1986 Yamaha XJ700x - 10amp Fuse Blown

Howdy - bike blows the 10amp fan fuse every time I switch the ignition switch from “off” to “run”. I stripped the bike to the frame, did a top end rebuild and did a little customization but all electrical is original minus a few led upgrades. Everything else on the bike works as it should. I thought maybe a starter issue before checking fuses but bypassing solenoid will turn starter no problem. Any ideas?

 

It could be a short circuit, overloaded circuit or a faulty component or connection. Check the wiring to the fan make sure you have a good connection. Do you have the original Yamaha fuse box? Change it for a blade one. @Rooster53 might chime in he knows a lot about electrical systems.

 

It is the original fuse box behind the cluster panel but they are blade type fuses. Fuse terminals look to be in good condition but I’ll run through it again. What’s strange is the fan fuse terminal is only giving a reading of 10.8 volts when switched to run. Battery charge is at 12.5 and I’m getting a consistent 12.5v reading on other terminals. I unplugged the fan and thermoswitch but fuse still blows. I’ll check for other issues here in a minute. I did spill coolant on the connects when filling the system back up earlier yesterday. Maybe coolant got in a contact and is causing a short? I’ll look at that too.

 

Sounds like you are on tract to finding it - That fuse is dedicated to fan circuit only so I would suspect a pinched / chafed wire along the way since you stated fuse still blows with thermal switch and fan disconnected.

 

I’ve basically pulled the entire harness to inspect and there is nothing standing out. I’ve got a copy of the wiring diagram and it looks power for fan also runs to the main switch, main fuse and starter relay. All areas are checking out. Could the short be coming from a joining wire? Looks like left and right front flashers/hot line tie into same junction at main switch.

 

***update*** I connected main switch, cluster wires and front blinkers back together as well as off/on switch and the fuse will now blow at the turn of main switch to on. Which has changed from main switch on, no issues, and then fuse blowing when kill switch turned to on. Should that be pointing me in a direction I haven’t picked up on? I test ohms on fan motor live wire from harness and I’m getting 0.0l so there is a dead short for sure but where is the question. I didn’t have this issue before I tore down and rebuilt the bike. I was able to use switches as they should and crank bike for compression testing with no issue. So whatever is going on it probably my fault but hard to find when everything looks alright.

 

That is either parallel or upstream to the fan fuse so that's not a good path to troubleshoot fan fuse blowing - unless you are mistaken on which fuse is actually blowing?? Are you sure you are not blowing the ignition fuse since you mentioned one time only with the kill switch set to run?

Live wire to where? ground? wire color? And is the "0.ol" supposed to mean zero ohms? You have to be careful trying to ohm circuits particularly when they power light bulbs as the filament will read a very low resistance.

If you disconnected the fan and the thermal switch and it truly is the fan fuse blowing then it just can't be anything else but a damaged wire shorting to chassis or to another wire.

I can't remember if it is the XJ700 but one of the XJ's you can easily attach the wrong connector to the TCI and cause the ignition fuse to blow - check the TCI connectors if you had them off for any reason.

 

So I found that old thread and it is the XJ700 that you can easily hook the side stand relay wiring to the TCI and cause the ignition fuse to blow

https://xjbikes.com/forums/threads/blown-ignition-fuse.47583/

 

*Final Update* Found the issue. After removing 75% of the harness, testing all connects and ohms and volts to every possible component.. it was the kill switch. The moto shop I bought the bike from soldered a split wire in the kill switch before they decided to sell it as a parts bike and I guess when they reassembled the kill switch, they put the 3 prong copper contact plate in upside down so it was making constant contact with all three prongs wether off or on. Flipped it around and no more fuse blow. Le sigh

 

So just for clarification it was the "ignition fuse" that was blowing?

 

What is a "split" wire? Do you mean they "spliced" a wire into the kill switch wiring, and if so where did it go?

