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Electrical problem on Mercruiser 120

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  • Electrical problem on Mercruiser 120

    1973 Mercruiser 120- I can't get the motor to start. The coil wire won't arc to ground when cranked. I just replaced the coil. Just to check myself- There are two sets of wires coming out of the wire bundle. One which is "hot" and connected to the positive terminal of the coil. Another which carries no charge and is hooked to the negative terminal along with the negative wire from the distributor. With the ignition on, I'm getting 12 volts off of the hot wire before I connect it. Once connected it only reads 4 volts. I also get 4 volts off of the center terminal of the coil but even less at the distributor end of the coil wire.I saw one diagram where only the negative distributor wire was on the negative post of the coil and both other wires were on the pos. post. The hot wire came from the starter solenoid and the other went to a harness. I tried this set-up but had no luck that way either.Shouldn't I be getting 12 volts at the distributor end of the coil wire or am I just confused?Where should I go from here?Thanks in advance-


  • #2
    Re: Electrical problem on Mercruiser 120

    there should be two wires going to the positive side of the coil.. one is the main feed from the ignition switch and is normally a resistor wire.. the other is the bypass wire this feeds the coil a full battery voltage while the starter is being cranked and bypasses the resistor wire which powers the coil during normal running....the wire from the negative side of the coil goes off to the distributor where it is switched by the points and the whole lot earths thru the distributor body..trog100

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    • #3
      Re: Electrical problem on Mercruiser 120

      Hi Mesa11, welcome to iboats!!Did it start before you replaced the coil? What color are the wires you're refering to? Do you have a tachometer in the dash? Did you adjust/replace the points?When testing the 'pos' coil wire (ignition on) make sure the points are open - still only got 4 volts?Aldo
      Watch this space......................

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      • #4
        Re: Electrical problem on Mercruiser 120

        just to add.. two wires going to the positive one wire to the negative..ignition on and points closed should produce about nine (roughly) volts at coil positive..ignition on and points open should produce battery voltage.. no current is being passed by the resistor wire and it will only drop voltage if its attempting to pass current..voltage at coil positive while the engine is being cranked again would be battery voltage.. this would be whatever voltage the system has left after the voltage drop caused by the heavy drain from the starter motor..trog100

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        • #5
          Re: Electrical problem on Mercruiser 120

          Thanks for the help. Unfortunately I had to stay late at the proverbial salt mine today so haven't had a chance to get to the boat. Please check back in a day or so.To answer the questions: It didn't start before I replaced the coil. I had the coil tested (it was bad) because I couldn't get the proper voltage there. I haven't changed the points because I didn't think that would keep the coil wire from arcing to a ground. If you are saying it can I will definitely do that. I've got a new set here. There is no tach in the boat.The hot wire is yellow(ish) the other is brown. Both are actually two wires ending in single round connector end. The negative wire from the distributor I made out of 10 gauge wire and soldered on connector ends. I tested it and it definitely has no breaks. Again- thanks for the help. I'll report back asap. I hope to get into it Thursday if the weather holds.

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          • #6
            Re: Electrical problem on Mercruiser 120

            Have fun in the saltmine,Actually,what happened that you felt the coil needed replacing?have you seen the motor run? then it stopped running on it's own,or was something else done first?Too many years ago,I couldn't find out why a customers boat wouldn't start.Finally he mentioned that it quit coming into dock,when shifting to neutral.I found the shift interrupter switch was the cause.So tell us all you can,in the best order you can.DHP

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            • #7
              Re: Electrical problem on Mercruiser 120

              Yes, the points can definately cause the coil to not "arc". (By that I take it you mean you are holding the main spark plug cable just off the engine block, then cranking the engine.) Be sure when you do that that you are holding it near a bare part of the block not painted - a bolt head, etc. The points simply act as a switch for lack of a better explanation. If they are dirty or corroded, they will not work as a switch, therefore the coil will not fire. You may want to consider replacing the points with a Pertronix kit - I did that on my Dad's Mercrusier 120 and it is much more dependable.

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              • #8
                Re: Electrical problem on Mercruiser 120

                "The points simply act as a switch for lack of a better explanation"there isnt a better explantion fisher.. thats exactly what they do do.. they simply switch on and off the current flowing thru the coils primary winding.. what is known the the dwell angle is just the on (closed) period the remainder of the time is the off (open) period.. during the on period the current goes thru the coils primary and then thru the points (switch) and earths off thru the distributor body..what is known as "timing" is just when they do this..folks over complicate this simple function.. there really aint any need to..

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                • #9
                  Re: Electrical problem on Mercruiser 120

                  I finally got back to the boat and remain frustrated!I've put in new points, new condenser, adjusted points to .022, new coil, new plugs, new wires. The voltage measures as you all describe in your responses. However, when I put the thing back together and crank- the voltage from the center coil wire just stays at 4. Voltage at the positive terminal is nine and twelve like you all indicated. If I open the distributor and manually open the points there is a nice blue spark that jumps them. I can't seem to get this to the coil wire even though the negative wire has no opens.The rotor and cap have only about 20 hours on them and look perfect with absolutely no corrosion.By the way, the boat does in fact have a tach. I just never noticed it before.The boat ran fine when it was put up last fall.What am I missing?

