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Old 08-24-2008, 06:29 PM
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A4 original ignition schematic (basic version)

[UPDATED 26 August 2008: new diagram after double-checking the actual wiring installation (original image had been from the boat model's manual). This lead to a couple of minor changes in the text too.]

This is the ignition/charging circuit on the Atomic 4 on our "new" old sailboat that I am refitting. Disclaimer: These are my personal notes, and I am not a mechanic. Please correct me if there seem to be errors. By the way - I am not entirely sure if the wire sizes shown here are correct. I meant to check before posting this image.


1. The starter, alternator, coil and distributor are all attached to the engine.
2. The ammeter, ignition switch and starter switch are on the ignition panel in the cockpit.
3. The wiring for FG and AJ are in the wiring harness that runs from the engine to the ignition panel.

A - Large post on the starter solenoid. Hot with 12 volt electricity from the (+) post on the battery. This post is "hot" all the time when the battery selector switch is on whether or not the engine is running or the ignition switch is pushed in. So are the terminals at J, H and B.

BC - Ignition switch in the cockpit. Pull the handle out to close the circuit, allowing electricity to flow from the battery to the coil at D (+ terminal) and the ignition panel gauges. Accidentally leaving this switch pulled out when the engine is not running is not a good thing (constant power to the coil and maybe other appliances, such as an electronic fuel pump).

EF - Starter switch in the cockpit. Press the spring-loaded button to close this circuit and release it to open the circuit. Pressing the button allows electricity to flow to G, the small post on the starter solenoid. That activates the solenoid, closing an internal circuit that allows very large current to flow from A to the starter motor. Releasing this starter button opens the FG circuit, shutting off the starter motor.

The ammeter measures the balance of current flowing from J to H.

When the engine is running:

IH - I think that current flows from I to H and then back to A and the batteries through the ammeter, charging the batteris, and supplying the engine and house loads with current as required.

CD - As long as the ignition switch is pulled out, current flows from C to D, supplying the coil with electricity. The coil boosts the current to high voltage and sends it back to the spark plugs via the distributor.

Alternator to D - That yellow wire from the Transpo external regulator is called "Ignition" in Transpo's literature. I don't know the function of this wire (senses battery voltage?). The regulator needs to know the voltage state of the battery. Does it get that information from this wire, or from IH? The other Transpo external regulator wires (not shown) are Green (Field, inside the casing), Black (Ground) and Red (Isolation diode).

Disadvantages of this venerable old system, which has served well for more than 30 years on this boat:

1. Long wire runs for the alternator output: goes all the way back thorugh the harness to the ignition panel, and then forward through the harness to the batteries. Lot of connections that can get corroded too.
2. House loads combined with engine loads. The house circuits are not separated, so dirty electricity from the engine can get into the house system (starting spikes, noise), and house loads are supplied by the same battery that supplies the engine's systems. In addition, power to the house loads has to run to the ignition panel in the cockpit and then back to the distriubtion bus in the cabin, a long wire run with more connections than necessary.
3. The 1-2-Both-Off battery switch requires understanding of how the system is wired and works, and does not protect a discharged battery from draining a charged battery if they are combined.
4. The separate switches for ignition and starter require understanding by the operator, and run the risk of burning the coil when the engine is off (and running an electronic fuel pump when the engine is off, if there is no oil pressure saftey switch or similar device installed).
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Last edited by rigspelt; 08-26-2008 at 05:22 AM.
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Old 09-15-2008, 05:21 PM
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Then I found this thread, with a much better schematic:
1974 C&C 27
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Old 09-30-2021, 12:36 PM
keithems keithems is offline
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Question ammeter view

first let me add to the kudos you've received for your wiring diagram -- especially useful to me since it's based on a c&c27, and i sail a c&c30-mk 1

a question [pls. don't laugh too loudly at my ignorance]:

does your diagram show the ammeter as viewed from the front of the panel or the rear?

i assume you're showing the rear, since that's where the connections are, but i'd like to be sure.....or maybe it doesn't matter?

and a few more q's:

does the ammeter have to have a ground wire attached to the bolt in between the 2 connecting posts?

must a wire be run between the 2 posts, as shown in your diagram?

also -- is there an easy way to test the ammeter? mine has never worked since i bought this boat way back in 2007. i know, i know, but i only use the engine at most to get in and out of the harbor, and i can easily do without it, except for in and out of the marina / slip. so i can always tell if the batteries aren't charging -- eventually the sound level goes way down and the lights dim -- for that reason i only run on 1 battery at a time, and i just bought one of those jump starters from wally world for around $35.

so i'd like to see if there's any point in even trying to get the ammeter to work, before i just go ahead and replace it with a voltmeter..which i see most of the afourians consider the way to go

thank you in advance...
[1976 c&c 30 mk 1]

Last edited by keithems; 09-30-2021 at 12:41 PM. Reason: typos
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