Unless flipping that part removed a short from ground that switch shorts contacts to work, so it wouldn't blow a fuse no matter how it was assembled. It may not deactivate the ignition though, which would be a bad thing as it is part of the safety circuit.

 

well - void all that bs/I debated deleting my account and never speaking of this but I did have the safety relay plugged into the TCI. But it was 100% blowing the fan fuse and not the ignition fuse. Unless they labeled it wrong in production. Fan fuse is the left and ignition is on the right of cluster and the left labeled fan fuse was blowing every time. My battery is too low now to test anymore so I’m throwing it on the charger and will come back with an update. I am assuming relay going where it needs too and ignition going to TCI will correct the problem but right now I’m only getting 8v to fuses with low battery.

 

Thanks so much for posting that picture, I have looked far and wide to get a shot of that.

I see what you are saying that if you were to flip the cover and mount it the fan fuse would be on the left. Maybe what Yamaha did was label it such that you wouldn't have to think too much and the fuses match what you see with the cover up - so ignition on the left. It looks like they thought ahead knowing it would be confusing so that if someone did use the wrong logic at least the 10 and 15 amp fuses would be appropriately fitted.

Please don't delete your account, this is live and learn experience for us all, and although mistakes might have been made I always view that as a positive learning experience of getting to know the bike better.

 

No problem - let me know if there are any other pics you need. I’m sure I’ve got them.

Well if it is backwards - it sent me on a wild goose chase. Hopefully that’s the case and I’ve been actually blowing the ignition fuse. Will test once battery is charged.

And I appreciate that. I rely too heavily on this forum to actually delete my account and yes once the bruised ego heals it is definitely a positive learning experience haha

 

What you said just clicked - when looking at the cover while off, that will represent fuse lay out. Ignition is left. I was thinking it would be as the cover sits assembled.

 

That's certainly what I would have assumed - assembled with the label corresponding to the fuse it is directly over and NOT mirrored.
And that's the way any normal rational person would do it. The Japanese engineers were certainly that.

 

*Update* I confirmed it is mirrored. So hunting down a short from fan was a waste of time. The relay to TCI and TCI to relay was blowing the ignition fuse and now doesn’t blow since corrected.

*New Issue* Starter button does not crank the bike. No click from solenoid when attempting either. So far here are my test:

• Starter button does not show voltage to the blue/white wire that connects when pressed

• Cranking works when 12V is applied directly to the relay trigger wire (so the relay, solenoid, starter, and wiring to the starter seem to be fine)

• confirmed and cleaned the button contacts, but the signal still dies when pressed
  
  Starter Relay, Starter Motor, and Battery: GOOD
  • Relay works when triggered directly with 12V
    
    
  • Starter spins when jumped
    
    
  • Battery shows healthy voltage (~12.6V+)
  • Neutral switch is working
    
    
  • Safety system likely not the problem as I bypassed this with no change.
    
    *Problem Area*
  • When I press the starter button, voltage drops to ~57mV when reading blue/white wire that also connects to starter solenoid down the line.
    
    Connecting wire at solenoid is reading proper voltage.
    
    What I’m thinking:
    
    
  • • Not getting sufficient power in
      
      
    • Or internal resistance/failure under load
      
  I see that the starter button is not passing voltage at all. I don’t think the starter button is being fed proper 12V power to begin with but the kill switch is? None of the starter button wires show a voltage more than 50-200mv and they drop when button is pressed. Should I diagnose upstream or connect to a 12V ignition source and feed it straight into the starter button's input wire and see if it’ll crank? If it does I might just bypass and add a push button start somewhere else.. any ideas?

 

*update* I placed a jumper wire from 12v supply with kill switch on and then to blue/white starter wire and the bike cranked. So is this telling bad starter button/contacts and that’s what I need to replace or repair? Picture attached:

 

*Final Update and step by step for anyone else that falls into my shoes*

@Rooster53 thank you for the help and let me know if you see anything I need to correct below.


