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On engines with compression release valves, the recommended method for testing compression is turning the engine in reverse direction. I'm working on a lawn tractor (with a Kholer K241AQS), so there aren't a lot of fancy electric systems. What complications would arise from simply plugging my battery in backwards? (Being DC, my starter should run in reverse direction.)

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    Using a chassis for a ground is one thing, but using it for the hot side is asking for a fire.
    – cory
    Aug 29, 2016 at 19:35
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    @cory, why? I mean, what component would cause a fire if the electron flow is reversed?
    – anonymous2
    Aug 29, 2016 at 19:44
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    When you do whatever, ensure you disconnect the battery before you start. This could physically disasterous if something should happen and the starter engages going forward.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Aug 29, 2016 at 20:42
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    You never turn it backwards for a compression test, where did you get this information?
    – Moab
    Aug 29, 2016 at 20:55
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    @cory It's not easier to short to ground, because the only things which are "grounded" are the things which were formerly "hot" - and which are presumably insulated. The fuse is also still close to the battery, and will still work in exactly the same way; the only real reason to not fuse the negative side of the battery is that the usual wiring layout makes it difficult to isolate circuits on that side. The confusion likely comes from using "hot" and "ground", which are terms from the AC domain where the "hot" is a voltage relative to the earth/ground - which is everywhere and hard to avoid.:)
    – dannysauer
    Aug 30, 2016 at 21:16

4 Answers 4

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This engine is equipped with a voltage regulator. Depending on the exact construction of the regulator a reverse current would be disastrous, especially the high current from a starter battery enter image description here

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  • Thanks, @Myself. This was kind of what I was looking for. So flipping the battery around would blow up my voltage regulator?
    – anonymous2
    Aug 29, 2016 at 19:43
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    And since there would be lots of heat generated, you could melt or fry other things as well. Connecting your battery backwards, even on something this straight forward is asking for trouble and fried parts.
    – cdunn
    Aug 29, 2016 at 20:06
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    Most possible, yes. There are some kind of voltage regulators around who are immune against it, but we cannot be sure about the internal construction of the regulator. Besides, nobody will give you a guarantee that there are no other traps hidden.
    – Martin
    Aug 29, 2016 at 20:07
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    All automotive-grade electronics must be designed to protect against reverse polarity on the battery - it's one of the standard parts of making something automotive-grade electronics. It's possible that your lawn tractor isn't using a standard off-the-shelf alternator, but it seems unlikely they'd design it themselves at a much-increased cost, when they can just buy a standard item.
    – Graham
    Aug 30, 2016 at 12:16
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    On designing for reverse polarity: electronicdesign.com/power/…
    – bwDraco
    Aug 30, 2016 at 18:01
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Why not just put a wrench on the crankshaft? Ignoring the electrical repercussions, I fear damage to the starter, too. Besides, if the starter on this tractor uses a sprag clutch (and probably most other designs), it wont engage backwards, anyway.

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  • Good point, @justinm410. I believe my motor does have a sprag clutch.
    – anonymous2
    Aug 29, 2016 at 19:42
  • And BTW, I would opt for a rope around the flywheel, as I'll be better able to get the motor going fast enough for a good compression reading. Just means taking the motor covering off.
    – anonymous2
    Aug 29, 2016 at 19:57
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Looks like the starter drive is the typical spring loaded drive gear that engages when the starter motor turns in the "forward" direction, then disengages if the engine starts running and overdrives the starter gear forcing it to disengage. So if you run the starter motor backwards, the starter gear won't engage at all, and the starter will just spin. And without being loaded down by the force needed to spin the engine, it may spin too fast and become damaged.

So, even if your electrical system could handle it (and it almost certainly can't), the starter won't run the engine backwards.

enter image description here

(image from partstree.com)

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Even if there are no polarity-sensitive electronics to damage and the starter motor was able to engage, it is unlikely that the motor would rotate backwards anyway.

Most car starter motors (in fact all that I have worked with) do not use permanent magnets, instead opting for coils for the stator. I suspect the same may be true for your lawn tractor.

Therefore both the stator and rotors are using coils from the same feed and reversing the polarity of the feed reverses the direction of the magnetic fields on both the stator coils and rotor coil - the result being that the motor will rotate in the same direction as before anyway.

On a permanent magnet motor, you effectively just reverse one magnetic field (the rotor) when reversing polarity, and that it why they will rotate in the opposite direction.

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  • Good point, though in my case it is indeed permanent magnets - it's an old tractor without many of the modern features.
    – anonymous2
    Aug 30, 2016 at 11:35

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