# 25 HP Kohler



## Len (Jun 19, 2008)

Having a problem with a 25HP Kohler engine. It has the electronic advance on it. Anyway it will start just fine but as soon as it starts it cuts off. It is an electrical issue I’m sure of that. When I bypass the safeties by unhooking the white wire as the manual says same thing runs for a sec and loses fire. I can take a wire from the key switch or from the Battery and hook it to the red wire going to the to the advance module or from the battery to the purple wire that comes from the B+ side of the rectifier and the motor runs perfect. I replaced the rectifier / regulator and still the same runs for a sec. and shuts off. I’m getting 30 volts AC from the alternator, I hooked up a 6 amp bridge rectifier from Radio Shack in place of the factory rectifier and the motor runs perfect. Bought another new factory rectifier and same results motor runs and shut off. I know the motor starts on the battery and run on the alternator but can figure this one out. Any Ideas would be great


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## 30yearTech (Sep 14, 2006)

You can go to the www.kohlerplus.com site and download a service manual for your engine.

I have never run across a similar issue, it does sound baffling. Does your carburetor have a fuel shut of solenoid? Perhaps it's the culprit and you may be loosing power to it, causing the engine to shut down.


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## Len (Jun 19, 2008)

Thanks for the reply. 
The Carburetor solenoid is working fine that was one of the first things I checked. I have the wiring manual it’s been some help. But I’m not sure what else to do. I guess I will pull the flywheel and check the wires on the alternator


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## 30yearTech (Sep 14, 2006)

What is the model and spec number off of your engine??


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## Len (Jun 19, 2008)

I will get it Monday and post it. The motor belongs to a buddy I work with and it is at his house.


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## Len (Jun 19, 2008)

Model CV740S

Serial 3330820221


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## 30yearTech (Sep 14, 2006)

Len said:


> Model CV740S
> 
> Serial 3330820221


Need the Spec number (different from serial number)


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## Len (Jun 19, 2008)

Spec Num. CV740-0Q21


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## dawgpile (Apr 28, 2008)

I'm a little confused by your statement about running a wire between the battery and the violet wire(B+) from the reg/rect module and things working. Looking at the wiring diagram, it appears the pos batt lead connects directly to the violet lead at the starter. Connecting a wire as you describe is merely parallelling the existing connection between these two. (For reference, I'm looking at Figure 8-18 in the service manual). Am I understanding your statement correctly? If so, then it would appear there is perhaps a poor connection between the 3 leads at that starter terminal. (Similar comment applies to the other connection you made)

Also, have you checked the two diodes which appear to be embedded in the harness? Likewise, have you either replaced or jumped across the fuse which appears to be down by the starter? Fuses are notoriously poor from a reliability standpoint. 

Some food for thought...


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## dawgpile (Apr 28, 2008)

Len..just a further comment. I looked a little closer at the schematic representation in Figure 8-3 and note the following. My comment about the wire between the battery and violet wire still stands. However, when you connect a wire between the battery and the red wire of the advance module, you are in fact going 'around' one of the embedded diodes. Again, notwithstanding the comment about the battery to violet wire, I'd check the embedded diode with an ohmmeter. (Check 'em both while you're at it) It could be open.

Let us know!


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## 30yearTech (Sep 14, 2006)

dawgpile said:


> Also, have you checked the two diodes which appear to be embedded in the harness?


According to his spec number, his engine has the 15amp alternator, so it does not have any diodes embedded in the wiring. The diodes are all in the rectifier/regulator. 

Len,
Have you checked to make sure the factory regulator has a good ground, since it screws into the plastic blower housing, there is usually a ground wire or strap that grounds to the engine housing. Check and make sure you have good continuity from the rectifier to ground and that you are getting 12 volts dc from the B+ terminal on the rectifier.


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## dawgpile (Apr 28, 2008)

30YT...do you know that it doesn't have the embedded diodes from experience or from another source? Your comment implied that since it had a 15 amp alternator, it doesn't have them. I'll be the first to admit that I have little Kohler experience. I was using the service manual to provide guidance, and it indicates in there that all 3 variants(15, 20, and 25amp) have the embedded diodes whether it has fixed timing or variable. (I made the assumption, perhaps improperly, that the diagram shown in figure 8-18 of the CV17-750 service manual applied to his engine!) Is there a 15 amp variation you are aware of not shown in the service manual? I couldn't find any other. By the way, the embedded diodes I'm referring to are not used for rectification. Rather, they are used for blocking/switching purposes. I agree the rectifier assy also has diodes in it and they are used for rectification of alternator voltage.

I appreciate your insight and any light you can shed on this for me will be helpful!


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## 30yearTech (Sep 14, 2006)

dawgpile said:


> 30YT...do you know that it doesn't have the embedded diodes from experience or from another source? Your comment implied that since it had a 15 amp alternator, it doesn't have them. I'll be the first to admit that I have little Kohler experience. I was using the service manual to provide guidance, and it indicates in there that all 3 variants(15, 20, and 25amp) have the embedded diodes whether it has fixed timing or variable. (I made the assumption, perhaps improperly, that the diagram shown in figure 8-18 of the CV17-750 service manual applied to his engine!) Is there a 15 amp variation you are aware of not shown in the service manual? I couldn't find any other. By the way, the embedded diodes I'm referring to are not used for rectification. Rather, they are used for blocking/switching purposes. I agree the rectifier assy also has diodes in it and they are used for rectification of alternator voltage.
> 
> I appreciate your insight and any light you can shed on this for me will be helpful!


