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Suddenly no ignition at all?? 1979 Yamaha XS750 Special

6811 Views 4 Replies 3 Participants Last post by  Retired Guy
Well, i'll take some blame right away, because certainly this has to be my fault in some way or another.
I was replacing my carb boots, and also decided to very carefully remove some excess hardware from my 1979 XS750 Special, such as rear fender and some reflectors, which involved relocating a little bit of wiring.
Nothing was disconnected though, but it appears that somehow something was maybe not a healthy connection and officially lost its continuity during the motions.

The bike won't attempt to start. Battery is good, all lights, horn, signals (except one busted bulb) work fine, but when you press the engine start button (yes the engine kill switch is not pressed) there is no electronic or mechanical reaction. Simply nothing. However, to rewind even further, when i first turned the key after doing the original work today, i got no electronics at all. No battery light at first key location. I had to go through and reseat a bunch of wire junctures and then the basic electronics started working again. So now i have electrical power but not getting any startup.

I had this similar symptom with this bike once when the run switch mechanism wasn't properly grounded (to the chassis since it had fallen loose during another repair), but that was easily discovered and remedied. Startup has been great ever since. I of course checked and reseated that wire to or three times now.

But i've got zero "ignition".

I've checked all the fuses with a continuity tester and they're all fine.Thought maybe i had made some bad contact and blew something, but this doesn't seem to be the case.

Any ideas what this could be?

I have done digging around for this already, looking for something that may have come loose. I found one single loose connection, however, it appears to have been taped up by a previous owner. So, it may have already been disconnected before my problems began and might have nothing to do with this. I looked up green-yellow wires in service manual diagram and they all appear to be related to turn signaling, not startup. I can't quite trace where it goes since it's under the tank and my tank is full so it's a pain, but i might revert to that.

However, the loose wire is right by the ignition system. Seems like a possible suspect.

Any ideas?


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That wyre next to the rear brake hydraulic bits, could be the brake light switch.
I recall they are yellow. The starter relay can quit. Turn key, lights action, press button nothing, as in no crank.
If you have crank but not fire, which is usually no power to the coils, it can be dirty connections. Which includes the kill switch, ignition switch, the multi wyre plug to the ignition module, a connection just behind the steering head, the stupid magic box that controls the headlight turning on. Ignition is often red. Start at the battery and follow that wyre.

When I fire Dark Vader my 81 Midnight special, I will eliminate all the silly connections. Just the key for ignition on off.
They seem to run better the more useless stuff you cut out.

That ignition module is in a great place to get dirty. Mine are under the seat, but still can have dirty connections. Seems age is all that is required.

Just wanted to post here that this problem was solved! It was a poor connection on the downstream side of the fuse just prior to Engine Cutoff switch. Was able to find the issue with voltage testing and replace the connection with a fresh one. Electronically we are back to normal. Still got lots of stuff to do to the engine but things are already dramatically improved after a bunch of basics have recently been covered.
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Electrical problems can be such a pain to discover. Congratulations on getting it to percolate again! :)
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