Both coils gone at once?

Hey guys.

Since a couple of days ago the bike has been running ever so slightly funny when going slowish. Between about 2-4k revs it was a bit spluttery, hardly noticeable though.

Last night I had no spark on plugs #3 and #4. Then no spark on any of the plugs. Called out the breakdown service and me and the mechanic had a look. The left hand side came back and it started. Managed to get it running and up to 8000 revs then it just died again. No spark whatsoever again.

The mechanic started testing the connections and the bike is fine everywhere up to the coils. He said that it might be possible that both the coils have given up sort of one after another, but one thing he didn't check was the pickup coils. I'm gonna take a look at these today, but unfotunately I'm stuck 50 miles from home with a pretty limited set of tools. I'm pretty lucky I could crash at a mate's place nearby. Rang up the local Yamaha dealer and he said definitely check pickup coils first.

Is there anything else I could check while I'm trying to get the bike going?

 

I searched the forums for almost 20min looking for a link to a place that has XJ manuals online. I did this because I knew nothing about a '91 Maxim. After finally tracking the website down I couldn't find any mention of a '91. Did a Google search for a '91 Maxim. Nothing. Clicked your link, saw your description. Its an '84. -Please- change your sig. </RANT>

http://www.manualz.info/Manuals_pdf/YAM ... Manual.pdf

I would be very surprised if both coils died at the same time, and in the same way; they have lasted 24 years so far.

Cyl #1 and #4 are on one coil, #2 and #3 are on the other. If you had spark on #3 and #4 (half of each pair) I would think your coils were good. You said that at first you had lost it on 3 and 4, then all, then 1 and 2 came back. Very odd.

From what I understand (someone correct me if I'm wrong) Even though #1 and #4 fire at the same time, one of them is 'wasted', as that cyl is on the exhaust step of the cycle. So both sides of the coil fire at once. Why would the same 'side' of both coils go bad at the same time?

The printed version of the manual linked to above (PG 143, section 1) says that there are two pickups, one for cyl #1/#4 and #2/#3.

Figure out what 3 and 4 share, that 1 and 2 also share and you'll find your problem.

In fact, the only thing I can think of that they share is the TCI.

There is no way to test the TCI. But if the TCI is firing the coil at all then the coil should create spark on both #1 and #4 at the same time. That does bring it back to the coils.

Basic debug methods... check the simplest things first. Connections (loose/shorting wire somewhere?), plugs (Maybe your plugs are damaged?) , etc.

 

There is one way, and that's to swap it with another one or swap it to another bike. With that option unavailable it might at least be worth having a look IN IT. You can remove the screws and pop the cover and check for obvious signs of corrosion or broken/bad solder joints. I have read posts on here where folks successfully repaired a flaky TCI unit that way. You didn't mention whether or not it was wet, rainy, misty out when you experienced the issue. If so, it COULD actually be the coils...

 

Thanks for the replies guys.

It was raining when the bike broke down. Was raining all day yesterday. And Saturday, when the bike started to show the first signs of going wrong. Perhaps I'm wrong about it firing on one plug of each coil. What I am certain of is that at times it was running on two cylinders, which was why the recovery mechanic suspected the coil(s) were at fault. When the engine was running, there's not really a way I could see which cylinders were firing. It must have been running on all four cylinders to rev to 8000 rpm. Maybe it's loose wires. I too find it very odd that both coils have gone bad at once. Not really an option to change the TCI box at the moment. I think the only thing I can do is check for corroded/missing/bad connections and see if I can bring the bike to life to get me home.

There aren't any fuses in obscure places or anything I might have overlooked are there?

 

If there was a low voltage condition from the TCI, etc could that cause only 1/2 the coil to fire? I've never seen a schematic for a coil so I'm not sure if that is possible. If the two plug wires are connected to the same place on the coil then that wouldn't be the case.

 

The coils have a single, not-center-tapped, secondary winding. To complete a circuit, current in the secondary winding must go through one plug wire, across the gap of the plug (firing it), through ground, across the gap of the other plug (firing it), and up the other plug wire back to the coil.

The only ways one plug connected to a coil can fire without the other firing are:

1. If the gap of one plug is bridged

or

2. If one of the plug wires is shorted to ground somewhere

or, maybe

3. If the coil has an internal short to ground somewhere along its high tension winding

2 seems fairly unlikely to me, but would be somewhat obvious if it was the cause. 3 seems extremely unlikely to me, as such a failure would be more likely to kill the coil entirely.

 

If the gap was bridged, what would cause it to fail, self correct, and fail again? I eagerly away news of Pernig's progress.

 

The reason I asked about moisture is that the stock coils are known to develop microscopic cracks that can allow moisture to affect the coil's performance. The problem ususally comes to light (at least initially) in damp, rainy, foggy and/or wet conditions. There have been a few posts about this, RickCoMatic has a "fix" but I don't remember what exactly it was. I'll try to find it.

 

I would be interested in finding that fix also. My bike runs like crap in the rain.

 

It's not a Fix.
It's just "Open Case Surgery" to install New Plug Wires.

I think the Coils are going to be like the Fuse Panels.
There is something about the way they are made that deteriorates in all of them.
And, we are probably entering the early part of when that deterioration process causes these Coils to misfire.

