Did wrongly connected wires fry my new coil?
 

EDIT: The answer is NO!

If I connect the coil wires back-to-front, will this fry the high voltage winding in my coil?
Car is a 1986 930 with the 6 pin CDI.

I've had intermittent ignition problems lately that eventually resulted in total failure to start (fuel OK, but no spark).
I checked all the usual stuff (connections, green wire, power at CDI, grounds, output from distributor sensor, wires to coil, cap, rotor, HV leads, etc) and couldn't find anything wrong.
Suspicion fell on my PermaTune CDI. It passed all the bench tests as per their website except for capacitance check across terminals 1 and 5 (31/1 and 15) so I assumed this CDI was the problem so I replaced it with a known good Bosch CDI.

Still no start, so I started looking at the coil. It had OK resistance across the LV terminals but was open circuit between the HV centre terminal and both "+" and "-" terminals.
Ah ha! Off to local parts shop for new coil ... MSD Blaster 2 (#8202).

Hooked everything up and ... no start. :confused:

I wasn't 100% sure that I had the coil wires connected correctly, so I switched them around to see if it made a difference. No joy.
I just checked the MSD coil and it is OK across LV terminals but is also an open circuit between the centre terminal and both "+" and "-" terminals. :(
This indicates a blown HV winding.

I am reasonably confident that my original problem was a dud coil.
I also know that at one stage, I had the two coil wires connected wrongly on the new coil.
So the ONLY question is, did I fry my new coil when I had the LV wires connected back to front?

As far as the LV connections are concerned, I have now got doubts about which is "+" and which is "-".
When I first fitted the new MSD coil, I just assumed that the brown/white wire was ground so I put it on the "-" terminal.
Since then, I checked an old photo from when I rebuilt this engine a couple of years ago and the original connections before I dismantled it clearly show the brown/white wire on "+" terminal. :confused:

So before I try this with another coil, does "A" on the wiring diagram on coil = "+" or "-" ?

http://forums.pelicanparts.com/uploa...1329111258.jpg

On my '78 White goes to Positive, Brown with white trace goes to negative on the coil.
From your drawing white goes to positive.
Lookng again at your drawing, the Brown with white also goes to ground through the common point at 31/1, also indicating the White goes to Positive on the coil.
A=Positive

If you connect a coil backwards (polarity) it might damage the CDI but not the coil.
Coils are meant tobe fed by the positive voltage of the system (except for British cars where all bets are off...LOL)
Also...with the windings inside some coils....you might not get any voltage at all because you are shorting the positive voltage to ground.
Some coils have no internal connection to ground...so they might give you high voltage when wired backwards.
Hope this helps.
Bob

"If you connect a coil backwards (polarity) it might damage the CDI but not the coil"

Not correct! It really doesn't matter how the the wires are connected.
Most likely the secondary resistance wasn't measured correctly.

"Some coils have no internal connection to ground...so they might give you high voltage when wired backwards."

Also not correct!

There's always a connection between the primary and secondary or arcing would continuously occur
internally to the coil.

Thanks Loren, I was hoping you might comment.
I will recheck my measurements again tonight (for the umpteenth time!).

I can't remember the exact numbers right now, but I think it was a resistance of about 1.8 ohms between A and 4 for the MSD coil and about 1.2 for the original (not Bosch), but open circuit between 4 and both A and B on both coils.
Hence, it seems that the HV windings are kaput.

Is there any type of fault elsewhere in the system that could the HV windings to blow?

Let's get the nomenclature correct.

A, in CDI-speak, stands for "ACHTUNG" and this is the 460V pulse from the CDI.

4 is the secondary

B is the ground, in your case, a reference ground from the CDI box itself. Bosch does this to make sure the ground doesn't float, they want a good connection back to the CDI, and then the CDI itself is grounded.

All right. Primary measurement between A and B = _______?
Secondary measurement between B and 4 = _______?

Check here for coil data; Diagnostic,
under Typical Ignition Coil Values.

Additional coil data (MSD, Perma-Tune & small Bosch silver) has been added
recently.

Hate to burst the bubble Loren...but I have had coils that the neg post was directly connected to the case of the coil.
In that condition...if you put the output of the CDI or even 12V to that terminal...you would have a direct short to ground (providing the coil was mounted to a grounding surface).
Bob

"Hate to burst the bubble Loren...but I have had coils that the neg post was directly connected to the case of the coil."

Right, so it was a bad coil and NOT a good coil that the comment was in reference to!

New MSD 8202
Primary measurement between A and B = 0.6 ohms (1.4 - 0.8 lead resistance)
Secondary measurement between B and 4 = 5520 ohms

Old "Flame Thrower"
Primary measurement between A and B = 1.4 ohms (2.2 - 0.8 lead resistance)
Secondary measurement between B and 4 = 8750 ohms

The pics below show the story.
Loren hit the nail on the head when he suggested "Most likely the secondary resistance wasn't measured correctly."
I did my first measurements on the 200 ohm scale when I should have been using something higher, like 20K. I am still coming to grips with this digital meter after my trusty analog multimeter died last year.
I presume I have read it correctly this time.

