Debugging no spark

[emschwar]

I'm not sure about the connection here, (probably none) but I just fixed my brake lights, and now my bike won't start.

As I say, I'm pretty sure it's not relevant, but I just removed, cleaned, and replaced the front and rear brake light switches on my '82 GS750T. Now, when I try to start it, nothing happens. I removed a spark plug but left it connected to the cable, and when I hit the starter, I don't see a spark. So now my question is, how do I debug this problem?

I've definitely got power -- the lights all work fine (including the brake light! ). I haven't run the bike in about two weeks, but before that it was starting and running just fine, so my first thought is maybe I knocked something loose somewhere, but I'm not sure where to start on this problem. Any thoughts?

[SqDancerLynn1]

Recheck for disconnected wire. DOUBLE check the fuses

[Texas Don]

These bikes are sensitive to losing their fuel level in the carb bowls. If you let it sit for 2 weeks, you may have nothing more than a lack of fuel. Try putting the petcock on prime for a minute, then starting.
The proper way to check for spark is to ground the plug while you're turning the engine over. Just laying it out of the hole won't get it done. I have a really simple tool that I purchased for less than $5.00 new from a lawnmower repair shop. It is a light that plugs into your plug wire and ground. When you hit the start button, it will light up if there is current flow. Easy. You might want to look into getting one.
Don

[hikermikem]

I removed a spark plug but left it connected to the cable, and when I hit the starter, I don't see a spark.

You have to lay (ground) the plug against the top of the engine when you do that to get a spark across the electrodes...

Don't hold it while you perform this test unless you like getting zapped

[hikermikem]

The proper way to check for spark is to ground the plug while you're turning the engine over. Just laying it out of the hole won't get it done. I have a really simple tool that I purchased for less than $5.00 new from a lawnmower repair shop. It is a light that plugs into your plug wire and ground. When you hit the start button, it will light up if there is current flow. Easy. You might want to look into getting one.
Don

Ooopps - Sorry Don, I only read half your post. You had it covered nicely

[emschwar]

You have to lay (ground) the plug against the top of the engine when you do that to get a spark across the electrodes...

Yeah, I did that-- it took me a couple of tries, then I realized that duh, the electricity has to go *somewhere*. No dice, though.

[emschwar]

These bikes are sensitive to losing their fuel level in the carb bowls. If you let it sit for 2 weeks, you may have nothing more than a lack of fuel. Try putting the petcock on prime for a minute, then starting.

I tried that; alas, it didn't help.

The proper way to check for spark is to ground the plug while you're turning the engine over. Just laying it out of the hole won't get it done.

I'm sorry, I didn't think to mention this. I did in fact ground the tip of the plug against the engine while cranking it, and got no spark out of it at the time.

-=Eric

[Texas Don]

Then your problem is pretty straightforward. You need to check and see if you have 12v at the coil on the primary side when the bike is cranked. If yes, the coils or plug wires or connections are at fault. If not, then you most likely have a poor connection in the ignition switch circuitry.
Don

[emschwar]

Then your problem is pretty straightforward.

Awesome! I love a straightforward problem.

You need to check and see if you have 12v at the coil on the primary side when the bike is cranked.

Okay... I'm no stranger to the multimeter, but I'm pretty inexperienced at working with motorcycles. If it's not too much trouble, can you walk me through this process?

-=Eric

[Texas Don]

Sure. First you have to remove the gas tank. The coils sit under it on the center of the frame. Remove the small wire from one coil and place your positive lead in the removed wire end. Negative lead to a ground anywhere on the bike. Ignition on and crank the bike. See if you have +12 volts. Do the same for the other coil. The wire color that you're looking for is orange/white if memory serves me correctly.
If you have 12 v input to each coil, and the plugs aren't sparking, you have a bad coil. If you don't have 12 v, then you need to backtrack on the schematic and see where you lose it. What exactly were did you do to repair your brake light btw?
Don

[emschwar]

Sure. First you have to remove the gas tank. The coils sit under it on the center of the frame. Remove the small wire from one coil and place your positive lead in the removed wire end. Negative lead to a ground anywhere on the bike. Ignition on and crank the bike. See if you have +12 volts. Do the same for the other coil. The wire color that you're looking for is orange/white if memory serves me correctly.

Thanks, that was very clear; I'll check it out first thing when I get home tomorrow.

What exactly were did you do to repair your brake light btw?

Heh. Well, the switches were what was wrong, actually; the contacts on the handlebar switch were very corroded, so I took it off and cleaned it with some 000 steel wool, and it worked again. So figuring I was on a roll, I checked the rear brake switch, and it turned out it wasn't working right either. I removed it entirely and looked it over. It turned out to be fine; I think there just wasn't enough tension from the spring that was connected to the brake pedal. So I reinstalled it (or rather, my wife reinstalled it; her hands are smaller than mine, so she could more easily get in there), and afterwards the darned thing wouldn't start. The brake light works fine, though. :-)

Thanks again for your help!

