Scag SFZ48-26BS hard start when hot/starting issues

nullrout

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  • / Scag SFZ48-26BS hard start when hot/starting issues
So here is what I have going on.

SFZ48-26BS, sometimes it will just click (not turn over) when you go to start it. Most of the time you let go of the key, try again and it start just fine. This is an on, again off again issue. It doesn't always exist. What does exist ALL the time is is it will not start on its own when hot. Let's say you mow for 20 minutes then shut it down. You get nothing when you turn the key no click, no crank. It's like it's not engaging the starter at all...but here is the kicker, if you hook it up to a jump box or jumper cables, it will fire right up when hot and not hesitate or miss a beat...I'm at a loss, I would say starter, but it will start will extra voltage. It runs fine when it's hot, you just can't start it hot. I also don't know if the hesitation to start when cold is related or not to the hot start issue.

I'm thinking the hard start when cold issue is the starter solenoid but I'm not sure. I've also heard that not starting when hot could be a coil issue as the coil could have too much resistance when hot to fire the plug, but it runs great when hot, it just doesn't like to start...Thoughts on any of this?

Thanks,
Dan
 

bertsmobile1

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  • / Scag SFZ48-26BS hard start when hot/starting issues
Cranking circuit is battery , solenoid ( just as a contact to save wire ) fuse, start switch, PTO switch, parking brake, Solenoid trigger.
If the solenoid has 4 terminals ( 2 heavy switched ones & 2 thin control wires ) then there might also be a switch on the ground side of the solenoid as well.

Working perfect when cold but not when hot is generally a dirty contact, heat increases the resistance to a point where a circuit can not be maintained or in some cases physical expansion can cause contacts that are slightly burned not to come into contact.

Apart from cleaning everything, substitution with known good ones or bypassing one at a time is the way to diagnose.
You might like to give the following a thought.
It was written for a no-start situation full stop but should also work on a hot engine.

I like to start from the starter motor and go backwards .
Do the following 5 tests, regardless of the results from an or all of them as there can be more than one problem and you want to isolate where the problem lies.
Elimination of individual parts is important so you know by the end, the battery, solenoid & heavy power circuits are all in good order.

1) try to jump the starter motor directly from your car or truck.
Starter turns = starter good

2) do the same directly from the mowers battery
Starter turns = mower battery good
No turn = duff battery, recharge it & try again.

3) check for voltage ( + 12V ) at the solenoid trigger wire with the key in start position
3a) same with ground trigger wire ( 4 wire solenoid ) or body of solenoid ( 3 wire solenoid)
( I like to test V from the battery hot terminal to ground terminal rather than ohms as they give funny readings )

4) leave ground jumper in place ( from step 2 ) & try key start.
Starter turns = power connection good but ground connection suspect ( most common )
Confirm it by trying again, extra ground removed
I run a secondary ground from the grounding bolt to one of the starter mounting bolts & paint over both with liquid electrical tape.

5) Remove the trigger ( thin ) wire / wires from the solenoid.
Ground one & bridge from the hot terminal to the other.
Starter cranks = solenoid good.
Solenoid is not polarity sensitive, BUT THE WIRING IS so make sure you remove the thin control wires.
Note a thinner wire on the hot terminal is not a control wire. It is the main power feed to the mower.


From here on things become very mower dependant as starting circuits are getting changed all the time.
Basically the power goes in a loop from the hot side of the solenoid ( saves wire, no other reason ) through the fuse to the B terminal on the key switch then to the PTO switch then to the parking brake switch then to the solenoid trigger switch , easy peasy after you grow the 3rd arm. Use a test lamp and follow the power.
However a lot of mowers with a 4 pole solenoid, run a secondary ground control circuit to the ground solenoid wire through the lap bars.
Then to stop this interfearing with the normal safety function of the ground kill, it goes to a relay with the ground as the switched connection.
These are a PIA as the + control wire to the relay comes from the power loop above and the ground side of the control comes via the normal cut out functions of the lap bars.
Be very careful because if you have a system like this and accidentally send 12V down the ground loop you can fry the magnetos on some circuits.
 

nullrout

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  • / Scag SFZ48-26BS hard start when hot/starting issues
Cleaning contacts is certainty an easy and cheap fix so I'll start there. I just don't understand the two scenarios: (Cold) turn the key = nothing, turn the key again = nothing, turn the key again = crank and start. OR (Hot) Shut off, turn key = Nothing, connect jumper cables and turn key = starts like normal.
 

mcdonell

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Mine was the battery. I keep my Scag on a tender charger so it is fully charged when cold. When the battery got old, I had starting problems when the mower was hot. But it would jump off fine. So a new battery fixed the problem. I assume the 12v mower clutch was draining the old battery too much to start motor..
 
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