Anyone know the cranking amps for a riding lawnmower?
« on: May 28, 2011, 10:59:09 AM »
I need to build a temporary remote starter box for my riding mower using a momentary switch and need to know what current rating the switch should be.

Mower uses a 14.5hp B & S engine, standard 12V battery which tests at 12.75V.

I'm guessing at least 12A, but if the starter motors resistance is <<1ohm then the figure gets way higher.

This particular engine doesn't start super fast either so maybe 5 or 6 seconds will go by at full pull.

Anyone ever built a remote starter for a lawn tractor?


Re: Anyone know the cranking amps for a riding lawnmower?
« Reply #1 on: May 28, 2011, 11:24:26 AM »
Cranking current for cars is something like 100A+ IIRC. Batteries are good for this because they can put out huge current for short time. I would research the data sheets for battery used on the mower for some ballpark max current needed.

Don't only half-ass tune your drums. Visit to hear what properly "cleared" drums sound like.


Re: Anyone know the cranking amps for a riding lawnmower?
« Reply #2 on: May 28, 2011, 02:45:51 PM »
Can you not get at the starter motor to measure the resistance?

Re: Anyone know the cranking amps for a riding lawnmower?
« Reply #3 on: May 28, 2011, 03:46:20 PM »
From the hot bolt to ground yes.  Sometimes I get 1.2r steady, sometimes 0.3r.  I wouldn't be surprised if it dropped further and gave peaks in the ballpark of the figure John mentioned.  Those static resistance numbers suggest 50A.

I know people have made "plunger" starters for lawn tractors but I can't seem to find any info on the net. Best switch I've got on hand is 10A.  Best to stick with jumper cables for the time being.


Re: Anyone know the cranking amps for a riding lawnmower?
« Reply #4 on: May 28, 2011, 09:16:17 PM »
A marine grade momentary switch rated at 30 amps is good.  The starter switch on any tractor is not more than that.

Re: Anyone know the cranking amps for a riding lawnmower?
« Reply #5 on: May 28, 2011, 09:58:09 PM »
Cool. Thanks Mike.   


Re: Anyone know the cranking amps for a riding lawnmower?
« Reply #6 on: May 28, 2011, 11:01:58 PM »
> Cranking current for cars is something like 100A+ IIRC

Yes, even 200A, I have seen 400A on specs.

2002 Accord MT starter should pull 90 Amps NO-load. The stall current will be far higher.

As a wild stab, a 15 horse engine might start with 1/10th the ooomph of a 150 horse, so 10A-40A. OTOH the starter fights each cylinder one at a time, a 150 horse 4-banger only has 40 horse per cylinder, so the peak current may be 1/2.5 or 40A-100A. On the third hand, cars must start in sub-zero weather, a grass-mower only far above freezing. Accepting JR's "100A" estimate for small warm car, probably not over 40A.

> marine grade momentary switch rated at 30 amps

Surely good for a lawnmower. Put 40A through a 30A-rated switch, it will work many times. The rating assumes possible re-start far out to sea with a squall coming; if the mower burns its switch by the back fence you walk to fetch the jumper-cables for a sparky-start.

> switch I've got on hand is 10A

Will work once or a dozen times. If you have a box-full, it might get you through for a while. Possible worst-case is that it welds itself, starter won't stop starting, battery drain and wear on the pinion (very worst-case: motor rotor bursts from over-speed).

Permanent(?) solution: auto-parts, ask for "starter solenoid for a 1966 Ford Mustang 289CID". Ford used the same ~~200A relay from the 1940s into the 1980s. Because it is SO common, it is not expensive and often in-stock. Ground (B-) the frame. Battery+ to "Batt". Other big lug to starter. Now apply +12V to the small terminal nearer "Batt", it THUNKs and the starter should whirr. Drawback: this oversize relay pulls about 1A by itself, not insignificant added load on the pint-size battery.

The key feature is two 1/4" -copper- bolts and a 2"x0.1" copper disk. The disk shorts the bolts. Disk, so that each new start tends to be on a different part of the edge to distribute the damage over 6 inches rather than a 1/4" spot. Two contacts avoids a fat flex-lead and gives some redundancy against welding (one side may weld yet the other side may break). A mower-hack can be less robust.
« Last Edit: May 29, 2011, 11:53:12 PM by PRR »

Re: Anyone know the cranking amps for a riding lawnmower?
« Reply #7 on: May 28, 2011, 11:47:41 PM »
Thanks for the good tips PRR.   I'm actually just building an off board box to function as 'sparkless jumper'.   I'm keeping it off board for the very reasons you cited.  Another concern was that a weld during mowing could override the safety switches.  Last year I ran over a yellow jackets nest and literally leapt straight up off the mower by pure adrenaline reaction.  When MTD gets around to sending me the ignition schematic I'll hopefully be able to rule out safety switch issue vs possible burnt out ignition switch.


Re: Anyone know the cranking amps for a riding lawnmower?
« Reply #8 on: May 30, 2011, 01:27:23 AM »
Play with the switch on the blade-lever. On my Murray (same class of machine), the mounting was lame, it liked to work loose 1/2" so it never felt the disengage action. Took a lot of ductape to keep it in proper place. It isn't the cheap 25 cent electric switch, but the half-cent mechanical mounting.

> sending me the ignition schematic

It may be in the owner's manual? I went to MTD's site, Owner's section, Manuals, entered the Model Number of the first lawntractor listed on the Products page, and got a 20 page PDF with a schematic. Snip attached.

Yes, they are ALWAYS drawn for minimum comprehension. Sometimes unclear about NO/NC contacts versus lever position. Some recollection of how it used to work helps.

