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RPM drop with brake depress

3K views 17 replies 8 participants last post by  WSC 
#1 ·
Strange thing happening to the Thunderbird lately. When I engage either the foot brake or the lever the RPMs drop about 50 or so. I have it perfectly idling at 1100 RPM but it drops to approximately 1050-1025 when brake is engaged. Battery is less than a year old and is reading great voltage at between 12.3-12.6 volts. I did the Gill coil to coil stick conversion the other day but it was doing it before that. Any ideas? I don't recall this happening prior to a few weeks ago.
 
#9 ·
....voltage at between 12.3-12.6 volts
That's with the engine not running right? When running it should go up to around 14 volts I believe. Try a voltage test on the battery terminals while the engine is running, then pull the brake lever and see what happens.
 
#14 ·
Correct, those measurements are with the engine off. I will test it while running and measure the current draw when I depress the brake. Assuming it is dropping, what would be an acceptable drop and why would it affect RPMs?
Alternator LOAD. It takes energy to turn the alternator. More if there is an electrical load, as in it is charging. The rear brake lamp is relatively high wattage causing a significant current draw (unless you have changed to LED).

- Jay
 
#11 ·
It shouldn't drop at all I would assume, maybe half a volt max! But my guess is you'll find something very odd like not going over 12 volts while running - ie not charging!
 
#13 ·
I was able to "hear" my turn signals in my engine before my alternator went south. Very subtle but I don't think it was my imagination. A week later my bike stranded me with a bad alternator.

New/used alternator and all is well no noticeable change with brake lights or turn signal now. Had the Battery Tender battery in both before and after. (loving it so far)
 
#16 ·
I was going to respond to this post immediately, but held off till I actually had put a digital volt meter on my battery before making a comment.

I have a similar multi colour LED on my bike which shows various changes in the charging circuit at tickover. My lights will drop the voltage enough to indicate under charging at tickover.

At rest my battery indicates 12.4 volts. It is not on a charger over the summer months, so this is its charge after being left unused for six days.

Start the bike and I have these readings as I let the choke off, e.g it climbs down from 2500 rpm as the choke is backed off

2500 14.4v
2000 14.4v
1500 14.4v
1000 14.4v

Now the interesting part, all readings at 1000 rpm

14.2v brake lights on
14.0v 30w spotlights switched on
13.7v main beam 55w switched on
13.2v main beam plus spotlights 85w (combined) switched on
??? if I switch the indicators on, the voltage climbs and falls in line with the flash

Rev the engine past the tickover speed to 2000 rpm and the voltage drops caused by the various power drains become negligible.

Take no notice of a drop in rpm as the brake light is applied, unless you are looking to make it a problem.
 
#17 ·
Thanks a ton for the test and feedback.

I am seeing 14.3v after start and idle at 1100 rpm. Applying either brake I drop to 13.7v and to 1050 rpm. No clue if this is an issue but certainly will not stop me from riding! If anyone has an idea of this is within 'normal limits' let me know. Cheers all
 
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