1995 BMW R100RT Coil Problem (I think)

[Scarletswirl]

My 95 R100RT is running on one cylinder. I have accomplished the following troubleshooting so far:
1. Checked that fuel is flowing to both carbs. It is.
2. Installed new spark plug wires, new spark plugs. No change.
3. With both spark plug wires connected and spark plugs out, it appears that only one plug is firing, the Left.
4. With both spark plugs installed, shorted Right plug, no change. Shorted Left plug, engine quit. (Confirms step 3)
5. Swapped the leads at the coil. and repeated step 4. Now engine quits when Right plug is shorted.
6. Checked resistance across coil secondaries and got 13 Kohm. Not in spec according to Hayes, but identical to a brand new Dyna Tech coil.

Before I replace the coil, has anyone seen this before, where only ONE side of the coil goes bad? Does the fact that the resistance value across the secondary is not in spec on the new Dana Tech coil mean it's bad (even if it's brand new)?

One last question. It appears that there are wires from every harness on the bike that come together right above the coil. Does anyone know how to disconnect them, or can the coil be replace without disconnecting all those wires?

Any hint appreciated

Marc

 

[20774]

What is the resistance from one spark plug lead to the other lead?

 

[Scarletswirl]

What is the resistance from one spark plug lead to the other lead?

22.78 Kohms measured with a Fluke 87 V. When you subtract the 10 Kohm from the wires, that means approx 12-13 Kohm across the secondaries. This is an OEM coil, in fact probably the one that came with the bike.

Additional info:
I checked the Dynatek specs and 13.5 K across the secondaries is correct for the 1.5 ohm primary model, which is the one I bought as a spare some time ago. So this should be a viable substitute if it turns out to be the coil, as I suspect.
I'm using a Wedgetail ignition system. I've had it for over a year and it has given me zero problems.
According to snowbum, when swapping spark plugs wires at the coil transfers the problem, it is either a coil or (more rarely) a module problem. Thing is, the module is fairly new, the coil is nearly 30 years old...

 

[20774]

That 22.78K ohms sounds pretty darn good! I'd be looking elsewhere.

 

[88bmwjeff]

If I’m reading this correctly, you have a spare coil you purchased a while back. If that’s the case, I would swap coils. It’s only time and it would tell you whether or not the coil is an issue.

 

[oldcamper]

If I am not mistaken, the coils are wired in series and BOTH spark plugs should “fire” at the same time, with a waste spark on the cylinder not needing it. Simple and maybe good for reduced emissions too.

OEM coils on at least the mid to late 80’s airheads sometimes developed cracks, and would short out internally when the bike was ridden in the rain or after a good washing. I’m wondering if this happened to your later model airhead coils.

I’d swap the coils out next as long as you have the spare set already.

 

[Scarletswirl]

Jeff, I've come around to your way of thinking even though I acknowledge Kurt's comment that the resistance across the secondaries sounds normal. My hesitation about the swap was that I wasn't sure the Dynatek coil I have on hand is compatible with the Wedgetail, but I just found out that it is. Today I'm going to try and do the swap.

Oldcamper, I'm aware of the "cracking" problem in the early two-tower coils, but according to everything I've read so far, the problem was solved around 1989. My bike is a '95, so it should not be affected. As far as riding in the rain, the answer is no, and the garage is air conditioned. On the other hand, humidity was very high yesterday when the problem first surfaced, so I can't rule it out. Even after removing the tank the coil is pretty hidden, so I won't be able to inspect it properly until I remove it.

As an additional data point, when I run the stater with the plugs out but connected, I can see that one plug makes a nice fat spark while the other is much weaker if it sparks at all. If I swap the plug wires at the coil, the problem transfers to the other side. This just confirms that only one coil outlet is working properly even though the resistance across the secondaries is apparently normal.

Thanks to all who contributed. I'll report on the results of the coil swap, keep your fingers crossed
Marc

 

[Scarletswirl]

Finally replaced the coil, started her up and noticed the idle being low. However, it responds to both carburetor idle screws, so it's definitely running on both cylinders, with smooth acceleration. Tomorrow I'll take it out for a decent ride, then reset the idle and synch the carbs. But, all in all, it seems that the problem is solved.