Well at least I have now satisfied myself that the coils are OK :rolleyes:
Looks like it's back to the garage for another round of testing again.
Loren, thanks for the link to your company website - very enlightening - and thanks for persevering with electronic knuckleheads like me, it is appreciated. I prefer a wrench any day than having to deal with this black-art stuff!

And of course the answer to the topic question is NO.

http://forums.pelicanparts.com/uploa...1329222454.jpg

http://forums.pelicanparts.com/uploa...1329222477.jpg

http://forums.pelicanparts.com/uploa...1329224932.jpg

http://forums.pelicanparts.com/uploa...1329222561.jpg

http://forums.pelicanparts.com/uploa...1329224916.jpg

Great that the coils have been eliminated as a problem source.

Now to determine the real problem. Since you have most likely checked all the wiring
and connections, then that leaves only power and the distributor signal. Use a test light
to check for power, which is a little better than a meter because it loads the power
source. Next, measure the distributor signal wire first with an ohm meter (~ 600 ohms).
Then use AC volts to measure the actual signal voltage (~ 100 - 200mvAC). If both of
those are O.K., then the CDI unit is bad.

Bill, many thanks for posting the pictures of testing the coil with the multimeter. I'm a total novice to using the meter but your clear pictures have helped me rule out the coil in my no start issues. Next time your in Brissy, I owe you a beer.

Bill:

I led you to the information on how to determine if your coil was good or bad several days ago. Why the need to start another thread (same as your first) and ask the same questions. I see that others chimed in and gave good answers as well.

Not intending to screw anyone around - I think it was just a timing issue with the two threads. I started one on 930 forum but didn't have any response initially so started one here where there is more traffic. Then I started getting responses to both threads which sort of overlapped. A few days earlier, I thought my problem was with CDI, so there are threads related to that as well and not surprisingly, some reponses to those threads also cover similar territory to the coil threads.

Some great info provided by several posters, so I guess you know what I'll be doing on the weekend!
Your time and shared knowledge is very much appreciated.

I still haven't figured out why there's no spark. :(
Here's where I am at ...

• both of my CDI units (Bosch and Permatune ) are OK, they test perfectly on a shop's test rig
• I have two coils and both have correct resistance on primary and secondary
• the distributor signal is about 0.15VAC when cranking and up to 1VAC on the bench connected to a drill
• power to the CDI is good at all key positions - tested with a 55w bulb
• all wires check out OK for continuity and earth leakage
• a replacement harness for the coil wires, distributor green wire, power and earth didn't make any difference
• still no spark at plugs or coil center terminal

After checking everything umpteen times I've run out of ideas. :confused:

what about the engine ground? What about the plugs?

"both of my CDI units (Bosch and Permatune ) are OK, they test perfectly on a shop's test rig"

You verified this, right? And it they where tested with YOUR coils, right?

"I have two coils and both have correct resistance on primary and secondary"

That type of test is not an absolute test for a coil, i.e. They still could be bad.

YOU need to setup a bench test with a CDI unit and one of YOUR coils AND your distributor.

This sounds like an interesting project for Sunday. I'll build a rig with CDI, coil and dizzy and report back.

I understand your frustration. It sounds like you have done a ton of testing thus far.

Did you use my ignition system troubleshooting spreadsheet?? I'll send it to you if needed. IM me if you need it.

Did you test your distributor rotor?? The rotor has a resistor epoxied into it. Measured the resistance to ensure that it has continuity. I assume that you might be getting voltage from the coil but not to the plugs since the rotor has failed. I hope this is your issue and that this fixes your problem.

My opinion is that if your CDI tests good on the bench then it's good. If your green wire is good and the pulse coil in the distributor is good.. Then the ignition coil is the next likely culprit. I don't know why a coil that tests good by measuring resistance wouldn't fire.. Maybe Loren can elaborate on this???

"Did you test your distributor rotor??"

The test should be just at the coil output for a spark, and not at the distributor yet.

A bench test is simple:

1. Get a battery and connect +12 volts to the #15 (center pin when unit with fins
down) top row.
2. Ground unit case to the battery neg pin.
3. Ground one of the coil's terminals to the unit case.
4. Connect the other coil pin to the top right pin (fins down).
5. Connect the distributor to the left upper & lower pins.
6. Place the coil wire about 10-15mm from the unit case.
7. Sparks should occur as the dist, shaft is turned.

"I don't know why a coil that tests good by measuring resistance wouldn't fire"

Internal HV breakdown