Mercruiser 140 Ignition short

[bobofthedeep]

1985 Merc 140....just got it almost free...beware those bearing gifts...<br /><br />Problem is when the ignition switch is turned on, the fuse blows. The fuse is connected from a 12V source under the dash directly to the hot side of the ignition switch. Turning the switch on with a test light shows the light lights dimly for about 3 seconds and blows.<br /><br />Tested the coil, as the ignition wire comes off the switch and goes to the coil. Coil shows .3 Ohms across the primary. If I remove the lead going to the dashboard ignition switch, and hotwire from the battery (with a fuse lol) the fuse blows same way (dim test lamp). I have isolated the stall microswtich out of the circuit, same thing. New points/condenser: same thing. There is a wire going to the starter purple/yellow stripe I think (colorblind) removed that, same thing.<br /><br />Anybody have any thoughts? I wanna go diving.<br /><br />Bob

 

[Don S]

Re: Mercruiser 140 Ignition short

Somehow free boats seem to mean the one giving it away had things so messed up he couldn't get it fixed.<br />I would start by checking the wiring under the inst. panel. My guess is something is just hooked up wrong in the process of the previous owner trying to install of fix something.<br />Here is a link to the Merc Service manual wiring diagrams for a newer model, but the wiring on page 4E-2 and 4E-5 is the same as yours and will help you find what is hooked up wrong.<br /><br /> http://www.boatfix.com/merc/Servmanl/13/13E4R2.PDF

 

[Uncle Dave]

Re: Mercruiser 140 Ignition short

Welcome Deep Bob<br /><br />Don is likely right about something being hooked up wrong.<br /><br />You are using good logic to isolate the problem. Just keep unhooking until removing the one that is causing the fuse to blow.<br /><br />Also is the fuse the right size. HOWEVER , do not put a larger one until the short is found or the right size is verified for sure.<br /><br />Post us back with what you find.<br /><br />Uncle dave

 

[rbezdon]

Re: Mercruiser 140 Ignition short

The coil should closer to 3 ohms, not .3 ohms. This alone wouldn't do it as this ignition system should have a ballast resistor. Make sure it is OK and not by passed.<br /><br />The ignition switched +12 goes quite a few places like radios, dash lights and every other device in the boat. You likely just have a short. You can isolate the circuits and use the ohm meter to track the short. It could even be a defective alternator field winding. Disconnect the circuits one at a time and measure resistance +12 to ground.

 

[Dunaruna]

Re: Mercruiser 140 Ignition short

When working alone, sometimes it's difficult to use a test meter. Connecting the two probes from the test meter over a long length of circuit or trying to read the meter while you are wriggling connections etc. <br /><br />There is an easier way to test for a short. If you have no luck with Don's or Ron's suggestions, come back and post, I'll describe how to test for a short using a simple tool that you can make very cheaply.<br /><br />Aldo

 

[bobofthedeep]

Re: Mercruiser 140 Ignition short

I checked under the dash, in fact it is isolated out and the fuse still blows (fused wire from battery directly to + on coil...<br /><br />What is the ballast resistor for? I think I found a gray wire which I think is resistive. What should the resistance be? Again, what does it do?<br /><br />re the coil...3 ohms....I thought I read .3, which is certainly closer to a short lol...<br /><br />Bob