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post #1 of 4 (permalink) Old 10-28-2020, 10:48 AM Thread Starter
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Bench testing a riding lawnmower engine

I've got a nice looking 24HP Briggs ELS engine removed from a mower and would like to see if it runs. Came off a big cub cadet riding mower. Anyone have a written procedure about the temporary wiring that needs to be done....or maybe can answer my questions below.....

I put in fresh oil and a new filter already.] Plugs are good, carb was dry and looks ok. The engine wiring harness connector is intact.

Its easy enough to hook up a battery ground, and then touch a positive lead to the starter solenoid....I did that and the engine turns over as it should.

Next step: The fuel shut off solenoid and the coils must be powered for the engine to run.

I'm concerned about the charging system because in a number of engines there are charging diodes among the harness wires with the steering wheel housing....Will the anything on the engine portion of the charging system be damaged if it's not hooked up?? Should I leave the voltage regulator disconnected? There aren't any safety shut off features on the engine itself, right? All those are on the mower itself....like PTO no start, seat safety switch, and so forth.

I checked Youtube but didn't find anything.....
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post #2 of 4 (permalink) Old 10-28-2020, 09:49 PM
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The fuel solenoid need 9V or better to remain open and a transistor battery will usually hold one open for a few hours .
The coils are MAGNETOS & thus are self energising so you just hook a wire to each of them ( one wire each )
DO not connect them and leave them hanging
To stop the engine touch them to the crankcase to ground them out.
They are ground wires & must be either open circuit ( run ) or ground ( stop )
If you have connected any sort of battery to them then the coils will be toast.

Remember the torque reaction
The engine must be bolted down to something solid.
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post #3 of 4 (permalink) Old 10-29-2020, 12:09 PM Thread Starter
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bertsmobile....I was about to try to correct my original ignition coil statement when I saw your [correct] post.....thanks.


I'm thinking through the charging system connections. The stator [similar to that in an auto alternator] connects to the voltage regulator, the regulator needs to be grounded to operate, and any diode rectifying is likely done by diodes mounted [off the engine] in the wiring harness in the steering column/hood....I know from previously working on 120 amp alternators on marine diesel engines, manufacturers warn you NOT to run them open circuit.....

One alternative for bench testing my lawnmower engine without any mower body is to wire up the regulator, with its own ground, and connect the output to the 12volt battery. I'm unsure if I can just leave the regulator disconnected, leave the stator wiring hanging free and clear, and will post when I think I have figured it out.

Found an excellent stator and regulator test video here:

[Output of the stator alone, unregulated, on this particular Briggs is only about 26 volts....nothing to damage the coils.]

Bottom line: It's safe to run a Briggs engine with the stator wires or the regulator output disconnected or hanging open. Likely the same with Kohler. [This IS different from my experience with my boat diesel alternators]. No need to hang on the regulator and wire it all up for bench testing......

[This video says diodes are in the regulator, not aboard the mower....Maybe Kohler does it with diodes aboard the mower....don't remember where I came across that..]


For those wondering why I bothered posting here, I have been buying, repairing and reselling and modest number of riding mowers each year to keep myself busy...an auxiliary benefit is that my wife won't come down to the garage when I am working...cussing.

So far I have been avoiding rusty mowers that have been sitting but with good engines and wanted be sure I had the right wiring procedures to test run them when disconnected from their mower body wiring harness....trying to diagnosis old mower safety switch malfunctions is [so far] too much of a hassle.

Last edited by Robin; 10-29-2020 at 12:38 PM.
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post #4 of 4 (permalink) Old 10-29-2020, 11:25 PM
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You are still thinking in car terms
Mower coils are magnetos .
All rectification & regulation is done before the plug between the engine & mower .
Google "Toro Riding Products Demystification Guide " they are a free download from Toro but you need the Toro file numbers.
Briggs & Stratton put out a nice little book called "Engine Alternator Replacement Guide " Worth getting as well, also free off the web .

Mower engines mostly need no external power to start & run apart from the carb solenoid where fitted
Some have an engine management system and you cn tell thoce because they will have 2 or 3 small thin wires going into the coil .
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