coil question revisited

[edheiser]

I was reading old posts early this morning and reread this one of mine
https://forums.bmwmoa.org/showthread.php?89368-Now-no-spark!-Coil-tests-bad
The bike still runs fine, but my question remains a mystery to me. Perhaps someone now knows the answer, or I missed the answer (possible) in the post.
Where do you measure to get 7-8K ohms on a coil like this.
Or perhaps the answer is nowhere and the manual is wrong.
Just curious.

 

[happy wanderer]

I was reading old posts early this morning and reread this one of mine
https://forums.bmwmoa.org/showthread.php?89368-Now-no-spark!-Coil-tests-bad
The bike still runs fine, but my question remains a mystery to me. Perhaps someone now knows the answer, or I missed the answer (possible) in the post.
Where do you measure to get 7-8K ohms on a coil like this.
Or perhaps the answer is nowhere and the manual is wrong.
Just curious.

If you are talking about the RTP coil I have one here and had the same problem. I measured zero at the secondary contacts. This could not be right on a bike that was running when the coil was taken off. I decided to melt the solder and get it out of the RF case it was in. What I found was a coil with the same part number as my normal RT coil but it was NOT the same. There was a green ground wire coming out of the epoxy that connects to the RF metal case and also the secondary terminals again measured zero confirming that my initial measurement from outside was correct on a known good coil. As a point of intrerst my normal RT coil measures about 7.8K ohms which is spec.

You should be aware that the BMW Factory manual states 15K ohms and this is WRONG.

So the only thing I can come up with is the output terminals on the RTP coils are somehow insulated via an air gap and thus the secondary cannot be measured in the usual way. That means that testing it can only be done on a coil test jig or on the bike!

Since the RTP coil requires very expensive connectors on the high voltage wires and I got it as a spare thinking it would plug right in... well now it's just garage art sitting on my bench as a curiosity. I soldered it all back together in case someone needs one at some point but it's useless to me. I ain't buying RTP wires just to use an RTP coil that is no better than the stock one for my bike.

Hope this helps. I was really mystified too. What that happens my default action is to start taking things apart for better of for worse. I can't stand not knowing...

 

[PGlaves]

If you are talking about the RTP coil I have one here and had the same problem. I measured zero at the secondary contacts. This could not be right on a bike that was running when the coil was taken off. I decided to melt the solder and get it out of the RF case it was in. What I found was a coil with the same part number as my normal RT coil but it was NOT the same. There was a green ground wire coming out of the epoxy that connects to the RF metal case and also the secondary terminals again measured zero confirming that my initial measurement from outside was correct on a known good coil. As a point of intrerst my normal RT coil measures about 7.8K ohms which is spec.

You should be aware that the BMW Factory manual states 15K ohms and this is WRONG.

So the only thing I can come up with is the output terminals on the RTP coils are somehow insulated via an air gap and thus the secondary cannot be measured in the usual way. That means that testing it can only be done on a coil test jig or on the bike!

Since the RTP coil requires very expensive connectors on the high voltage wires and I got it as a spare thinking it would plug right in... well now it's just garage art sitting on my bench as a curiosity. I soldered it all back together in case someone needs one at some point but it's useless to me. I ain't buying RTP wires just to use an RTP coil that is no better than the stock one for my bike.

Hope this helps. I was really mystified too. What that happens my default action is to start taking things apart for better of for worse. I can't stand not knowing...

There probably is an air gap in the coil. Maybe shielded and intended to suppress radio interference. I am reminded that K75s have an air gap in the plug wire connectors so they cannot be read for resistance as they will always read open unless something gets melted.

 

[edheiser]

Hi again

If you are talking about the RTP coil I have one here and had the same problem. I measured zero at the secondary contacts. This could not be right on a bike that was running when the coil was taken off. I decided to melt the solder and get it out of the RF case it was in. What I found was a coil with the same part number as my normal RT coil but it was NOT the same. There was a green ground wire coming out of the epoxy that connects to the RF metal case and also the secondary terminals again measured zero confirming that my initial measurement from outside was correct on a known good coil. As a point of intrerst my normal RT coil measures about 7.8K ohms which is spec.

You should be aware that the BMW Factory manual states 15K ohms and this is WRONG.

So the only thing I can come up with is the output terminals on the RTP coils are somehow insulated via an air gap and thus the secondary cannot be measured in the usual way. That means that testing it can only be done on a coil test jig or on the bike!

Since the RTP coil requires very expensive connectors on the high voltage wires and I got it as a spare thinking it would plug right in... well now it's just garage art sitting on my bench as a curiosity. I soldered it all back together in case someone needs one at some point but it's useless to me. I ain't buying RTP wires just to use an RTP coil that is no better than the stock one for my bike.

Hope this helps. I was really mystified too. What that happens my default action is to start taking things apart for better of for worse. I can't stand not knowing...

I remember you from last June and you having a spare RTP coil that had no OHMs reading either. I never heard of an air gap in the secondary, but it makes sense this must be the case. Funny though, my Ebay replacement non shielded coil had no reading either,but works fine. BMW sure does do it "their way". I'll just use the old "If it ain't broke, don't fix it" method.

 

[edheiser]

Hmmm

There probably is an air gap in the coil. Maybe shielded and intended to suppress radio interference. I am reminded that K75s have an air gap in the plug wire connectors so they cannot be read for resistance as they will always read open unless something gets melted.

So why would BMW give an OHMs reading for a coil that can only be tested with 12v present. OHMs can only be read without current present, at least on my cheap multimeter. Just a moot point I guess.