Coil voltage question.

[briwill]

I've got a 90 Reneill with a 5.0 OMC Cobra engine. I've been having trouble starting the engine. The carb was rebuilt and appears to be okay. I suspect a weak spark? I'm measuring 5 volts at the coil when the key is in the run position. I thought the resistor wire should be putting out 7-8 volts. Is that correct? Is it a bad resistor wire? If so, does anyone know where that resistor is located?

 

[Don S]

Re: Coil voltage question.

Do that check with the engine running.
You need to know what the voltage is when cranking, so watch the meter when the starter is engaged to see. The voltage should be higher because you have a wire coming from the starter to put cranking voltage to the coil for starting.
When was the last time you had new points, condenser, plugs, cap, rotor installed the DWELL set and the timing reset?

 

[briwill]

Re: Coil voltage question.

Thanks for the feedback. The boat was tunned up at the end of last season. New cap, rotor, plugs, points, etc. It was running good a few weeks ago and now it's just really hard to start when cold and hot. I was considering replacing the coil with a high performance coil like the MSD Blaster 40,000v unit. Do you know if that will work with my ignition system? I'd like to start adding performance mods if they will increase performance, starting and fuel economy but I don't know how much of that stuff is marine friendly.(ie, won't go boom)

 

[Don S]

Re: Coil voltage question.

Changing coils isn't going to help if you don't have the proper power to it.
Check things out and find your problem. Don't just throw parts at it and hope for the best.
Test the voltage to the coil while cranking and while the engine is running.
From the voltage you measured earlier, you may have a bad Battery or loose, corroded connections on the Battery causing your problem. If it cranks slow because the Battery is low for any of the above reasons, then it's going to be hard to start.
Find the problem----- Fix the problem------then play with your performance parts.

 

[jmoorepghpa]

Re: Coil voltage question.

One of the handiest tools you can spend less than $15 on is an inline spark tester. Lysle makes a very good one and its available at most auto parts stores - don't get the cheapo they won't tell you as much. Check all your voltages like the guys said then install the tester between the cap and the coil wire. Stay clear of moving parts and crank the engine over with a remote starter or have someone do it for you. You will be able to see in the plastic chamber of the tester if you have weak, erratic or no spark. You can test a good engine to see what you are looking for. You may have a bad starter--- yes, a bad starter. On some models when you turn the key to start the power from the ign switch to the coil is interupted and as the solenoid on the starter kicks it send a full 12 volts to the coil to compensate for the reduced voltage due to the draw the starter pulls during cranking. Once the motor starts and you release the key the switched voltage from the resistor is restored. A good indication of this is if the motor cranks and strarts just as you give up trying and release the key. Also resistors usually are good or bad-- that simple. Occasionally if they have a flaw they can heat up and shut the motor off due to increased resistance but don't cause a cold start issue. Good clean connections!!!!!

 

[Boatin Bob]

Re: Coil voltage question.

Just to clarify so you don't go looking for it but there is no resistor on this engine, the resistance is provided by the wire itself. If you think that is the problem after verifying everything else previously mentioned you can run a temporary wire around it to bypass it which will give the coil a full 12 volts to run with, won't hurt anything to just do it as a test.

 

[SuperNova]

Re: Coil voltage question.

What you are actually measuring is the voltage drop of the coil--the correct way to measure voltage at the coil is to disconnect the hot lead then measure voltage at the hot lead. You should have 12 volts or battery voltage with the key in the run or start position. There is NO resistor or resistor wire, they are only used on points ignition systems and I believe you should have an electronic(breakerless) system. The resistors were used to allow the points to last longer by reducing arcing. The fact that you have a 5 volt drop across the coil--I'm not 100% sure, but that seems a little high. The coil may be partially open creating a high resistance in the primary circuit. If you measure resistance from the + terminal on the coil to the - terminal on the coil you should only get like 8 ohms or in that neighborhood, if I remember correctly, somebody else may have better recall or exact specs. If it's bad you'll get in the Kohms or Mohms. You can check the secondary side by going between the plug terminal at the top of the coil and the neg terminal also. That shouldn't be very high either. Hope this helps a little.
--P.s. The resistor wires don't limit voltage; they limit current(amperage). The voltage is a constant (12V from the ignition switch) and ohms law states that if voltage stays the same, as resistance goes up, amperage(current) drops.

--Stan

 

[Boatin Bob]

Re: Coil voltage question.

Stan....I think you may have made an assumption here but unless that ignition has been upgraded later (no indication from owner) a 1990 Cobra 5.0 came from the factory with a points ignition. As the resistor wire is in series with the coil ohms law states that the total voltage is the sum of all the voltage drops in the circuit so in this case the resistor wire does reduce the voltage to the coil. When the key is on start it provides 12v direct to the coil through an alternate path, when key is in run the 12v goes through the resistor wire thus dropping the voltage at the coil.