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                  • #10
                    Re: Electrical problem on Mercruiser 120

                    Did you recently replace the points? If you did, you did not set gap/dwell correctly. You just said operating the points manually creates good spark. Spark occurs when the points open. Therefore they apparently are not opening when the cam turns. Rotate the engine so the rubbing block on the points is at the high point on a cam lobe. Set the points to spec. Replace the cap & rotor and let 'er rip.

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                    • #11
                      Re: Electrical problem on Mercruiser 120

                      On the positive side of the coil find your tach wire. Disconnect it and re-try to start the engine. Iv seen shorted tachs do this many times.

                      Comment



                      • #12
                        Re: Electrical problem on Mercruiser 120

                        hello if you get a nice spark at the points when manually toggling them quit hunting primary wireing problems. do as suggested about rotating the engine until the point rubbing block is on the high point of the distributor cam and double checking your gap. a quick test of the secondary side, while the dist cap is off and the key is on, remove the high tension lead from the cap. hold it 1/4 inch or so from a metal surface now toggle the points with a screw driver again to make them "spark" if the coil and high tension lead is good you will see a spark from the high tension lead to ground. if your not using a pair of insulated pliers and holding the lead in your hand you may even get to feel it . the secondary side is a high voltage ,low current. when the points open no current flows thruogh the coil primay windings, this causes the magnetic field generated by the previously flowing current to collapse. as it collapses it induces a voltage in the secondary winings. voltage is electrical pressure similar to water pressure in a pipe. current is electrical flow similar to water in a pipe. as this high pressure builds it pushes a small flow. goues from the coil secondary tower,to the dist cap tower, through the dist cap tower electrode to the rotor button across the rotor to the terminal on the cap and down the wire lead to a plug and from there commits suicide by jumping the plug gap. and make sure the rotor is on. you would not believe how many times I have listened to an engine crank and crank after a tuneup and when I walk over to inspect, the rotor is laying on the deck.
                        no tech questions by PM, they wont get answered.
                        you have to be trusted by the people that you lie to .

                        Comment



                        • #13
                          Re: Electrical problem on Mercruiser 120

                          I get a spark from the high tension lead when I toggle the points manually.The rotor is in. I've set and re-set the point gap. I've even tried it a little wide and a little narrow. Still, when I crank it put together there is no charge coming from the high tension lead. It stays right around 4volts.Is there anything else it could be or am I simply mechanically incompetent all of the sudden?

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                          • #14
                            Re: Electrical problem on Mercruiser 120

                            It sounds like your getting a short while cranking, disconnect the tach wire from the neg side of the coil (that's the one that DOESN'T go to the distributor), if it runs o/k, the problem is in your tach or tach wiring (kudos to Trent!). If it is still low voltage while cranking - time to trace the positive coil wire and find a possible faulty ballast resistor.Aldo
                            Watch this space......................

                            Comment



                            • #15
                              Sign up today
                              Re: Electrical problem on Mercruiser 120

                              To start with while doing all this sparking make sure have no gas fumes.Do not try and messure the output voltage of the coil with a volt meter. To start with must be set to ac to messure but when it fires it will destroy your meter unless you have a high voltage meter lead for 10,000 volts or more.Make it simple to troubleshoot. Pull the distributor cap off. Pull the center high tension lead form the coil out of the center of the distributor cap. Run this high tension lead to the top of a spark plug. Ground the spark plug or the spark plug ground lug with a jumper to a good ground.Now turn the key on and open the points with a screw driver and a spark should jump every time the points open. Now turn the engine with the starter. Watch the rotor and make sure it turns. As it turns spark should jump at the plug. If the rotor turns and you get no spark then make sure the points open and close. On the lobe of the cam should have a gap of .018 to .022 inches. Points must close and open for the coil to fire. If you see points open and close and get no spark then hook up a battery charger with ground to the block and positive lead to coil positive lead and try again. If it sparks your either have a low battery in the boat while cranking or a problem with wireing to the positive side of the coil. Many batteries will read 12 volts with no load and less than 8 while cranking. Do not run off the battery charger for a long time as coil may over heat.If it sparks all the time while cranking then it could be a bad distibutor cap, Bad rotor or so far out of time the spark will not jump the gap between the rotor and spark plug post in distributor cap. If the rotor does not turn then have a problem in the distributer or engine block.By this time you should know where the problem is.Distributor to coil - lead normaly is a 16 to 18 guage fine wire. More than 14 guage is over kill.Good luck
                              Have some fun and enjoy the Great Outdoors.
                              Love to fish out of Bodega Bay, California.

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