Author Topic: Voltage draw on '78 CB750f (Read 5097 times)

SurfinBird · « **on:** April 19, 2015, 01:06:06 PM »

I read on a different forum that a large drop in voltage when the key is turned would indicate a bad ground. What drop can I expect turning the key, not starting, at the battery on a functional charging system? I've had a charging issue forever and everything else checks out. I'm interested to see if this tells me anything.

hsas.69 · « **Reply #1 on:** April 19, 2015, 02:15:52 PM »

A large drop in voltage could be a number of things from just a bad/loose connection to a short somewhere. What problem exactly are you trying to find/fix?

SurfinBird · « **Reply #2 on:** April 19, 2015, 02:25:19 PM »

I'm not getting more than 11.9 at the battery at any rpm. The battery was at 12.57 when I dropped it in, it fell to 11.6 when I turned the key (not starting the engine). Just did 10 miles and the battery is at 12.3

hsas.69 · « **Reply #3 on:** April 19, 2015, 02:33:06 PM »

Is the battery new or used. 12.57 is just a shade low. 12.6 is fully charged. The battery voltage will drop a little with the key on due to powering the headlight, taillight and front marker lights assuming your lights/wiring is stock and possible posing ignition depending on kill switch position. If all your getting is 11.9 at the battery at any rpm I would clean and check your connections and then be looking to the charging system.

SurfinBird · « **Reply #4 on:** April 19, 2015, 03:06:59 PM »

I am actually getting a hair over 12v @ 4k. After another 10+ miles keeping in over 4k I'm getting 12.29, so losing voltage still. How low can the battery get and still run, so I can gauge how long I can ride. What connections should I be checking? The wires are original and from what I can see they looks good. My fuses are 1/4" shorter than the stock ones, would that have any effect? they are the correct voltage.

flybox1 · « **Reply #5 on:** April 19, 2015, 03:32:37 PM »

Your starter won't turn, but you can kick it as low as 9v. Maybe even lower.

SurfinBird · « **Reply #6 on:** April 19, 2015, 03:45:29 PM »

Thanks flybox, good to know. I haven't used the started since this problem popped up, plus it's more fun kicking it, starts first time, every time.

SurfinBird · « **Reply #7 on:** April 19, 2015, 03:52:28 PM »

What are your thoughts on leaving the battery on the bike and using a trickle charger. I have a 60 mile round trip commute and would love to use the bike again (commuted on it for 2 spring/summers). I usually give the battery a couple hours rest before running it, is that nessecary? I'm trying to avoid taking it to a shop to fix the charging issue, but if I can run it until I figure it out or cave and take it in, I'd like to ride it.

madmtnmotors · « **Reply #8 on:** April 19, 2015, 05:05:39 PM »

I would start by inspecting, cleaning, and/or replacing (if damaged/corroded beyond repair) the primary connectors to the fuse box, regulator, and alternator. I usually replace the bullet connectors on the three yellow wires from the alternator under the front sprocket cover. These high amp connectors are also subjected to high heat and tend to suffer. Most of the original connectors at this location that I've encountered tend to be fused together and typically have to be cut apart. Vintage Connections makes easy work of these. This targeted approach helped me resolve issues with my main fuse failing:

http://forums.sohc4.net/index.php/topic,137351.msg1549191.html#msg1549191

hsas.69 · « **Reply #9 on:** April 19, 2015, 05:10:37 PM »

I would agree as well as checking/cleaning main battery connections. If your still having problems you might check the points gap in the voltage regulator. You also should check voltage at the battery with the engine running at different rpms while not riding. To see what the charging system is doing not just seeing what voltage is after a short ride. These bikes don't really charge until 2500 or so. And it will take more than ten miles to fully charge the battery back up.

SurfinBird · « **Reply #10 on:** April 19, 2015, 05:31:36 PM »

Thanks mad, i already changed the block coming out of the alternator, it was fused together so I had to cut the wires, is there another spot where there are bullet connectors? Do i need to remove the sprocket cover to get to them?

hsas.69
I checked and i'm getting 11.96v at 3k rpm, and 12.06 at 4k, well short of the 14.5v i'm supposed to be getting. I have continuity on all the yellow wires from the alternator, rectifier tests as good and so does the regulator. I used twotired trouble shooting guide and ran a jump wire past the regulator and got the same results. I will check the connections to the fuse box, the main battery connections look fine, no corrosion at all.

hsas.69 · « **Reply #11 on:** April 19, 2015, 06:30:24 PM »

What about continuity between the three yellow wires to each other or ground? And if you bypassed the regulator you should have gotten somewhere near 15 volts. If the stator and rotor are in good shape.

SurfinBird · « **Reply #12 on:** April 19, 2015, 06:41:14 PM »

If I remember correctly I had continuity between all yellow wire and none to the ground. I got under 12v bypassing the reg. another user said I should have a strong magnetic field at the alt cover. All I have is a very weak field. It can't hold a small 10m wrench, his showed the field holding a good size 3/5" wrench.

hsas.69 · « **Reply #13 on:** April 19, 2015, 07:07:43 PM »

Sounds to me like your stator is shorted to itself causing it to basically have less windings which means less magnetic field equalling less charging.

SurfinBird · « **Reply #14 on:** April 19, 2015, 07:09:41 PM »

Is a new stator the fix?

hsas.69 · « **Reply #15 on:** April 19, 2015, 07:13:27 PM »

From what you have told me that is what I would say. Hopefully someone can chime in and confirm that for us due to the stator being quite pricey.

