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Slow cranking motor

Printed From: The Fishing Website
Category: General Forums
Forum Name: The Outboard Clinic
Forum Description: Anything related to outboard maintenance here....
Printed Date: 19 Mar 2019 at 4:52am

Topic: Slow cranking motor
Posted By: HERD
Subject: Slow cranking motor
Date Posted: 29 Dec 2015 at 1:41am
I have a 5 year old Mercury 90HP 2 stroke 3 cylinder motor. After pumping up the fuel blader I tried starting motor in driveway today after cycling that battery on a trickle charger for the last couple of weeks while waiting for a new trailer axle to arrive. With water running through tell tale,  I pressed in the  ignition key while turning it. The motor cranked over as I would expect it with a freshly charged battery, but never fired up.  I then turned the ignition key a 2nd time and the motor would barely crank over. I thought that the battery was at fault and went and located my booster battery which has enough grunt to crank over my 6 cyl 4.2 petrol Nissan Safari.  I connected this to the battery and turned the ignition key once again. It craned over fine. I then released the ignition key before the engine fired. I then turned the ignition key again and engine would barely crank over.
I have tried with a 2nd alternate battery with same results, I have cleaned battery contacts.
Where else do I look? 
Would the fault be related to a heat build up in the solenoid (or something else) that prevents enough currently getting to the starter motor?

Regards Herd.

Posted By: MacSkipper
Date Posted: 29 Dec 2015 at 5:54am
Run a light (nav lite?) while cranking see if it dims excessively when slow cranking ( may be a battery/isolation switch problem after all?  Had a broken battery cable (broken at lug) causing this from poor assembly.  Difficult to see as cables are sealed t prevent water getting in but felt weak at break.
Try tapping starter motor where brushes are, starter relay while a mate cranks - most likely a wiring fault.  Other thing to try is manually powering starter solenoid from battery rather than using ignition switch.  Need to isolate fault. 
Does motor have a pull cord option?  If so remove plugs and check it pulls over freely?  Or try turning over with spanner and plugs removed.

Good fishing trip nothing breaks, great trip catch fish.

Posted By: HERD
Date Posted: 29 Dec 2015 at 6:28am
Interesting you mention watching a navigation light to see if it dims. I noticed the dial lights dimmed on each of the 2nd cranking attempts when the engine barely turned over. To me this indicates the current and resultant energy is not getting  to the fly wheel to turn the motor over but is being drawn off the battery. What puzzles me why it fails on each of the 2nd attempts to start the motor and not on each of the 1st attempts at starting the motor.

Regards Herd.

Posted By: MacSkipper
Date Posted: 29 Dec 2015 at 7:13am
Hmmm - interesting the more info you have the more likely to track fault.
If dimming lights and slow crank 2nd attempt - indicates a battery problem (your backup battery may not be connected properly to crank thru battery leads?) or high load or poor connection to battery (remember poor connection - high resistance/ load for battery - do any terminals wires get hot after several attempts?  If faulty starter relay (not triggering starter motor) say then lights would stay bright.
 To rule out battery/wiring can you hook up backup battery direct to starter motor (remove connection to boat battery) and crank okay?  or to switched side of starter relay so if relay. starter ok will crank fine and start? Be careful as plenty of current from battery.  If 5 year old motor then most likely a wiring/battery problem.

Good fishing trip nothing breaks, great trip catch fish.

Posted By: Steps
Date Posted: 29 Dec 2015 at 9:27am
To me this indicates the current and resultant energy is not getting  to the fly wheel to turn the motor over but is being drawn off the battery.

If all connections are good, and assuming they have all been checked.. and thrown in more than adequate battery 'power'.. eliminates battery.
That leaves either the solenoid or the starter armature faulty.
Generally a faulty solenoid will just click rather than engage.
Which leaves the starter its self....
I do not know If mercs have an inherent armature problem, but if like older chevies, classic symptom of what ppl call "starter header heat"....If an engine has too much initial advance its harder to start.. which loads up the starter armature with very high initial current draw... this eventually cause 'dry' connections in the starter armature. The only way to check an armature is in a growler which very few workshop now have and if they do they dont know how to use it.
Another way to check is heat up the armature in an over till  quite hot to the hand and check it for continuity
Baddly worn bearings/ bushes or brushes in bad condition will cause and do the same thing.
As can a baddly worn contact disk in the solenoid... but that is something caused with huge use and age and bearing bushes tend to go out 1st.
This motor is not that old .

This is a long shot.. If the battery cables are not tinned marine grade, and corrosion has got inside there will be a big power/ voltage  drop between each end.
Electricty runs down the outside of the strands in the wire... NOT down the center, therefore if the outside of the strands are corroded the resistance in the wire increases causing voltage drop.. and as they get hotter the resistance increases thru the corroded strand surfaces.
Check voltage at battery and again at the starter end of the main battery cable... or do as suggested above run a known good set of jumper cables

Posted By: Joker
Date Posted: 29 Dec 2015 at 11:18am
I suspect that the boat battery is stuffed internally. Putting a jumper onto the original battery wont help if its collapsed inside and the current doesn't flow through.
Disconnect it and try with a different battery.

