Just bought a nice 78 Honda 750 hondamatic, cheap, because no alternator output. Runs OK on the battery. I pulled the Alt. cover and everything looks OK, nothing black or dark brown, wires OK. I did continuity test of the wires to ground, zero, except the red and blue wire was about 4.0. The yellow to yellow were all about 0.4 even. Checked continuity of the V reg wires, unplugged, zero on the red and yellows, the two greens were 1.1 to ground. This alternator does not have brushes, it does have the copper wire coil that is stationary in the center, as is the stator. Nothing checks grounded. Anybody got any ideas? Thanks
The 750 SOHC stator has field coils (electro-magnets) instead of permanent magnets. The rotor is magnetized by the field coils and sweeps its magnetic field past the stator windings where far more electricity is generated than what was used to energize the field coils. The alternator itself has a reputation as being bullet-proof. The voltage regulator and rectifier are more frequent causes of trouble. The red and green wires go to the rectifier, not to the voltage regulator. Each diode should show near-zero resistance in one direction and near-infinite resistance in the other direction. I recall that the diodes are in stacks, but I really forget what it looks like in there so I'll stop. The voltage regulator is that squarish "can" near the rectifier. It has only white, green, and black wires (or a green, white, and black sticker by the three spade terminals on the side/bottom of the regulator. The voltage regulator makes the alternator's field coils stronger or weaker depending on the voltage sensed at the regulator. You can take the cover off the regulator and see the coil in there (another electro-magnet). Once that coil gets a certain voltage, it attracts the arm and its contact which puts that wire-wound resistor into the circuit. mcma111 is correct that the alternator isn't able to overcome a dead battery. EDIT: I'll go measure the resistance between different terminals of a working regulator and edit this again later this evening.
The battery does seem to be in good shape but the meter reads 11 to 12 volts given some RPM. I trust the hydrometer if I can test each cell. There isn't a corrosion problem as far as I can see, so far. The bike was stored in doors. I will look next to the regulator and let you know how it goes.
On one working example of a 750 voltage regulator, black and white show about 0.4 ohms. The green spade to either of those is about 35 ohms. Probably lots of variation between examples. Even though you don't have a general corrosion problem, look at the contacts inside the regulator; they are critical to charging. But I quit too soon (above) on the rectifier. Any yellow wire should be able to pass current to red, but not to green. How this shows up on your meter depends on the polarity of your meter.
Thanks Grinnin, the voltage reg. was mounted in a way I couldn't see the can. I opened it and it looks very clean and OK. There are two small black items that may be fuses on that board near the V reg. I'll check those in the morning. I also see the diode stack. I can see the individual diodes. Thanks again, I'll get back to it in the AM.
I can help if you post a diagram. also.... meters.... if you are checking diodes the meter needs to be on a special diode scale if it is a digital. And, most meters auto range on the ohm scale & will read "K" ohms or even "Meg" ohms so be careful about what scale it's on because numbers will come out no matter what. and last but not least... many of the manuals for older bikes have electrical readings that were made with analogy meters. the pure resistance measurements (like coil ohms) will be the same but any circuit with a diode or transistor in it will be different. it has to do with how the meters are different and the characteristics of solid state devices
Typically with those motorcycles it's a bad rectifier, which is a Wheatstone bridge type iirc Anyone have a schematic?
think I found one... http://oldmanhonda.com/MC/WiringDiagrams/MCwiring.php bottom of the page CB750A if this is it then yes, as mentioned above the exciter field is fed by small carbon brushes which are prone to wear. pull the connector from the regulator and measure ohms between the white wire and ground. the rule of thumb is this type of field should have a resistance about 1/2 the system voltage (6 ohms for a 12V system)... the meter should be between 4 and 15 ohms (not K ohms). if so, then the brushes are good and the field coil is good. if less resistance, it may be shorted, if more ohms then it has bad connections, if it reads infinite the circuit is open (broken brush or wire or coil). If this all checks good but the generator does not make good voltage then jump the white wire straight on to the battery plus... this will give full field and voltage will rise with RPM.... be careful, the unregulated output can go to 40 volts or more. if the regulator has contact points, run a business card through the closed contacts to shine them up... if really bad they can be filed but they won't last long until the crust up again to test the stator, pull the plug on the rectifier. ohm out each pair of the 3 yellow wires. they should read low, like .3 to maybe 2 ohms between each paring. high resistance is wires & connections (bad connections, corrosion). any reading of infinity is an open circuit (bad). also read each lead to ground.... that should be open (no path) if there is a path, the stator is shorted to ground. to test for a short between windings is harder. the easy way is to connect a light bulb between each pair and run the engine. the light should light and get brighter with RPM up to system value. (or you can measure the voltage... it will be AC, not DC) to test the rectifier, select a diode scale. pull the connector on the rectifier. put the meter NEGATIVE lead on the rectifier's output lead (that goes to battery plus, looks like a red/yellow wire)... now touch each of the 3 stator input points. they should read continuity,,, usually about 450 ohms, depending on the meter. reversing the leads should read infinite... no path. now put the POSITIVE meter lead on the rectifier's ground output (looks like there are 2) and touch the meter negative lead to each of the 3 phase inputs... they should read a diode number. reverse the leads and it should read infinite
No brushes in the alternator. None. The field coils and the generating coils are both stationary. The rotor is energized by the magnetic field of the field coils instead of being energized electrically. I failed to pick up on one measurement by YOUNZ. I cannot tell if you measured the white wire into the alternator (it's on a separate bullet connector, not on the gang plug). The white wire to ground should be 7 or so ohms. The white wire to green wire (another separate bullet connector at the same place) should be the same. Green to ground should be zero (it's grounded on in the alternator), but if you're getting zero on white you have a shorted field coil.
