Topic: main fuse and coils getting warm

[northernrebel]

hello.

i have a 78 cb750 with dyna s ign. w/stock coils. the main fuse(15A) gets really warm, almost hot to the touch. the fuse hasnt blown(it did blow a 5 amp i put in by accident). the only other warm part i can find are the coils and their immediate wiring. this is with the ignition off and the key in the on position.  ive never noticed this before. any suggestions? 
thanks
jason

[ev0lution7]

sounds like you need to ohm out your coils and check them 3ohm coils can draw ALOT...

it also sounds like you need a "higgins fuse box" that way you if you blow one you can get one at a auto store!

[TwoTired]

You should be able to hold your thumb on the fuse without injury.  However, if there is oxidation on the fuse clips, or  if the clip aren't pinching the fuse ends tightly, or if the the wrong length fuse was installed where the ends don't make complete contacts, then the connection becomes resistive.  If you pass current through a resistive connection it generates heat.

The dyna draws more power from the coils than the stock points do simply because it leaves the coils on for a longer duration.  The extra power must go through the main fuse.  The stock fuse is sized for the stock bike electrical load, and normally runs at  about 2/3 of its rating, or about 10 amps.  The dyna will increase this power draw about 150%, and make the fuse run warmer, too.  The fuse melts when the current gong though it generates enough heat to melt the junction.  So, whenever it is passing current there is some heating and the closer you get to its rating, the warmer it gets.  If you've added a higher wattage headlight, the fuse will run hotter, too.

[Mdub]

Funny, I just had some electrical issues that lead me to account for my "electrical budget", and found that my green Dyna 3 ohm coils draw 4~5 amps when the kill switch is in the 'run' position.
Having just had some electrical gremlins/issues of my own, I thought I'd chime in

Unless wired otherwise, you shouldn't see any power at the coils when the kill switch is activated.
You want to power the red fused wire that comes from the Dyna pickup plate harness,
with the black and white stripe wire that comes from the kill switch.

You might try an ampmeter in series with the black lead of your multimeter (set and plugged in for amps measurement of course) on the + terminal of the battery and the red lead of your MM on the + battery cable to the bike.
Turn the key on and you should see a difference between 'run' and 'stop'
Your field coil draws about 1.6 amps (7.2 ohms @ 12v) so account for that.
If you see much more than 6.5 amps draw or so with no lights on you gotta short/leak somewhere.

Don't know what kind of shape it's in, but as an aside, if it's anything like mine, when I started adding Dyna coils, headlight relays, GPS ports etc. etc., my 37 yr old harness said "ENOUGH" at which point one of the three yellow wires out of the stator had had enough of the resistance (that became a big problem with the added loads) that it finally overheated and melted itself into the plug! Not enough can be said about cleaning/replacing and packing connections with dielectric grease (silicone grease is a great substitute)
I wound up replacing all the coneectors between the stator and the reg/rect. (that seemed to be the worst of it).
Also, the main ground to the frame is often overlooked and should be removed, scraped, greased and tightened.
All these little things add up huge.

Try this, find the black wire going to the rectifier and , with the key on, take a voltage reading between the black and negative on the battery, and compare it to the battery voltage. A 2 volt drop is not uncommon!
Of course if your pulling amps through the coils with the key on and ignition switch off, you have other problems!

[GammaFlat]

I've owned 5 bikes that had the three fuse arrangement and all but one had some level of heat damage to the 15 amp fuse holder.  They were in different stages of melting. 

Some of this is caused by things folks add the electrical budget like bigger coils.  Other problems arise on their own.  The fuse holder metal parts go through heat cycles and lose their tensile strength over time.  It's pretty easy to tell that they've lost springiness by comparing their "strength" with the 5 and 7 amp holders with your fingers.  They won't "grab" the fuses as hard as TT points out.  It becomes a problem that feeds itself. 

Other problems are the connectors.  HondaMan has pointed out to me that years ago, self-serve car washes used lots of lye.  Motorcyle folks used these car washes for their bikes.  The lye finds its way into the connectors and corrodes the crap out of them.  Take some of your 6 and 8 conductor connectors apart (rectifier, fuse holder and engine to wiring harness) and examine them.  Don't be surprised if they're a major pain to get apart (and that's not a good sign).  White and crusty =  no good.  All connectors are fair game for a look see (bullets all over the place). 

The Dyna S (as TT and HM have pointed out) saturates the coils more (longer "on") and therefor adds to the electrical budget (TT's terminology ). 

If you're looking for more hand warmers, the rectifier should do the trick as well. 

John

[Hush]

Like TT said, clean your fuse box up and the fuses too,I use a wee steel wire project brush to do mine.
That buildup is just adding to the load, also if you have half a day spend it wisely cleaning all your connectors and coating them with Dielectric grease, it makes the connection remain clean and conductive.