Mercruiser 470 intermittent no spark condition - Lots of diagnosis in thread!

[Oshawapilot]

Ok, 30' aft cabin cruiser with 2 Mercruiser 470's. Both engines run great and strong...but the port engine is suffering from intermittent no spark issues.

In the last 12 months it's received a new distributor cap, rotor, points, condenser, coil, plugs, and wires.

Both carbs rebuilt from the ground up this spring and the engines run absolutely perfectly now. This is definitely NOT a carb or fuel starvation issue.

The engine randomly goes from starting and running beautifully, to an absolute zero spark condition - it started doing this late last season but the boat just went back in the water after it's winter on blocks..and it's got much worse. Sometimes it starts fine when cold but loses spark when it gets hot, and then today..even stone cold, no spark.

I had suspected the coil on that engine so we swapped the coils from engine to engine, and sure enough the problematic engine started right up, as did the other one after the fact (with the suspect coil, argh), but I thought maybe it was a heat related failure and it had just cooled down enough when we were doing the work that it started working again.

However, with that in mind as a possible culprit, installed a new coil today, and still no joy - no spark. Moving the coils from engine to engine again did nothing either, so the coil is 100% ruled out as being suspect anymore. Polarity is confirmed correct on the coils.

I have confirmed that the shift interrupt is working properly and both the helm shifter control cables as well as the shift cable headed to the outdrive on that engine were lubricated last week. When coming in for docking on the last outing (with both engines running great) we did a lot of shifting in both forward and astern and the shift interrupt behaved perfectly without even a hint of a stall.

It's the intermittent nature that is making this a real bear to diagnose. Engine would not start (or even sputter or cough) today for the life it, and confirmed no spark. But we could go back to the boat tomorrow and it could very well start without any hesitation at the first touch of the key.

Ideas on where to start looking?

 

[Bt Doctur]

install a panel light at the + side of the coil ,motor dies, light is lit , you have power to the coil .
So it must be after the coil. Inspect the neg side wire where it enters the dist. should go thru a rubber insulaltor .
Only other wires on the neg side are shift interupter that supplies a ground to stall the motor during shifting and a tach wire.

 

[Oshawapilot]

Thanks, yes, a broken/wonky wire somewhere is on my list of things to check.

I did some more thinking and am wondering about the shift interrupt - although the mechanics of it are working 100%, I was left wondering tonight if the actual microswitch itself is possibly the culprit. If it was sticking in the closed/triggered position it grounds out the ignition system and could also be a culprit for a no-spark condition as well, correct?

I think the first thing I'll do is disconnect the wires at the terminal block where the shift interruptor is hooked up and if the engine suddenly roars to life, well, I'll know where the issue is.

 

[Bt Doctur]

you can do that for testing, can also disc the tach feed but remember it will be difficult coming out of gear in the water

 

[Scott Danforth]

Check the ignition wire from the key all the way back to the + side of the coil

i would simply hot wire the coil and go for a run. If that works, you juse eliminated the points and ground wire.

by the way, it is almost never the coil

 

[kenny nunez]

Have you tested the “by-pass” circuit ? Connect a volt meter to the + or bat side of the coil when the problem starts and check that you have 12 volts when cranking . In my years with my shop that was one of the most hard starting causes.

 

[Oshawapilot]

Thanks all - hadn't considered the ignition switch as a possible point of failure. FWIW when the switch is toggled to the on/run position (before turning to the start position) all the related gauges at the helm for the port engine do come to life and report as would be expected (zero oil pressure, tach settles to 0, battery voltage is as expected, etc) so I hadn't considered it as suspect. Is there a completely separate wire that runs from the ignition to the ignition/coil, or is the fact that the gauges at the helm come to life properly something that would rule out the ignition switch itself?

And yes, I will be probing with the test light high and low..and will come prepared to run new wire if needed.

It was my brother in law who was doing the work/diagnostics last weekend so I wasn't there to help, but am planning to go up to the boat with him on Wednesday. Although he is doing really great and learning a ton about all things old boat engine related (willingly, or not, LOL) he didn't have a test light when he was there...so even walking him through things on the phone it was limited what could be tested.

I just want to be prepared with all possible ideas before we get back there Wednesday.

Huge thanks to everyone for their ideas and brainstorming. I'm more confident than ever it's going to be something stupid simple in the end and the issue will be solved permanently, but I must admit it has eluded us so far.

 

[kenny nunez]

There is a circuit that goes from the “I” terminal on the start solenoid that supplies 12 volts to the coil when cranking. The contacts in the solenoid become carbonized which keeps them from making contact. The solenoid is located under the intake manifold.

 

[Bt Doctur]

upload images

 

[Oshawapilot]

Thank you, the schematics are very helpful.

If this does turn out to be a simple issue with an intermittent 12V circuit leading to the coil I may just run a new one from somewhere else on that engine (or heck, the other one) where I can get a solid switched donor 12V feed from - the red/purple wire directly at the engine wiring harness itself (which according to the schematic should be switched) would work.

