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Coil connection question

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Roger_67MG

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I just received my new Lucas BLB101 coil for my '67 MG Midget 1275 with positive ground. The old coil and wiring diagrams are causing me a bit of confusion.

The old coil, which turns out to be Bosch, has one terminal 1 and -, the other terminal is + and 15. The diagram notes SW and maybe CB (it is hard to read) for the terminals. The new Lucas is marked + and -.

The diagram traces SW via a white wire to eventually the ignition switch (with a routing through the tach). The other wire connected to "CB" (or whatever it is) go via white/black wire to the distributor.

But, is SW + or -? Is "CB" the other, or is it a duck?

Does anybody know??
 
SW is switch and CB is contact Breaker.

For a positive ground car CB is positive and SW is negative.
 
Re: Coil connection question - Answer found

Gang,

I found the answer in Restoring Sprites & Midgets by <span style="font-style: italic">Grahame Bristow</span>. On his diagrams on p.89 he shows that on the positive ground cars the coil's connections are SW/+ and the other is CB/-.

And now... we will try to start the car!

Well, maybe tomorrow.
 
Re: Coil connection question - Answer found

Ye of little faith, go for it now!

Mark
 
Re: Coil connection question - Answer found

And after trying to start the car we have.... Failure! This time we have discovered that the oil filter cup gasket was installed incorrectly and was cut part way through.

As soon as the oil system was pressurized there was a nice blow-out and oil flowed down the outside of the cup. A new gasket is on the way. When it arrives we will try again.

If it isn't one thing it is another.
 
ChrisS said:
SW is switch and CB is contact Breaker.

For a positive ground car CB is positive and SW is negative.
Click to expand...

Oh, oh.... the diagram could be wrong, eh?
 
Whenever I have seen coils so marked, they were originally designed for positive ground cars. As such SW meant "switch" and received the white wire. The CB connection was for the "contact breaker" (a.k.a. 'points'). When the polarity of the car was switched to negative ground the two coil connections change also.

I doubt they changed the coil markings when reproducing a "classic" piece. As such, consider the polarity of your car: Positive ground, SW = white wire, CB = white/black wire. Negative ground, SW=white/black wire, CB = white wire.
 
dklawson said:
Whenever I have seen coils so marked, they were originally designed for positive ground cars. As such SW meant "switch" and received the white wire. The CB connection was for the "contact breaker" (a.k.a. 'points'). When the polarity of the car was switched to negative ground the two coil connections change also.

I doubt they changed the coil markings when reproducing a "classic" piece. As such, consider the polarity of your car: Positive ground, SW = white wire, CB = white/black wire. Negative ground, SW=white/black wire, CB = white wire.
Click to expand...

Yep, this sound like the way to go. All I have to do is get the oil filter back on and then I will take a crack at this.

I have been tracing wires and checking connections. I discovered some problems that seem to be mostly from the car being wired wrong when I got it. But as I move along it gets closer and closer to being correct.

I think... :wink:
 
At the risk of starting up a long discourse, which wire goes where on the coil is not a big deal. It will work fine either way.
 
jlaird said:
At the risk of starting up a long discourse, which wire goes where on the coil is not a big deal. It will work fine either way.
Click to expand...

While I too, when trouble shooting, have had success with hooking up a coil "backwards", and driving the car, after an examination of the related electrical drawings, found that when hooked up backwards, the coil only gives half the voltage (or thereabouts) for which it was designed.

Perhaps a better designation than POSITIVE and NEGATIVE, SW and CB would be HOT and GROUND?
 
Hooked up backwards it probably gives more than half but probably not full spark. Possibly worse problems are that the coil will overheat and the plugs will burn or erode much faster. Backwards makes the spark jump from ground to the sparkplug center electrode instead of the other way around. This takes a tiny bit of metal with it from the "adjustable" piece, which is at ground in the engine. The center electrode is specially hardened to minimize the problem when the spark jumps the right direction. I suspect it also has a detrimental effect on the points too. So unless you want to spend extra time and money replacing plugs, coils and maybe points, it is a good idea to be sure the coil is wired for proper polarity.

