Troubleshooting Kohler 12RES Problem

sniffdogJune 13, 2010

I have a Kohler 12RES generator with an RDT 100 Amp Auto Transfer Switch.

Last week I noticed that my generator did not kick on for the weekly exercise test and that the red Source Available Light on the ATS was flashing rapidly. I tried starting the generator from the front control panel and the engine would not crank. When I put the generator on Run, all I heard was a rapid clicking noise and then nothing. No faults were displayed on the generator control panel. It did display a CC1 which indicates crank cycle 1 was attempted.

After troubleshooting per the manual, I discovered that the battery was dead  a 2 year old DieHard Group 26 which I thought was odd. I took the battery to Sears and they tested it  found it was bad. Bought a replacement, installed it, and was able to start the generator from the front panel. I cleared the fault on the Automatic transfer Switch per the manual and tried to run the generator remotely from the ATS panel using both the Exercise and Test buttons. In both cases, the generator started, ran for a little over a minute, and then shut down. When the generator stopped, I heard that rapid clicking noise again and then nothing. I looked at the front panel on the generator and noticed that it received a Stop command from the ATS. Generator Source red LED flashes rapidly after each start and then shut down. I repeated this process 3 times and same result each time.

The flashing red LED indicates one of two faults: Failure to Acquire Standby Source, and Failure to Transfer. Since both tests I ran were with no load transfer, I believe the first fault listed is the one. This fault occurs if the transfer switch does not sense voltage from the generator set within 78 seconds after signaling the generator to start.

I checked the output voltage on the generator terminal block and found that one side is putting out between 125 to 126 volts, while the other side is only putting out about 26 to 29 volts. It appears that the ATS is sensing that the wrong output voltage is being produced by the unit and automatically stopping the generator  as it is supposed to.

I attempted to adjust the output voltage by programming the controller and could increase and lower the side outputting normally from 125 up to 138, and then back to 125 volts. But the other side did not change  remained at a constant 29 volts during this test.

Does anyone have any idea which components on the generator could be causing this problem and how I can safely isolate which component is bad? It seems like it could be a bad voltage regulator and if so, I wonder if that component be replaced without replacing the entire controller? I am hoping this isn't something horribly wrong with the unit.


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It is probably not the regulator, as a regulator issue should affect both sides of the phase, not just one. Look for an open connection between the generator end and the terminal block. A loose terminal or bad breaker etc. If you dont find anything, time to look inside the generator end.

What kind of meter are you using? If it is a digital meter, the 29 volts is probably just inductive voltage. You may benefit from an old fashioned analog meter, Wiggy or test light.

    Bookmark   June 13, 2010 at 7:59PM
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thanks - I am using a digital meter. I found out that there is a Silicon Controlled Rectifier device inside the generator controller box. This device is controlled by the main computer and is used to adjust the output voltage of the generator.

I pulled the unit and ran through a series of tests using a procude I found in a Kohler Marine Generator service manual that happens to use the same type of ADC contoller that is in my unit. My SCR unit failed one of the test steps - so I ordered a new part. The service manual said to check this device (plus fuses) before replacing the main computer. I am hoping this 50 dollar part fixes the problem.

Once I get the part installed and tested, I will post the results. I will also check all winding and controller cables to make sure none have vibrated loose.

Along my searches on-line, I found a great source for Kohler generator parts: (or ASAP parts)

I was able to order the SCR as well as a Service Manual for the Kohler 12RES generator which I have been looking for quite some time. Once I get that manual I will be able to perform more detailed troubleshooting and maintainence. It seems like Kohler works extra hard to prevent the DIYer from doing anything more than changing oil and filters on the residential generators. The problem I have found is that the authorized Kohler service companies in my area charge exorbitent rates to come to the house just to look at the box, let alone repair it.

Even if the SCR isn't the issue, I want to pinpoint the problem so if I need to call in a service company I can tell them exaclty what the issue is.


    Bookmark   June 14, 2010 at 11:09AM
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Out of curiosity more than anything - is the dead side of the phase the same one that was a bit low when you installed the set?

    Bookmark   June 15, 2010 at 4:43PM
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I replaced the SCR module but that did not fix the problem. I also checked all wire connections and fuses again and found nothing wrong. I decided to call an authorized Kohler service company and they are coming Monday morning to look at it.

It is very odd that the ADC controller on the unit is not sensing an under voltage condition and issuing an alarm code, yet the transfer switch controller is sensing the wrong voltage and is issuing the stop. I am wondering if the ADC controller on the generator is bad.

I don't remember what the readings were after I installed the unit and did the startup checks.

    Bookmark   June 22, 2010 at 1:40PM
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I would love to know what the Kohler tech discovered. I am having the same exact issu on a unit that is located in a VERY remote location, so getting a tech there is almost impossible.

    Bookmark   July 1, 2010 at 4:27PM
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Had a similar problem on a marine kohler. After two weeks with a tech and taking apart the whole back end, replaced the scr module and controler unit, also getting funky readings from windings, turned out to be a fuel restriction problem. Aparantly makes the fancy controler go haywire. It's worth a check.

    Bookmark   May 6, 2011 at 6:41PM
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Ryan Raber

I'm having the same problem with my 12RES did you find the problem? thank you!


    Bookmark   March 23, 2015 at 6:56AM
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