I could even run a jumper from the positive terminal at the coil from the other engine for that matter, although I guess there's a risk of overloading the wires doing it that way as they are a fairly low gauge.

An issue with the starter relay could also be at play here as that engine doesn't exactly crank with any vigor like the starboard engine (which is actually *further* from the start battery) does, but I can't honestly say I'd be too excited about pulling the intake manifold to access it. The engineer who thought THAT was a good idea deserves a swift kick in their private bits.

 

[nola mike]

Thank you, the schematics are very helpful.

If this does turn out to be a simple issue with an intermittent 12V circuit leading to the coil I may just run a new one from somewhere else on that engine (or heck, the other one) where I can get a solid switched donor 12V feed from - the red/purple wire directly at the engine wiring harness itself (which according to the schematic should be switched) would work.

I could even run a jumper from the positive terminal at the coil from the other engine for that matter, although I guess there's a risk of overloading the wires doing it that way as they are a fairly low gauge.

You don't want to run 12v directly to the coil except for diagnostic purposes. You get 12v from the slave solenoid only when cranking as kenny nunez said. Once you're done cranking the coil voltage is fed through the purple resistance wire, stepping it down to 9v or something. The red/purple wire is not switched. If you tried to tap onto it from elsewhere you'd potentially bypass the neutral safety switch. Positive coil to positive coil I don't think would work either, unless you cranked both engines at the same time. Basically, you need 12v to the slave when in the "start" position, coming through the neutral switch, which then feeds 12v to the coil. In the "run" position, there is no more 12v from the slave, and your 12v goes through the purple wire, to the choke, and then through the resistance wire to the coil. I'm tired and it's late, but I think the above is correct...someone will correct me if not.

 

[Oshawapilot]

Ok, thanks - wasn't aware of the fact that the voltage to the coil is reduced once the engine is running - I'm still learning the finer points of oldschool ignition systems.. That does indeed change my thoughts/ideas a little.

FWIW, once the engine IS running it runs perfectly fine, but it just acts totally dead (as mentioned, no spark/no signs of life whatsoever) when cranking. Accordingly, I suspect that the resistance wire from the electric choke lead (as per the wiring schematic, thanks again for that, VERY helpful!) is actually OK, but the lead coming from the start relay that feeds 12V to the coil during cranking may be the culprit.

Does that make sense?

I will bring my multimeter as well as my test light to do some further diagnostics from the voltage perspective as well.

So, where to "borrow" a new source of 12V to the coil during cranking to make sure the coil is receiving 12v as it should?

- Right at the main positive lug of the starter itself? It may not be pretty, but it would get the job done.

- Alternately, the red/purple wire at the main engine harness looks like it's the trigger from the ignition switch itself that closes the start relay. According to the schematics that shows it would provide a solid 12V keyed source during cranking only. My only concern there is the amp draw - ignition switch shows that it's a 20A circuit, would the start relay as well as the coil draw during cranking exceed that? It's pretty small gauge wire to the coil (14 or 16 from the looks of it) so it can't draw a whole ton, but the start relay itself may. This kind of points me back to using the starter lug itself as the source of power during cranking. Either that, or install a relay, but then the wiring gets messier.

The only reason I'm looking at alternatives is, again, because of the fact that I really don't want to pull the intake manifold off to replace the starter relay. I would probably cut all the wires and re-route them to an external starter relay before I would pull the intake to service/replace the factory one.

 

[Bt Doctur]

Even without the 12v during cranking the motor will still start with the normal ign system.So before you go and reinvent the wheel just slow down.
When the motor dies do you have 12v at the + side of the coil, Y or N
The Yellow/red coming from the ign switch activates the slave that activates the main solenoid. If you turn the key to ":start" and nothing happens or you hear a soft click, suspect the slave relay.
The Red/purple wire is the main 12v feed to the solenoid then when activated sends power to the yellow/red that goes to the starters solenoid.
There are no electrical items mounted on/under the intake manifold , the solenoid is mounted under the exaust manifold

If all else fails, get a ballast resistor for a Chrysler and a jumper wire with alligator clips, .Remove all the wires from the + side of the coil and hot wire it direct to the battery thru the resistor.Stays running Y or N
Y is a harness issue,N is an ignition issue, coil, coil wire to dist, points, condensor, tach

 

[Oshawapilot]

OK, good news, returned to the boat last night with the schematics and a much better understanding of things in hand and the engine is alive.

Found a few culprits:

- The 12 V feed to the coil from the start relay was completely disconnected

- The resistance wire I’m going to assume is nearing end-of-life because the coil is only getting slightly above 5V with the key on (engine running or stoppped) which probably wasn’t helping the spark situation.

- The connector inside the coil on the problem engine was corroded - looks like some moisture made its way in there.

- A few plug wires were a bit loose fitting, adjusted the connectors and snugged everything up.