Yep, zero on the white wire to ground! That's the white wire from the copper wire spool in the alternator! Son of a gun, I thought zero to ground was good. Oh well, I guess I'm in the market, if anyone has one. Would rather deal here than on flea Bay. Thanks for all your efforts, guys.
Before shopping you may want to remove it and measure it on the bench. It could be abraded insulation where the wire goes through the case or pinched inside the case. The white and green wires go beside the rotor on the inside so there's extra opportunities for a short in wiring instead of in the field coil itself. You need to remove a largish side case to look. After the side case is off, the field coil is held in place by th three screws that are prominent on the outside of the alternator housing. I'll also look to see if I have a spare field coil.
Oops. Sounds like you don't need the directions that I gave in my last post. Please excuse me. I DID find a spare field coil. 7.6 ohms. The woven wire protection is in good shape, but the tube from inch 5 to inch 8.5 is stiff (but not broken or cracked). I don't know all the differences in engine cases between 750K and 750A. I wanted to let you check the photo to make sure this field coil matches yours. The photo doesn't show well that the rectangular grommet where it exits the case is skewed to exit at an angle.
Isee you are reading from the red wire to the white. I just checked mine in the same manor and got the same reading. Prior, I was testing from white to ground and got zero. So I guess mine is OK also. Guess I need to go back to looking at the diodes and regulator.
something in the generator system has to move in order to generate... there a permanent magnet on the crank, rotating in the middle of the field coil yes? there are a number of brushless designs out there but they all work something like this: a permanent magnet induces into a pickup coil & generates electricity. that electricity goes to a regulator and the regulated output induces the stator. Edit: looks like the diagram I posted does not quite match.... there is no red wire off the field in the one I posted
The flywheel on the end of the crank is a heavy cylinder that rotates around the center coil and is surrounded by the stator coil, both coils are stationary. The steel cylinder isn't magnetic. It seems the coils are OK. Now, it's back to checking the regulator/diode side. I'm sure I could somehow bypass the regulator/diode system and check the alt. output, but I don't want to harm the coils in the process. On a car, I would just energise the field terminal to get output, if the coils were OK. I hope I don't have to check every wire for continuity from the alt. to the reg system because other systems use the same color wires. Oh, the joys of an unknown old motorcycle!
I measured from green to white, not from red to white. Do you have a red wire on the field coil or is that a typo? There ARE red wires on the rectifier. If you get 7 or so ohms measuring wire-to-wire, but get 0 ohms measuring white-to-ground when the parts are in place, it could be that the white wire shorts when it's assembled. But then again it sounds like you know what you're doing but just need help with this odd alternator.
No. I stole this diagram and photo from Practical Machinist. Part 7 is the field coil which is bolted to the engine case. Part 6 is the set of generating coils which is bolted to the engine case. Part 3 is the rotor. It is steel, but it is not permanently magnetic; it is magnetized only when the field coil is energized. It does NOT have wires going to it; this diagram must be cut from a more comprehensive one. The left photo is the rotor alone. The field coil is magnetized so that left is North and right is South and that the wave pattern in the rotor causes the magnetic field to switch from N to S to N to S . . . . But the rotor is not permanently magnetized, but only pulls the magnetic field from the field coil. The right photo has the rotor beside the nested field coil and generating coils. The rotor spins in the space between the field coil and the generating coils. So many motorcycles use permanent magnet alternators and generate the maximum amount of electricity all the time then dump some with the regulator. In 1969 Honda tried generating only what was needed by changing the strength of the field coil to generate only the electricity that was needed. EDIT: To correct polarities.
That is the type of system I have. The red to white wire was a typo, it's the green to white wire. If I manually close the points in the regulator, I should get output from the alt., correct? But that doesn't necessarily mean the reg. is defective, does it?
More resources for YOUNZ I have a Honda-published manual downloaded from this page: http://forums.sohc4.net/index.php?topic=17788.0 I BELIEVE that I have the third one after SOHC4 Motorcycles, but I'm not going to download again to check because it's 1,200 PDF pages. It has the CB750 plus a general Honda manual plus many magazine articles and reviews. Even though it doesn't list 750A, it gives many specs to help interpret electrical measurements.
