I Flushed $1,200 Down the Drain on Generator Ignition: A 5-Step Starter Relay & Spark Plug Checklist

Who This Checklist Is For (And How I Wish Someone Had Given It To Me)

If you’re looking at a 20kW Kohler generator that won’t crank, or you’re spec’ing out a new install and wondering about the starter relay and the spark plugs, this is for you. I’m an installation tech handling generator service orders for about 6 years now. In my first year (2017), I personally made (and documented) 4 significant mistakes on this exact topic, totaling roughly $1,200 in wasted budget, rush shipping, and sheer embarrassment.

This checklist is the result of those mistakes. I now maintain our team’s pre-start checklist to prevent others from repeating my errors. I can only speak to domestic residential and light commercial installs. If you’re dealing with a 100 kW industrial unit or a marine generator in a saltwater environment, the calculus might be different.

There are 6 steps here. Step 3 is the one most people skip, and it’s the one that cost me the most.

Step 1: Verify the Failure Mode Before You Touch a Tool

Don’t assume it’s the starter relay. I learned never to assume the proof represents the final problem after an incident in September 2022. A client’s 20 kW Kohler (SDMO series) was dead. Dead silence. My gut said “starter relay.” The symptoms were textbook. I ordered the part, drove out, swapped it. Nothing.

Turns out it was the pulse fuel pump. It had failed *closed*, flooding the carb and hydrolocking the engine. The starter tried but physically couldn’t turn over. The relay was fine.

Action Item: Before ordering a single part, determine if the engine is locked, or if the electrical system is dead.

• Manual crank test: Disconnect the spark plug wire (Torch AC7R or equivalent). Try to turn the engine over by hand with a socket on the crank bolt. If it won’t move, it’s mechanical (often fuel or seizure), not electrical.
• Battery voltage check: A weak battery can mimic a bad relay. The Kohler controllers are finicky—below 11.8V under load, and they act dead.
• Listen for the fuel pump: On most standby Kohler units, when you hit the “manual start,” you should hear the pulse pump click for 2-3 seconds. No click? Start your diagnosis there, not at the starter.

Step 2: The “Click” Test vs. The Multimeter Test

So you’ve confirmed the engine isn’t locked, the battery is good, and the fuel pump clicks. You hit start, and you hear a single click from the control box. That’s the control board relay. It’s not the starter relay. The starter relay is usually the cube-style relay in the junction box near the battery.

I made the “what are the odds?” mistake here. I knew I should test the relay with a multimeter, but thought “it’s a Kohler, they rarely fail.” That was the one time it failed. Skipped the final review because we were rushing. $400 mistake for an unnecessary truck roll and a loaner generator while I waited for the correct part.

Action Item: Test the starter relay before you replace it. Here’s how.

How to Test a Starter Relay with a Multimeter

Set your multimeter to continuity mode (the diode symbol or the speaker icon).

1. Identify the pins. On a standard 4-pin Bosch-style relay (common on Kohler units, including the 20RESA models), pins 85 and 86 are the coil. Pins 30 and 87 are the switch.
2. Check the coil resistance. Probe pins 85 and 86. You should see a specific resistance (usually 70-100 ohms on these relays). A reading of 0L (open) or 0.0 (short) means the relay coil is fried.
3. Test the switch. Probe pins 30 and 87. You should read 0L (open circuit) because the relay is at rest.
4. Now, the step I forgot: Apply 12V from a battery (or the generator battery itself) across pins 85 and 86. While it’s energized, test pins 30 and 87 again. You should now hear a click and see 0.0 ohms or a very low resistance (under 0.5 ohms). If it stays open, the relay is mechanically stuck.

Most guides stop here. Mine doesn’t. I assumed the relay was good because the coil tested fine. But the contacts were pitted. It passed the bench test but failed under the higher starting current of the 20kW unit. See Step 3.

Step 3: The Load Test You’re Probably Skipping (This Cost Me $400)

Testing a relay at rest isn’t enough. The starter on a 20kW Kohler pulls easily 80-100 amps briefly. A relay with slightly burnt contacts will pass a multimeter’s test current (like 1mA) fine. Under load, the connection arcs, the voltage drops, and the starter just buzzes or clicks.

