Topic: Chasing down an electrical/charging system problem - 1978 CB750A Hondamatic

[Stage 2]

Hello SOHC gang!

Fair warning, this is a rather long post with a lot of information. Here goes ...

I am working on a "resurrection for resale project" of a 1978 CB750A Hondamatic. This bike was acquired in what appeared to be above average overall condition considering its time of hibernation which was allegedly around 20+ years. It had been kept covered in a clean and dry building during this time.

Anyway, I have this bike back in running order and have taken the bike for a couple of "test" rides. It runs and goes well, but it has been discovered that it is not charging the battery. The battery is a new model correct size battery. This battery is presently charged and showing 12.34 vdc. With the bike running we were only getting around 12.3 at just above idle to maybe 13 volts at the battery with an RPM at approximately 3k (there's no tach on this bike hence the RPM approximation) We could not get this reading to rise up to the usually recommended 14.5 vdc at anytime or any RPM. Of course, I am also thinking with the battery at 12.34 volts, and if the voltage regulator is working correctly, we might not get the 14.5 volts at 3k RPM. Nevertheless, after a couple of short rides the battery is depleted enough that it will no longer crank the engine.

The stator windings read consistently at .7 ohms with no shorts to ground. The field coil tests at 7.2 ohms with no shorts to ground. I have found that there is only about 10.11 volts going to the white wire input of the field coil when the ignition switch is turned on. I get 10.5 volts to the black (voltage sense) wire at the voltage regulator with ignition switch turned on. If I understand this charging system correctly, the field coil needs to have 12 volts in to energize the stator to charge the battery. Am I correct in this thought process?

We have also found that the red wire going into the ignition switch will read whatever the battery voltage is, but the black wire coming out will be around the 10.5 volts. The ignition switch was tested away from the bike wiring harness. It was bench tested and connected directly to a 12 volt source ... 12 volts in we get 12 volts out ... so I believe the ignition switch is not the problem. With the ignition switch connected again to the harness, the red wire reads 12.16 vdc with the key off. I have been trying several tests today so we are starting to see a slight battery voltage drop. Anyway, with the ignition switch on, the red wire reads 10.58 and the black wire reads 10.32 vdc. These readings are with the Start/Stop switch connected. With the black wire from the Start/Stop switch disconnected these readings at the ignition switch are; Red wire 11.38 - Black wire 11.14 vdc. These readings are as of today which is AFTER we have already disassembled the Start/Stop switch and thoroughly cleaned the contacts and reviewed the wires in this Start/Stop switch. There was a minimal amount of tarnish/corrosion on the contacts of the Start/Stop Switch before the cleaning. 

I have found a rather detailed charging system test procedure on this SOHC/4 forum that was posted by "Airborne 82nd" several years ago on May 14, 2014. I am aware that this procedure is not necessarily specific to the "Hondamatic" model. Nevertheless, I have followed this test procedure as it is laid out. Based on this particular test procedure, I have received correct results for each step until I get to the section titled "Resistance testing of regulator wires". The first step says ... "While the bike is not running disconnect (-) battery lead and Black regulator wire.  Check for 0 resistance between (-) battery wire and Black regulator wire. When tested this way I am currently getting a reading of 1.9 ohms. The only way I can achieve 0 resistance is if I disconnect the black wire from the "Stop/Run" switch and the pod that houses the fuel gauge, turn signal indicator, neutral light etc. I am aware that the black wire in question is the 12v switched wire that powers the indicator lights in the previously mentioned "pod". Because of this, I do not know how a 0 resistance measurement can be obtained if these components remain in the circuit. I am wondering if this part of the procedure may not necessarily apply to the Hondamatic model.

We are much obliged if you have read my novel this far! Other than this charging system problem, this bike is in pretty nice overall condition and it is actually more fun to ride than we expected! We would really like to correct this charging system issue. Any input, thoughts, tech support, suggestions or miracles will be sincerely appreciated. Once we have it corrected we will gladly share what we have learned with anyone else that finds a similar issue.

