Brake Light Anomaly

2009 ST1300 Police
Friend said that my running and brake light seemed dimmer than it should be. I checked and running light does seem a bit dim but brake light doesn't seem too bad. I did notice however that when activating either the front or rear brake that my digital on board volt meter drops .4 volts and returns to normal upon release. This is consistent weather or not any or all accessories are on or off at any RPM.
History:
Purchased 10/16 with 8500 miles
volt meter reads 14.1 at idle, 14.2-14.3 at rpm. Has not been confirmed with a handheld at battery
police lights self installed on dedicated circuit with fuses, relays. Trigger is the rh Police switch
Volt meter and gps added and connected on quarterlet harness
Clearwater Ericas added on dedicated circuit with trigger at quarterlet harness.
Heated gear harness added on dedicated circuit.
I do recall that posi taps (removed) were on the brake wires for assumabibly an additional brake light (removed before purchase)
I do not know if decreased brightness or voltage drop was present at purchase as the bike has been parked for six months for additional work and maintenance since its 1800 mile maiden ride home.
Ive checked brake voltage drop on my DL1000 also with dual 21/5V rear bulbs and the loss was only .1V
Hence the anomaly

Any thoughts would be appreciated
Roger

Check the ground at the light.

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I'd also have a look at the wire where the positaps were. You could have some broken strands a/o corrosion.

Trapperdog said:

I checked and running light does seem a bit dim but brake light doesn't seem too bad.

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This is pretty standard for the OEM lighting on an ST— anemic taillights but decent brake lights. That doesn't mean you don't have a problem but even in best cases the ST's taillights often pale in comparison to other bikes. If you could compare it to another ST that would be a better test.

Trapperdog said:

police lights self installed on dedicated circuit with fuses, relays.

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You might pull the fuses on those lights and check the lights in question for changes in brightness. And check all the wiring and connections for the tail/brake lights.

Dave.David said:

Check the ground at the light.

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Will do, I'll have a look at the schematic first

st1300r said:

I'd also have a look at the wire where the positaps were. You could have some broken strands a/o corrosion.

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That was my first thought, if there are fewer wire strands for the 42W draw, the transfer to heat might cause an excessive V drop. That's a bit above my pay grade though

ST Gui said:

This is pretty standard for the OEM lighting on an ST— anemic taillights but decent brake lights. That doesn't mean you don't have a problem but even in best cases the ST's taillights often pale in comparison to other bikes. If you could compare it to another ST that would be a better test.
You might pull the fuses on those lights and check the lights in question for changes in brightness. And check all the wiring and connections for the tail/brake lights.

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That's somewhat good to hear, I'll check it against another. I'm not too concerned about the illumination aspect, but more of the .4 Voltage loss upon activation.
durring the bikes decommission I did pull all the fuses, cleaned the spades and checked them with a Volt meter

Once you have fixed the bad ground connection. Then switch over to Red leds. They will use one fifth the amount of current and cause less of a voltage drop. Check out the red/amber swicthback thread and/or add an extra brake light...

Not sure exactly where your meter is located, but if you're getting more voltage drop than expected at the meter, that sounds like a dirty connector or positive terminal battery connection well upstream of the brake light, because that's where the excess voltage drop is being measured.

If you have a corroded ground connection, that's going to increase the resistance in the path, and lower the current draw, which is the opposite of what your symptoms suggest. Not saying you shouldn't check the ground connection as well, it could also explain a dimmer than usual bulb due to lower current draw, but it wouldn't explain the voltage drop.

I'd also check the voltage at the battery terminals when illuminating the brake light, perhaps your battery is on its last leg and the extra voltage drop is due to the battery itself. I think you said you have other accessories running that might rule out a weak battery, but it only takes a second to check it and make sure.

dwalby said:

Not sure exactly where your meter is located, but if you're getting more voltage drop than expected at the meter, that sounds like a dirty connector or positive terminal battery connection well upstream of the brake light, because that's where the excess voltage drop is being measured.

If you have a corroded ground connection, that's going to increase the resistance in the path, and lower the current draw, which is the opposite of what your symptoms suggest. Not saying you shouldn't check the ground connection as well, it could also explain a dimmer than usual bulb due to lower current draw, but it wouldn't explain the voltage drop.

I'd also check the voltage at the battery terminals when illuminating the brake light, perhaps your battery is on its last leg and the extra voltage drop is due to the battery itself. I think you said you have other accessories running that might rule out a weak battery, but it only takes a second to check it and make sure.

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Good points, thanks.
The meter is dash mounted and not a direct battery hook up so its accuracy can't be trusted and a hand held at the battery will certainly be warranted as well as a battery load test. Connection checks will also ensue when time allows

I sort of had that issue on mine, seemed to have a large voltage drop when brake lights or any other load was added at the battery for testing purposes. What I found on mine was a poor fitting connector that joins the alternator subharness to the main harness. I think it's an 8 gauge cable, and the connector had gotten hot and degraded. I ended up splicing the cable with a new crimp an solder/heatshrink. Voltage drop much less now. This was after I had spent considerable time checking voltage drops on both sides of many circuits to narrow it down.

gsforster said:

I sort of had that issue on mine, seemed to have a large voltage drop when brake lights or any other load was added at the battery for testing purposes. What I found on mine was a poor fitting connector that joins the alternator subharness to the main harness. I think it's an 8 gauge cable, and the connector had gotten hot and degraded. I ended up splicing the cable with a new crimp an solder/heatshrink. Voltage drop much less now. This was after I had spent considerable time checking voltage drops on both sides of many circuits to narrow it down.

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I'll bet "considerable time" is an understatement! Hopefully I won't need to delve into things that far but good info just in case

Trapperdog said:

I'll bet "considerable time" is an understatement! Hopefully I won't need to delve into things that far but good info just in case

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Yes for sure let's hope Mine was more of an entire system voltage drop and the brake light was the load that seemed to aggravate the issue the most if that makes any sense.