70's Merc 1150 115HP Intermittent Ignition Failure

[RedDelPaPa]

Hello folks. We have a mid to late 70's Merc 1150 115HP I6 2 stroke outboard motor that suffers from intermittent ignition failure. I'm not precise on the year of the motor, but I can find out more about it later. The boat and engine are immaculate, and have been well taken care of.

Here's the scoop. When it runs, it runs like a champ. No missing, no hesitation. But it will occasionally just die instantly just like you turned the key off. Running under load, high or low RPM's, or just at idle in neutral. It hasn't produced a pattern. I have found no way to induce it, it just happens at random. Also, when it dies, if you bump the key, it may fire right back up instantly like nothing happened, or, it may not start again the rest of the day. Or maybe it will start again after a few minutes, or a few hours. I have seen all of these.

I'm certain it's an ignition failure by how it dies. Just like you turned the key off. I'm also pretty sure it's dropping ignition to all spark plugs simultaneously. So I'm pretty sure the problem is somewhere from the coil back.

Here's what I have tried so far:

#1 I tried gently smacking the dashboard of the boat where the key switch is, thinking it was possibly vibration there causing it. Wasn't that.

#2 Took the distributor cap off just to have a peak inside for anything abnormal. Looked fine by me.

#3 Checked by hand all coil, distributor, spark plug wire connections. All were good. Greased and no corrosion.

I'm told by a long time friend who has been an auto mechanic for decades, that intermittent ignition failures like that are probably 8 times of out 10, the ignition module/CDI box/switch box, or whatever it's proper name is.

Being an intermittent problem sure makes it very difficult to test for. I would think it has to be in its failed state for any meter equipment to diagnose it. I'm likely looking at a trial by error process of replacing the most likely and cheapest parts first then running it to see if the problem still persists.

Based on the info I've given, what do you all think?

Thanks,
Nate

 

[Chris1956]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

Nate, Merc CDI Switchboxes do not have a history of intermittent failure. Usually they work or don't. The ign trigger inside the distributor, can be intermittent, however, it usually fails shortly after it starts that pattern.

Let's look for cheaper causes/fixes. First start with the quick disconnect plug. Put a voltmeter on the white wire next to the red wire on the switchbox. This wire must have +12VDC for you to have spark. Now with the voltmeter reading +12VDC (key on), wiggle the quick disconnect plug. Turn the motor side-to-side and see if you lose voltage. If so, that is the problem.

 

[RedDelPaPa]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

Hi Chris. And thank you for the reply. What exactly is the ignition trigger you speak of inside the distributor? What does it look like?

On that quick disconnect plug, that makes sense and has a high possibility in my opinion. Thanks for the info. I will check into these and report back.

Thank you,
Nate

 

[RedDelPaPa]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

Ok guys, what do you make of this? In my seloc manual, it states the ohm reading for the trigger coil inside the distributor should read between 1.1 and 1.4 Kilo-ohms, or 1,100 to 1,400 ohms. 11-14 on the Rx100 scale. Instead, I am getting a reading of 4.3 mega-ohms! Double checked with 2 different meters. If I'm not mistaken, this trigger coil is way out of spec, and it's a miracle the motor runs at all.

Could you guys direct me to a replacement trigger coil for a 1975 merc 1150 115hp I6 2 stroke?

Thanks,
Nate

 

[CharlieB]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

Check this CDI part

http://cdielectronics.com/InstallSheet/134-3736.pdf

 

[CV16]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

Just a thought here. I had the same thing happen on an old boat I had. Turned out being the neutral safety switch in the control box. I spent lots of hours tracking that one down. An easy check is to just unplug the switch and take it out. Be careful though, it'll now start in gear! My wires were toast inside my controls, I redid them and never had a problem with it after that.

 

[RedDelPaPa]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

Ok guys, what do you make of this? In my seloc manual, it states the ohm reading for the trigger coil inside the distributor should read between 1.1 and 1.4 Kilo-ohms, or 1,100 to 1,400 ohms. 11-14 on the Rx100 scale. Instead, I am getting a reading of 4.3 mega-ohms! Double checked with 2 different meters. If I'm not mistaken, this trigger coil is way out of spec, and it's a miracle the motor runs at all.