In the meantime ... Hava Blast and Make It A Great Day!!! Respectfully, WK @ Stage 2

[scottly]

Anyway, with the switch on the red wire reads 10.58 and the black wire reads 10.32 vdc. These readings are with the from Start/Stop switch connected. With the black wire from the Start/Stop switch disconnected these readings at the switch are Red wire 11.38 - Black wire 11.14 vdc.

The different readings are due to different current draw with the stop/stop switch connected/disconnected. You more than likely have a problem with the fuse box, such as corroded clips on the main fuse holder.

[Don R]

  With the key on are the lights on too? It appears the A also has a few more power consuming features than an earlier K. Just FYI, The regulator and rectifier are different than a K, as is the amp outlet of the alternator. I parted out an A but it had been left outdoors for years and the wiring plugs were mostly dissolved. 
 I recently found my K1 had the red wire from the charging system not plugged in to the harness. Same result as yours. One end of the wire was hanging out with the spare blinker wire and the other was tucked in out of sight.

[Stage 2]

Thank you Scottly and Don R for taking the time to peruse my novel.

To Scottly - Per the previously mentioned "test procedure" that I found on the SOHC/4 Forum, I have previously reviewed the fuse box. It appears to be in very good condition with no noticeable corrosion ... with good connection at the fuses and the connector plug that connects the fuse box to the main wiring harness. I have also read about the fuse box connections being prospectively suspect in other forum posts. I am certainly not against taking another look! Is there anything more specific that you might recommend that I check for with the fuse box/panel?

To Don R - Yes, the lights are on with the key on. However, I have the headlight and front turn signals disconnected for the reported the test results I mention here. As I stated in my initial post, the CB750A I am currently working on has been store inside for allegedly its lifetime. I have had it inside as well. In general, the bike is in very good condition. Nevertheless, it remains a 45 year old bike with apparently a gremlin our two taking up residence! The wiring connectors are all intact and not all dried out as I have seen with several older bikes like this. I am unsure that I understand what red wire you are referring to that was not "plugged into the harness" on your K1. Could you elaborate on this a bit more?

I am reasonably sure that this problem will likely turn out to be something relatively simple. But for now it's being a substantial disruption of my mental state!

Thanks again and as always ... Hava Blast and Make It A Great Day!!!  Respectfully, WK @ Stage 2

[scottly]

Try measuring the voltage drop through the harness directly: connect the red meter lead to the positive battery terminal, and the black meter lead to the black regulator wire, and note the voltage with the key on, and the headlight on. (For now, just turn the stop/run switch to stop.) A meter reading of over 1 volt indicates resistance in the harness and all it's connections.
Please report your results.

[Stage 2]

Thanks again Scottly!
I stepped away from that project yesterday to basically just try to NOT think about it for a bit. (It didn't work!) Anyways, I had not though of your suggestion ... however I agree that it should at least prove to be an interesting way to determine whether or not the issue is a short to ground somewhere yet to be found. I have a few other obligations etc to manage today, but I will respond with my findings hopefully a little later today (10/30/2023). Thanks again for the continued responses and suggestions. In the meantime and as always ... Hava Blast and Make It A Great Day!!!
Respectfully,
WK @ Stage 2

[Stage 2]

Hello again Scottly,

I was able to get back to the project in question a few minutes ago. With the bike and my meter set up as you suggested ... headlight on, Start/Stop switch set to stop, red (+) meter lead connected to the battery (+) and the black (-) meter lead connected to the black wire going to the voltage regulator, then switch the ignition switch to the on position, I get a reading of 1.28 volts. So with that in mind and based on your suggestion of anything over a 1 volt reading, there is a resistance somewhere yet to be located. I believe my meter to be reasonably accurate, but even if I consider a 10% + or - voltage measurement error, this 1.28 volt reading is in excess of your suggested parameter. Any thoughts

I am usually fairly good at this sort of thing, but this one has been creating a bit more frustration than expected for sure!!!

