Not hard to tell it is winter in this neck of the woods.

Doing some searching on this site I have found some information on the coils, but it is scattered and actually doesn't totally address the topic. I also checked the Electrical Trouble Shooting Guide to see what it has for information. I haven't been able to find any information from BMW concerning what values should be on a new coil. It is pretty clear that the coil should have a certain primary ohm value to work properly with the ignition system and not cause harm to the computer/ignition electronics.

What I have found so far ( based on the troubleshooting guide on this forum ) is that on a K100 2 valve the primary ohms value was quoted as 2.6 ohms. The secondary was 12.6 K Ohms. This doesn't say if it is off the black coil, the grey coil or the newest coil with the red tips, new or used coil. So what I'm asking is for anybody that is interested to please provide readings that you might have. In your reply give the year of bike, build date if you have it, 2 V or 4 V, colour of coil, secondary readings if you have them and new or used coil. I realize the coils/ignitions are totally different between the 2 & 4 valve versions, but it might help somebody dealing with an ignition issue on one of these bikes.

My example. 1986 K100RT Feb/86 build date, grey coils, primary 2.3 ohms on both coils. No secondary reading as I didn't remove the coils. Bike runs fine, no ignition issues, plugs fairly new, original plug wires, original coils, bike has approximately 19,000 miles/31000 kms.

Thanks for your input.

90,000 mile/145,000 km black Bosch coils with brown tips, 1987 K100RS, the resistance measures (between the 2 high voltage terminals) 12.61KΩ and the primary coil measures 2.6 Ω.

All the coils for 2 valve K100 should be the same and when measuring check that there is no electrical connection between the HT and primary coil and also either winding and the centre core. Even a few meg ohms means there is a break down of the insulation and failure is not far off.

Ignition Off. OK I went out to the garage today and measured the coils again. I unhooked the primary wires and found the same reading 2.3 ohms for both coils primaries. I hooked the primary wires back up and then unplugged the plug wires for each coil and measured the secondaries. Coil # 1 first coil in the pair - measured 10.14 K ohms, Coil # 2 measured 10.02 K ohms. Also it would be nice to see some readings from the older coils black or grey as maybe they where all like that and that is why they changed them out on the later K100 2V.

Both primaries and secondaries are a bit low from the 2.6 and 12.6 Ohms ( from troubleshooting guide ). So is that the gold standard for these coils and what the readings must be or is there a tolerance range. I see the coils shown with these readings is a 4v coil ( used in the troubleshooting guide on this forum ). I know the 2V & 4 V coils are not interchangeable. So is there a different primary & secondary resistance value with the 2 V coil.

As a general comment on coils, anything I've read generally says that the primaries should be between .5 - 3 ohms and secondaries  6-12,500 ohms. This depends on the ignition system of course, points/electronic etc.

I would think that a 3 ohm aftermarket coil would work fine. Any of the others from Dynatek have too low of ohm value. Any thoughts.

This was from a 2014 post. Bike was a 1989 K100 not sure if a 2 v or 4 v, but it looks like a 2 valve model.

" Thanks for that Rick
Just to confirm, I have the black coils with the brown towers. I think they are the original coils??

Leads (1) 5.32kΩ  (2) 5.06kΩ  (3) 5.17kΩ  (4) 5.36kΩ

Coil 1/4 - Prim: 2.2Ω  Sec: 11.92kΩ   (No earth leakage)

Coil 2/3 - Prim: 2.3Ω  Sec: 11.83kΩ   (No earth leakage) "

That's just about the same as my 86 coils. So it would seem there is a bit of a ohm resistance range for these coils to operate in.

cycleman wrote:Bike was a 1989 K100 not sure if a 2 v or 4 v, but it looks like a 2 valve model.

". . . Just to confirm, I have the black coils with the brown towers. I think they are the original coils??
Coil 1/4 - Prim: 2.2Ω  Sec: 11.92kΩ . . .  
Coil 2/3 - Prim: 2.3Ω  Sec: 11.83kΩ . . ."
 
That's just about the same as my 86 coils.

The '89 coils had values 94% of the benchmark values in the troubleshooting guide. Your '86 coils had values 80% of those benchmark values. Those wouldn't seem "just about the same." Nevertheless, the values in this extract from the arcane but ever-popular BMW Diagnose—Testing Instructions for K Models seem to suggest more possibilities.   



You'll be revered for your pioneering work with Dynatek coils.  Exactly which model were you considering?

Thanks for that and that should be in the resources folder for these bikes. That is an interesting chart and shows that my coil values both primary & secondary are spot on. The plug wires are also in the range of 4.8 K - 5.3 K so I think I'll spend my money elsewhere. I'm pretty sure the coil shown in the trouble shooting guide is for a 4v as the primary connectors are different than on my 2 v units, and that could explain the slight difference in resistance.

As to Dynatek coils. I was going to go with the 3 ohm version, green in colour, Model DC1-1. They make two versions with slightly different angles on where the plugs wires connect and this one likely would be the easier to adapt. The other version DC6-1 would be harder to adapt. The first version would be easier to use the existing stock mounting bracket, just drill out the old coil mounting rivets. The BMW coils are rubber mounted, so to use the Dynatek you would have to add a ground to the mounting bracket.

BMW couldn't have found a hotter place to mount the coils, I'm not sure what they were thinking.