coil craps out intermittently

Curious about this one, it's a head scratcher [prob why I'm bald] I just took the 81 xj650 out for a spin, got 2 miles from home, the 1-4 coil craps out. This is the one I just changed for that reason. Before it was 90f outside, today only 60. Again I waited a few minutes and it started right up on all 4 ! I did cycle the master switch several times while waiting- beginning to wonder if the ignition switch is going on me. While it was running I pulled the 1 and then the 4 wire off, no change. :roll:
Any thoughts? would be appreciated!

 

Could be:

1) bad pick-up coil for that ignition coil channel:

Checking pick-up coils: the resistance across each pair of lead wires (at the TCI) should be as follows. NOTE: if both coils are out of specifications, suspect a pinched or shorted black ground wire, which is a shared ground for both of the pick-up coils on most models. It is very unlikely (although not impossible, especially in a case of improper jump-starting, etc.) that BOTH pick-up coils would expire at the same time!

A simple test to see if the coils are working, at all is to place a voltmeter (preferably an analog unit) across the Grey or the Orange wire to the Black wire. Energize the system and watch for voltage pulses as you rotate the reluctor past the pickup. This can be done by hand or with the starter.......we'd recommend using the hand method so that the pulses are slow enough to see. These "pulses" are what the TCI "black box" counts and interprets when "deciding" when to fire the ignition coils.

NOTE: the orange lead wire is the trigger wire for the #1/#4 ignition coils, while the grey lead wire is the trigger for the #2/#3 ignition coils.


120 ohms +/- 10% for all XJ650 Turbo models, XJ700 all models and XJ750-X models, XJ900RK, RL, N/FN, and F models, and XJ1100 models.

650 ohms +/- 20% for all XJ550 models, 1982-84 XJ650 Maxim, 1982 XJ650RJC Seca (Canadian, yics-engine), and all XJ750 models.

700 ohms +/- 20% for all 1980-81 XJ650 models and 1982 XJ650RJ (non-yics engine) models.




2) Bad wiring (loose connector, etc.) to the ignition coil.....check to see if you have about 12V to the coil when the key switch is on.

3) Bad (or going bad) TCI channel for that coil......assuming there is no problem in test "!" and "2" above, then it may be that a solder joint or electrical component is going bad in the TCI unit (it most likely happens as the electronics in the TCI warm up). Unfortunately there are no real useful tests for the TCI unit besides replacing it.

You can always try and swap the 1/4 and the 2/3 coils and see if the problem "moves" with the coil (in which case the ignition coil is the problem), or if it remains on the 1/4 side, then it's either the pick-up coils or the TCI unit.


TCI UNITS:

Yamaha (thankfully!) used a TCI (which stands for Transistor Controlled Ignition) system on all XJ-series bikes to control the coils, timing, spark advance, etc. A TCI unit is an "early" version of the now-common electronic control systems that are used on virtually all modern vehicles of almost every type, and even these early versions are completely maintenance-free and very rarely cause problems...........which is a good thing, because original TCI boxes are no longer available new.

When engine performance problems develops, many people immediately suspect that the cause may be within the "black box" workings of their TCI unit, which is unlikely. The factory service manual gives "instructions" for diagnosing TCI problems, and it basically says "test every other possible cause for your problem and if no other cause for the problem exists, only then should you "suspect" TCI failure, but before you buy a replacement, first try to find a known, working TCI unit from a similar bike and plug it in on the problem bike, and see if the problem goes away............"

Well!

There are three main problems that TCI units succumb to after years of reliable service:

1) bad solder joints on some of the internal components (known as "cold solder joints") result in the component pieces coming loose from the circuit board, and thus they can no longer perform their function reliably (or at all).

2) component failure......a blow-up transistor, a burned circuit trace, etc. This situation can develop if you have a short-circuit in your electrical system, or hook up your battery or jumper cables incorrectly, etc. TCI units do not like "big blue sparks" in the electrical system (except at the spark plugs, of course!).