- I also think the points were dirty as well despite being only a year old and with the other issues resolvedI still wasn’t seeing a solid spark with manual opening of the points. Shined them up with a bit of emery paper and that solved that.

Decent spark afterwards (well, after I took the distributor cap off a second time and put the rotor back on LOL, that was a bit of a duh moment) and the engine came to life in a flip of the key.

The engine isnt happy under heavy load however (missing) but I’m 100% sure I didn’t gap the points correctly as I forgot my feeler gauges, and gap is probably massively over-spec. It ran perfectly a few weeks back before this ignition gremlin came along so it’s gottta be something simple.

I will return with the feeler gauges next week and finish up, so that’s probably the culprit, along with perhaps still weak voltage to the coil...?

Whats the best solution to bypass the resistor wire with something more modern and get proper voltage to the coil while running? Or is it a matter of “it works, don’t mess with it” and should we just get another length of resistor wire and piece it in replacing the existing chunk?

The puzzle is nearing completion.

On a good news fromt, the starboard engine is running like a champ.

 

[nola mike]

The resistor is there to keep you from frying your points.

 

[Oshawapilot]

The resistor is there to keep you from frying your points.

I understand that, perhaps "bypass" was a poor choice of words - I meant "replace"...IE, replace it with something more modern that will restore the proper level of voltage to the coil.

I haven't looked but I'm going to guess that a Mercruiser dealer will want some insane amount of money for a direct replacement of the resistance wire when there's a $20 or under alternative solution out there that will likely do the job better when things are all said and done?

Anyhow, the engine has become more reliable again after all the work - I returned today and gapped the points properly as well as did a lot of electrical tidying in the engine bay. Unfortunately I was alone without anyone to manage lines in and out of the slip, so I couldn't take the boat out and actually test the engine under load. We are going out for a day cruise on Friday, so I'm hoping for the best.

ON a side note, cleaning up all the main electrical feeds to the engines (amongst everything else in the engine bay) seems to have made them happier as well, I notice they are both cranking with a little more vigour.

 

[Oshawapilot]

Argh, I typed a big long reply here earlier and then apparently I got logged out..and it was lost.

Anyhow, short version, reposted:

- Got points gapped properly, engine started like a champ and ran beautifully under load. Ran the boat on full step for the length of the lake (a solid 20 minutes) and it ran great.

On the way up the river into town (about 1 hour at slow speed, 10KPH/6MPH) the problem-child engine started to behave like it was load up occasionally. Slipped it into neutral, gave it a rev, back into gear, and continued on - issue went away.

On the way back down the river it started running really rough again and when shut down (I went to check for a loose ignition wire somewhere, there were none) it wouldn't restart again. Checked for spark at the coil wire and there was none again. ARGGGGHHH.

Eventually, perhaps 10-15 minutes later, without having done anything of particular note, it decides to start again.

Shut it down again to see if it's repeatable, and again, no start.

For diagnostics (suspecting low voltage to the coil, as diagnosed earlier, is what's causing this) I grabbed the booster cables and wired direct from the coil to the start battery. Engine started smooth as could be several times.

So, I suspect that the earlier diagnosed low voltage on the resistor wire is actually getting worse when the engine gets heat soaked, and/or may be intermittently acting up on top of that.

So, next question...what's the best modern-day option for replacing the resistor wire? Surely there has to be a better solution?

 

[achris]

3 options.

1. (My preferred) Do away with the points and put a Pertronix kit in. The kit includes instructions on running a full 12v wire to the coil. This also does away with the starter feed, and brings all the benefits of electronic ignition. Forget replacing points every year. Once the kit is in, the timing doesn't drift either. . And spark plugs last longer.... and so on.

2. Buy a resistance wire kit from Merc and bring the system bring to original.

3. Buy a Ballast resistor and fit that in place of the resistance wire.

Chris. ...

 

[Oshawapilot]

Yeah, ballast resistor is going to be the route to go. Pertronix maybe next year (it's been on my radar), but I know they've spent a ton of money on this boat so far this season so that can wait.

 

[Oshawapilot]

Following up...finally got the port engine happy for the most part. Low voltage issues turned out to be a series of things which explained why the gremlins were hard to track down.

- The engine start battery turned out to be nearing end of life...which was odd considering it passed a health test at the beginning of the season. Battery replaced.

- Ground from said battery was weak, old cables. Even with brand new battery the cranking was still not great. Doubled up with a second ground and suddenly it snapped to life.

- Resistance wire was problematic somewhere in the loom, but long story short proper voltage wasn't making it to the coil. Replaced with a ballast resistor and it's MUCH happier.

With all those issues taken care of...it's a whole new engine. Starts reliable every time, keeps running nicely. Finally!

Ironically just after getting the last of the gremlins out of the port engine the starter on the starboard engine crapped the bed. It had been giving some symptoms it wasn't super healthy, moaning and such during starting - suspect a bearing seized. Getting replaced probably today or tomorrow.