An “open” reading or low reading of 1 OHM or less is bad.  A good rotor reads 4-6 OHMs)

• Now test from any one ring to the steel base  (A good rotor should have NO continuity)

If your rotor tests good - test the stator:

• Identify the 3 stator wires (probably all yellow)
• Check all 3 wires for continuity (approximately .5 – 1 OHM)
• There should be no continuity to ground

Unfortunately, there is no field test for the rectifier/regulator; however they can be tested on a custom built machine at Rick’s Motorsport Electrics’ facility.

Once a diagnosis is made, call for your replacement parts and get the job done.  Both the stators and rectifier/regulator are brand new pieces.  The rotors are rebuilt units, requiring a core exchange.  All three components come with a one year replacement warranty.  For further information or to place an order, call Rick’s at 1-800-521-0277 or visit their website: www.ricksmotorsportelectrics.com.

Application chart of applicable motorcycles:

HONDA: 1979-82 CB650 Custom, 1982 CB650SC Nighthawk, 1979-82 CB750C/F/K/L, 1982-83 CB750SC Nighthawk, 1980-82 CB900C/F, 1983 CB1000 Custom

YAMAHA: 1981-83 XJ550 Seca/Maxim, 1981-83 XJ650 Seca/Maxim, 1975-83 XS650, 1985-86 XJ700 Maxim, 1981-83 XJ750 Seca/Maxim, 1983 XJ900 Seca

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