Weak spark

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ford141
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Weak spark

Postby ford141 » Sun Dec 15, 2019 8:20 pm

I’m trying to finish up my auction find 154 but I’m having trouble getting it to run. I am able to get it to fire occasionally but it won’t stay running and just misses and sputters when it does. I put new plugs, wires, cap, rotor and points in it and adjusted the points to 0.020 gap, but still the spark is so weak I almost have to turn the lights off to see it. I have a strong 12v to the coil, but I suspect the coil might be bad. Is there a way to check it?

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Re: Weak spark

Postby Eugene » Mon Dec 16, 2019 11:32 am

No good way to test the coil.

Coil wire about 1/4" from engine block. Ignition on, snap the ignition points open and closed while observing the spark. Weak spark, probably the coil.

Faulting condenser will also cause a weak spark. Significant spark across the points, condenser faulting.

Edit. The in tractor tests can indicate that the coil is functioning. If in doubt, test with a known good coil.
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ford141
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Zip Code: 16201
Tractors Owned: International 184

Re: Weak spark

Postby ford141 » Mon Dec 16, 2019 4:03 pm

Eugene wrote:No good way to test the coil.

Coil wire about 1/4" from engine block. Ignition on, snap the ignition points open and closed while observing the spark. Weak spark, probably the coil.

Faulting condenser will also cause a weak spark. Significant spark across the points, condenser faulting.

Edit. The in tractor tests can indicate that the coil is functioning. If in doubt, test with a known good coil.


That's what I suspected. I'll have to pull the coil off of my 184 and test it. I replaced the condenser along with the points so I don't think that's the problem. Thanks.

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Re: Weak spark

Postby SONNY » Tue Dec 17, 2019 4:48 pm

Use the right coil with resistor!! -- either internal or external! IF you have an external one on it now, better check it!-- I had 2 of them shoot craps this past year. If yours is internal, in the coil, it probably is fried. New condenser dont mean good one either! sub good known parts on a trial basis to narrow down the problem. You may find the wires are part of the problem too. Might want to check them.

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Re: Weak spark

Postby BigBill » Tue Dec 17, 2019 7:00 pm

What’s the voltage and amperage at the coil?

I’m not sure about the cubs wiring systems but with coils and points there was a bypass wire bypassing the resistor with full 12 volts for starting. With the built in resistance in the coil we have a full 12 volts starting and less amperage running. Since she starts them dies the low side of the coil maybe kaput. Put a jumper wire from the battery to the + side of the coil to see if it stays running. The ign coils have a primary and secondary system.
I'm technically misunderstood at times i guess its been this way my whole life so why should it change now.

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Re: Weak spark

Postby Eugene » Wed Dec 18, 2019 12:22 am

BigBill wrote:I’m not sure about the cubs wiring systems but with coils and points there was a bypass wire bypassing the resistor with full 12 volts for starting. With the built in resistance in the coil we have a full 12 volts starting and less amperage running.
Not on Farmall Cubs.
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Re: Weak spark

Postby Larry B » Wed Dec 18, 2019 9:18 am

The coil for a numbered cub is a 3 ohm coil. Usually referred to as a "12 volt" or "internally resisted" coil. Measure across the two screw terminals and you should get approximately 3 ohms. If you measure approx 1.5 ohms you will need a 1.5 ohm ballast resistor.
Some cars and tractors use a 1.5 ohm coil with a ballast resistor and are wired so that the resistor is bypassed when cranking. On cars this was usually done with a 5 wire starting relay. I have a system where i add a lawn tractor starting relay and a 1.5 ohm coil and a ballast resistor and can install a "hot start" resistor bypass system on a 184 without needing to get to the wiring under the dash. The amp draw for a 3 ohm coil is approx 4 amps at 12 volts. With engine running and charging at around 14.5 volts approx 5 amps.

ford141
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Re: Weak spark

Postby ford141 » Thu Dec 19, 2019 7:29 am

So, I swapped the coil from my running 184 and immediately had a nice hot, bright spark! I had previously checked to make sure I had a good 12+ volts to the coil, and it had 12.7. I had already taken off all of the connections and cleaned them too. The coil was definitely the problem. I ordered a new one with internal resistor, should be here in a few days. Good news is the tractor fired right up after that and ran good. Motor seems solid, no smoke or bad noises. I had previously got it to fire briefly right after I got it home but it didn't run long enough to really evaluate the engine. Now I need to finish the carb rebuild (I cleaned it out but it still needs new gaskets) and tune it up. Compression checked out at 125-130psi on every cylinder, so I think it should run great after everything is set up good.

Larry B wrote:The coil for a numbered cub is a 3 ohm coil. Usually referred to as a "12 volt" or "internally resisted" coil. Measure across the two screw terminals and you should get approximately 3 ohms. If you measure approx 1.5 ohms you will need a 1.5 ohm ballast resistor.
Some cars and tractors use a 1.5 ohm coil with a ballast resistor and are wired so that the resistor is bypassed when cranking. On cars this was usually done with a 5 wire starting relay. I have a system where i add a lawn tractor starting relay and a 1.5 ohm coil and a ballast resistor and can install a "hot start" resistor bypass system on a 184 without needing to get to the wiring under the dash. The amp draw for a 3 ohm coil is approx 4 amps at 12 volts. With engine running and charging at around 14.5 volts approx 5 amps.


Great info here, thanks! I'm going to test the old coil just for the curiosity of it.

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Re: Weak spark

Postby Don McCombs » Thu Dec 19, 2019 7:48 am

Good oil pressure?
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ford141
Posts: 33
Joined: Fri Feb 01, 2019 7:19 am
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Tractors Owned: International 184

Re: Weak spark

Postby ford141 » Fri Dec 20, 2019 11:06 am

Don McCombs wrote:Good oil pressure?


I haven't put a gauge on it but the light goes out as soon as you start cranking the motor over, so there's enough pressure for that.


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