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Thread: 1972 R75/5 Voltage to Ground Question

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    1972 R75/5 Voltage to Ground Question

    I'm a new member, so thanks in advance for replies.

    I have a 1972 R75/5 Toaster. Can anyone tell me whether 13+ volts between the battery ground strap and the frame is normal for this bike? I'd like to know whether I should be looking for a wiring short. I was getting 0.56mA as well - but this disappeared after pushing/pulling the ignition "key" a few times. I've taken each turn indicator bulb and the rear tail-light bulb out, one at a time, and still get 13+ volts on the multimeter. Thanks.

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    rabid reader dbrick's Avatar
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    Quote Originally Posted by NigelS View Post
    Can anyone tell me whether 13+ volts between the battery ground strap and the frame is normal for this bike?
    Haven't had an Airhead in decades, but I'll bite:

    Doesn't the battery negative pole connect via a ground wire directly to the back end of the transmission? The back end of the transmission is bolted to the engine, which is bolted to the frame, so the potential between the battery negative pole and the frame should therefore be 0, not 13+ volts.

    13+ volts sounds like the battery potential. I don't see how it can be seen between the battery ground strap and the frame. Are you perhaps measuring from the positive terminal of the battery, and not the negative one?
    David Brick
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    2007 R1200R

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    Unfunded content provider tommcgee's Avatar
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    Quote Originally Posted by dbrick View Post
    Are you perhaps measuring from the positive terminal of the battery, and not the negative one?
    Easy to do, even for a trained electronic tech, but since it isn't what I'd expect to get for a reading, I'd double check the battery terminals AND the meter leads for polarity.
    Salty Fog Rally 2007, 2009, 2011, 2012, AND LOOKING FORWARD TO 2014!

    -Tom (KA1TOX)

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    Thanks David & Tom.

    David - yes, the battery ground strap bolts onto the rear side of the transmission housing etc. So like you, 13+ volts between the negative side of the battery and the frame, raises my eyebrows. Although I've read some bikes (not Toasters) can show 8 volts?

    Yes, I'm measuring the voltage between the negative terminal of the battery and the frame and the multimeter polarity is OK.

    This bike has a history of eating batteries ($$). I now disconnect the ground strap when the bike is parked. But with zero amperage reading now - I wanted to make sure I'm not about to chase a ghost of an electrical short where none exists.

    Thanks, N.

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    jimmy armour
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    battery draw

    disconnect the neg cable ,then put a 12 volt test lamp between the neg terminal on the battery, and the neg cable or earth if the lamp lights you have battery draw,check behind the front cover or under the gas tank for wiring faults ,hope this assists Jimmy

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    #4869 DennisDarrow's Avatar
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    You undoubtedly have a short someplace in your system. Could be a microamperage or heavy duty. What you are seeing is the battery dumping a load into ground. Back long ago a sure fire test for a short in the vehicle was to barely touch the ground cable to the battery. If you got a spark of any kind it was a definate sign of a short. This is of couse done with all systems shut down and in the darkest situation possible.........Time for you to start running on ohmeter through the various systems to trace the leakage........Good luck......God bless........Dennis

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    Unfunded content provider tommcgee's Avatar
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    Quote Originally Posted by jimwjarmour View Post
    disconnect the neg cable ,then put a 12 volt test lamp between the neg terminal on the battery, and the neg cable or earth if the lamp lights you have battery draw,check behind the front cover or under the gas tank for wiring faults ,hope this assists Jimmy
    He has a multimeter, why would he use a test light?

    I'll go along with the rest of what you suggest and I'll even go along with he test light method if you have no meter. Visual checks will prolly be what finds the problem.
    Salty Fog Rally 2007, 2009, 2011, 2012, AND LOOKING FORWARD TO 2014!

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    Thank you all for your suggestions. I'll post the outcome when I get it sorted.

    Regards, N.

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    It sounds like there is a voltage leak somewhere: that is if a multimeter connected between the ground strap and the frame is showing 13V potential. I would disconnect the positive lead from the battery and make sure the voltage potential for the ground goes to zero - as a check. If the voltage is zero between the ground strap and engine with the positive battery cable disconnected but shows 13V with the positive lead connected - then there is a voltage leak.