Now a few pieces of info for future reference

1. If your bike has a fairing, like an RT or RS, the job is a real knuckleduster. A few things that help:
a. You may have to remove the ignition module, this will give you a little more room to maneuver. If you're using a Wedgetail ignition, you'll probably have to remove the voltage regulator first.
b. Get yourself one of those ratcheting "close quarters" drivers, I would have gone bonkers without one.

2. The late airhead dual tower coil (0.7 ohm primary) is still available at EuroMotoElectrics. It also costs over $200, but it is a direct replacement: two screws to remove it, two screws to reinstall it. (plus a couple of electrical connectors). Easy! In the picture below, the original coil is above, the Dynatek below. Both coils are upside-down and the Dynatek is already bolted to the bracket. Note that the outlets are side-by-side in the original and front-and-back on the Dynatek.

3. Dynatek makes 0.7, 1.5 and 3 ohm (primary) coils with identical form factors that are compatible with late model ignition modules as well as the Wedgetail ignition. Rick at Motorrad Electrik recommends the 0.7 phm model. He also recommends the Enduralast version of the same coils, which are apparently produced in the same factory and to the same specs but cost about $20 less.

4. Dynatek and Enduralast coils are not exactly a direct replacement. First off, they need a bracket (more on that later). Second, they have two screws in lieu of the spade connectors (the bottom screw is visible in the photo). This means you have to install a couple of quick disconnect adaptors or remove the female quick disconnects that come with the bike and install ring connectors instead. The first method is less work and more practical, but make sure the adaptors are oriented correctly or they will interfere with the bracket or other surrounding stuff. This may sound weird, but once you have eyeballs on the parts it will be obvious.

5. There are two types of bracket, and they both cost about $10 each. The older one is made by Dynatek (I think) and the newer one by Enduralast. I read that the newer one is better because it has longer "legs" and minimizes interference with the contacts. I had the old one and it worked ok. The hardware provided is SAE, so I replaced it with metric. For the anally retentive amongst us: 2 M5 x 16 mm and 2 M5 nuts (bracket to frame) and 2 M6 x 22 to 25mm with 2 M6 locknuts (coil to bracket). I tried to mount the coil to the bracket first, and then install the whole thing as a unit, but it's hopeless. So I had to mount the bracket to the frame, then the coil to the bracket. It's a tight squeeze and it's difficult to get a good grip on the M6 mounting bolts. When you're done mounting it, you still have to reconnect the primary and spark plug wires. The rear spark plug outlet comes out of the coil very close to the lower fairing mounting bracket, so it's a challenge to get the wire in nice and tight.

6. IMPORTANT: when done, check the resistance from one spark plug contact to the other to verify it's not an open or ultra-high resistance circuit. Ideally, it should read 10Kohm plus the coil secondary spec, which in my case was 13.5 Kohm. in fact, the value I got was very close to 23.5 Kohm.

Again, thanks to all who responded
Marc

 

[88bmwjeff]

Glad to hear you found the problem. BTDT, with regards to the coil on my 88 RT-it’s a royal PITA with fairing on.

 

[AntonLargiader]

I just use the stock Bosch dual-output coil. It fits, it works, it lasts. Yeah it's a little more money but no one has ever complained.

 

[oldcamper]

Thanks for the photos. Good to hear you found the problem.
Did you notice any fine surface cracking on the old coil?

 

[oldcamper]

I think lower primary resistance coils deliver a hotter secondary spark but the additional current is hard on mechanical points and are best used with electronic ignition systems.

 

[Scarletswirl]

Anton, agree. But the Dynatek was what I had on hand...
Oldcamper, no cracks. I had the later design coil that was not prone to cracking like the early ones. My research indicates that the problem is most likely in a quasi-undetectable break in the secondary coil that affects one side only. Sounds impossible, but I was assured that it happens. When you run the microscopic current of a voltmeter through it, it passes is without problem, but it can't pass the full current necessary to generate a good spark. As proof, I offer the fact that often times, when I ran the starter with the plug out (but connected), it would spark a few times, then quit. Even when it sparked, it would be visibly weaker than the fat blue spark on the other side.
And yes, the lower resistance coil (0.5-0.7 ohm) would have been better with my setup.
Next time I have a reason to remove the fuel tank I'll think about replacing the coil (again)