DELCO 'MAILBOX' COIL

[Terry Wiegand]

I know that there are some really sharp folks out there when it comes to early automobile electrics.  I have several of these old Delco coils and my question(s) is how can these things be checked using a multi-meter and know for sure that they are good.  I have scanned the page from the Delco Instruction Book for 1916 Buick Models D44 thru D47.  The coil has a primary winding and a secondary winding.  If a person looks closely at the diagram, they will see that the primary and secondary circuits are tied together.  Here is my first question for everyone - if I use the multi-meter and check for continuity on each end of this coil and I get a good reading - will this mean that this coil is good?  There are a couple of guys up in the Twin Cities area that have the capability of re-winding these coils, however, if it turns out that it is not necessary, then I would not want to have that done.  The coil in the following photos has the ends removed for inside illustration purposes.  I am really hoping that someone out there can shed some light on these old coils so that we all can know what is going on with them.

Thanks for the help.

 

Terry Wiegand

South Hutchinson, Kansas

[Terry Wiegand]

Here is a photo of the coil with the ends removed for inside view.

 

Terry Wiegand

South Hutchinson, Kansas

[Terry Wiegand]

Here is a photo of the other opened up end.

 

Terry Wiegand

South Hutchinson, Kansas

[Bloo]

A multimeter will not be enough to tell you whether an ignition coil is good. Even though these run on switched DC, they are essentially an AC device, an autotransformer.

 

If, with that diagram you posted, you measured continuity of every winding, and you found an open one, then it's bad. If it checks good it could still be bad. A shorted turn will completely ruin a coil and the continuity will check fine. I see there is a condenser inside too. Since it deals with fairly high voltage, the capacitor (condenser) test in a DMM won't help much either. An old fashioned capacitance bridge, such as used by vacuum tube electronics hobbyists could conceivably check the condenser.

 

A better approach would be to build some kind of a jig to run a set of points, and see what kind of spark you get.

 

Edited March 12, 2020 by Bloo (see edit history)

[Terry Wiegand]

Bloo, if I'm understanding what you are saying correctly, a person would need to have a power source (6 Volts) going into the coil and a way to check the output on the other side?  Please elaborate a little more on that.  Thanks for the information.

 

Terry Wiegand

South Hutchinson, Kansas

[Brian_Heil]

Check it with an Ohm meter.  Any run-of-the-mill multi meter will work. 
 

Lots of ‘how to’ on Google 

 

Just pick a resistance value for 6V not 12v coil. 

 

When a coil is manufactured this is how it is checked.  You can do the same. 

Edited March 13, 2020 by Brian_Heil (see edit history)

[Morgan Wright]

What Brian said. I was getting 3000 Ohms on the secondary circuit, seems to be good enough. For the primary circuit I forgot how many Ohms I think maybe 6? Or 60? I totally forget but just follow the diagram to figure out where to put the test wires to check the ohms.

 

You don't need a power supply to test Ohms, just the little battery in the ohm meter.

[Brian_Heil]

And now a fun story to make you all smile

 

I’m a new brass Buick owner (1911) and they have magnetos. A new animal to me. Mine came with a spare which the previous owner said was no good.  This is a high tension mag which means it does not need a power source.  It generates its owns low voltage and then multiplies it too like a coil does. (A low tension mag needs a battery to supply the low voltage)

 

First thing I do is dig it out of the box and set it in my lap and give the impulse coupling a twist.  
 

To quote my late Father

 

‘Well son, I bet you don’t do that again.’
 

Not sure what the specs for a high tension mag are but I can verify it can jump a spark through a pair of jeans and boxers.  

[Bloo]

4 hours ago, Terry Wiegand said:

Bloo, if I'm understanding what you are saying correctly, a person would need to have a power source (6 Volts) going into the coil and a way to check the output on the other side?  Please elaborate a little more on that.  Thanks for the information.

 

Terry Wiegand

South Hutchinson, Kansas

 

A battery and a distributor, something to spin it, a plug wire and a spark plug, grounded through a wire back to the battery. Coil needs to be grounded, too in this case (per the page you posted), because the condenser is in there.

 

Or you could forego the distributor and just ground the distributor terminal on the coil by dragging a grounded wire across it. Make it do what it does on the car, make spark!

 

If it passes an ohm test, that would be a very positive sign, but I wouldn't call it good until I have seen it make a nice hot  spark. A shorted turn or two would make it quite bad, and that wouldn't show in an ohm test. There is also a condenser in there that is about 90 or 100 years old. They don't last forever.

[Morgan Wright]

2 hours ago, Bloo said:

 

If it passes an ohm test, that would be a very positive sign, but I wouldn't call it good until I have seen it make a nice hot  spark. A shorted turn or two would make it quite bad, and that wouldn't show in an ohm test. There is also a condenser in there that is about 90 or 100 years old. They don't last forever.

 

If there was a shorted turn or two, the ohm meter would read  ∞

[Bloo]

1 hour ago, Morgan Wright said:

If there was a shorted turn or two, the ohm meter would read  ∞

 

Infinity would come with an open coil, not a shorted one.

 

A wire in a given size has a known resistance per foot. A shorted turn would reduce the resistance of the coil by whatever resistance that one turn of wire had. It would be a miniscule difference, and likely not able to be noticed with an ohmmeter, but it ruins the coil.