-=Eric

[emschwar]

Sure. First you have to remove the gas tank. The coils sit under it on the center of the frame. Remove the small wire from one coil and place your positive lead in the removed wire end. Negative lead to a ground anywhere on the bike. Ignition on and crank the bike. See if you have +12 volts. Do the same for the other coil. The wire color that you're looking for is orange/white if memory serves me correctly.

Here's what I saw- with the + lead on the orange/black wire, and the - on the frame, I saw ~12V as soon as the switch was turned on. When the starter was pressed, I got ~8V. Interestingly, when I grounded the - lead to the engine, I got nothing at all. That smells like a grounding problem, but I'm baffled as to how it's possible-- if nothing else, the engine mounts should ensure the engine and the frame share a ground.

-=Eric

[Texas Don]

Here's what I saw- with the + lead on the orange/black wire, and the - on the frame, I saw ~12V as soon as the switch was turned on. When the starter was pressed, I got ~8V. Interestingly, when I grounded the - lead to the engine, I got nothing at all. That smells like a grounding problem, but I'm baffled as to how it's possible-- if nothing else, the engine mounts should ensure the engine and the frame share a ground.

-=Eric

Okay, you've got 12v to the coils. A drop in voltage when you hit the starter is normal, although I'm not sure that the drop should be 4 volts. That sounds excessive so you may or may not have starter issues too. The ground on your engine is through a wire from the engine to either the frame or the negative pole of the battery. Your engine is mounted with rubber shock mounts which are effective insulators, hence the ground wire. Check for it. It should be a #12 black wire connected to a bolt on the engine case. My GS1000 has it on the right side of center beneath the carbs. Since you have 8 volts at the coil, you should have a spark, even a weak one.
The points open and close to cause the coil to discharge its potential through the plugs when they close and recharge the coil when the points open. Pull the points cover off and watch the points as you crank the engine to see if they are opening and closing as the crankshaft turns. You may want to avail yourself of the coil relay modification detailed on Bikecliff's website. It uses a relay to send 12v directly to the coils so that you don't have a large voltage drop when you apply the starter. Did you check to see if the secondary of either coil is shorted or grounded? Is any plug firing at all or did you just check one? You have 2 coils so if both are not firing, logic would indicate the problem is elsewhere. Pretty high odds against both coils failing together. You should be able to check for continuity from the breaker points all the way to each coil when the points set for each coil is closed. You should not read continuity when the points are open for each coil.
Keep me posted.
Don

[BassCliff]

Hi,

You'll find the following article, and others, in the "Electrical Odds and Ends" section of my website. Maybe they'll help.

Testing The Ignition System
by Mr. Matchless

Properly localizing a problem with the ignition is sometimes a bit confusing and the following below should of assistance:

• Basic test, remove spark plugs. Fit them to the plug HT leads and ground them to the engine. Turn engine with starter and see if plugs spark. If any one spark plug does not spark swap it out. If the spark seems good on all 4 plugs, the ignition system is very likely in order. If spark is not present or very weak proceed with the following tests.
• Remove the tank, left side cover, seat and signal generating unit (pickup) cover at bottom right of engine.
• A good habit is to check the coils and igniter to see if they do not get very hot once the ignition is switched on, as this will most likely be the indication of a failed component of faulty connection or wire.
• If the spark is weak but present, inspect the HT leads and plug caps. Suspect coil wires and spark plug caps, or voltage at the coils and thus the battery condition. It could also be due to coils with partially shorted windings, but do not jump on this cause immediately, and they may get hot.
• Overheating coils with no spark may also be due to them getting a permanent full ground either from a faulty igniter or a grounded and pinched wire. Disconnect the coil plug connectors and proceed with tests.
• Measure the battery voltage directly across the battery terminals. If lower than approximately 12.6V first charge battery fully before proceeding.
• Next measure voltage over the coil connector plug, orange/white wire and the battery negative terminal. If lower than 12 Volt inspect the wiring for poor contacts and localise cause of voltage drop.
• Again measure directly across the battery, but pull off the spark plug leads to prevent engine from firing and swing with starter. While starter is turning the voltage should stay at least above 11 Volt. Also swing the starter with the headlights on to see that the voltage does not drop significantly at the coils while the starter plus headlights load the battery, which could prove that even if your battery is fully charged, it cannot give full or sufficient current and is on its way out or your starter may be drawing excess current (usually unlikely if starter is spinning at full speed) and pulling the battery down. If it drops much lower, charge battery fully or have it load tested and replaced if faulty.
• If all is well up to here you can assume your battery and the positive feed to your coils are in good order.
• With the coil plug connectors disconnected, use an ohm meter and measure the resistance of both windings on each of the coils. Exact resistance measurements are not too important, but continuity of the windings close to the approximate resistance values given indicates that the coils are in good condition. It must be noted that it is possible for the coils to only show up a fault when at higher operating temperatures, but this does not happen frequently.
  Ignition coil resistance:
  Between the two HT plug caps of the same coil, secondary HT winding, approximately 30 – 35 K ohm
  Between the orange/white and white on the first coil and orange/white and black/yellow on the second coil, primary winding, approximately 2-5 ohm. If this test is within limits you likely have two good coils.
• Locate the igniter and disconnect the plug with the blue and green wires coming from the signal generator (pickup) at the right bottom of the engine. Test the resistance across these wires coming from the signal generator pickup coils, it should be approximately 250 – 360 ohm. If this is in order you have proved the pickups and the wires to be good.
• The little back box or igniter is now tested as follows. With the ignition on, kill switch on, test for 12 Volt DC between the orange/white and the black/white wires in the plug going into the igniter. This proves that it is getting the correct voltage.
• The next step is to prove whether the igniter is powering your coils. Remove all 4 spark plugs and connect the HT lead caps to a spark plug #1, #2, #3 and #4 which is grounded and located to enable you to see the spark. Ensure all the connector plugs are back in, except the one with the green and blue wires from the signal generator.
• To simulate the small voltage generated by the pickup coil, prepare an ordinary 1.5V dry cell with two wires red for positive and black for negative. Connect the negative black wire from the dry cell to the blue wire on the connector plug going into the igniter. Switch on the ignition and kill switch to power the igniter and briefly touch the red wire from the dry cell positive to the green wire connector going into the igniter. You should see a spark on plugs #1 and #4 when you touch the wire and on #2 and #3 when you remove the wire. If this works your igniter is in a working condition.
• One other check that is often overlooked. Run the bike at night with all lights off in a dark area and check that no sparks are jumping from the HT leads to the frame or tank. If so they may need replacing.
• At this stage if all tests have passed you MUST get a spark at all the plugs when everything is reconnected and the engine is turned with the starter.
• If a spark that was previously missing has mysteriously appeared, have a very good look at your wiring and connectors, as something may be making intermittent contact and corrected itself temporary when you moved the wires.
• If there is a spark, but the engine will not fire, it could be due to ignition timing, valve timing or fuel problems. Ignition timing and valve timing problems are more likely on a bike that has not run yet after being disassembled and more unlikely to be the cause if the bike was not taken apart and was running before.
• A quick test with a few drops of fuel directly into the cylinders before replacing the spark plugs and then trying to start will prove it to either the fuel system or ignition system.
  1. If the engine then starts and runs for a few RPM's the ignition is correct and you need to look at the fuel and carburettor side.
  2. If the engine does not fire, but occasional gives one load pop through the carburettors or even exhaust, it may be igniting when the valves are open, the valve timing may be a problem, cam chain or swapped wires from pickup – plugs firing at wrong time.
  3. Other reasons could be extremely low compression, too tight valve clearances, thus not closing fully (usually unlikely on all cylinders at the same time, but plausible), carburettor faulty or dirty, fuel line, filter or petcock faulty. Filter can cause an airlock if not positioned properly to allow free gravity feed.
  4. Then finally the ignition timing must be set properly. At less than 1500 RPM connect a timing light to #2 or # 3 spark plug and check that the timing marks line up for 2-3 on F, then move to #1 or #4 spark plug and check that marks line up for 1-4 on F. Check the advance by pushing revs up to 2350 RPM and the timing marks should both line up on the 45 mark behind the signal generator mounting plate through the sight hole at the top. The three mounting screws can be loosened and the mounting plate can be moved slightly in the elongated holes to meet this.
  
  

Thank you for your indulgence,

BassCliff

[emschwar]

Okay, you've got 12v to the coils. A drop in voltage when you hit the starter is normal, although I'm not sure that the drop should be 4 volts. That sounds excessive so you may or may not have starter issues too. The ground on your engine is through a wire from the engine to either the frame or the negative pole of the battery. Your engine is mounted with rubber shock mounts which are effective insulators, hence the ground wire. Check for it. It should be a #12 black wire connected to a bolt on the engine case. My GS1000 has it on the right side of center beneath the carbs. Since you have 8 volts at the coil, you should have a spark, even a weak one.

Okay, the ground problem was a red herring; I was only trying on the top of the cylinder head cover, which I guess is insulated from the rest of the engine by a gasket? Anyway, I could get power from the coils when grounding to the engine, so that's not it.

The points open and close to cause the coil to discharge its potential through the plugs when they close and recharge the coil when the points open. Pull the points cover off and watch the points as you crank the engine to see if they are opening and closing as the crankshaft turns.

I think my '82 GS750TZ doesn't have points. At least, in my Clymer, the section on 1981 and later bikes makes no mention of them. I won't swear to it, but I can't find anything on the bike that looks like points.

You may want to avail yourself of the coil relay modification detailed on Bikecliff's website. It uses a relay to send 12v directly to the coils so that you don't have a large voltage drop when you apply the starter. Did you check to see if the secondary of either coil is shorted or grounded?

I checked the negative connection to the coils against a ground, and got 0V, so I don't think I'm getting a short there.

Is any plug firing at all or did you just check one? You have 2 coils so if both are not firing, logic would indicate the problem is elsewhere.

I checked all 4 plugs, and can't see a spark on any of them. I agree completely; the odds of them both failing seems, well, unlikely.

-=Eric