    If so, I would first disconnect the alternator diode board and see if the potential stays zero when reconnecting the positive lead. This approach of reconnecting the positive battery terminal after isolating a specific section of the electrical system is a reasonably safe and effective way to troubleshoot such problems. If yours has fuses, trying this with each fuse removed individually should also be done. Good luck!
    Stan

    AH# 13238

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    jimmy armour
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    voltage potential

    hi tommcgee,to give you an answer,while you can have 12 volts potential, on the meter it may not light a 12 volt bulb,

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    R100GS, '89 Guenther's Avatar
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    I was getting 0.56mA
    0.56mA =~ 7.5 Watts, yikes! That'll drain the battery completely in about 30-40 hours.

    Measured the amps between which points?

    When you say "frame", where exactly do you connect to the frame?

    By all means you should have zero volts between the battery minus pole and "the frame".

    /Guenther

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    Thank you all for your input. Consensus is I have a pretty massive short somewhere, which fits with my previous experience of flattening two batteries so badly they could no longer be charged.

    For Guenther - the 0.56mA & 13+V readings were originally taken between the disconnected battery ground strap and the engine casing (or carb, or frame bolt - didn't matter which). The amperage disappeared once I pushed & pulled the ignition "key" a few times. But the leaking voltage has remained constant.

    Stan - the voltage did drop to 0.10V when the red ignition wire was disconnected from the positive terminal on the battery (with the starter cable still attached).

    I started to unwrap the wiring harness from the positive side of the battery - following the red ignition wire forward towards the starter protection relay. It's clear from the melted state of at least one wire (green/black) that this bike has had a severe episode of electrical overheating, at some point in its history. I could spend a lot of time re-stringing wires, but I reasoned I'd never be entirely sure there wasn't a problem elsewhere in the harness. My bet is that somewhere, the insulation on two wires has fused together. I'm surprised there haven't been any other electrical issues - runs OK, lights etc. work. So I'm going to bite the bullet and replace both the engine and main frame harnesses. Then proceed cautiously and check for leaking voltage while reconnecting everything per the factory wiring diagram, check the diode board etc. etc.
    Last edited by NigelS; 10-02-2012 at 04:00 PM.

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    Unfunded content provider tommcgee's Avatar
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    Quote Originally Posted by jimwjarmour View Post
    hi tommcgee,to give you an answer,while you can have 12 volts potential, on the meter it may not light a 12 volt bulb,
    I get that -- the meter also measures current if you know how to use it. My major problem with test lights is that they don't tell you how much.

    I've been an electronic tech since the sixties, US Navy trained. These days I repair church organs for a living. One time, there was low AC voltage in the church, around 85 volts, and the power supplies in the organ couldn't come up and hold regulation. The church called an electrician to meet me there. He immediately plugged in a test light and told me the voltage was fine.
    Salty Fog Rally 2007, 2009, 2011, 2012, AND LOOKING FORWARD TO 2014!

    -Tom (KA1TOX)

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    Diode Board - Mounting Bolts Short the Battery?

    I think I'm getting a clue to what's going on. I replaced the diode board a while back with one made by Wehrle (pic attached). While it looks identical to the original factory board, closer inspection reveals that two of the mounting holes are insulated with non-conductive washers (per my old one), while the other two holes include pressed-on metal washers that are attached to the chassis of the board. Since the positive wire from the battery attaches to a spade terminal on the diode board chassis - then as soon as the top two mounting bolts are inserted there's a direct connection between the positive side of the battery and the body of the engine. The old diode board had all four mounting holes insulated.

    Has anyone else run into this problem on a 1972 R75/5 when replacing the board? I guess I could try and modify the top mounts?

    N.
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    It's been a while since I looked at my Bosch diode board, but I got rid of the rubber mounts, added solid metal mounts, plus added a grounding harness which IIRC connected to the upper mounts. That says that you want the frame of the diode board to be very well grounded.

    Snowbum discusses the Service Bulletin that BMW released that described how to install the wiring harness. Here is what he says about the bulletin and the picture that was on it:

    "Facing the timing chest, the upper left and upper right diode board mounts were each a connection point for this 'wire assembly', and these wire connections were made onto the mounts."

    So, it sounds like you want positive connection between the mounting points of the board and the engine case.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
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