Need somebody smarter than me
 

Friend of mine has a 48 Mercury stock engine, drive only a mile or so when it warms up go to restart it starter relay clicks starter is drawing amperage does not rotate engine at all.Checked cables and ground connections.Jumped power directly to starter terminal , installed new starter same result.If the car sits for couple hours fires right up.Load tested battery reads o.k I'm stumped engine runs fine drives great?

Re: Need somebody smarter than me
 

Try tapping on the starter lightly? Maybe brushes are sticking?

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Sounds like you did check bypassing the solenoid...but I fought a similar problem for a long time. Then I touched the solenoid, and smelled burning bacon and heard screaming. I realized that I had discovered RESISTANCE!

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May have a dead spot in the armature and looses contact on that spot. Not unusual.

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Replaced the starter with a new one , the flywheel and gear on old one was good . Didn't think it might be some kind of alignment issue since it only does it when it gets hot .Both starters have the same problem, do you ever shim starters in flatheads?Jump starting it didn't help so thought battery wasn't the problem.

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Just curious, have you got a good ground cable from battery to engine,
and from engine to body & from body to frame ?







.

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Cordbob, do you have heavy enough cables ? I use 2/0, may seem like over kill but my 48 starts when hot, you could also have a weak coil. Just some thoughts.

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Battery is grounded to the intake of the motor, all cables are stock .Ignition is not an issue the motor does not rotate at all, fires right up when it does.

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I hear you, but grounds are always the place to start. :)

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What you got is lack of current...warm motor with higher compression needs more power to turn over.
So either cables/solenoid have resistance or the battery doesn´t have enough cranking amps.
I would take a volt meter and measure the voltage at starter and battery while cranking first.
If it´s good at battery and low at starter follow the cables backwards until you find the voltage drop.

Re: Need somebody smarter than me
 

Thanks for the info .This car is a 100% stock Dearborn winner , owner wants to keep it original I'll try big ground and power ccables to trouble shoot it .Never seen a battery act up when it gets hot but I'm leaning toward that being the problem. Keep you posted on the outcome.

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Battery voltage was 6.43 , dropped to I think it was 4.5 or so.Thanks Flatheadmurre I will try that.

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I've got three grounds on mine and used heavy cable with soldered ends. No problems cranking motor. Good place to start.

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The original car had the proper sized cables. But even if the existing cables look good that does not mean they are carrying the required current. This is both the "+" & "-" side! Are all the connections clean? Like has already been said, if the starter is not turning over (and it is a good battery and starter) then there is a current problem somewhere. Have you tried bypassing the starter solenoid with a jumper cable when it is in this hot/no start condition?

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Not turning over means that you don't have enough voltage to push enough current through a resistance; its one of those three.

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From what I remember when I had my starter rebuilt - the rebuilder said that there is alignment bumps in the starter pieces (male & female). Make sure they are aligned perfectly or it will cock the rear bearing resulting in binding and no starting when hot. When it's cold, there's enough armature clearance and it will start. Just something to check since you have two starters doing the same.

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Ck your cables closely, they could be corroded under the insulation where you cannot see it. this will cause high resistance, especially when warm, and cause the problem you describe. good grounds , and more than one, cannot be stressed enough on a 6 volt system, along with the proper gauge wire as others have mentioned.

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If the 6.43v good battery voltage drops to 4.5v under attempted warm start and the original and replacement starter don't turn the engine as well as jump starting not turn the engine, you have a mechanical issue, not an electrical issue.

Can a cylinder be filling with fluid temporarily, increasing the torque needed to crank an incompressible cylinder, and that fluid be released from a hot engine as vapor or ??
In any case, forget the grounding and cables and direct your search to the engine loading the starter(s) down.

You initial thread stated incoherently "drives only a mile or so" perhaps a water leak quickly overheating ?? don't understand but I'm thinking that may be related.

Re: Need somebody smarter than me
 

Starting from the battery...a battery is designed to deliver a maximum amount of instant amps (cranking amps) this is set by the surface area of the battery plates.
If you are trying to draw more current out of the battery then it´s designed to deliver you will have a voltage drop...this doesn´t mean the battery is bad...it´s just to small for you aplication !!
So if the battery voltage drops to much cranking it may just be to small.
So when we have a correct size battery that will deliver the needed current it´s time to make sure it gets to the starter.
This is when ohms law will work against us.
Power of a starter is voltage x current thats why truck starters has become 24V to limit the needs for current and giant cable areas.
With only 6V available we will need twice the current of a 12v starter to achive the same power.
Now we measure over the starter (between case and cable bolt) while cranking, the ideal would be to have the same voltage as from battery but resistance will keep that from happening.
So we got an unwanted voltage drop on the way from battery to starter...how do we identify the problem ?
Grounding issues...measure from battery +(positive ground) to starter case while cranking.
Solenoid issues...measure between solenoid input and output while cranking.
Feed cables...measure between battery - and solenoid input or solenoid output and starter while cranking.
Take notes of the voltage readings and you will find the place where the voltage drops.
Taking it step by step will identify the problem.

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The first thing I would do when the warm engine failed to crank is to put a socket on the crankshaft pulley and see how easily my 1/2" drive ratchet can rotate things.