[scottly]

While the 1.28 volt drop is on the high side, it's not really a smoking gun. Do my favorite quick-and-dirty test of the regulator and field coil. With the key on, the heads of the three screws that mount the field coil should have enough magnetism to support 5/16" bolts, assuming you have recharged your battery. If the field is being sufficiently magnetized, the charging issue is on the output side of the system, involving the stator and rectifier.
 

[Stage 2]

Thanks again Scottly for the continued input.

I was kinda thinking along the same line as you with the 1.28 voltage drop not being a HUGE deal. Nevertheless it does exist. Although I am aware of your suggested "quick-and-dirty test", I have not tried it with the bolts as you mention. I tried it with a 10 mm wrench. There was some magnetism there around the case cover in general, but I recall not really touching the wrench to the bolts as you describe. I will review and/or recharge the battery as I have been switching the electrics on and off a lot during this sequence of testing. I will then give the "quick-and-dirty test" a go. I'll let you know the results a bit later.

Additionally, last night I took a closer look at the fuse block/box. I have it removed from the harness and with the fuses removed from their respective clips. I know I reported previously that the fuse connectors seemed to be reasonably clean and corrosion free. Unfortunately, I may have been incorrect in that assessment! There is some corrosion and/or other less than perfect fuse connective surface evident with the fuse clips. Additionally, I have discovered that apparently at some point the main fuse had been rather hot. On one side of the plastic block that holds the main fuse clips, there is some distorted looking plastic and the clip itself is not in the same position as the other clips on the same side of the block. What I will call the plastic "divider" for the one side of the main fuse clip is gone completely! The clip still makes contact with the fuse, its just off to one side a bit. As an experiment, I was checking to see if there was any resistance between the wiring in the connector plug and the respective fuse clip for each circuit. Depending on how I position my meter lead alligator clips onto the main fuse block clip, I could find a resistance reading of in excess of 3 ohms which I realize, if correct, cannot be a good thing! However this has been a rather inconsistent test. I have some low pressure resurfacing and refinement equipment here in my shop that I am going to thoroughly clean up the fuse connector clips and see if or what changes this makes. I'll let you know after that.

Thanks again and as always ... Hava Blast and Make It A Great Day!!!
Respectfully, WK @ Stage 2

[scottly]

Your fuse box has had the magic smoke let out of it! What does it look like on the back side? I would upgrade to a modern blade type fuse box.

[HondaMan]

Scottly requested my joining...
The main fuse in your holder gets hot because the chromate plating on these older glass-fuse fuseholders was a 10-year-life designed product. Today it makes a pretty nice resistor, which melts the clips' plastic mounts and often melts the fuses, too. Then they look like they 'blew' for no reason. The original fuse type was SFE, and they have large filaments with very short wasp-waist sites in them, which have a 10-year lifetime from brand-new. As the result, most of these bikes today, once discovered in hiding somewhere, immediately lose their fuses when restored to life.

The SFE fuse needed for these fuseholders was obsoleted worldwide in 1996, which was originally made to withstand bouncing and rattling while carrying current. Modern AGC fuses (i.e., instrument-grade types) just fall apart when shaken while carrying currents like these, not rated at all for vibration. Sometimes riders have slipped in an ABC (appliance-rated) fuse to get home, but they are too long and only 50% of their end cap meets the fuseclips, so they just heat up and fall apart, too.

To combat this (circa 2006, here in these forums) I created the ATC Blade Fuse holder for these bikes. It's a drop-in part, but changes the fuses to the normal automotive versions you can find everywhere (because these is so much electronics in modern vehicles, all using these fuses). It holds a bunch of extra fuses, too, which can be handy.  If you'd like one, PM me for details?

[Stage 2]

Thanks for joining in the fun HondaMan!!!

I appreciated the information on the old fuse connections and fuse holder design. That is certainly something for which I was unaware in such detail anyway. The main fuse that is currently in the "Main" fuse position is an SFE. The one that is now and was there when I started on this adventure in gremlin hunting, is actually a 20 amp. I know that a 15 amp is supposed to be there. For now I want to see if I can locate the problem and correct what that might be before I add any more parts just guessing.