Perhaps I am not comprehending what exactly you are referring to.

Figure 8-18 for the CV17-75 service manual shows no diodes inline in the charging system, only the regulator. I know from experience and training (school) that there are no embedded diodes in the power leads from alternators that use regulators. 

Any inline diode in the alternator power leads will "block" current in one direction, and if used on an ac circuit will basically rectify the current. This is the system employed on the 3 amp charging circuit, crude but effective, for the simple purpose of charging the battery. This type of charging system can also be found on many Tecumseh and Briggs and Stratton engines.

Which diodes are you referring too?


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## 30yearTech (Sep 14, 2006)

dawgpile,

I think I see the light, or rather what is was you were referring to. I was talking about the charging system, and you were talking about the SAM wiring.

I think you may be on the right track as to the problem, I was just wanted to make sure the charging system was working properly, before continuing on to other possibilities.

My apologies


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## dawgpile (Apr 28, 2008)

BTW, it appears the purpose of those diodes is to allow both the starter solenoid to engage and the carb solenoid to be on with the key in the 'start' position as you'd expect, and when the key is returned to the 'run' position, the carb solenoid must remain on while the starter must shut off. In other words, they are used to isolate the starter solenoid from the carb solenoid. Can't wire them directly together or the starter would remain engaged in the run position.

Also, 30Y, perhaps you thought I was referring to the lower amperage charging systems which have their rectification diode embedded in the wiring and hence your comment about being a 15 amp system.

I believe the 15, 20 and 25 amp systems have the embedded diodes in addition to the rect/reg assy. They are needed if the engine uses a carb solenoid. Not needed if no carb solenoid but I didn't see any reference to this.

Let me know what you think... Thanks!


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## dawgpile (Apr 28, 2008)

Looks like we are typing at the same time!

Ignore this if you see the diodes in 8-18. They are just barely visible below the connector. Fig 8-3 shows them more prominently in schematic form.

If the upper diode in fig 8-3 were 'open', the engine would start just fine, but as soon as you moved the key to the run position, it would shut off the carb solenoid and then die. It wouldn't necessarily die right away since the kill wire is not grounded. It would die as a result of fuel stoppage. This is what I think is happening in Len's case.

The way to test this is simple. Turn the key to the run position, and using a voltmeter, measure the voltage right at the carb solenoid connector. It should be about 0.6V(the drop across the diode) less than the battery voltage if the diode is OK. If nothing, then the diode is bad.

Let us know Len!


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## 30yearTech (Sep 14, 2006)

Looks like were were typing at the same time again, so I will just edit this last post instead of making another.

Looking at the diodes you are talking about, it looks like they are both installed on "Red" lines suppling power to the SAM and Carburetor Solenoid. I don't see one inline to the starter relay. 

I believe these diodes are in place to prevent feed back from the power source and charging system. It looks like one side supplies battery power when starting, but once the engine is running power generated from the alternator, should keep everything functioning. I originally thought power was being lost to the carburetor solenoid, but Len states that it works ok if wired directly. Fact is, both the SAM and Carburetor solenoid are supplied by the same line, so if voltage is lost, this could affect both the ignition and fuel delivery on this engine. 

This could indicate a problem with the connection from the B+ side of the regulator, either at the connector, or a broken circuit through the diode. Don't You Think???

Len,

What kind of output are you getting at the B+ terminal of the regulator when the engine is running.


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## dawgpile (Apr 28, 2008)

Yes 30Y, I agree with you. Power is in fact lost to BOTH the SAM and carb solenoid if the upper diode in fig 8-3 is failed 'open'.

Len, as I mentioned in my previous post, start with everything off. Turn the key to the RUN position, not START. With the key in the RUN position, measure the voltage at the carb solenoid connector, or at the red wire connector of the SAM(Spark Advance Module). You should measure about 12 volts or so. If you don't measure any voltage there, move back to the ignition switch. You should measure 12 volts on two of the three red wires connected to the switch. One of those reds is the source from the battery/charging system(actually comes from the fuse down by the starter solenoid). It's 'hot' all the time, including when the key is in the OFF position. The other 'hot' red is the one going from the ign switch to the embedded diode which then feeds the SAM and carb solenoid. The third red with no voltage is the one which is only 'hot' when you turn the key to START. It feeds the second embedded diode which also connects to the SAM and carb sol but don't worry about this one. I think it's fine.

If you have voltage on two of the three reds, but not down at the SAM or carb solenoid(with the key in the run position), this points to an embedded diode being open. If you only have 12 volts on one of the reds, with the key in the RUN position, this points to an ignition switch problem, as opposed to an embedded diode problem.

Len, let us know if you follow this analysis. If you haven't done so, download the free service manual and you can reference the diagrams we've been mentioning. Take a few measurements and post the results. My first bet is that one embedded diode is failed 'open'. If it's good, my second bet is the ignition switch. Let us know!


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## gaftop1 (Jul 1, 2008)

i had a kohler 25 and the only problem i had with the thing was the sam going out (7 of them). i took it to my dealer and the only kohler engines he had on his bench were the kohler engines that had the sam. all had sam problems)
i lean toward the sam as your problem.
i guess i'm just  just a little ticked at kohler.
their engine now resides at the bottom of the intracoastal waterway. rust in peace.


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