3) another common failure of these units occurs if the TCI is continuously grounding the ignition coil (i.e. its output driver is shorted). You can verify this situation with an ohmeter with the following tests:

- disconnect the 2-pin plug at the ignition coil and measure from the orange or grey wire to the chassis. You should see very high resistance. This should be pretty close on both the working and non-working channels.

- if you read a few ohms of resistance or less, then the TCI is bad. A shorted driver will also make the ignition coil run very hot and may ruin the coil.

4) dirty external terminal connections.


Bad solder joints can be repaired by someone who is skilled at that sort of diagnosis and repair, and even individual circuit components can be replaced, but it's tough to find someone in the modern world of "pitch-and-plug" skill-sets who actually has the skill and patience to do this type of work. Yamaha gave absolutely "zero" electrical specifications for checking the condition of the TCI units, besides the afore-mentioned "check everything else first" type of diagnosis.

But you can perform a simple set of tests to determine whether your TCI unit is good or not, without having a second, known good unit to install in place of the suspect unit. Although these instructions were written for XS owners, the exact same thoughts apply to the TCI units on the XJ-series of bikes:

http://home.earthlink.net/~randyrago/da ... 0faq's.htm


All of the XJ-series TCI units are of the "4RO" style as described in the above article.


And if the above isn't enough, if you feel the need to get medieval with your TCI unit, well, then it doesn't get much better than this:

http://www.jetav8r.com/Vision/IgnitionFAQ.html





IGNITION SYSTEM OVERVIEW:

Before we get into the list of components within your ignition system, it may be useful to explore the basics of the ignition design used on these bikes, as this knowledge may help you to better recognize, troubleshoot, and repair performance problems with your engine that you think may be due to these components.

The ignition system actually begins at the left end of your CRANKSHAFT, since the rotational position of the crankshaft determines the position of the pistons and of the camshafts. Obviously, since the purpose of the entire ignition system is to deliver a high-voltage spark at the plugs at exactly the proper instant----meaning, as the piston approaches Top Dead Center of the compression stroke----then the ignition system must "know" what the position of the crankshaft is in order to transfer that information (via electrical signals) to the major components: the PICK-UP COILS, then onto the TCI UNIT, to the IGNITION COILS, via the PLUG WIRES and through the PLUG CAPS and finally, onto the SPARK PLUGS.

But it all begins at the crankshaft, which has a flat metal ROTOR DISC bolted onto the left side snout, and which hides under the left side, round "Oil Pump Cover" (also called a "YICS" cover on YICS-equipped engines). This spinning rotor disc has a small magnet embedded within it's outer tip, and as that outer tip rotates past the fixed magnets within the PICK-UP COILS, the interaction of magnetic fields triggers a small voltage in the pick-up coil wires that lead to the TCI UNIT.

Note that since the rotor disc is fixed in position and spins along with the crankshaft, this rotor disc "knows" the position of the crankshaft at all times. And since the pick-up coils are bolted in place, and are thus stationary, whenever the spinning rotor passes by a fixed pick-up coil, and thus triggers it to send a voltage signal to the TCI, in this way the TCI unit thus also "knows" where the crankshaft is, rotationally-speaking, and thus where the pistons are in relation to Top Dead Center and when their spark plugs need to be fired.......

Also note that since there are only two pick-up coils for your four-cylinder engine, that each pick-up coil is actually providing the "firing signal" to the TCI unit for two different cylinders. In these engines, one pick-up coil is responsible for sending the signal to the TCI that eventually leads to the spark plugs firing off for cylinders #1 and #4 at the same time, and the other pick-up coil sends the message to the TCI unit to fire off spark plugs #2 and #3, again, at the same time.

