Mercruiser 3.7/470 coil resistance wire question.

artificialreef

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Good morning folks. I am trying to figure out a voltage problem with my coil. Motor is a 1983 3.7/470. I was reading the resistance to the coil I have twins. The good motor is within spec, 1 to 2 ohms. The bad one is 4 ohms. I have the 1 wire alternator. The resistance wire kit is 84-94227A2. The diagram on page 3E-13 of service manual # 3 shows 3 wires at the choke.
1. A stainless steel core purple wire.
2. A copper core purple wire.
3. A black wire.

On my motor there are only 2 wires at the electric choke. A red wire and a purple wire. I tried cutting out the purple wire and putting the new resistor wire in it's place but the existing purple wire is what supplies 12.5 volts to the coil so no go.

Any advice would be greatly appreciated. I dont think it was jacked with by the po because i have twins and the second one is the same way. Did this motor come with a coil with the resistor built into it also?

Thank you.
 
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Re: Mercruiser 3.7/470 coil resistance wire question.

The black wire is to ground the electric choke.

I am a little confused in your question? What is it that your doing, replacing the resistance wire?

The copper purple wire at the choke gives the choke 12v from the ignition switch. The coil gets it's power from 2-sources depending on the position of the ignition switch.

It gets reduced voltage while the I.S. is in "run" position, thru the purple resistance wire which is run off the choke.

It also gets full battery voltage while the I.S. is in the "Start" position, from the purple/yellow wire which is connected to the starter solenoid. When the key is released to the "run" position, the purple/yellow wire no longer gets full battery voltage from the solenoid, it will now show the reduced voltage it is getting thru the resistance wire, as they are both connected at the coil.
 

artificialreef

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Re: Mercruiser 3.7/470 coil resistance wire question.

I was going to replace the resistance wire. But the schematic shows 2 purples and one black to the choke (total of 3). I only have 2. 1 red and 1 purple. On both motors. One runs fine.
 
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Re: Mercruiser 3.7/470 coil resistance wire question.

the wire run between the choke and the coil should be the resistance wire, the other wire will be the ignition wire to power both.

The choke must be grounded by touching the engine and doesn't have the black wire in yours... think mine as well.

coil picture.jpg
 

artificialreef

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Re: Mercruiser 3.7/470 coil resistance wire question.

I checked the termininal on the choke to grd. 4 ohms. On the good motor it is 18 ohms to grd. I tried running it with the wires not connected to the terminal. I guess thats not good because no grd. Is that 4 ohms ok even though the other motor is 18?
 

nola mike

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Re: Mercruiser 3.7/470 coil resistance wire question.

The choke terminal with the purple wires isn't a ground. Still not exactly sure what your question is or what problem you're having...
 

artificialreef

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Re: Mercruiser 3.7/470 coil resistance wire question.

4 cylinder 3.7/470. Runs a little rough in the low rpm range. I pulled plug wire 1 no change in idle. (replaced 1)Pulled plug wire 2 no change in idle. (replaced 2) Pulled plug wire 3 stalls. (replaced 3) pulled plug wire 4 stalls (replaced wire). This leads me to believe cylinders 1 and 2 are not combusting.
Compression test results.

1. 139 lbs.
2. 152 lbs.
3. 145 lbs.
4. 140 lbs.

Leak down test.

1. 5 %
2. 26 %
3. 12 %
4. 18 %

There is fuel going down the throught of the carb. Nice spray. There is spark. Checked firing order thats fine. Dwell in spec. Timing in spec. I have the 4 basics for a running motor. Fuel, air, compression, spark. Dont know where to turn next. Was looking at the coil circuit with the resistor. Maybe the spark is not strong enough. I read in a previos thread that could be caused by the resistance wire. I dont want to lead the string if you folks think i should look elswhere as to why 1 and 2 are not firing.
Thank you.
 

nola mike

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Apr 22, 2009
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5,024
Re: Mercruiser 3.7/470 coil resistance wire question.

Have you checked for spark in cylinders 1&2 specifically? Did you try switching plug wires? Have you changed the cap and rotor?
 

artificialreef

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504
Re: Mercruiser 3.7/470 coil resistance wire question.

I am thinking something in the coil to choke circuit. I have spark but maybe its not strong enough. Like i stated above when i ohm out the tab of the choke to ground i get 4 ohms. I dont think the choke is working but could i just pull the terminal off and run like that to test? When i do that, no change. Still runs rough.
 
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Re: Mercruiser 3.7/470 coil resistance wire question.

I think you are chasing a ghost with the choke. If that was part of the issue it would effect the other cylinders. Bad plugs, wrong gap, cap, rotor, bad connection with one end of the spark plug wires.... or a bad wire itself, that's what I would personally eliminate first.
 
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Re: Mercruiser 3.7/470 coil resistance wire question.

If you think the choke is part of your issue, then switch carbs between your two engines and see if the problem follows the carb..

Lets get those 470's wet!!! ;)
 

artificialreef

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Re: Mercruiser 3.7/470 coil resistance wire question.

Eric 3.7 You nailed it again. Just wanted to thank you. I retrofitted a carb i got in a swap that wasnt really the correct one but it fired up and runs like new(not coast guard approved and will fix the original carb). Thank you your imensness. I think i am officially a 3.7 club member with twin 1983 3.7's. Running cool and smooth. Still have my points (kind of dig the old school points, and bought an old school snapon dwell meter on fleabay) I will poke that weep hole before i go out. :)
 

artificialreef

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Messages
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Re: Mercruiser 3.7/470 coil resistance wire question.

I dont really think it was the choke more so than it was the carburetor rebuild i did. I probably botched it since it was only my second and i think i botched my first too. Also when i pulled the original carb i notice the intake manifold pattern did not match the carburetor base gasket. The intake was a big rectangle and the gasket was 2 holes. That means the section of gasket between the 2 butterflies is flapping around in the intake and that cant be good. When i retrofitted the test carb i fabbed a gasket without the center seperation that the stock had to more closely resemble the intake. She runs great. I think when i replace the original carb i will modify the stock carb gasket like i did with the test carb.

Anyone experienced what i am talking about with their 3.7 intake to carb gasket?
 
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