Could you guys direct me to a replacement trigger coil for a 1975 merc 1150 115hp I6 2 stroke?

Thanks,
Nate

Actually, I'm told now by a tech that these readings are invalid because they were not done with a DVA device. So, back to placing bets again it looks like.

 

[RedDelPaPa]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

Check this CDI part

http://cdielectronics.com/InstallSheet/134-3736.pdf

Yes sir! that's it right there. IF, that's what's actually wrong with my motor. Can anyone tell me the odds of this being the failing part based on experience, history, and how I described my problem? Isn't it more likely that a failing trigger will cause irratic spark and allow the engine to still run rather than shutting her down just like the key was turned off?

I have verified that the motor gets 12v directly from the big main leads from the battery terminals and not any of the smaller accessory type leads. Also, I now know how to jumper the switchbox to bypass the key switch. We've now ran the sucker in the driveway for near 8hrs and can't get the sucker to fail like it was. Damn it. I know it will **** out as soon as I was in a position where I really needed that motor running. Like, trailering the boat in the wind. BTW, serial number of this motor is: 4239480 Switchbox PN: 332-2986A21 Distributor PN: 393-3736A23

Thanks,
Nate

 

[RedDelPaPa]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

Just a thought here. I had the same thing happen on an old boat I had. Turned out being the neutral safety switch in the control box. I spent lots of hours tracking that one down. An easy check is to just unplug the switch and take it out. Be careful though, it'll now start in gear! My wires were toast inside my controls, I redid them and never had a problem with it after that.

Hello, I think the neutral safety switch is just a starter interlock to prevent you from cranking the engine with it in gear, isn't it? I could be wrong. Does it actaully tie into the 12v ignition circuit at the switchbox?

Thanks,
Nate

 

[RedDelPaPa]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

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[Gomer50]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

The trigger can do just that but not for very long,she then wont do anything.Been there done that
But before investing in a new trigger/distributor check to make sure all your wiring from the ignition switch to your internal harness is in good shape.

 

[CharlieB]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

Running on the trailer and running in the water are very different, the driveway doesn't rock and roll.

Retest the power to the ignition box again, both the 12 V terminals while wiggling the wire harness to the motor, the pins in the main plug sometimes 'wear' a little and can momentarily loose connection, killing the motor.

The cure is to take an exacto knife and carefully slide the blade into the 'slot' on the side of the male pin to slightly enlarge it, making a better connection.

 

[RedDelPaPa]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

The trigger can do just that but not for very long,she then wont do anything.Been there done that
But before investing in a new trigger/distributor check to make sure all your wiring from the ignition switch to your internal harness is in good shape.

Yep. Very thoroughly checked all that before replacing the trigger. Wiring harness is in tip top shape, quick disconnect is in great condition and secure. Jumpered the switchbox to bypass the key switch. Only thing left to do now is take her for a spin and see what happens. Thanks for the reply. Nate

 

[RedDelPaPa]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

Running on the trailer and running in the water are very different, the driveway doesn't rock and roll.

Retest the power to the ignition box again, both the 12 V terminals while wiggling the wire harness to the motor, the pins in the main plug sometimes 'wear' a little and can momentarily loose connection, killing the motor.

The cure is to take an exacto knife and carefully slide the blade into the 'slot' on the side of the male pin to slightly enlarge it, making a better connection.

She'd fail running in the driveway too. The quick disconnect plug is in great shape, clean, lubed, and tight. Thanks for the reply. Nate

 

[RedDelPaPa]

Re: 70's Merc 1150 115HP Intermittent Ignition Failure

BTW, the motor is a 1975 Merc 1150 I6 outboard, almost mint condition, low hours, and 125psi across all cylinders. Spent most of its life sitting covered up in a garage. Have a new water pump to swap in first before taking her to the lake.