I don't know how nor what they are made of. Earlier, a Member said there are PCB's inside the Coil Pack.
That's enough to keep me out.

Weather it's actually the Coils or the Wires is unknown.
But, I think there is enough "Coil Not Firing" Posting to not get to the bottom of it and find-out what it is that's beginning to be a headache for so many.

 

Thanks for all the input guys. Not got to the bottom of this yet but I'm home safe and the bike's with me. I have a multitester and a better selection of tools and no pressure when I look tomorrow which will benefit a lot.

The guy who I bought the bike off is an independent mechanic/dealer/racer. He gave me and the bike a ride home in his van. We tried to get it jump started as the battery is nigh on flat now, and for a moment it wanted to start when he was playing with the killswitch. I've seen this on single cylinder bikes before where the killswitch kind of lets the bike run if you bump or kickstart but not let the electric starter start when there is damp in the switch itself and have cured it by dismantling, WD40 and reassembling. I'm going to pore over wiring diagrams tonight.

The fact that I've got it firing and I've had it running points to corroded or dodgy wiring to me. Cracks in the coil could be a possibility, but it seems very unlikely that the coils would both blow within a minute of each other and then start working again intermittently.

So tonight I'm going to look at the wiring configuration for the ignition system, particularly around coils/pickup coils/killswitch areas, stick the TCI indoors to dry and whack the battery on charge. We'll see what tomorrow brings.

Thanks for your input everyone, you've got my brain going and are keeping me going . I will keep you updated.

 

SQLGuy: would you run that by me again, something there just don't sound right

 

Short story: when one of these coils fires, the completed HV circuit goes through BOTH plugs - they are fired in series, not in parallel. So, if you were to disconnect the plug cap from #1, #4 would stop firing too.

However, if a plug's gap is bridged (as happened to me the other day) that completes the circuit for that side, so the other plug will still fire.

Here's the full circuit:

One end of HV Coil - plug wire - 5K resistor (in plug cap) - plug center electrode - arc - plug ground electrode - ground (through the cylinder head) - other plug ground electrode - arc - other plug center electrode - 5K resistor (in plug cap) - plug wire - other end of HV coil

The energy in the HV winding of the coil comes from the collapsing magnetic field when the TCI module turns off the driver transitor that pulls the coil primary winding to ground.

This reminds me, though: those plug caps are a bit problematic. Most bike shops stock the NGK LB05F (for #1 and #4) and YB05F (for #2 and #3) resistor caps, which are good replacements. They run $4 or so a piece, and screw on. You can't easily replace the wires, but you can replace the caps.

 

I don't think the Ignition Coils fire that way.
That does not sound right at all.

 

Care to elaborate on that? My background is, admittedly, not a mechanic's one. I majored in computer engineering and worked for many years as an electronics tech and avionics tech (anything from component level repair of computers and monitors to black boxes on F-15's).

On my bike I no-longer use the stock coils anyway, I've instead replaced them with GM Saturn coils that are being directly driven by Microsquirt:





Nonetheless, I'm quite sure about the electrical theory and the current flow in these devices. The schematics from Haynes manual also backs this up. Which part(s) of my explanation don't ring right with you?

Cheers,
Paul

 

plugs from each coil are in parallel, if #1 wire is pulled off #4 fires alone
the ground connection for the coil secondary is the mounting bolts, that's why the outer laminations are stainless, they need a good connection to the frame/ground

 

The Series part.
That part.
Losing #-1 causing #-4 to misfire.
That must be incorrect.

If that were true; then wouldn't pulling a Plug Cap off a running engine to detect a Bearing Knock cause the other cylinder to misfire?
It doesn't.
The Engine will run on 3 Cylinders just fine.
As many of us know, who have had to deal with only one cylinder misfiring.

 

OK guys, then, explain me this:

I still have my old coils, and they worked fine before I removed them. I just measured one with my meter (a Fluke 89IV). From one plug cap to the other measures 22K Ohms. From either plug cap to the lamination core, or to either of the low voltage contacts, measures open.

If there is no measurable connection between the high tension side and the core, how is the ground return for the spark supposed to be through the core?

Look at the schematic in the Haynes manual: there's no electrical connection between the HV side and anything else.

Pulling a cap off a running engine doens't cause the other cylinder to misfire; it causes the other cylinder to not fire at all. The engine will run on three cylinders, like with a bridged plug gap, and it will also run on two cylinders. I wouldn't call either of these conditions "fine", but it will run.

-------------------------------

I just tried a simple little experiment, that you can try as well: pull one cap and put a spare plug in it. Let that plug sit against the block and crank the engine. You should see a strong spark there. Now pull the corresponding other cap and leave it as far from everything as possible. Crank the engine again... you may see some very weak spark on the loose plug (since it's hard to insulate 40KV) but you certainly won't see the strong spark you had when the other cap was connected; you may see no spark at all. If the plugs were firing in parallel, removing the other cap would actually make the first spark stronger.

 

You should remove the plug caps when testing the coils.

 

while i ponder that tell me how the voltage jumps one gap goes through the engine and out the other plug without electrocuting you, since the head is at
coil secondary potential
is the engine isolated from ground?
once the path to ground is made the circuit is complete, it can't come back out and keep going