Although this may seem odd at first, the mechanical arrangement within the engine of the crank throws, and thus the rods and pistons, as well as the camshaft timing, allow this situation to proceed without a problem; in fact, when one of these "paired" cylinders (for example, #1) is approaching Top Dead Center of it compression stroke----and thus is in need of a spark from its spark plug----it's "mated" cylinder (#4) is also approaching Top Dead Center, too.........but on its exhaust stroke.......and so even though cylinder #4 gets a spark at its spark plug, there's nothing in the cylinder to combust, and thus it's a "wasted" (yet harmless) spark that occurs in cylinder #4.

Obviously, the exact same situation occurs in the mated pair of cylinders #2 and #3.

In fact, the whole system is known as the "wasted spark" system, since one of the two sparks that always occur at the same time is "wasted" on a cylinder that is on its exhaust stroke............


Anyway, to continue our journey: when a pick-up coil is energized by the passing magnetic field of the spinning rotor disc, it send an electrical impulse signal to the TCI. Therefore, the TCI unit now also "knows" the position of the crankshaft (and thus of the piston). Using other sensor information.....primarily, the rotational speed (RPM's) of the engine......the computer chip in the TCI is then responsible for calculating exactly when to send a "message" to the proper IGNITION COIL to release it's energy to the proper cylinders. And note that we said "cylinders" (plural), since just like the pick-up coils, one ignition coil also sparks two cylinders at once (part of the same "wasted spark" method discussed above).


The ignition coils use a rather small (12V) input on their primary side to product a large (20,000V +) amount of electrical energy on their discharge (spark plug wires). When the TCI unit determines that "the time is right" for a particular coil to fire, it grounds that coil, which collapses the small magnetic field inside the primary side of the coil, which thus induces a large electrical field in the secondary (plug wires) side of the coil, which then rushes to ground (the plugs are grounded to the head). This electrical energy rushes down the non-resistive PLUG WIRES, through a resistor in the SPARK PLUG CAP, and finally jumps a small gap in the spark plug electrode on it's way to ground, and thus the spark occurs that fires the air-fuel mixture (in one of the two cylinders being sparked at the same time), things go boom, power gets produced, and you're on the way down the road........


By the way, the firing order for the 550 thru 900 engines is 1-3-4-2, with the two center pistons coming up while the two outer pistons are going down.



Now, What Could Go Wrong?:

Well, remember, all of these components are basically used to transmit knowledge of where the piston is within it's four-stroke cycle, and to be able to deliver a spark at the appropriate time to each cylinder to take advantage of the compressed air-fuel mixture in the cylinder (during the compression stroke). So it sometimes help to understand these component pieces as being mainly responsible for providing this stream of precise information, and the electrical-spark making (at the spark plug gap) as merely being the natural (according to the laws of physics and electricity) end-result of all this information handling and processing.


Now, unless your crankshaft was installed backwards (an impossibility, by the way!) or the camshaft was installed improperly (or if the camshaft drive chain has skipped a tooth----not a common occurrence, at all), then we can safely ignore all of these issues. And since the spinning rotor disc used to trigger the pick-up coils is bolted firmly in place at the end of the crankshaft, it can be ignored for troubleshooting issues.


But the pick-up coils themselves can go bad over time, and even though it's rare, it can happen. No pick-up coil signal out----along their wires to the TCI unit-----means no signal into the TCI unit, and the whole system breaks down. Which means the small wires from the pick-up coils to the TCI had better not be pinched, shorted to ground, or broken internally, or the message just won't get through.

NOTE: if you are having intermittent or difficult-to-diagnose ignition system problems, it may be that the wire leads from the pick-up coils (under the left round crank-end cover) have become pinched where they exit the cover and have worn through their insulation.


Of course, if the TCI unit itself is defective, then even if it is getting a signal from the pick-up coils, it is unable or unwilling to process the signal, or the signal gets processed incorrectly, and either the signal doesn't get sent to the ignition coils at the proper time, or doesn't get sent at all.