On my Murray, if I got off the seat, the engine died, UNLESS I had the brake jammed and the blades disengaged. If I wantd to run-over or cut-off my toes, I had to hold the seat down while dismounting. There was another interlock with the starter. On my Sears there is (was) a gearbox switch which had to be in N to work the starter.

It would be unwise to bypass these switches. Partly because the logic requires an NO and an NC contact for each function, and your brain can't keep it all straight how to wire bypasses. (The Sears was fairly simple and some previous owner bypassed everything for me. Start in gear with blades engaged while off the seat? Slice your toe and run over the stump? Sure!)

The core question is: how do you kill the engine?

Old-old true Magneto always runs unless you short the points to kill it.

Some mystery-modules, you must apply battery to get spark, break battery to kill.

It appears that some current engines emulate the old short-to-kill logic. You see in the plan below that the ignition switch does nothing when in "run". It isn't a run switch, it is a kill (and start) switch.

Now note what combinations of seat blade brake gear should or should-not start or run. Puzzle out the logic how to do this by grounding the magneto or opening the start relay wire.

Re: Anyone know the cranking amps for a riding lawnmower?
« Reply #9 on: May 30, 2011, 02:28:57 PM »
Excellent find PRR! - Thank you for posting this.  I guess MTD must have gotten more thorough with their manuals.  Mine is an older ('98) model and the manual is very basic with no ignition schematic or switch location maps.  The help desk has light techs who can only repeat what is in the basic manual.  You have to ask for a "level 2 tech" to get a schematic.  They are not as easily reached but the schematic you posted has the same wire code colors as mine and the same #  and type of safety switches.

Troubleshooting has pointed to either ignition switch or safety switch issue.  When turning key to start position I get a clean 12V at the ignition switch feed wire that attaches to solenoid (wire pulled off of solenoid). **when I initially checked for this I got no voltage under same conditions.  Most of the time it would do nothing when key started, a few times it would.  Perhaps a loose/faulty wire or safety switch that isn't resetting.  Tricky to pinpoint because I can't get it to do one thing all the time.

From other reading I've gathered that the brake safety switch on these is prone to malfunction due to the lightweight metal frame, causing the switch plunger not to reset properly.  Bigger problem - this switch is underneath the deck and will require getting the mower up on a lift and/or removal of the blade deck.  Apparently the frame may have to be hammered back in to position.

Q: If I can get it to not key start ( = no voltage at the iginiton switch solenoid feed wire), does jumpering a straight wire from "S" on the ignition switch to the solenoid tab look like a safe way to isolate whether it's a safety switch( or wiring in the vicinity) issue?   It appears this would bypass the safety switches for temporary isolation purposes.  I haven't checked whether there is voltage at the "B" terminal on the ignition switch under the 'no voltage on switch wire to solenoid feed' yet to rule out that stretch of wiring.  Today there is voltage and it's key starting ok.



Re: Anyone know the cranking amps for a riding lawnmower?
« Reply #10 on: May 30, 2011, 11:12:47 PM »
My dogs don't use the tractor, neither does anybody else here except me. And unlike where I had the Murray, there's no children around. So my Sears already has the start-safety bypassed. However I must say this is un-safe. Do as I say, not as I do. 

On yours, yes: a jumper from switch S tab to starter relay small terminal appears to give start-anytime with electrical safety (your toes are another matter), ASSUMING there is no short-to-ground in the wiring to the interlock switches. If you can remove the wires at S and relay and run your own wire, that would be best.

SAFETY aside: you "need" the 20A fuse to B, wire from S to start relay, wire from magneto to M, from G to frame (so you can kill the engine), and it is nice to have Afterfire to L. Alternator feeds battery _though_ 20A fuse. Start motor loop is unfused. All that other stuff just reduces the number of ways you can mangle your toes.

You can't go in reverse with the blades engaged?? Yes, I know that mowing backward is dangerous, and probably rates high on accident statistics, but unless the gear lever knocks the blade lever FOR you, it's just annoying. (And even then.... some corners, I do half my mowing on the backstroke.)


Re: Anyone know the cranking amps for a riding lawnmower? New
« Reply #11 on: May 31, 2011, 12:01:07 AM »
> no voltage at the iginiton switch solenoid feed wire

Jiggle the switch (of course). Take it out and glare at it: sometimes that helps. Inspect terminals for rot and corruption. Shake it: loose bits inside? Can you get it apart? Probably not; anyway the springs will jump out and vanish.

There may be a universal switch; decoding the connections may not be easy.

Ready to bodge?


12V 1A pushbutton from Red/Wht (fused) end of fuse to start relay: Crank

Magneto Yel wire to 100V 1A toggle switch to ground. "ON" is really "no-run" so mount it upside-down.

If the POP! at shut-down is distressing, change the no-run switch to 2P2T wired so in "run" one pole connects fused+12V to Afterfire, and in "Kill" the other pole connects Magneto to ground. Get it right, don't dump +12V to frame. (Have spare 20A fuses.)

If there are lights, and you care, they will work just switched to fused+12V. If you leave them on, the battery will drain. Many mowers instead have a separate output from the alternator: lights work (switchable) when engine runs but not from battery.
« Last Edit: May 31, 2011, 12:06:33 AM by PRR »


Related Topics

  Subject / Started by Replies Last post
11 Replies
Last post September 13, 2004, 04:02:22 AM
by t-wurst
14 Replies
Last post April 25, 2005, 06:07:06 PM
by mcs
4 Replies
Last post December 02, 2008, 04:17:20 AM
by CJ
18 Replies
Last post March 10, 2010, 10:12:30 PM
by toobdood