Battery Voltage Too High Detected
What does KOHLER-28 mean?
The KOHLER-28 (Kohler RDC2) diesel fault code means: Battery Voltage Too High Detected. This is a moderate severity code.
Common Symptoms
- RDC2 controller display shows fault code 28
- Red or yellow fault light is illuminated on the controller panel
- Generator shuts down or refuses to start and holds this fault
- Battery may feel warm or hot to the touch
- You notice a separate battery charger plugged in near the generator
- Weekly exercise cycle fails to complete and the unit sits in fault
- OnCue Plus monitor or Mobile Link app shows a battery voltage fault alert
Probable Causes (Ranked by Likelihood)
- Float charger regulator stuck in bulk-charge mode, pushing voltage above 15V continuously Very Likely
- External battery charger or trickle charger left connected and overcharging the generator battery Very Likely
- Failed or shorted internal battery charging circuit on the generator control board Likely
- Wrong battery installed with a higher voltage rating than the 12V DC system requires Possible
- Corroded or loose negative battery cable creating a false high-voltage reading at the controller sense wire Possible
- RDC2 controller board voltage sense circuit failure causing an inaccurate high reading Less Likely
- Alternator on a connected vehicle circuit back-feeding into the generator battery terminals Less Likely
Step-by-Step Diagnostic Procedure
Set your multimeter to DC Volts. Touch the red probe to the positive battery terminal and the black probe to the negative terminal. A healthy standby generator battery should read between 12.6V and 13.8V DC at rest. Anything above 14.5V DC while the generator is not running is abnormal and confirms overcharging.
Check whether an external battery charger or trickle maintainer is plugged in and connected to the generator battery. If yes, disconnect it immediately, clear the fault on the RDC2 controller display panel, and retest.
Locate the generator's built-in battery charger, typically a small board or module near the control panel. Inspect the wiring harness connector for corrosion, burnt insulation, or swollen capacitors on the board. Any of these point to a failed internal charger.
With the generator in OFF mode, measure DC voltage at the battery again after 30 minutes with all chargers disconnected. If voltage drops back below 13.8V, the battery itself is healthy and the charger circuit was the culprit.
Inspect both battery cables end to end. Look for green or white corrosion at the terminals and at the controller wiring harness connection points. Tighten all connections to hand-tight plus a quarter turn. Clean corrosion with a baking soda and water solution and a wire brush.
If voltage stays above 14.5V with all external chargers removed and the generator not running, suspect the onboard battery charger regulator or the RDC2 controller board sense circuit. At this point, measure voltage directly at the B+ sense input wire on the RDC2 harness connector and compare it to the battery terminal voltage. More than 0.5V difference means a wiring or sense-circuit fault. Diagnosing this further requires a wiring schematic. Call a certified Kohler technician.
After correcting the root cause, navigate to the fault menu on the RDC2 controller display panel and clear code 28. Switch the panel to AUTO and confirm the READY indicator lights up without the fault returning. Run the generator manually for five minutes and recheck battery voltage to confirm it stays below 14.5V.
Frequently Asked Questions
What does Kohler code 28 mean?
Code 28 means the RDC2 controller measured the battery voltage above the upper limit it expects, typically above roughly 15V DC. This usually happens because a battery charger, either the built-in one or an external unit, is pushing too much voltage into the battery instead of dropping into a safe float charge.
Can my generator still run with code 28 active?
It depends on timing. If the fault trips while the generator is running, it may continue briefly but will likely shut down to protect the battery and control board. If the fault is present at startup, the RDC2 controller may block the start entirely. Reliability is reduced, so treat this as something to fix promptly.
How much does it cost to fix Kohler code 28?
If an external charger is the culprit, the fix is free. Cleaning corroded terminals costs under $10 in supplies. A replacement internal battery charger module is roughly $80-$180 in parts if you install it yourself. If the RDC2 control board itself is at fault, professional repair or board replacement can run $300-$600 with labor.
Will the generator start the next time the power goes out?
Possibly not. If code 28 is stored and uncleared, the RDC2 controller may block an automatic start when utility power fails. Do not assume the generator will come on during an outage until you clear the fault and confirm the READY indicator is showing on the controller panel with no active faults.