Engine Scag / Kohler engine has me stumped

aloha

Forum Newbie
Joined
Aug 31, 2011
Threads
3
Messages
9
I have a Scag Turf Tiger ZTR with the 27 h.p. Kohler CH740 engine that has died and defies troubleshooting. The grass is growing and I am desperate for suggestions.

The Scag is 6 years old, 560 hours, and has been completely reliable up until now. 3 days ago I had just started mowing, and about 3-5 minutes after having started the engine the engine backfired very loudly and immediately died. (I had just started a right turn after having mowed a straight line for about 2 minutes when it happened, probably just a coincidence, but it did happen at the instant I started the turn.) When I looked at the engine the air hose from the air cleaner was off the carburetor, presumably blown off by the force of the backfire. When I crank the engine it fires and sounds as if it is going to run, but when I release the key from the start position the engine it dies.

I have confirmed fuel is being pumped and the carburetor fuel shutoff solenoid is operating correctly. I pulled spark plugs and one was damp with a gas odor. Visual inspection of the carburetor shows nothing amiss, linkages are working correctly, although the throttle valve moves little from the full open position, which is where I normally run it. It appears that the fuel supply is not the cause.

I have checked that there are no obstructions to the air flow and attempted starting with the air cleaner removed. I find no problem with the air flow.

I have replaced both spark plugs. I have used a spark plug tester that shows both cylinders are sparking.

The local Scag guy said he has seen similar symptoms with a broken valve push rod. I pulled the valve covers and all push rods appear sound and straight. Pushing on the rocker arms showed all valves moving freely.

I have gone through the troubleshooting steps in the engine service manual, http://www.kohler-engine-parts.opeen..._REV1010EN.pdf, and find no clues in those things I can do without special tools and which seem even remotely possible. I'd like to get a timing light on it, but it appears that would require pulling the engine from the Scag to get access to the timing marks, and with the engine not running except by the starter turning it there might be little to be learned.

The local Scag guy has 2 to 3 weeks of repairs backed up, so I am pretty much on my own. And having determined gas apparently ok, air apparently ok, spark apparently ok, and the main mechanical components apparently ok (at least those that can be seen without seriously tearing into the engine) I have run out of ideas.

So, someone please show me that I am an idiot and have overlooked something easy. Or even something not so easy. All ideas are welcomed. Thanks.
 

earthworm

Well-Known Member
Joined
Jul 4, 2011
Threads
20
Messages
307
Probably too late..
A compression test will reveal any broken push rods, or should somehow....plus this should be visual...Check the valve lash adjustment.
 

aloha

Forum Newbie
Joined
Aug 31, 2011
Threads
3
Messages
9
Thanks for the suggestion.

Compression is about 143 psi in both cylinders. Although the service manual does not say what it should be, that sounds about right. I pulled the valve covers. All push rods are intact and appear straight, valves move freely when the rocker arms are pushed.

I have concluded that this must be an electrical problem because the only difference between the spark plugs firing and not firing is the position of the ignition switch -- they fire when the switch is in the start position, stop firing immediately when the switch goes to the run position.

Today or tomorrow I should receive a new ignition switch and a relay/diode. Logically, the problem seems like it has to be in the ignition switch, but testing the diode indicated it had a short. Because all parts need to be shipped in I figured that I needed to order both rather than risk letting the grass continue to grow if the first part did not fix it. I will report on the results.
 

webuyanymower.com

Active Member
Joined
Aug 28, 2011
Threads
0
Messages
88
I think you are correct it does sound like an electrical fault rather than a mechanical engine failure. As far as timing light goes the flywheel is keyed and the ignition timing is pretty much safe although it is not impossible to shear the key it would be unlikely.
The most likely candidate would be the relay on the ignition circuit they do tend to fail at around 3 years so anything after that is a bonus.
 

aloha

Forum Newbie
Joined
Aug 31, 2011
Threads
3
Messages
9
As previously posted, I thought I had the problem narrowed down to either the ignition switch, since the spark plug stopped firing when the switch went from start to run position, or a diode, since my multimeter reported the diode was bad. Replacements for both parts arrived today. I tried the new ignition switch and new diode (which part includes a new relay), but there is no change in the symptoms.

A quick recap:
The engine fires and runs with the ignition in the start position, but the spark stops instantly on changing the ignition from start to run.
Disconnecting the wire for the kill circuit makes no difference.
All safety switches check out ok.
All wiring connections appear good.

I am well and truly stumped by this. There is an electronic module that apparently does some magic involving the ignition switch and safety switches, but the manuals give no clue on how to test it, and it is too expensive to get a new one on the off chance it is bad. Troubleshooting is complicated by the Scag and Kohler wiring diagrams not exactly matching what is on the machine (even though wiring is all original).

All suggestions will be most welcome. Thanks.
 

aloha

Forum Newbie
Joined
Aug 31, 2011
Threads
3
Messages
9
Oh, man, do I ever feel foolish, but I have learned something: Never trust a visual inspection to determine that a fuse is good.

The first thing I had checked when the mower stopped was the fuses. They both looked good. After having done a jumper wire test that got the engine running, I put a multimeter on the fuse for the ignition circuit. No current was getting through it. It still looks good, but I replaced the fuse and the mower now runs.

So, a visual inspection may tell you that a fuse is blown, but don't trust it to determine that a fuse is good.

I sincerely thank everyone who offered up comments and suggestions.

A new problem has surfaced, but hopefully I can sort it out myself. Again, thank you.
 
Top