Posted By: pjc
Date Posted: 29 Dec 2015 at 3:52pm
I am no expert but i presume battery is as old as motor?if battery is low trickle charging over 2 weeks will not charge put jump pack on and turned once?jump pack is trying to put energy in to stuffed battery,try a new battery or put jump pack directly on to cables and disconnect old battery completely.maybe take battery out of car,do not turn motor over just see if lights dim, try pull starting motor with car battery in place as you need to complete the circuit.

water water everywhere,how many fish does it hold?

Posted By: Muzzfishing
Date Posted: 29 Dec 2015 at 3:57pm
When you have tried  all the above get the compression checked as one cylinder could be picking up and stopping the motor from turning over freely. This happened to a guy I know. Dealers swapped starter motor a few times still no better so traded the motor on a newer model 4 stroke.   Dealer did some more checking and found one cylinder was causing the problem.  

-------------" rel="nofollow">     A Good Skipper Keeps the water on the outside of the boat.

Posted By: pjc
Date Posted: 31 Dec 2015 at 6:03pm
any update on problem?

water water everywhere,how many fish does it hold?

Posted By: HERD
Date Posted: 04 Jan 2016 at 12:03am
The bad weather on the Hibiscus Coast delayed weekend work on sorting the motor. Had the opportunity this

afternoon for about an hour before heading out to a BBQ. Found split heat shrink around the HT crimp at the +ve

 terminal of the battery isolating switch. T ok the spilt heat shrink off. The crimp looks secure with a bit of green

 muck on the exposed copper wire. The HT cables are marine grade.Then decided to remove the fly wheel

 cover and try turning the shaft after disconnecting the battery and removing the spark plugs. The shaft seems easy to turn over. I was supprised at what the spark plug looked like. Its a BUZHW-2 NGK Surface Discharge Spark Plug. Never seen a spark plug like it before in my 60 years, 17 cars and 3 outboards.(Up until now a local service agent has done all servicing on motor, Once I had removed all 3 spark plugs and found they all looked the same I realised that normal the electrodes that form the cap had not corroded away but were all part of the Surface Dsicharge Spark Plug design. I cleaned these up and placed them back in the respective cylinders. I connected the hose to the engine flush tube, turned the water on, connected the +ve HT cable from my battery booster to the +ve side of the starter motor (with boat battery disconnected). The -ve HT cable from my battery booster was connected to the -ve cable which is normally connected onto the -ve battery terminal.. Once the booster was switched on, the motor turned over freelyand when the ignition key was turned on, the motor roared into life. However, I could not repeat the start a 2nd time. Instead, on the 2nd attempt to start the motor, the HT leads became very warm and the motor would not crank over. At this stage I had to pack up and attend a BBQ so issue not solved although it looks like a faulty starter motor.

Regards Herd.

Posted By: MacSkipper
Date Posted: 04 Jan 2016 at 7:04am
Thanks for the update disappointing that starter failed after only 5 years?  I guess one option is to take boat and starter to good shop tell them what you have done get him to look at it and advise on it? Or take starter off and get a electric motor reconditioner to look at it?  Boat shops will be busy at this time of year.

Good fishing trip nothing breaks, great trip catch fish.

Posted By: Steps
Date Posted: 04 Jan 2016 at 10:27am
However, I could not repeat the start a 2nd time. Instead, on the 2nd attempt to start the motor, the HT leads became very warm and the motor would not crank over

HT leads.. I assume u mean the battery cables not the HT (high tension voltage spark plug leads(????)
If one or the other cables gets hot that quick it means there is a bad connection ...
Try bypassing the battery cables, espec that u mention u gave that green crap there...See my post above about corrosion on the stands of cable/ electricty goes down the outside not thru the center of a copper strand.
Or in other words eliminate the cables as the issue before digging into the starter or solenoid.
If the cable has been corroided for some time.. not noticed.. the higher currents drawn thru the starter solenoid contacts would be the most likely issue
Again my post above.
Or could just be build up of crap on the commutator (the bit on the armature the brushes run on) and around the brushes.
A simple and effective way to re turn down the commutator if dont have a lathe...
A leather boot lace...a strip of around 320+ grit wet dry sand paper.  Wet down the sandpaper wrap around the commutator with kero, wrap the boot lace 2x around.. then pull each end and spin the sandpaper.

mark any alternator/ starter cases/ body before disassembly its very frustrating to re assemble, pull the brush retainers out the fine the case is out of whack with mounting bolts.

Also stuffed armiture bushes/ bearings will cause the same issues.. but generally need to be quite stuffed.

Posted By: CEEBEE
Date Posted: 14 Feb 2016 at 3:59pm
Using a digital multimeter place one lead on the _ve battery and the other on the starter body and set to 2 volts. Crank engine / should be less than 0.2v
Repeat for +ve lead on the +ve battery and +ve starter terminal. Max = 0.2v
If higher than that means there is a high resistance fault somewhere, Just a matter of "probing" from battery forward until resistance fault is located.
Also using dig meter hold across battery whilst cranking set at 20v and see how much it drops.


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