Actually, coils should be marked + and -. That way there could be no confusion.
 
As stated above, the coil polarity/connections do matter if you want "normal" component life and an acceptable spark.

I got flamed rather badly on the A-H section of this board when I suggested it wasn't important. I was directed to several sources that confirmed the importance of the coil connections. Yes, the coil will work either way... but you pay a price for connecting it wrong.
 
This is all very interesting. Another question; What about the dashboard components if the coil is connected incorrectly? I notice that the tach is between the switch and the coil. (Of course, that is on cars with an electronic tach, not the mechanical one.) Can there be any trouble there? :confuse:
 
Your first concern with the tach is polarity. Make sure you have the right "ground" tach for your car... or convert it (there are links on the web showing which components to reverse).

With the RVI tachs the ignition will work regardless of which direction the wires to the tach are connected (as long as the wires are continuous). However, tach troubleshooting guides often mention swapping the two white wires on the tach if the tach appears unstable or is reading incorrectly. The later RVC tachs are voltage pulse sensing and connect to the coil with a single wire on coil (-). Coil (-) is the only place a voltage sensing tach connects when a standard ignition is fitted.
 
dklawson said:
Your first concern with the tach is polarity. Make sure you have the right "ground" tach for your car... or convert it (there are links on the web showing which components to reverse).

With the RVI tachs the ignition will work regardless of which direction the wires to the tach are connected (as long as the wires are continuous). However, tach troubleshooting guides often mention swapping the two white wires on the tach if the tach appears unstable or is reading incorrectly. The later RVC tachs are voltage pulse sensing and connect to the coil with a single wire on coil (-). Coil (-) is the only place a voltage sensing tach connects when a standard ignition is fitted.
Click to expand...

OK, this makes sense. I am following the original wiring diagrams for the car, positive ground, with the exception of the coil, which wasn't clear on the issue. But that is sorted out now.

Minutes ago I received the new started solenoid and some other odds and ends, so if all goes well I will see about getting this thing started. Of course, this has been going on for a couple of weeks. I've been tracing wires, checking connections, and testing voltages. One step at a time!
 
:crazy:
I am about ready to go nuts. The new coil, Lucas, on my positive ground '67 is only marked + and -, not SW and CB.

So..... based on what everyone has been saying, and looking at some more diagrams, I believe that the white wire should go to the - terminal and the wb wire should go to the + terminal.

How does that sound, everybody?

One of the books I have shows early coils marked SW and CB with later coils + and -. Needless to say, I look forward to any confirmation you might be able to provide.


One note: In an earlier message I referenced the ignition system drawings in Grahame Bristow's book. It turns out that there is a rather unfortunate error on those drawings. All 8 drawing for the range of cars inclusive of the MK 1 Sprites to the 1500 Midgets shows the battery positive ground. No wonder I was having trouble sorting out details.
 
You are correct. If you have a positive ground car, the coil (+) terminal should be connected to the points (white/black wire). This gives the coil positive terminal a path to the positive side of the battery through the points. Obviously this leaves the white wire which would connect to the coil (-) terminal.
 
Right. The + side of the coil goes to the positive side of the battery and the - goes to the negative. That's why it makes sense to mark the coils that way.
 
dklawson said:
You are correct. If you have a positive ground car, the coil (+) terminal should be connected to the points (white/black wire). This gives the coil positive terminal a path to the positive side of the battery through the points. Obviously this leaves the white wire which would connect to the coil (-) terminal.
Click to expand...

Ok we are really on the trail now!!
 
Ok, right. The problem is tracing it on the diagram, but this puts 2and 2 together.

Geeze, and to think I used to design dimming systems. :cheers:
 
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