Scag Tiger Cub blowing fuses.

[Mr Mower Man]

I have in the shop right now a Scag Tiger Cub (model # STC48A-20CV, serial # 9261745) powered by a CV20-65597 Kohler. It's blowing one of the two 20 amp fuses, and I haven't figured out why just yet. Hopin' y'all had a better idea than me.

The fuse will last for maybe 2 or 3 minutes while mowing before it blows out. I discovered that this particular fused circuit is used by the charging system and the fuel solenoid. The other fused circuit (starting, PTO, hourmeter) is OK. One strange symptom I discovered is that the charging system doesn't work when the PTO is activated, but as soon as it's deactivated, the battery is charging again. The fuses have been blowing when the PTO is activated. The customer says he recently replaced the PTO switch himself not very long ago (exactly how long, I don't know). The switch he has is the 8-terminal Delta switch (Stens #430-798) which is the correct aftermarket replacement.

The charging system itself is fine. Stator puts out plenty of AC volts, and the regulator passed all tests (I tried a new regulator just for the heck of it -- no difference). My guess is that when the the PTO is activated, the charging current is being shorted to ground somewhere. Wouldn't that account for the battery not charging and the blown fuses? I can triple check, but the DC wire from the regulator looks good and doesn't appear to be touching ground anywhere. The most likely culprit seems to me to be this enigmatic little box called the interlock module (reference #17 in parts diagram). This thing looks like it's a part of every circuit in the machine. I don't really know the operation of this little jewel (over $100), and I don't know how to test it besides trying a new one. But I'd like to be reasonably certain that's the problem before I buy one.

I verified that the fuses are supposed to be 20 amp, and the charging system is rated at 15 amps. 14.8 amps is the highest reading I was able to get out of the charging system. The only other component I haven't comfortably ruled out is the relay. But I've never seen a relay cause a problem like this.

What do you guys think?

[Luffydog]

Sounds like a short try Looking at the electric clutch wiring. Sounds like the clutch heating up and then blowing the fuse. Clutch Is prob going out.

[KE4AVB]

Check your pto clutch current draw. It should be between 4-7 amps normally (coil resistance approx. 1.7 - 2.8 ohms). I have seen partially shorted clutch coils to pull as much as 15+ amps and still work. I had one that would blow 15 amp fuses but not the 20 amp; that was on a JD 737. The customer only knew something was wrong as the battery discharge light was coming on.

OGURA GT2.5-MC04 is a 12v 50 watt pto clutch.

[bgsengine]

Also check if the clutch is adjustable - the clutch air gap is important too to how much amps they draw..

[KE4AVB]

Also check if the clutch is adjustable - the clutch air gap is important to how much amps they draw..

BGS, I think your going need to explain this to me as I don't see how that going affect the amperage once the clutch is engaged. I can see how it could cause slippage at engagement and affect the clutch braking on disengagement. Other than that the mutual inductance should not be affected once engaged.

[Mr Mower Man]

I don't see how a short in the clutch can possibly be the cause. If that were the case, the other 20 amp fuse would be blowing. The fuse that's blowing is used by the fuel solenoid and charging system. But I'll check the resistance between the two wires anyway, to be a good sport. I actually have run into that problem before, with the ohms being too low and the clutch pulling too much current, knocking out the fuse. But this is the wrong fuse. I'm still suspecting that interlock module. Anyone familiar with the operation of that part?

[Mr Mower Man]

OK, you guys are smarter than me, and I'll be leaving my previous reply on here for posterity's sake, though it makes me look about as dumb as I am.

I measured the resistance between the two PTO clutch wires, and I got either 0.6 or 0.7, both of which seem too low. I'm waiting to hear back from a tech support guy at Ogura (he's helped me before), but I'm pretty sure my reading is below spec.

I also tested the operation of the charging system with the PTO clutch unhooked. When I activated the PTO switch, the battery was still charging, whereas it had stopped before! And just to be sure, I tried it again with the PTO clutch plugged back in, and then I had the problem again. Looks like my PTO clutch needs to be replaced, huh?

I was pretty confident the clutch couldn't be the problem, but I was obviously mistaken. This machine has two 20 amp fuses, and I would've expected a faulty PTO clutch to be blowing the fuse in the circuit that powers it. But this one is blowing the other fuse somehow. I'm at a loss right now as to how to explain that one.

[bgsengine]

Also check if the clutch is adjustable - the clutch air gap is important to how much amps they draw..

BGS, I think your going need to explain this to me as I don't see how that going affect the amperage once the clutch is engaged. I can see how it could cause slippage at engagement and affect the clutch braking on disengagement. Other than that the mutual inductance should not be affected once engaged.

takes more amps to generate the magnetic field necessary to pull it across the larger gap against the spring tension - same way a bigger spark plug gap causes higher amps required in the ignition secondary. (think in reverse) Have had experience with that over the years back when Deere, Wheel Horse and Cub Cadet were getting electric clutches and they mostly all had adjustments (3 bolt or 4 bolt, had to adjust to .010-.012 gap) - we'd see them start to blow fuses due to the increasing amp load, and adjusting the clutch air gap fixed that issue and reduced the amps draw. Not sure as I can explain the theory behind it correctly, but am sure it *does* happen... have actually measured that back when I was a "newbie" and my boss was explaining it - one slow winter we had one of them in and he was showing me with the ammeter how the clutch air gap affected the amps draw..
With most of the more modern clutches now though I rarely see that (and most of them are not adjustable anyway)

Anyhoo, it is probably a moot point if the fuse that blows is not carrying the load to that circuit - but if that fuse blowing is preventing the clutch from engaging, then it *IS* carrying power for that circuit somewhere. - So that is something for the O.P. to check on - when the fuse blows, does the PTO stop working? (assuming the engine stays running)

IT is possible the PTO engaging is causing the engine to shift (bad ground) and then the PTO is grounding by back feeding through the engine circuitry, thus blowing a fuse (have seen that before too on isolator mounted engines with a broken ground strap - Actually caught a plastic covered choke cable on fire once! - Electric clutch was grounding through choke cable to the dashboard.)

[Mr Mower Man]

Yes, the clutch would still work after the fuse blew. I basically figured out which functions were part of the circuit with the blown fuse by removing the fuse and noting what stuff stopped working. That's how I came up with the fuel solenoid & charging system. I was also able to engage the PTO with that fuse removed (without the engine running).