install electronic ignition kit in a Mercruiser 120 engine?

[75starflight]

What Pertronix part number do I need for a 1978 Mercruiser 120 (4 cyl.)?

iboats.com lists a number for Sierra and Mallory, but not Pertronix

Is it a Pertronix 1146A?
http://www.go2marine.com/product/90148F/electronic-ignition-conversion-kit-delco-4-cyl-1146a.html

Just looked in pertronix's ign. book it is 1146a. what ever you do do not buy the flame thrower coil they recommend, stick with the oem coil.

[Rich_V174SS]

That would be the correct one, as long as it's for the Delco 4 cylinder distributor. I have seen the ones from Sierra and they are Pertronix units but with a Sierra part number, #18-5297. I bought one for one of my customers and still have yet to install it for him. (pic below)

I agree to stick with the OEM ignition coil, no need to change that.

[Neutron68]

I'm an electronics guy by profession, and I'd like to understand how the ignition system is wired.
First, I want to establish what the correct schematic is.

I found a simplified points ignition system schematic using Google Images.  
Anyone know if this is correct for a Mercruiser 120?

[Rich_V174SS]

That drawing is not quite right because the breaker points don't connect to the positive terminal of the ignition coil, they go to the negative side. Maybe this will help, just drew it up for you:

[Neutron68]

But, the ignition coil has 3 connections (2 bolts on the side and one in the middle).  
You only have 2 in your drawing.  Could you add the 3rd?

see:
http://www.marineengine.com/newparts/part_details.php?pnum=MER392-806529A+1&pass_title=392-806529A+1+%3A+COIL+KIT-IGNITION+O+-+Part+replaced+by+-898253T24&replacement=-898253T24

http://www.marineengine.com/newparts/part_details.php?pnum=MER898253T24&ptype=&Engine=&Model=

I'd also like to see the distributor and spark plug location in the circuit.

[Rich_V174SS]

I left the center one off because it's the high voltage lead that goes to the center terminal of the distributor cap, which then branches off to the 4 spark plugs by way of the rotating rotor underneath. In terms of wiring the circuit I didn't feel it was necessary because that to me is a given. I've included it to the coil here:

[Rich_V174SS]

Here's a modded version showing the wiring of the Pertronix in place of the points and condenser.

[Rich_V174SS]

Looking at your engine pic you posted recently I see you don't have an electric choke but you can still wire the positive red wire from the Pertronix to the front side of your ballast resistor, not the back side that goes to the coil.

[Neutron68]

I left the center one off because it's the high voltage lead that goes to the center terminal of the distributor cap, which then branches off to the 4 spark plugs by way of the rotating rotor underneath. In terms of wiring the circuit I didn't feel it was necessary because that to me is a given. I've included it to the coil here:

Thanks for adding the 3rd connection to the coil!
I have an electronics background, not an engine or automotive background.
My goal is to see of FULL schematic of the ignition system so I can understand it from an electronics standpoint.

I found some schematics for the insides of an ignition "coil", which is really a transformer, in electronics terminology.  
Transformers have to at least 2 coils (windings) inside.



There's a really good description of how the points system works, complete with an animated graphic of the distributor.
http://auto.howstuffworks.com/ignition-system4.htm
It also includes a very important piece of information - spark is not generated until the points (which are normally closed) are opened.

From the schematics, I can now see that when the points open, the capacitor (condenser) is allowed to charge from 0V to 12V, pulling a pulse of current through the primary coil of the transformer, which then induces a high Voltage in the secondary coil of the transformer, which is then passed on to the spark plugs.

[Rich_V174SS]

I too have a background in electronics so I've got an understanding in how this works. As one of my hobbies I repair antique tube radios and have a collection from the 1930's and 40's.

Here's a simple breakdown of how it operates:

Illustration 1) points closed completing the circuit through the ignition coil's primary winding. A magnetic field begins to build within the coil.

Illustration 2) points open breaking the connection and stopping the flow of current through the coil. The magnetic field collapses inducing a high voltage spike through the coil's secondary winding which is sent to the associated spark plug. Collapse backlash is absorbed by the condenser and also prolongs contact point life.

[Neutron68]

Any idea what circuit is inside the Petronix "sensor"?

An SCR, TRIAC or FET that is triggered by the Hall-effect sensor inside?

[Rich_V174SS]

That I don't know, maybe it's a trade secret. 
However, if you were to purchase the Pertronix Ignitor II for 3 times the price of the basic Ignitor I've heard it automatically adjusts dwell time and coil voltage depending on the operating conditions for optimum efficiency.

[thedeuceman]

I too have a background in electronics so I've got an understanding in how this works. As one of my hobbies I repair antique tube radios and have a collection from the 1930's and 40's.

hijack alert...
i would like to see a few pix of that

[Rich_V174SS]

Umm ..... ok, just a teaser.

They are as follows:
1934 Emerson Console and 1942 Philco table top rado
1940 Zenith Console
1938 Zenith Shutter Dial (Robot Dial) Console

And yes, they are all functional.

I even have a 1947 Fada (RCA) Television that works, it's got a 10 inch round picture tube, B&W of course.

[thedeuceman]

so is that a "tuning eye" tube on top of the Emerson ? you have it wired in as an add on ?
they all look vary nice, did you have to work on the cabinets also ?

[Rich_V174SS]

Yes, that is a "magic" tuning eye on the Emerson. I added it to the set but didn't want to install it into the cabinet so I built a separate box for it. I wired it with a connector so it can be removed at any time without disturbing any of the circuits. All the cabinets are original finish, I haven't had to do anything to them except a little furniture polish. The grill cloth on the 1938 Zenith is a reproduction, all the others are original.

[Neutron68]

I have a 1938 Zenith 12-tube floor model almost identical to the last one you have pictured. 
I first saw one at the Pavak Museum in St. Louis Park and decided I wanted to own one!
I bought mine already refurbished at Great Northern Antique on Bloomington Ave in South Minneapolis.

[Jerry]

My friends Tim and Tom Moen sell a bunch of them on E-Bay.

[Burnin Daylight]

OMG!!   My Grandpa and Granny had one of those in their living room.  I remember him sitting and listening to the "rasslin" (wrestling) and the news while he smoked his pipe.   We had to sit and be quiet during that or Granny would take one of those little switches off the walnut tree and whelp us with it.  I always thought I could out run her, but I couldn't!

[Rich_V174SS]

I have a 1938 Zenith 12-tube floor model almost identical to the last one you have pictured.  
I first saw one at the Pavak Museum in St. Louis Park and decided I wanted to own one!
I bought mine already refurbished at Great Northern Antique on Bloomington Ave in South Minneapolis.

Hmmm.... So if you have a 1938 12 tube model that's similar to my 9 tube model 9-S-262 then it's either going to be a 12-S-265 or a 12-S-267, pictured below respectively. Neither are mine but I wish they were part of my collection. I really liked those models. They are quite collectible and can be pricey if already refurbished/restored. I bet you paid over 1k for it.