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Voltage drops between Battery & Ignition Sw.

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Old Oct 16th, 2013, 10:12 PM     #1
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Voltage drops between Battery & Ignition Sw.

My 1989 FLHTCU has 12v at the battery & the post on the starter, but drops to 11v at the ign switch. Lights & ign are turned on, but engine has not been started.

It also drops another volt thru the Ign Sw, so I know I need to replace the switch (only 10v going to the lights). The guy who sold it to me had run an air horn thru the Ign Switch instead of using a new relay circuit and ruined the Ign Sw.

The only thing the wiring diagram shows in the circuit from the starter post to the Ign sw. is a 30amp CB (circuit breaker) marked "Main". 'Seems to me a 1v drop across a CB is too much. Anyone know where this CB is & how do I get to it?

It's not under the seat or in either side panels, so I'm guessing it might be somewhere in the fairing???

Am I right? If so, where in the Fairing is it and how do I get to it to check and/or replace it? If I'm wrong, please send me in the right direction.
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Old Oct 17th, 2013, 06:57 AM     #2
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Re: Voltage drops between Battery & Ignition Sw.

I believe that is the same as my old '86. If memory serves, there are four of those breakers located in the fairing. Remove the outer fairing and you will be staring right at them.
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Old Oct 17th, 2013, 09:01 AM     #3
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Re: Voltage drops between Battery & Ignition Sw.

Quote: Originally Posted by Jennmarr View Post
I believe that is the same as my old '86. If memory serves, there are four of those breakers located in the fairing. Remove the outer fairing and you will be staring right at them.
Jennmarr has you covered here, disconnect the battery from the bike and charge it fully while you do your work
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Old Oct 18th, 2013, 07:37 AM     #4
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Re: Voltage drops between Battery & Ignition Sw.

ThanX Much!. I'll take the fairing off and replace the CB. I expect that the 30 amp one from the Battery should be somewhat near the other four 15 amp ones shown on the wiring diagram.

Unfortunately, the Harley dealer said the CB won't be in until after I leave town for a trip, so it'll be after Thanksgiving when I'll get to finish it.

I'll post the results when I get back here to FL in Dec. (Great riding season! No rain & not much cold weather.)
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Old Oct 18th, 2013, 06:35 PM     #5
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Re: Voltage drops between Battery & Ignition Sw.

If you only have 12.0 volts at the battery you need to charge it before you do a voltage drop test. at 12.0 volts the battery is completely discharged so any results of any electrical test are invalid. A fully charged batt should be 12.7 volts
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Old Oct 18th, 2013, 08:43 PM     #6
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Re: Voltage drops between Battery & Ignition Sw.

U R right. The battery was mostly discharged. It was 12.7v until I turned on the ignition/lights. So, I did the test with a battery charger on the battery. When I disconnected the headlight, the 1v drop went away. But with the charger on & headlight on, it still should not drop a whole volt across a CB, so I know there is a problem there. There is either a bad connector in series with the CB or a CB with too high a resistance. I will meter it out and find out, but I suspect the CB & ordered it..
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Old Oct 18th, 2013, 11:59 PM     #7
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Re: Voltage drops between Battery & Ignition Sw.

The best way to test for voltage drop on the feed side of a circuit is Hook one of your DVOM to batt pos post the other one to your ignition switch. If it reads 1.0vlt you have a 1volt drop, if it is .10 you have 1/10 volt drop and so on. rule of thumb you should never have more then 1/2 or.5 volt drop in the total feed circuit you are testing. Even at 1/2 volt can cause problems. If you do have that big a drop work you lead back in the circuit across junction blocks or breakers until the drop goes away and that is where the problem is.

If you test it the way you described you may have 11 volts at switch but what was batt voltage with switch and light on? it may have dropped down also due to batt be discharged or not being able to hold a load.
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Old Oct 20th, 2013, 05:26 PM     #8
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Re: Voltage drops between Battery & Ignition Sw.

Right again. I find the best way is to set up the condition (IE Battery Charger on Battery, ign. switch on, & lights on) & then do the measurements starting with the battery. Anytime U change anything, U R not comparing apples with apples but with oranges and any readings become suspect..
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