Coil resistance?

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Tinlizzy

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Hi again folks,
This is in regards to a 71' 1150 (115 hp) Merc.

I am trying to troubleshoot an ignition problem and I am on the coil atm. I am doing a simple ohms test.

1. "pos" and "neg" lead wire on the Rx1 scale = 0 (which is correct)
2. Either of the primary leads and coil tower on the Rx1000 = 0 ( according to the shop manual it requires between 28 to 34)

Am I correct in saying this coil is bad? :)
 
Barnacle_Bill

Barnacle_Bill

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Re: Coil resistance?

Between the pos and neg terminals should read less than 1 ohm.

Between either pos or neg to tower should read open (no continuity

Between tower and ground should read less than 75 ohms
 
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Tinlizzy

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Re: Coil resistance?

Thanks Bhile for the reply.

I just tested the tower to ground as well and I read an open circuit.

The coil is definitely at fault here :)
 
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Chris1956

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Re: Coil resistance?

Tin, Across the primary winding, you should have 1 ohm or less. Tower to either primary wire should show some resistance, but not be open. If the coil has a metal case, there should not be any continuity from primary or tower to it. Coils in general do not fail often.
 
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Tinlizzy

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Re: Coil resistance?

Thanks guys for the replies.

Chris, the tower to either of the primary leads show an open circuit. Is it safe to assume a bad coil still?


I know the shop manual specifies 28-34k Ohms between the primary to the tower. My readings show completely open. The needle will not even move on the meter.
 
Barnacle_Bill

Barnacle_Bill

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Re: Coil resistance?

My Seloc manual says its suppose to read no continuity. But if you have the factory manual I'd believe it before I would Seloc.
 
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Chris1956

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Re: Coil resistance?

Tim, I have a factory manual, and I will try to remember to look it up later today. BTW - I once owned a cheap analog ohmmeter and it could not tell the difference between 400 ohms resistance and a dead short. I hope your meter is better. I threw that one out!
 
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Tinlizzy

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Re: Coil resistance?

Thanks again guys.

Hehe, my meter is a well built one, not on the cheap side. And yes Bhile its the factory manual.

I would appreciate the lookup Chris :)
 
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Chris1956

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Re: Coil resistance?

Tin, I looked in my Mercury Service Manual (vintage 1978) for the specs. This manual specifically covers all Mercs in the 1974-78 model years.

The ignition coil (P/N A-339-5288A2 and A-339-5748A2) for the distributors used on the 90-150 HP Inlines have the following specs:

Primary winding .02 - .04 ohms

Secondary winding (tower to negative primary winding or ground) 570 - 732 ohms.

Merc recommends you use a R X 10 setting for checking the secondary winding.

Good luck....
 
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Tinlizzy

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Re: Coil resistance?

I do appreciate your time and help Chris.

My specific year is a 1971 though with coil A-332-2983A5.

..So on that note I will replace the coil with a used one. Here in CT we have a dealer who just sells used parts as well as rebuilt ones. Its called the outboard exchange if you ever need any cheap parts :)

He quoted me $40 for a coil, which is not bad compared to $160 new.
 
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Chris1956

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Re: Coil resistance?

Tin, I think it likely that your OEM coil will have the same specs as the later years, since the Dissys and switchboxes are the same.
 
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Tinlizzy

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Re: Coil resistance?

Ok Chris :)

My secondary still would not fall into those specs either. From the tower to the ground or neg primary lead I read a completely open circuit.

...Just to be on the safe side hehe....the spot to test the tower is the hole where the HT lead comes out of the coil which goes to the dissy. Correct?
 
Barnacle_Bill

Barnacle_Bill

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Re: Coil resistance?

Make sure your probe is making contact with the metal down inside the tower. Sometimes thats hard to do. I always used a piece of an old coil wire to be sure. You can also clip the lead to small screwdriver.
 
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Tinlizzy

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Re: Coil resistance?

...Ok thanks Bhile I am correct in my testing then. I used an old HT lead I had lying around. :)
 
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Chris1956

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Re: Coil resistance?

Tin, If you have an "open" on the secondary, you have a bad coil. Do you have one of those old large ugly red ones?

You could consider a used coil, if you can find one locally. Sierra will have a new replacement coil, which will be more like the newer version that mounted on the crankcase next to the carbs under the Dissy. You may need to invent a method to strap it to the front cowl support, like your original one.

BTW, I have two inlines which are currently parts motors. I have enough parts to create a '73 850, but not the will to do it. I may part both of them out.
 
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Tinlizzy

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Re: Coil resistance?

Haha yes Chris, its an "ugly red" one. :)

I picked up a used coil today, and I will install it tomorrow. And hopefully its cures my problem.

...850 eh? Hmm I might be interested. Where abouts are you located? (You can send me a private if you like). Not that I dont have 3 other motors of various HP lying around waiting to be started;)
 
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Tinlizzy

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Re: Coil resistance?

Thanks guys, I have changed the coil and it seems to run great :)
 
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