# Lawn tractor won't start unless jumped



## edsaws

Its got a 18 hp briggs opposed cylinders. Starts no problem when jumped. Otherwise it turns real slow like the battery is dying. It has a new battery that reads 12.75 volts. I checked and clean the cables connections no help. Could it be the starter? Any suggestions?

Thanks, Ed


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## 30yearTech

When you jump this unit, where are you hooking up the jumper cables?

If your hooking it up to the battery, then you should have your battery load tested, as it may not be delevering the required amperage to the starter motor.

Since it starts ok with a jump, then the starter motor should be alright. The problem lies in either the power supply (battery) or delivery (cables, solenoid, connections, etc...)


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## bobotech

How much does the battery read when you are cranking it?

A tractor engine should only take a small amount of amps to turn over from the battery.

If the tractor battery is known good and the electircal system is known good on the tractor yet it requires a external jump to be started, it possibly could be the starter.

I'm thinking back to when I used to be professional mechanic and a dying starter could still start the car when it was jumped (a combo of say 500 Cold cranking amps from the battery and whatever else the jumping car provides) yet when it was jumped by the battery alone, it wouldn't start up the car. 

So it is still possible that the jump start is enough to overcome the strong resistance in a failing starter.

The big test is to see how many volts are there when its being cranked.

Well that's how we would test starters when I repaired cars.


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## bobotech

I would venture to say that its not the starter, i just don't think that it can be totally ruled out.


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## 30yearTech

Most small engine electric starters require a maximum of 18 - 35 cranking amps to operate, even the smallest lawn and garden batteries provide 165 cranking amps (5 times what is required to operate the starter), given the short run of the battery cables and the cranking amps available, it is unlikely that a "weak" starter would need the additional amperage of a jump start, as the electrical system is capable of delivering 5 times what is required, providing the battery is in good condition and all the connections are also good.

If the starter had a dirty or worn commutator, weak magnets or a bad bearing this would show up when the unit was jumped as well, if the starter is not dragging when it is jumped then it is most likely not the starter. These starters do not have the heavy guage field windings like automotive starters and they would not hold up to high cranking amps for very long.

If you feed additional amps to an electric starter that is overloaded, you will eventually burn up the field windings. Check and make sure that there are not other factor that may also be loading up the engine and requiring more effort to crank. (such as a mower deck that is not fully disengaging, etc..)

Keep in mind that if you are not getting sufficient cranking amps to the starter, continued cranking can cause the field windings to overheat and short out. Then you will need to replace your starter.

The starter cannot be completely ruled out unless it is fully inspected and tested, but based on your description I think the starter "should" be alright for now.


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## bobotech

Wow, I didn't know that garden starters only needed 18-35 cca compared to the batteris 185. Yeah, that totally makes my suggestions moot.

Like I said, I'm an old car mechanic and was applying automotive logic to a problem that won't occur in the small engine world.

Thanks for the info, 30year, explained a lot to me!!


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## edsaws

Ok today I started to get into it. I rechecked voltage still 12.75v then cranked it without the plugs in and it dropped to 8.4v. Even without the plugs in it it didn't seem to be cranking all that fast.


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## TechLarry

The only thing that makes sense to me here is the battery terminals being dirty.

That would account for it starting normally when jumped, and why it won't start on it's own even with a good battery.

Nothing else really makes sense.

Dirty terminals would allow the jumper cables to work, but prevent full current from the battery itself getting to the starter.

-Larry


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## bobotech

8.4 volts when cranking? Now again, this is coming from an old car mechanic, not a small engine mechanic. 8.4 seems just a tad too low and would be drawing too much amps and dropping the voltage too much. If I remember correctly, we didn't like the voltage to drop below 9.5 volts or so while cranking.

Agreed on the battery terminals being dirty could be a major factor. 

Try cleaning them all off with wire bursh and maybe some baking soda and water mixed (or coke).

They also need to be darned tight too. If yoiu can twist the terminals then they are too loose.


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## 30yearTech

The terminals on the battery cables can also have high resistance where they are crimped on to the cables, over time moisture can get into them and cause corrosion inside the crimp. Some times you have to cut off the terminal and replace them or the cables themselves. Cables on some riding mowers are very light guage and sometimes marginal at best delivering current. Also do not rule out the solenoid, they have contacts inside that arc and over a period of time build up resistance at the contact point.

Briggs service manual states that voltage should not drop below 9 volts when operating the starter under a load.


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## edsaws

Ok I going to aquire some new wire and termials tonight. I'm hopeing its that they are original although they don't look to be bad. Termials are clean. I do have a spare seloniod to try if wires don't help.


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## kallsop

I had the same problem with my 28N707 single. Turned out to be the valve timing was way off (because a valve cap was missing) and the compression very high. I chewed up the flywheel plastic ring gear before I knew it. Installed the aluminum gear replacement and fixed the valve timing and the engine now starts easy again.

Did you try removing the spark plug and starting the engine? If that turns easy, your battery / starter system might not be the problem.


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## edsaws

Ok heres an update I charged the battery. Replaced wires going from battery to selonoid and ground. Got it running but was running like crap so I got a rebuild kit for carb. from this forums sponsor http://m-and-d.com/?src=hbytlk. Rebuilt carb put it back on motor gave it a couple of cranks and it fired right up. Hasn't run this good in a few years. I checked the voltage with throttle wide open and I was getting around 17.5 volts. Does this sound right?


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## bobotech

17.5? That sounds a tad high in automotive terms. I think the max should be around 13.7 volts or so but maybe 17.5 is acceptable in tactor terms.


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## 30yearTech

It does sound high voltage wise, however the voltage will drop when a load is applied, all the testing in the service manual give amp ratings rather then voltage ratings for the unregulated power. If your engine has a voltage regulator/rectifier then the output voltage is too high.


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## murph518

edsaws said:


> Its got a 18 hp briggs opposed cylinders. Starts no problem when jumped. Otherwise it turns real slow like the battery is dieing. It has a new battery that reads 12.75 volts. I checked and clean the cables connections no help. Could it be the starter? Any suggestions Thanks Ed


I had the same problem and after many hours of fixing my tractor I always try and remember the first rule, which is to check the easiest thing first! I won’t bore you with the details but after half a day I found out my positive terminal was loose cleaned it up reattached it tractor started fine. I hope this saves people lots of time!


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