1. Ignition Fuse Blowing

• Thought my fan fuse (10A) was blowing — turns out the fuse box diagram was mirrored, and it was actually the ignition fuse that kept popping (photos above).

• Root cause: I had accidentally swapped the TCI and side stand relay connectors — they use identical plugs and sit close together.

• Once I corrected the plug locations, the ignition fuse stopped blowing completely.



1-2. TCI and Side Stand Relay Swapped

• The TCI unit and side stand relay have the same connector type, making them easy to mix up.

• Swapping them caused a short or incorrect power routing, which was killing the ignition fuse and interfering with ignition system behavior.

• After correcting the plugs, power stabilized and circuits began working as expected.



3. Starter Button Not Triggering Crank

• Pressing the starter button would cut the headlight, illuminate oil and fuel light (normal to conserve power for crank), but there was no crank and no relay click.

• Voltage test showed:

◦ Yellow/white wire (input to switch): 11.6–12.5V

◦ Blue/white wire (output to relay): only 40 millivolts when button was pressed



4. Weak Continuity Through Starter Button

• Tested continuity across the starter switch — showed high resistance and poor contact.

• Jumper test: Sent 12V directly to blue/white at the starter relay — starter cranked immediately.



5. Final Fixes

• Starter button is internally corroded or worn — not passing enough current to activate the relay.

• Temporary solution: jumper wire bypass to crank the bike.

• Permanent fix: replacing the starter button or installing a clean external momentary switch.


Outcome

• Ignition fuse issue resolved (caused by connector swap)

• Starter system now working

• Bike cranks reliably

**for clarification the two red and white power lines running to kill switch I soldered separately, heat shrinked each and then wrapped together with electrical tape. Hard to tell that from the photo**

 

This suggests that there is a problem with the safety circuit starter cutoff relay or its controls, or the wiring.

In normal operation one side of the starter switch (L/W wire) should be 12V via the starter cutoff relay closed contacts but only if the bike is in neutral OR the clutch is pulled in AND the side stand is up. When the starter button is depressed the switch grounds the L/W wire, which then energizes the starter solenoid

So another area on the XJ700 that can be easily miswired is the solenoid - it should look like this:

 

Confirmed starter solenoid is wired properly and I tested safety mechanisms and all check out. Manual test and continuity testing while using jumper wire to crank bike over.

Crank Or No Crank

Neutral, stand down, no clutch

✅ Yes

In gear, stand down, no clutch

❌ No

In gear, stand up, clutch pulled

✅ Yes

In gear, stand up, no clutch

❌ No


Just to clarify how I’m seeing that it works on the bike:

• The red/white (R/W) wire sends 12V into the starter button from the kill switch circuit.
  
  
• The blue/white (L/W) wire sends 12V out to the starter cutoff relay when the button is pressed.
  
  
  • If all safety conditions are met, the cutoff relay passes 12V to the starter solenoid, and the bike cranks
  
  
  
  
  
• So it’s not grounding L/W — it’s passing voltage to it (?)

Here’s what I tested:

• Got ~12V on R/W at the switch (so power coming in was solid).
  
  
• Pressing the button gave me only millivolts on L/W — meaning the switch wasn’t passing power.
  
  
• Ground path was clean (confirmed via continuity back to frame and battery ground).
  
  
• I jumped 12V directly to the L/W wire (bypassing the switch) and the bike cranked immediately.
  
  But with my most recent jumper crank, the bike continued to crank until I turn keyed ignition off which tells me you’re right that the starter cutoff relay is bad/going bad/faulty or the solenoid is sticking… and if the relay is bad internally or not pulling current due to worn contacts, the starter button could seem dead even if it’s working. Which would make sense because the starter button was working before I disassembled the bike. I wonder if I damaged relay when having it plugged into TCI connector. I’m going to play on the safe side and replace solenoid, cutoff relay and switch here….