But even if the pick-up coils and the TCI are performing flawlessly---which they usually are----if the ignition coils are not getting enough voltage input to them, or, if they are not able to multiply the small incoming voltage into a much higher output voltage (that is, after all, their main purpose in life), then we have a problem.

Worn-out ignition coils, or coils that perform poorly or get cracks in their outer cases (and thus short out when moisture enters their internal shells), are a common cause of ignition system problems.


And even if the coils are performing properly, if the spark plug wires have an internal break, or an external break, and thus prevent the coil electrical output from reaching the plugs, then nothing good is going to come from all this activity.


Of course, the spark plug Resistor Caps also play a role. They have a tiny resistor embedded within them, and the purpose of the resistor is to provide, of course, electrical resistance to the flow of electrical energy. There are a couple of reasons why some resistance is necessary-----it helps eliminate electrically-generated Radio Frequency Interference "noise" (RFI), it provides a "cushion" against instantaneous electrical energy pulses (which is really hard on small, fragile electrical components, such as the capacitors and transistors in the TCI unit), and it "slows down" the passage of the electricity through the spark plug, thus providing a sort of "electrical Viagra" to spark at the plugs, allowing the electrical discharge (and thus the spark) to last a bit longer (instead of being instantaneous), thus promoting more-better and fuller combustion of the air-fuel mixture.

In fact, outside of the RFI suppression issues, it is the control of this "spark burn time" that is really the most critical issue, especially on bikes that are jetted a bit lean to begin with........as increased electrical resistance in the secondary circuit will increase the spark firing or "burn" time, and that longer burning spark assists in the more complete combustion of harder-to-fire lean fuel mixtures.


But those resistors in the spark plug caps........or in the spark plugs themselves, for those engines that use resistor plugs........do wear out over time. And when they do, their resistance increases, which means that they provide more resistance to the flow of electricity than is needed. So that 20,000 volts of electrical energy, instead of being discharged at the spark plug gap in a rather short (10 milliseconds) amount of time, gets "spread out" over a much longer period of time, and gets reduced in voltage, too. So when plug caps or plugs "go bad", they rarely fail to the point where no spark occurs at all, it's just that the electrical output is being "spread out" over such a long period of time that the energy being created in the spark plug gap is so low that it's not enough to fire the fuel mixture completely (or at all)........and that's what leads to hard starts and poor performance (and reduced gas mileage, too).

By the way, as you may have figured out by now, a spark plug that is contaminated ("fouled") by carbon or oil deposits, or one which has too large of a gap, fail to operate properly mainly because such situation can greatly increase the electrical resistance characteristics of such a plug.........and now you know why that's not a good thing.


A very good review of the issue of electrical resistance in ignition systems can be found at:

http://www.ultralightnews.com/enginetro ... dplugs.htm

Although the above article references the ignition systems in ultra-light aircraft, the same concepts apply to all ignition systems.


Okay, so that's your nickel tour, and although it's not as detailed as it could be, hopefully it's enough to get you started. A good voltmeter (also called an ohmmeter) is an invaluable friend when trying to track down ignition system problems, as you must make sure that the "information" between components is actually able to travel from Point A to Point B properly, and that the individual components are, electrically-speaking, able to process and transmit the electrical information properly.


A good companion write-up to this subject and some trouble-shooting guidelines can be found at:

http://xjbikes.com/Forums/viewtopic/t=21932.html

 

Age.
The Coils might be OK, ... but the Wires are breaking down.

The Wires are connected to the Plug Caps by a sharp, Self Tapping Screw.
Over time that Screw cuts into the Plug Wire Core and causes a point of resistance or and intermittent open short.

Trimming-back the Plug Wires --> IF there is enough slack, ... will restore the connection.

The Coils might be failing on you.
Coil Failure is on the list of things to expect after the 25 year mark.

 

Can a failed coil cause 1 dead cylinder?

 

No, but a failed spark plug wire, wire cap, or plug can.