[Brian_Heil]

I agree that finding one single winding shorted would be near impossible to find but having one less winding when you have over 1000 in the secondary is going to have a minuscule effect on the coil output.   But if the short was removing a significant number of windings from being active, then you would be able to find that with the meter and these inactive windings would impact the coil output. 
 

And that’s the trick, what is the correct resistance value range for this coil?  Best I could do is use 6v values for a modern 6v coil unless Delco published these values in a service manual back in the day. 
 

And I don’t disagree, if you could energize the coil and collapse the field like it operates in the vehicle and read the output on an oscilloscope then you would know it’s true performance. 
 

Am I missing something?

Edited March 13, 2020 by Brian_Heil (see edit history)

[Brian_Heil]

Or Terry could put it in his lap......

[Morgan Wright]

I don't see how you can have a short in the winding and still have continuity. If the wire is broken and shorted on the winding to the left of it, it would be open on the right side and the whole winding would be open.

[dibarlaw]

The coils are wound on a bobbin type of form. They would have many layers and some times the short happen between layers. Making part of the coil ineffectual. Many years experience of trying to rejuvenate 1920s antique radio audio transformers. Much finer wire gage. The same principal of one to10 step up ratio of coil output voltages.

[Morgan Wright]

And if part of the coil is ineffectual the ohmeter would read  ∞

[Jack Worstell]

Is everyone using the same notions of "short"  and "open"   ??

To me an open is a break in a wire........so that an ohm meter reading would go to infinity.

 

And a short is where a wire touches another conductor such that there

is an alternate/inadvertent/undesirable  path for current flow

Ohms would depend on between which points in the circuit are being measured

but in general ohms would not go to infinity.

 

Of course there could be an open in a wire circuit such that one ....or both........ ends

of the wire could end up touching an alternate/inadvertent/undesirable

 conductor  providing another path for current flow.

 

When I hear "open" I think of infinity ohms.    But when I hear "short"

I don't think of infinity ohms.

 

My point is this.....using the terms open and short requires a common understanding

of what these terms mean

 

So far as testing coils...I agree with Bloo.....the only complete test a coil is to make

it produce a spark.     Even if the ohms in the primary and secondary circuits 

check out OK.....the internal insulation ( trying to control thousands of volts)

might not hold up under load.  And if the coil also includes a condenser......

most multimeters cannot be used to test condensers.......and in my experience

condensers can measure OK but still be unreliable under load

 

When I want to check out a coil...I check the primary and secondary windings

for correct ohms.    And then if this checks out OK

I still try to install the coil in a car and see if it will produce a fat spark.

 

My two cent's worth

 

Jack Worstell

[Brian_Heil]

Another way to check a coil is to gap a plug to 0.200 and see if you get a good snapping blue spark. 
 

if so, 0.035 gap should not be a issue 

 

Have also seen coils break down once they heat up with under hood temps. 

[Mark Shaw]

Terry,

    Just connect a good spark plug that is grounded to the negative side of a 6V battery.  Connect the negative to the negative side of the coil and touch the positive side with the positive lead for just an instant to see if you get a spark from the spark plug.

[Alan G]

Not to hijack your post Terry but I am looking for this type of coil if anyone knows of any it would be greatly appreciated

 

 

[Larry Schramm]

An auto coil is nothing more that a transformer.  You have a primary winding and a secondary winding.  The number of windings for each depend on the voltage being stepped up or down.  On most older transformers the insulation for the copper wire was varnish.  If the varnish deteriorates, the wires can touch together.  This could cause a "short" where the current flow does not travel the full length of the wire to get the maximum step up or step down of voltage.  Your could still have a resistance, but it would be less than the original specification.  IT WOULD NOT BE  ∞.   It would have some resistance.

 

If the wire on one of the windings broke apart, ie. no continuous connection from one end of the wire connector to the other end/ between the terminals, that is an open.  Then you would have a reading of ∞.  The coil which is a transformer would not work.  That is true if there is a break in either winding. 

 

One of the issues with old coils is that when they heat up they can develop an open in one of the windings and you end up in tennis shoe mode.  Exactly what Brian Heil said earlier.   

 

I personally run a modern coil on my vehicles with a built in capacitor.  I want to eliminate one driving variable of an old part. 

 

My vehicles are made to be driven and not sit on a parking lot. 

[Morgan Wright]

I think it's unbelievable that the varnish they used could last 100 years and still insulate the wires, especially at such high voltages. I would love to know what sort of varnish that was.

 

Can't be shellac, right, but old phonograph records are made of shellac and I have lots of them that are 100+ years old, some of which look brand new and play well on the record player. Shellac doesn't deteriorate unless it gets wet. I've seen 100 year old furniture that still looks shiny, they always used shellac made from bug secretions.

 

Maybe I should get a spare coil so I don't have to go "tennis shoe mode".

[DonMicheletti]

Sometimes a faulty coil is not the coil at all, but a bad built in condensor / capacitor.. 

Adding an external capacitor might make live again

[Bloo]

You would probably have to disconnect the old one to get it to work well. Maybe you could stuff the new one inside? They have become smaller.

 

It would be pretty easy to test though, based on that manual page posted earlier. If you just don't ground the mailbox, that takes the condenser out of the circuit. Then you could hook a condenser from the points wire to ground, like in a "modern" car.

 

 

Edited March 18, 2020 by Bloo (see edit history)

[DonMicheletti]

I tried it with just adding an external condenser with success.

 

Longevity...unknown