1974 115HP Johnson only firing on 1 cylinder

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kshelly

Petty Officer 2nd Class
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Mar 9, 2004
Messages
124
1974 115HP V4 Johnson. Using an automotive in-line type spark tester, it appears my engine has only 1 cylinder firing. I ohmed-out my coils, and all seem OK. I did a search of previous threads and using that information I'm leaning toward a power pack. I have the Service Manual specific for my engine, but it requires a "Neon Test Light" to test the Power Pack output. Anyone know of an alternate method using a voltmeter or other common device to test Power Pack output? I could stick my tongue on the terminals and have wifey turn the key :)
 
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kshelly

Petty Officer 2nd Class
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Mar 9, 2004
Messages
124
Re: 1974 115HP Johnson only firing on 1 cylinder

I took some voltmeter and resistance readings, but I'm not sure what readings are "normal". I was expecting to read 300VAC to the power pack from the charge coils, but I only get 50VAC. Would this be a normal voltage output for the charge coils while cranking? I was expecting to read some DCV at the ignition coils, but read 0. I ohmed-out the sensor coils and charge coil. Charge coil should be 835-985 ohms, but was 622. I'm lost. I guess since resistance is low on charge coil, I should replace it, but I don't think repalcing the charge coil is going to fix having just one cylinder firing. Any wizards out there have any input???
 
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kshelly

Petty Officer 2nd Class
Joined
Mar 9, 2004
Messages
124
Re: 1974 115HP Johnson only firing on 1 cylinder

Thanks seahorse. I've been to that site. If I could only determine if 50VAC from charging coils during crank is normal, I'd be in business. I have the Johnson shop manual specific to my engine, but they use a special tester (m80 neon test light) to test the coils & power pack. Without voltage specs, I'm sunk :(
 
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kshelly

Petty Officer 2nd Class
Joined
Mar 9, 2004
Messages
124
Re: 1974 115HP Johnson only firing on 1 cylinder

OK, frustration has turned into $ spending. I'm going to replace two of the ingnition coils since the outer plastic coating is cracked. I'm going to replace the Power Pack since I'm basically clueless as to my real problem (at worse I'll have a spare Power Pack lying around). I'm going to replace my ignition switch since the key falls out real easy when running, and then I'm going to cross my fingers and hope the thing runs!!!
 
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ledgefinder

Ensign
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May 2, 2002
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916
Re: 1974 115HP Johnson only firing on 1 cylinder

You should get 150V from the charge coil. That's the amplitude of the voltage signal, but you'll need an oscilloscope, or peak-reading voltmeter, to measure it. A 'regular' AC voltmeter won't work because the signal from the charge coil is not sinusoidal.<br /><br />You can rule out the ignition switch by disconnecting the black/yellow wire from the powerpack. Watch out - you'll have not way of killing the motor then & will have to use the choke. Could cause serious injury or death.<br /><br />I wouldn't replace parts (expen$ive) just yet. What's the motor doing? (That is, in what manner is it running poorly?).
 
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ledgefinder

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May 2, 2002
Messages
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Re: 1974 115HP Johnson only firing on 1 cylinder

My factory manual says 630 ohms +/- 50 for the charge coil resistance (terminals 7 & 8). Sensor coils 8.5 +/- 2.0 ohms. <br /><br />The CDI electronics applications chart says 150V peak for the charge coil output (to the pack), and 125V peak for the pack output (to the ignition coils). CDI is a well-respected aftermarket ignition products manufacturer (aka "Rapair").
 
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kshelly

Petty Officer 2nd Class
Joined
Mar 9, 2004
Messages
124
Re: 1974 115HP Johnson only firing on 1 cylinder

Thanks for all the replies. Ledgefinder, that's some deep info on the amplitude/sinusoidal stuff!!! I did find out my resistance reading for the charge coil is OK. All the sensor coils check OK too (resistance, anyway). I get no voltage readings at all to the coils while cranking. I'm figuring that since one cylinder fires, the power pack has failed and is not delivering the needed voltage to the other 3 cylinders. <br />I'm actually only replacing the ignition switch because the key falls out easily while in the "run" position. I don't think it has failed electrically.
 
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