Another question that I still have is this ...
Should there be at least 12 volts (DC) at the black wire at (or going into) the voltage regulator when the ignition switch is turned on?

Interestingly, you answered a question I was going to ask Scottly regarding where to find a blade fuse block with a newer blade fuse set up to replace the OEM Honda glass fuse set up without having to create something "custom" on my own. Your idea is interesting and once I can locate and correct the currently still existing problem, I will be in touch with you. We'll go from there. Honestly, it just might be the old fuse box that is the problem!!!

Thanks again and as always ... Hava Blast and Make It A Great Day!!!

Respectfully, WK @ Stage 2

[scottly]

Honestly, it just might be the old fuse box that is the problem!!!

Your old fuse box is toast!!!

[RAFster122s]

HondaMan's solution is plug and play and a very professionally done job of soldering and heat-shrink'd connection at fusebox with properly sized connectors and connector block for the harness. It saves you the hassle of building the sub-harness yourself if you install new wiring and connectors.

[Stage 2]

Thanks for the input RAFster122s!

I may need to look into that more seriously.

In the meantime ... Hava Blast and Make It A Great Day!!!

Respectfully, WK @ Stage 2

[HondaMan]

Another question that I still have is this ...
Should there be at least 12 volts (DC) at the black wire at (or going into) the voltage regulator when the ignition switch is turned on?

Maybe try this to see if it points in the direction of the bug- ground the voltmeter (like at the frame ground) and turn on the power, then check the voltage on both side of the fuseblock's main fuse, but at the connector. See how much voltage drop is occurring across the whole fuseblock subassembly. This should tell you the electrical condition pretty well. A good system with good connectors should only drop about 0.25v unless something is drawing a lot more current than it should. Fuses have some resistance to them (which is what makes them melt for overcurrent). Then, if you have one, install a jumper (like w ire or similar gadget)  across the contacts in the connector to bypass the fuse and see if this improves the voltage at the regulator. The Black wire at the regulator is supposed to show the regulator what the bike's voltage actually is: when below 12.1 volts it should tell the regulator to be full ON by applying full +12 volts to the field coil in the alternator.

BUT...all this said: the little bullet connectors inside the engine's left-side cover in the 750A tend to burn more than in the regular 750 engines because of the lower- impedance field coil in this bike. These little bullets were designed to live 10 years, but 50 years ago...today the zinc plating on them is now a pretty decent resistor, and tends to lower the current the regulator may be trying to send to it. The modern replacements are made from brass and should last our lifetimes: it is a rare 750 engine I rebuild that doesn't need new ones (and gets them).

(Edit update 11/4: I noticed that something truncated this message. It turned out that somehow, the words color=white got added all by themselves into the middle of it all, so the rest of the message was invisible. Cool! But hard to read.)

[Stage 2]

Hello again SOHC/4 Owners ...

Yesterday I tried to post basically a similar message. I clicked on the "Post" button thinking my message was "Posted". However, when I came back later to see if there were any other replies, for some reason my message seems to had vanished!  ()   Anyway, I will give it another go.

I want take a moment to thank everyone (specifically - Scottly, HondaMan, Don R, RAFster122s) that has made a contribution towards helping me with tracking down my electrical/charging system gremlin(s) on the '78 CB750A Hondamatic. I have utilized and/or at least considered each suggestion that has been made here. Some of the suggestions I had already tried, but when reminded of something that I had already experimented with, I would still review my notes again to see if we were basically "on the same page".

As it has turned out, one of the first suggestions that was made by Scottly has ultimately been found to be the majority of the problem culprit. That being that the fuse box fuse clip connections were a bit less conductive than I had originally reported. There were a few other electrical points of contact that were cleaned up along the way as well, but the fuse box seems to be the primary culprit!

