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Wiring trouble on an 08(mostly) Sprint ST


Well-known member
Mar 28, 2022
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Palm Coast, Fl
All right peeps and peepette, I've got this sprint ST almost completely built now and am trying to figure out why I can't get the bike to start up. When turned on with a hotwired "Ignition Plug" I get tail lights and turn signals and can hear a few relays clicking but the fuel pump won't prime and the starter won't even click. Not really sure where to start with this one, I have checked all the connectors and everything engine side is plugged in, wiring harness side the dash, headlights O2 sensor, and one mystery 2 and 3 pin plug arent connect. Writing this out I think that 2pin plug may be the crank position sensor which probably solves my problem :lol3 (I'll report the finding tonight after work) the other plug is up front in the dash area and I am not sure what that goes to yet.
Thanks for letting me talk this through and thanks in advance for any help/advice offered!
(and its not stolen, just pieced together from a wrecked bike and many slightly tipsy nights on ebay :freaky)
Almost forgot the most important part, bike pic and whats been done!

Got her as a "ready to run" Speed triple turns out it was neither! engine was completely locked up due to a rusted solid cylinder and it was actually an 06 Sprint ST. Got her apart and engine dropped and got to work. Ordered a good used upper engine case as the cylinders are not there own jugs like I am used to and it was about 1/3 the price of getting some good sleeves. While waiting for those parts went town on her with the rattle cans and got her looking a good bit better, deliberately left the parts without a clear coat as it should wear through relatively quickly and give a real nice worn in look. The original bike had the front forks bent pretty bad so traded a little help and some parts with the neighbor for a pretty clean zx6R front end and mated that with a GSXR600 triple clamp I had lying around. only thing reqired there was a little trimming of the wheel spacers to fit the slightly narrower gsxr clamps. The current gas tank on there is the plastic sprint one but I also have a metal one that I need to get painted to match as a backup. Kept the stock exhaust as it just looks good with that triple tip and until I know she runs well I'd like her quiet, makes finding my mistakes easier:lol3

Also, sexy single sided swing arm 😍
Now that I am going over this with fresh eyes I think I've got 3 things to check

Crank position sensor, might be plugged in wrong moved to correct plug
speed sensor, multiple triangular 3 pin plugs, may have them mismatched verified in correct plug
tipover sensor verify it actually is good
TPS verify is correct verified good
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New twist! so I started with an 06 originally as I knew it did not require a dash to run, I had planned on adding my own later on. turns out the ECU I had was a newer style plug while the wiring harness I had was an oldstyle ECU plug, not a big deal, upgraded the wiring harness to an 08. Later picked up an 08 spare parts bike for almost nothing so big win right? Almost! so it looks like the 08 requires the CANbus to fire up. I do have one ordered, fingers crossed that will be the last piece of the puzzle needed.
Bad ground.

That's my usual response to an electrical problem. I hope your CANbus sleuthing pays off.
I wish it was a ground! I actually cleaned that off last night as well just to be sure.

Im thinking the CANbus makes sense as everything else looks properly connected per the wiring diagram and yet I still cant get the fuel pump to prime or the starter to turn over. Fingers crossed I'll know in a week!
9 out of 10 times it is a bad ground. Good luck ..if it weren't for the canbus I would think safety switches or relays.
All right so put a fresh battery on her and.....she still turned over super slow. So I went and cleaned the grounds again and swapped to a spare starter I had and same thing. Why must she hurt me like this so close to being done lol
It looks like I can, I ran out of time before heading into work today so hopefully I'll know later tonight. If it works that way it implies the relay is bad correct? If so I can even swap in the spare relay I (think) I have and see if that fixes the problem
All right still no dice, I have ordered a OBDII scanner so fingers crossed that'll help pinpoint the issue/area to look into. Since then I have;
-rechecked the ground
-rechecked all fuses
-rechecked all wiring harness plugs
-fresh battery
-swapped in a spare starter (no change)

Up next is to;
-Check the starter button for corrosion (has continuity)
-RH switch plug (has continuity)
-Found one forum post about headlights causing a similar issue, maybe plug in a set of spares I have to help rule out
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I think it is a slow crank issue? Maybe yank the plugs shoot some marvels or wd40 in the holes. Spin it over with no load. Check the spark and hopefully reset the brush to commutator contact in the starter....make sure to ground the plugs!
So I do have spark, I am currently able to fire the bike up no problem as long as I've hooked up the booster pack. It requires it for every start otherwise I get a crazy slow crank. Im still not at all sure why lol but she lives!!!!!!!
Motorcycle batteries have always caused me some issues. The batteries are difficult to load test properly. Sometimes I have used voltage drops on the starting circuit or just the headlight. If the headlight is sucking more than a a volt per 15 seconds... battery is surface charged or sulfated.

The riding mower is the one that gets me the most on batteries.
Yeah I used this new battery to fire up my old Virago here and it was fine but I have read that the sprints are notorious for not liking batteries so I'll need to find some way to fix this problem. I wonder if I could just get a higher quality battery and it'll sort itself or somethng with higher cranking amps
Maybe run an amp meter and compare to spec. Alot of amps so crude clamp on or shunt style.

Willing to bet battery. My coworkers sprint and bonnie only liked top quality batteries. Watch the voltage drop while cranking. I thought the triumphs also have a low volt safety....hit the button and nothing...hit the button a 2nd time and it cranks. I watched this a few times at the end of the day at work.

My drz only likes good batteries. Slow crank equals roll start!
I agree with Danno, check starting amps. Is your alternator putting out correctly when the bike runs? You may want to also do a compression test and see where cylinder pressures have settled out after your rebuild. Did you rebuild starter?
Yeah checking the draw is up next, just found tthe multimeter that was miraculously exactly where I used it last :baldy
I hated my DRZ, I ended up putting the E model kickstart on my S model just so I wouldnt have to worry as much about it burning out stators, RR's, and batteries.

I am curious about the compression as well, I've run it twice for a few minutes each time and it feels and sounds healthy I am getting a little smoke out of the engine area somewhere but I think that was due to the O2 sensor being a little loose. I used the original starter that was with this bike as well as the starter out of a cheap spare parts bike so I am reasonably confident the starter isn't the issue but may rebuild the original to have a safe known backup.

Just roughed out the dash mount for her so its time to decide on which headlights I want to wire in (Old '05 R6, random 6" cruiser style, or twin bulb cafe style) and then make a windscreen. It's getting really close to being ready to ride!
A quick note about compression and starting...loose clearance intake specs can have crazy dynamic compression. I do this intentionally on some engines. All verified with a compression guage. I doubt this is your problem.

I am betting sticky engine and weak battery. Usually the commutator and brush contact is go or no go. It doesn't hurt to refresh the surfaces. But I have never done this to a motorcycle...maybe I should. My ct70 clone likes hybrid starting..kicker and the button. I can feel the kickstart shaft flexing. Sometimes I bump start the bike. Compression ratios are misleading. Only dynamic cranking compression matters and the starter has to overcome the pressure.
I think Danno is on to something.

Unless there's an auto decomp mechanism that's not operating properly.

Was servicing a fussy WR250F recently that was hard to start. Tried disabling the auto decomp to get a solid compression reading and the starter on the bike could not hardly crank the engine with the decomp disabled. Many bikes are this way.
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