Randomly Blowing the Ignition Fuse

I've been blowing the ignition fuse. This doesn't seem to happen with any warning, or with any regularity. Sometimes it will be at a steady speed on the freeway, sometimes while idling, sometimes it will be as I shut it off, like at a light or getting gas. Sometimes it's plenty warm, sometimes cold. Sometimes it will happen twice in a day...sometimes once a couple of weeks.

I've been through all the wiring checking all the voltage drops. There are no large voltage drops. The voltage at the fusebox is 12.02 (engine off) and at the other end of the circuit, at the starter switch the voltage is 11.41. Across the battery it's 12.14 and at the main fuse it's 11.77.

The ignition coils ohm out ok and the bike runs fine when it runs. Diode block seems ok, but the resistor in there is only at 10 ohms....seems awfully low to me but since I can't find the specs I can't say for sure. I can't find any bad wires in the circuit.

One time it blew early in the morning, and I could see the light get a little brighter right before hand, like a power surge. That one time it happened after I turned my right turn signal on, but it happens even if I consciously don't use a turn signal.

The starter works and the battery still holds 10v while cranking.

I did open up the IC unit, and it seemed fine, but there was a little bit of a rattle in there. I brought it inside to test it, couldn't find Polocks thread fast enough, and put it back. By the time I took it outside again there was no rattle.

I've had to put a 20A fuse in there a couple of times to get it home. Not my preferred method but it doesn't seem to blow a 20A. Also, I can generally put a 10A right back in it and be fine for awhile.

I'm pretty much lost....any ideas?

 

Check the condition of the connector from the alternator to the regulator/rectifier. See if it's melted or if the pins look burned.

You can also try taking a dowel, or plastic rod, or such to poke and move the harness wires around while the bike is running to try to see if there's somewhere where a bad connection, or damaged insulation, is making intermittent contact and blowing the fuse.

 

I've looked at that connector and it wasn't obviously burnt, but I'll do it again. I mostly focused on the parts that are after the fuse in question. I don't understand how that would cause the fuse to blow...but if I understood it all that well I wouldn't need help to fix it, would I?

I'll stick something on the wires while it's running and try to find it, too. Maybe I'll go somewhere dark an run it and look for the magic blue juice leaking out. That'll look shady down here near Mexico!

I forgot to mention, the first time it happened was right after I got done hand washing my bike....the second time was while riding in the rain.

Since then it doesn't seem dependent on humidity at all.

 

It's a semiconductor, so when you measure the diode resistance you will get infinite resistance one way and close to cero the other way - if so it's OK. If it's close to cero or infinite both ways it's dead.

When I have dismantled the ignition switch and looked carefully for loose connections there and everywhere else, I would go with the 20A and see what happens.

 

All the Fuse Ratings for the 650/750 are greater than 10A.

You should run:

IGN~20A

Lites, Sigs & Horn ~ 15A

 

Go look at the other current thread about a smoking diode block. He's posted pictures of the inside of his, and, besides the two rectifiers, there's definitely a resistor in there as well. I can't tell from the wiring diagram what the resistor is used for, though.

 

Well shoot....

I really hope you're right Rick, but now I'm really confused because I don't have a Horn fuse, but do have a Main fuse. The old fuse block still had the piece of paper that said what each fuse did....unfortunately I can't remember if it had the ratings on it though.

Right now I have

Main-20A
Headlamp-10A
Signal-10A
Ignition-10A

This is what Haynes says for all 650 and 750, with the exceptions that the 750's have a 5A Tail fuse and the US 750 has a 30A Main fuse.

And a aftermarket horn with it's own circuit that is 20, but that's not going to be in the book.


Pirok...according to my diagram there a resistor wired in with one of the diodes....it's not really series or parallel...it's more like a Y, 2 inputs, one output.

 

I re-read your post...do you mean that the following is correct...or at least ok?

Main 20A
Headlamp 15A
Signal 15A
Ignition 20A

Thanks a lot for your help!

 

I have to disagree. '81 XJ750rh Seca fuse specs are 30A for Main, 10A for Head, Signal and Ignition. I can post supporting docs later if needed. Right now I can only find J model manual online, and it has the same ratings (per wiring diagram on the last page). But as I just rewired my fuse block, this info is fresh in my noggin.

 

Check the resistance of the two circuits that take power and ground to the TCI. Each circuit leg including fuse, switches, etc. should be less than an ohm or two. If any more you have a bad connection imposing a high resistance and increasing the current draw through the loop or a frayed wire that is down to the last couple of strands. Even a dirty switch contact will have the same effect.