Because we have the capability to resurface a variety of metals using a low pressure blasting method, I resurfaced the fuse box fuse clips. Please see the attached pictures to provide a better idea of my description. After this process was completed, I was able to get the necessary voltage to the field coil so that the stator would then produce the output needed to properly charge the battery. A test that Scottly calls his "Quick-And-Dirty Test" was conducted after the fuse box clips were resurfaced. There was a positive result with this test. The attached picture of the 3 bolts hanging on the stator cover bolts is the evidence of this positive outcome. Before the fuse box clip resurfacing, the bolts would NOT remain attached. I still need to contact HondaMan to learn more about the ATC Fuse Holder he has created for these SOHC bikes. By the way, the resurfacing process that is exhibited in the attached pictures is something that we do here as part of our modest enterprise. We have a website that will provide a much more detailed explanation, pictures, etc. with a video presentation of the process planned for the near future (hopefully). I am unsure if I am allowed to post the website address here on the forum, so please feel free to DM me here for the site address if you have an interest.

In closing I am reporting that at this point the charging system is charging the battery and the bike has been reassembled. If the weather stays decent for couple more days. I will get it out for another spin. Thanks again everyone and as always ... Hava Blast and Make It A Great Day!!!

Respectfully,
WK @ Stage 2 

P.S. I was also using an older SOHC/4 Owners Club forum post that is dated May 14, 2014. The post was made by "Airborne 82nd" under the topic line "diagnosing charging system 1978 cb750k sohc"  . This is a VERY detailed testing process that I also found very helpful.

P.S.S. I guess the pictures I have been trying to include with my message may be a file size too big. Being somewhat forum posting illiterate, is my excuse for the separate posts for each picture.

[Stage 2]

Lets try this 

[Stage 2]

And again ...

[newday777]

Lets try this

Those turned out great!

[HondaMan]

Good job!

I miss Ken (Airborne 82nd), haven't heard from him in a long while. He's a smart guy, too.

[Stage 2]

Thanks gang for the kinds words on the resurfacing results. Yes, I agree HondaMan, Airborne 82nd seems to be exactly as you say ... "He's a smart guy, too."

Here's a couple more example pics showing the "Stage 2 Resurfacing & Refinements" process. Hopefully I can get them to work in one post!

Thanks again everyone and as always ... Hava Blast and Make It A Great Day!!!

Respectfully, WK a.k.a. Stage 2

[scottly]

Stage 2 looks a lot like vapor blasting?? Most guys just use a .22 caliber brass bore brush to clean the fuse holder clips.

[Don R]

  I had an experience with an 83? Yamaha 850, the voltage drop was so bad that the early electronic ignition would not make a spark. I cleaned a few connectors and wired in a blade fuse holder from O'Rielly's auto parts and that cured it.
  I swore off Yamahas and newer bikes after that. The guys that bought it acted like the police were hot on their trail, couldn't hand me the cash and go fast enough, didn't even have time to take the spare parts. The cash wasn't counterfeit either. 

[Stage 2]

Hey there Scottly and Don R ...

I apologize for the delayed response. After finally getting the Hondamatic back in decent order I have had to direct my energy (what little I have left some days) back onto other business.

Yes Scottly ... The Stage 2 process is similar to Vapor Blasting or Vapor Honing with the exception of it is not a "wet" process and has less of the issues associated with wet blasting. Less operational and maintenance costs, less issue with flash rusting on steel type materials, less residual media material sticking to the finished parts etc. Don't get me wrong, vapor blasting, when done properly, is quite effective too. The Stage 2 process is done at what is considered very low air pressure (usually around 15 to 20 psi) with minimal media requirements. One example of what we are doing allows us to do very delicate parts such old brass carburetor floats. We can process such things with air pressures as low as 5 psi. If that same thing were done at higher pressure, the float would likely be destroyed! In some cases where a replacement part, such as the float in this example is not available, the Stage 2 process is a great alternative.

Hey Don R ... I have read here on this forum that HondaMan apparently makes and/or has a blade fuse set up for the Honda CB's. (and possibly others ) I agree and as you have learned, that is very likely the best repair idea regardless the bike brand.

I am currently working on a project rebuilding a Honda Motocross bike engine for a young MX rider. Here's a couple of pics of the cylinder head that is getting some new valves before reinstallation.