Help! Bike won't start. Low voltage at coil

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Brett

XS650 Enthusiast
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Rutland, UK
(1982 XS650 with TCI ignition)

I can't start my rebuilt bike with my simplified wiring loom! It started and ran before I rebuilt it. I posted the wiring diagram here on my previous post:

http://www.xs650.com/forum/showthread.php?t=19007

It looked like I wasn't getting a spark, so I tested the voltage across the primary terminals on the coil, and there is three volts less than at the battery!

What could be causing that?

I tested the resistance across all the coil terminals and that's fine. I've had the rotor and stator rewound because there were some problems there. I've checked the Tricon kill switch and there's no resistance across it when on.

One strange thing: When I switch it off at the kill switch or ignition switch, after trying to kick it over a few times (it's kick only), it coughs back through the carbs or exhaust.

I really want get it started. I'm sure the wiring diagram is right. Anything else I can test?

Thanks
 
Check your grounding at your coils. The mounting lugs should be grounded to the frame. Check your battery ground at the battery, and where it attaches to the motor and chassis. It is important that the motor and the frame are well grounded, or else the frame can become a big resistor. You will get a voltage drop through the coil, but that sounds like a lot to me. Ensure your battery is topped up and has plenty of amperage available. The battery can read 12.31 volts or so, and immediately drop to around 9 or less with a dead cell or two. Measure the battery voltage with the igintion off, then switch it on, while still moitoring and see where the voltage goes. If it drops more than a volt or so, you have a dead cell on your hands.
 
Fantastic, thanks. I'll go through all those checks.

Should I be measuring the voltage at the coil across the two primary terminals or between one of the terminals and a grounded coil mounting lug?
 
To measure the battery performance, you can put the meter right on the battery. When the electrical system energizes, a drain is put on the system by everything that lights up. As far as the coils being grounded, make sure that whatever they are attached to (if it's the frame, then the mount tabs, if it is bolted on a bracket, then thru the bracket to the frame) is in good contact. Easiest is to check for a low resistance value between where your main ground from the battery attaches to the motor and/or frame, and the bolt that holds the coil down. Also check for low resistance between the motor and the frame. Especially when people have frames painted or powdercoated, the last thing they want to do is scrape some off for a ground path, but it has to be done somewhere.
 
Coils do not ground through the body. It may look like they do, what seems like a earth point on the mounts is for heat dissipation. Try running a hot wire direct from the battery to the coil, thereby taking everything out of the equation. ignition switch could be robbing? Or other connections. just need to move the meter till you find the fault, good luck. Cheers
 
Thanks guys. I've tested all the grounds and they're fine (the coil is grounded through it's mounting bolts even if it doesn't need to be)

I'm trying another new battery that I know is good. I've tested continuity for +12v at as many places as I can think of.

(With all the lights etc off) There is still a voltage drop of 2 volts (12.57v down to 10.3V) across the coil when I measure between the red/white wire at the coil and the orange wire at the coil. If I measure between the red/wire wire and ground, it shows 11.5 volts (So still a drop of one volt). I assume there should be almost no voltage drop at the coil.

I'm sure there is a mistake in my wiring - I can't understand why (after kicking it over a few times) it coughs back when I switch of the ignition/kill switch.

I'll keep checking.

I've attached my wiring diagram.

Thanks again
Rich
 

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There will be a voltage drop at the coil. The windings create resistance. Based on the resistance value of the coil's winding, you should be able to calculate what that drop should be. ( I admit that the math part of electricity is hard for me). Have you checked the voltage drop across your kill switch? Also, your cough could be caused by the ECU firing the coil to dissipate it's potential when you shut off, but that's a guess.
 

"Also, your cough could be caused by the ECU firing the coil to dissipate it's potential when you shut off, but that's a guess."

Thanks JD, I think you're probably right about that.

There's no voltage drop or resistance across the kill switch. All new parts inc fuses.

When the voltage is measured at the coil, it's before it goes through the coil, so the coil itself shouldn't affect the voltage should it?. (I assume I'm testing it correctly. Another thread on here said there should be about 12v at the coil, so I thought I should test it as it's not starting.)

I'll keeping checking and going through the wiring diagrams.
 
Have you had your battery load tested yet? 12 volts does not mean taht it necessarily has any capacity. With an electric start, you can meter the battery during the start, and see the drop as the current increases. If a battery drops below 10 volts during a start, it usually won't actually start the vehicle, and it usually indicates 1 or more dead cells. Either have the battery charged to top and load tested for actual amperage output, or check the specific gravity of the electrolyte in each cell. your battery, measured by itself, after a full charge should read close to 13 volts. 12.37 volts or a bit higher if it's sat for an hour and cooled off from the charging activity. Electronic ignition consumes more energy than most people think. If the battery you are using now came off a running bike, and started it just fine, then fair enough, it should have the capacity.
 
To check voltage at any part you test between the power input and ground.
In your diagram test between the red/white wires at the coil and TCI box, ECU as you have it marked, and ground.
If you get less than about .3 volts at the coil you have a high resistance somewhere between the battery and coil. It could be the main switch, the fuses, the kill switch or any of the wiring connections.
Leo
 
I'd ask pamcopete or retiredgentleman or some other electrical bright spark to look at your diagram. Your hot wire to the coil takes quite a journey!
 
Maybe your wiring is fine, after a second look. Like I said earlier, try running a hot wire direct from the battery to the coil see if it starts then.

You say no volt drop across the kill switch, what about the ignition switch, the 10 amp fuse and the 20 amp fuse, plus any connections on the route.
 
Thanks for all your advice, I think I've found the problem!

As I said, there was a 2 volt drop between the live wire at the coil and ground.

I took off the red/white (+12v) wire at the coil and measured the resistance between that and ground (with the power off). Obviously there should be no continuity but there was 75 Ohms resistance! (with meter set to 200).

I tracked it back to the ignition unit/ignitor (ECU). With the ECU off the bike, I measure 75 Ohms between the ground wire and the +12volt live feed! Even with my basic knowledge that means there is some sort of current flowing within the ECU between the live and ground - and I assume that's wrong.

Can it be fixed or do I need a new one?
Could I have broken it while I was soldering the conectors to the wires (the wires to the ECU have been shortened to 4inches before putting on connector blocks)
 

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