Glass thermal fuses will blow at close to their rating if held there long enough so you may have either 11 amps for a few minutes which slowly heats the fuse or 30 amps for a split second.

You can easily measure how much current you have in the circuit with a 30A analog panel meter (about $10 at Radio Shack). Wire it in series with the ignition fuse and gaffer tape to the handlebars. Use some reasonably heavy gauge wire and make sure the terminals are well insulated.

Then ride normally and keep an eye on it. If you see occasional sudden spikes under steady conditions you have a transient overload causing the fuse to blow. If on the other hand it is normally sitting around 10 to 12 amps and displays no other symptoms I would be betting you have a corroded connector or switch that is introducing an abnormally high resistance into the circuit.

 

iandmac...thanks...I'm not sure what circuits you mean. I put a 15A blade fuse in, doesn't ever blow so I'm thinking it's a problem with wiring...plus I live next to the ocean and am garageless....

Are you saying test from the IGN fuse supply side to the R/W wire on each coil?? Or across each connection? Or across connections on the TCI?

 

What we are trying to do is find a point in the circuit of unusually high resistance.

There are two parts of the circuit, the power side (which delivers 12 volts to the device) or the ground side (which takes the return current back to the battery negative terminal).

Our first tests will only determine if there is a high resistance point in either (or both) of these. That narrows it down a bit.

First disconnect the device (in this case the TCI) and identify the power wire (the black and white one) and the ground wire (the black one).

Now disconnect the battery positive and negative and measure the resistance of the power circuit and ground circuit. That is, first connect your ohmmeter across the black/white wire and the positive battery lead (for the power circuit) and then connect it across the black wire and the negative battery lead (for the ground circuit).

In both cases you should only see a couple of ohms, at most.

The bike has to be in normal running configuration, ie. main and ignition fuses installed, sidestand up, and ignition and kill switches in ON position. As a precaution I would also remove the other two fuses to isolate as much of the remaining wiring as possible.

When you get to this level of troubleshooting an accurate detailed circuit diagram is a must, as is a logical test method ...

http://www.xjbikes.com/Forums/viewtopic/t=32483.html

Let us know what you find.

 

On my 750 i went thru all the wiring connections with dielecric grease and went thru and sanded/cleaned all the switches and contacts with real fine sandpaper along with the coil/frame connections. I replaced the fuse panel with blade fuses. On the "main" 30amp circuit I replaced the fuse with an ATO 30amp circuit breaker instead of a fuse, BUT i can use fuses if the breaker(s) take a shit. So far i havent had any problems, I reccomend switching to breakers AFTER you clean up all the switches/connections on the loom. A breaker saves $$$ because you dont need fuses.. YMMV but its been workin great for me
.
Jay

 

That's not how electricity works. High resistance means low current draw, whether it's on the supply side or the ground side. Low resistance at the load, or something shorting around the load is where you get high current draw and blown fuses.

 

By the way, if you have a better multimeter, that can read up to 20 DC Amps, you can use it to directly read how much current your ignition circuit is pulling. If you don't have such a meter, it may be worthwhile investing in one.

Harbor Freight has one that will do this for $20.

The way to use it is to set the meter to DC Amps and the 20A range; usually you will need to connect the probes to the meter in different holes specific to measuring current; then remove the fuse and clip one test lead to one fuse clip (on the harness, not on the fuse you removed) and the other lead to the other fuse clip.

Basically, you're putting the meter in place of the fuse. When you start the bike, the meter will conduct current through the ignition circuit, but it will also tell you how many Amps are flowing. If you see this to be 5A or something, then you know there's an intermittent problem, like a frayed wire somewhere, blowing the fuse. If you see this to be more like 11A, then you know something is drawing too much current all the time. In the latter case, I'd suspect something like bad capacitors in the TCI causing it it to overdwell the coils.... which would also lead to the coils getting hot, and would probably eventually ruin them.

 

SQL...suppose it is bad capacitators...what is the solution? New TCI or new capacitators? Or new coils?

I might be able to borrow a good multimeter....I think mine only goes to 10A.

This electrical stuff is adding up...I think it's because I practically park it in the ocean. The neutral switch just started staying on and the right turn signal is intermittent as well. Haven't started on the neutral switch but i've been through the turn signal and can't seem to find any cause...but that's for another thread.

 

If the problem is the TCI overdwelling the coils, it's likely that it can be repaired. One of the moderators - can't remember which one - rebuilds TCI's... swapping out the old caps and repairing bad solder joints.