In Q1 2024, I had a job where the relay tested *perfectly* at the bench. I was installing it back in, thinking I was a genius. Then I used the multimeter with a headlight bulb. Connect the bulb to pins 30 and 87, jumper 85 to 86 with 12V. The bulb lit, but it was dimmer than a known-good relay.

Action Item: Do a simple load test. Use a 12V automotive test light (or a 55W headlight bulb) and compare the brightness to a known good relay. If it’s dim? The relay is toast.

Step 4: The Spark Plug Trap (Torch AC7R and the Gap)

Once you get it cranking, you need spark. The Torch AC7R spark plug is a very common OEM replacement for many Kohler engines (specifically the CH-series and some Command Pro engines). The mistake I made was trusting the “pre-gapped” label.

I ordered a box of 10 for our fleet. Installed one, fired it up. Ran rough. Misfired. I blamed the coil. The coil was fine. The gap on the AC7R out of the box was 0.045”. The spec for the Kohler CH730 engine in that 20kW unit was 0.030”. I skipped verifying because “it’s OEM.”

Action Item for Spark Plugs:

• Always check the gap. Do not trust the “pre-gapped” claim. It’s a mass-production part.
• Use a gap tool. Squeeze the ground electrode down. Do not pry it up on a platinum plug (the AC7R is iridium/platinum—prying ruins the center electrode).
• Anti-seize sparingly. On aluminum heads (which Kohler uses), anti-seize is good. Too much will foul the threads and mess with the torque reading. A dab, not a slather.

Step 5: The Pulse Fuel Pump (The Silent Killer)

This is a specific call-out for Kohler SDMO and some power generation models. The pulse fuel pump uses crankcase pressure pulses to pump fuel. It’s simple and reliable. Until it fails.

Remember my September 2022 disaster? The pump failed internally, letting fuel pass through into the crankcase. A hydrolocked engine forced me to waste a Saturday draining oil, replacing the pump, and changing the spark plug (Torch AC7R, properly gapped this time).

How to spot it without disassembly:

• Smell the oil dipstick. If it smells strongly of gasoline, your fuel pump is leaking internally.
• Look at the pulse line. Is there raw fuel in the rubber line that connects to the crankcase? That shouldn’t be there.
• If the engine is hydrolocked (manual crank test from Step 1), remove the spark plug. If fuel sprays out? It’s the pump.

Step 6: The Order of Operations (Or How to Avoid a Part-Cannon Approach)

I used to shotgun parts: “It doesn’t crank? Let’s try a new relay, a new starter, maybe a battery.” That’s a fast way to blow a $1,200 budget on parts you don’t need.

Follow this sequence:

1. Manual crank test (hydrolock check)
2. Battery voltage test (under crank load, if possible)
3. Fuel pump pulse test (listen and smell oil)
4. Starter relay bench test (coil resistance)
5. Starter relay load test (Step 3)
6. Spark plug gap and condition check (Step 4)

If you follow this list, you will not swap a starter when the issue was a $30 relay with burnt contacts. You will not order a new fuel pump when the problem was a $15 gasket kit. I’ve caught 47 potential errors using this checklist in the past 18 months. That’s about $5,600 in avoided reworks and wasted parts.

Common Mistakes & Final Warnings

Mistake 1: Assuming the multimeter tells the whole story. It doesn’t. A relay that tests good for continuity can fail under load. Always do the headlight bulb trick.

Mistake 2: Forgetting the engine ground. A loose ground strap to the frame mimics a dead starter. Clean and torque all grounds before you buy any parts.

Mistake 3: Over-gapping the AC7R spark plug. If the gap is too wide (over 0.040” on most Kohler standby units), the ignition module cannot generate enough voltage to fire under compression. You’ll get a no-start or a backfire.

Mistake 4: Ignoring the fuel pump pulse line. If the pulse line is cracked or clogged with oil, the pump won’t work. Inspect the rubber line before blaming the pump.

(Should mention: these prices are from memory and recent quotes. Verify current part costs from your local dealer. I’ve seen prices swing 15% in the last 6 months.)