merc 140 pontoon
Petty Officer 2nd Class
- Joined
- Aug 23, 2003
- Messages
- 129
Merc 140 - Problem 2 - Ignition Coil gets Hot<br /> <br />This problem is related to "Merc 140 - Problem 1 - Engine Overheats at High RPM" which I just posted, but I believe it is an independent problem.<br /> <br />When running at 2500 to 4200 rpm, my ignition coil gets very hot. When I throttle back to idle, sometimes the engine will kill, and then it will not restart unless I open the engine compartment and let it cool for 90 minutes, OR until I swap out the coil for a cool one.<br /> <br />I am familiar with the wiring for my 1974 Merc 140 (Clymer Manual, 1964-1985, pg 563). I can't stand the resistance cable design. It was giving me problems with increased resistance as it heated up, so I have bypassed it with a transom-mounted ballast resistor. The bypass removes the resistance cable from the circuit (not left in parallel). The ballast resistor is a Carquest RU10, which is a relatively constant 1.8 ohms across the temperature range. The "starting assist" wire from the starter, to provide 12V during crank, is still in place. My Pertronix Ignitor is fed 12V from a switchable power source (harness prior to ballast resistor), not from the coil positive post as my mechanic had installed it (it was shutting down at low voltages related to the resistance cable--very frustrating to locate that problem).<br /> <br />I have replaced my coil with a similar coil. The old one was an automotive-style "external resistor" model 503. The new one, cross-referenced to the marine part by a local auto parts store, is an automotive-style "external resistor" model 803. Both measure 1.2 ohms primary resistance, 7500 ohms secondary resistance, which meets the Mercruiser spec for Delco-Remy coils. If I need to change to a true Mercruiser coil, please explain the difference. The only difference I am familiar with are aftermarket epoxy coils for high-vibration environments.<br /> <br />Here are the things I have done:<br /><br />- change engine oil & filter<br />- gap & install new plugs .035"<br />- change stern drive gear lube<br />- replace temperature gauge & sending unit (old gauge inop)<br />- clean up wiring of gauges behind panel<br />- set timing (was 18 deg BTDC, now 6 deg BTDC)<br />- drop lower unit, replace water pump impeller, housing, base & gaskets<br />- replace fuel filter in fuel pump<br />- clean fuel filter at carburetor<br />- replace distributor rotor<br />- replace ignition coil<br />- replace old fuel with fresh, stabilized gas (no ethanol, ever)<br />- bypass resistance cable with new wire and 1.8 ohm ballast resistor<br />- re-route Pertronix Ignitor positive feed to 12V switchable source instead of coil positive post<br />- clean up wiring to engine compartment blowers and fishfinder<br /> <br />At this point, I think the coil overheating is either due to having the wrong coil, or due to too much voltage coming to the coil. So today I took voltage measurements again. The battery was somewhat run down, due to lots of cranking on the lake this weekend, so the voltages without the engine running may be a little low. The battery is on the charger now. I measured the voltage from the coil positive post to a good ground. I was also concerned about an overcharge condition, so I measured the "system voltage", from one large post on the starter solenoid (always energized) to the good ground. Here are the results:<br /> <br />ENGINE OFF, KEY IN "RUN" POSITION<br />Coil negative post grounded: 4.8V at coil positive post<br />Coil negative post not grounded: 5.3V at coil positive post<br /> <br />ENGINE CRANKING BUT NOT RUNNING<br />Coil negative post not grounded: 5.5V at coil positive post<br /> <br />ENGINE RUNNING, NEUTRAL<br />600 rpm: 8.5V at coil positive post, 12.4V system<br />1000 rpm: 9.5V at coil positive post, 12.5V system<br />1500 rpm: 9.7V at coil positive post, 13.0V system<br />2000 rpm: 10.2V at coil positive post, 13.0V system<br />2500 rpm: 10.5V at coil positive post, 13.0V system<br />3400 rpm: 10.9V at coil positive post, 13.0V system<br /> <br />Are the voltages with the engine not running (all around 5V) way too low?<br /> <br />I believe that the 13.0V system voltage confirms that this is not an overcharge condition. I have read that the voltage at the coil positive post should be 6-9V, so it would appear that I have too much voltage above 1000 rpm.<br /> <br />Some of the 140 wiring diagrams (Clymer pg 561 and 564) show TWO ballast resistors. I cannot locate information on the resistance (ohms) of these resistors. Is it possible that the ballast resistors act differently than the 1.8 ohm resistance cable, and two of them are needed?<br /> <br />I also removed the "starting assist" wire lead from the coil and checked its voltage. As expected, it is zero with the key in any position except "Start" (crank). However, when cranking, the wire did not always energize--sometimes it energized a second or two late, sometimes not at all, sometimes the entire time. The voltage I got varied from about 9 or 10V down to about 7V. The behavior was very intermittent. Is this a sign that the wire is bad, or of a problem with the solenoid? The solenoid was replaced two years ago.<br /> <br />Finally, the coil is mounted on a steel bracket which mounts to the engine block just aft of the distributor. I have heard of people moving the coil off the engine block, and I could aim some airflow from a blower onto the coil to cool it, but these would seem to be band-aids, and I'd like to fix the root of the problem.<br /> <br />Thanks in advance for any input.<br /> <br />Steve