 

[bruceb58]

Re: Coil voltage question.

The resistor wire limits both voltage and current. You get a voltage drop across the resistor wire due to the current flowing through the wire. The resistor wire is purposely sized to create a voltage divider effect which creates a set amount of voltage at the input of the coil.

You do not want to measure the voltage at the wire going to the coil with it disconnected. That tells you nothing because there is no current flowing. If you have a large resistance leading to that wire due to a poor connection, you will still read full battery voltage.

The 90 Cobra should have a point style ignition unless it has been upgraded.

 

[SuperNova]

Re: Coil voltage question.

Yes, you're right I did presume that by 1990 omc had caught up with the rest of the world, but I guess they are slower to embrace "new" technology than I thought. I knew there was a reason I never owned one. I guess we all have our own ways of diagnosing things and I appreciate your input. The only thing I'd add humbly is: what do you think you are measuring when you check for voltage at the coil with the engine running? Think about it and how voltage drop works for a second before you answer. Small clue: you can only measure the difference in voltage potential in the portion of circuit between meter leads.


Once you have the answer I'd also submit this: What happens if you have a bad ground side circuit?
--Stan

 

[bruceb58]

Re: Coil voltage question.

With the points closed, measure the voltage to ground from both the negative and positive terminals of the coil.

The positive terminal should read between 9 to 6 volts and the ground terminal of the coil to ground should be zero.

Stan is correct in that a faulty ground side of the circuit will effect the voltage you measure at the coil .

 

[briwill]

Re: Coil voltage question.

Thanks for all the advice. I discovered that my electric choke was not receiving voltage and not opening. Hence my running problem and hot start problem. No, I have not upgraded from the points type ignition and it drives me crazy. Does anyone have a good resource for upgrading my ignition to electronic? I think MSD makes a kit but it's very expensive. Also, I'm not sure if this engine has the "shift assist" which may limit my choices of what will work properly.

 

[DHPMARINE]

Re: Coil voltage question.

I have no idea what the electric modules run,but generally we install 1 set of points in an engine once every year or two.That kit runs about $15.00. And all dealers can diagnos it.

DHP

 

[jmoorepghpa]

Re: Coil voltage question.

I put a mallory unilite conversion in my pickup and it works like a champ. Its been in there for 8 years and I have never had a problem. The parent company for Mallory is Mr Gasket--- look them up on the web and see if they make a conversion for your applicaiton. Summit Racing is great to deal with and belive it or not there is a ton of distributor and ignition stuff on Amazon.com in the parts section and the prices are the lowest I have seen.

 

[Coors]

Re: Coil voltage question.

Mallory makes marine units, but the're about $450.

 

[Boatin Bob]

Re: Coil voltage question.

iboats has them here or you can also check out the Pertronix site but you need to do a resistor/diode fix to make their's work with the older ESA's. You can also buy a newer version of the ESA from Rapair that works just fine with the electronic conversions.

iboats OMC Electronic Ignitions

 

[Mazda3GT]

Re: Coil voltage question.

Hi there, I have a 1989 5.7L omc cobra all mechanical engine ( no electronic egnition or choke, has points and condensor etc...) I feel the problem may be due to weak spark. The last time the engine was running i had it idling and i noticed the tach gauge to be bouncing up and down about a 400 rpm gap, this was when it was idling with a little throttle (1200 rpm) I let the engine warm up, then I shut it off, came back a few hours later ready to go out and it wouldnt start. Turned over fine, but wouldnt catch, could smell gas so i doubt gas is the issue, I'm aware of flooding engines. I think the coil may be the issue I will check out what was suggested in this post when i go back up to the boat on the weekend. But what else do you think the problem could be? Thanks for your time.

Steve
1989 Doral Concept

 

[alaska_av8r]

Re: Coil voltage question.

Briwill try this link http://www.pertronix.com/ptx2007_cat/ptx07_marine.pdf

If it gives an option go with the Ignitor II....these guys know more about this than I do...but I believe the Ignitor one will burn up the unit if you leave the key on....while the ignitor II will not....

I installed one about 2 months ago and the boat runs and starts remarkably better. Price is appx $150 but varies greatly depending on the place you order it from.

You should also be able to get a 40,000/45,000 coil from them for around $40 as well.

Just make sure you correctly identify your distributor before ordering......

The resistor diode fix like they are talking about above.....for me ended up taking the old power wire from the coil (which was hot only in start position 12v) and the resistor wire (coming from automatic choke that provided appx 9 volts to run) and just tape them off (not used anymore) and install a new wire from the automatic choke that had continuous 12 volts to the coil.

hope this helps.....

 

[Don S]

Re: Coil voltage question.

Old Thread​

Mazda3GT
Start a new thread with your question, don't just tack on to someone elses thread. As you can see, your question may get overlooked completely

 

[alaska_av8r]

Re: Coil voltage question.

dope....thanks Don I didn't even notice it was an old thread.....