HondaMan · « **Reply #16 on:** April 19, 2015, 07:14:33 PM »

Quote from: SurfinBird on April 19, 2015, 05:31:36 PM

Thanks mad, i already changed the block coming out of the alternator, it was fused together so I had to cut the wires, is there another spot where there are bullet connectors? Do i need to remove the sprocket cover to get to them?

hsas.69
I checked and i'm getting 11.96v at 3k rpm, and 12.06 at 4k, well short of the 14.5v i'm supposed to be getting. I have continuity on all the yellow wires from the alternator, rectifier tests as good and so does the regulator. I used twotired trouble shooting guide and ran a jump wire past the regulator and got the same results. I will check the connections to the fuse box, the main battery connections look fine, no corrosion at all.

Maybe I should ask some other questions, which may lead to where the issue is originating...?
1. What is the wattage of your headlight? Is it stock, or an aftermarket, and if the latter, what wattage is the bulb?
2. What ignition coils are in the bike?
3. What type of ignition does it have: is it points, or Dyna, o something else?

Reasons for asking:
1. If the headlight is more than 65 watts high beam, it is overloading the system.
2. If the coils are 3-ohm type (or even lower), they are dragging down the system: stock coils are 4.3-4.4 ohms in the 750F bikes.
3. If the ignition is a Dyna or Pamco (or similar), they draw a lot more power than the OEM points setup.

All these things add up, and can drop the system voltage greatly.

Also, you will not get 14.5 volts on the battery on these bikes: they are made to peak out at 13.6 volts, at which point the regulator shuts off. This is a "voltage limiting" system, not a "voltage regulator" type system. There are some aftermarket voltage regulators out there that will raise this voltage, but they usually cause dried-out battery problems, or worse, in the long run.

Many times, these bikes accumulate things from previous owners that end up not being a good idea. Just one example: I just "fixed" a beautiful 1976 CB750F1 that had chronically low battery troubles for years. Even at mountain riding speeds, after 15 minutes in heavy (30 MPH) traffic upon his return the battery voltage would drop off until the headlight dimmed, turn signals stopped flashing, and his electric start would not work after he stopped for dinner. This had been going on for YEARS. The problems: someone installed an 80w H4 halogen headlight, 3-ohm Dyna coils, and a Dyna S ignition. Total wattage of just these 3 items was more power than the alternator can even make! The fix: installed a 55/60W H4 bulb, 5-ohm Dyna coils. He is planning on switching back to points and using one of my Transistor Ignitions as soon as he can find a spark advancer, so he can make a trouble-free trip to Florida this summer.

SurfinBird · « **Reply #17 on:** April 19, 2015, 07:26:56 PM »

Thanks HondaMan. As far as I knew it's all stock, but I'll check headlights and coils. Who knows what the previous owners did. The points are stock so I can take that off the list.

hsas.69 · « **Reply #18 on:** April 19, 2015, 07:29:33 PM »

Good points HM. I didn't really think about those things and they definitely add up. I tend to assume if not atates things are factory.
On a side note I can't wait to get my hands on your ignition box. Don't like the power consumption of the electronic ignitions on an already strapped electrical system. And points alone can be a bit of a hassle on a daily driven machine.

scottly · « **Reply #19 on:** April 19, 2015, 07:45:12 PM »

Disconnect the white wire from the regulator, and measure the resistance between the white wire and the negative battery terminal. Should be about 7 ohms. (test with ignition off)

SurfinBird · « **Reply #20 on:** April 19, 2015, 07:58:13 PM »

HondaMan, checked the headlight Bosch H4 Halogen, looks like i need a new headlight, what would you recommend

scottly
I'll check the resistance, i like to call my avatar "Sharkgull"

Retro Rocket · « **Reply #21 on:** April 19, 2015, 08:04:32 PM »

Quote from: SurfinBird on April 19, 2015, 07:58:13 PM

HondaMan, checked the headlight Bosch H4 Halogen, looks like i need a new headlight, what would you recommend

What wattage..? If its the correct wattage it's fine...

SurfinBird · « **Reply #22 on:** April 19, 2015, 08:05:03 PM »

I just realized that...jumped the gun on that one

scottly · « **Reply #23 on:** April 19, 2015, 08:22:06 PM »

Pull the 7 amp headlight fuse, and remove that load from the equation.

pamcopete · « **Reply #24 on:** April 21, 2015, 05:59:02 PM »

Quote from: HondaMan on April 19, 2015, 07:14:33 PM

Quote from: SurfinBird on April 19, 2015, 05:31:36 PM
Thanks mad, i already changed the block coming out of the alternator, it was fused together so I had to cut the wires, is there another spot where there are bullet connectors? Do i need to remove the sprocket cover to get to them?

hsas.69
I checked and i'm getting 11.96v at 3k rpm, and 12.06 at 4k, well short of the 14.5v i'm supposed to be getting. I have continuity on all the yellow wires from the alternator, rectifier tests as good and so does the regulator. I used twotired trouble shooting guide and ran a jump wire past the regulator and got the same results. I will check the connections to the fuse box, the main battery connections look fine, no corrosion at all.

3. If the ignition is a Dyna or Pamco (or similar), they draw a lot more power than the OEM points setup.

The current consumption for a set of points is 14.5 Volts / 5 Ohms = 2.9 Amps X 180/360 Dwell angle = 1.45 X 2 coils = 2.9 Amps

The current consumption for the PAMCO is 13.5 Volts / 5 Ohms = 2.7 Amps X 120/360 Dwell angle = 0.9 Amps X 2 coils = 1.8 Amps

The main difference between points and the PAMCO is the dwell angle. Also, there is a 1 Volt drop across the driving transistor. The power consumption for the electronics in the PAMCO is a negligible .020 Amps.

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Author Topic: Voltage draw on '78 CB750f (Read 5097 times)