My generac diesel generator only runs with prime button

If it is a Quiet Pac 75D, No AC voltage from Alternator.

My440.... Had that happen with a Honda 2000.. Would only run for a short time (it was the park's not mine) park workers were at the "Tearing their hair out" point when I spoke up and fixed it for 'em "Did you open the fuel cap vent valve?"

That's all it took.

Same problem I had would only stay running while priming. Discovered by accident it ran fine with the fuel cap off. Turned out to be a clogged fuel cap vent hole.

Hey, what can I say...I guess I am just a ""capitalist....and since my post was not addressed to you, you probably did the right thing by not reading it...probably would have bored you anyway

bounder39zman wrote:
THE PREVIOUS POST PRETTY MUCH COVERS ALL TH POSSIBLE PROBLEMS FOR YOUR SYMPTOMS. THE PRIME BUTTON IS ACTUALLY A PRE-HEAT (GLOW PLUG) SWITCH THAT ALSO RUNS THE ELECTRIC FUEL PUMP TO PRIME THE FUEL SYSTEM. THIS SWITCH WHEN ENERGIZED BYPASSES THE CONTROL BOARD AND APPLYS POWER DIRECTLY TO FUEL PUMP AND RUN SOLENOID. WHEN CRANKING, AS GENERATOR COMES UP TO SPEED AND A.C. VOLTAGE IS PRODUCED, THE CONTROL BOARD TAKES OVER, AND SUPPLIES POWER TO FUEL PUMP AND RUN SOLENOID. THEN YOU RELEASE START SWITCH, AND IT CONTINUES TO RUN, iF THE GEN SHUTS OFF WHEN YOU RELEASE THE SWITCH, THEN YOU LIKELY HAVE A BAD CONTROL BOARD, OR NO A.C. POWER BEING PRODUCED FROM GENERATOR. lIKELY CAUSES OF NO A.C. POWER ARE FIELD BRUSH WORN OUT OR STUCK, NOT CONTACTING ROTOR SLIP RINGS, BAD VOLTAGE REGULATOR, OR (WORST CASE)OPEN OR SHORTED WINDINGS IN GEN ROTOR OR STATOR. YOU CAN"T JUST JUMPER THE GLOW/PLUG/PRIME SWITCH, FOR VERY LONG, OR YOU WILL OVERHEAT GLOW PLUGS AND DESTROY THEM.
THIS IS A VERY DIFFICULT SYSTEM TO DIAGNOSE, AND AS O.P. SAID PARTS ARE GETTING TO BE SOMEWHAT OF A CHALLENGE. IF GEN END HAS GONE BAD (ROTOR OR STATOR) SET IS PROBABLY NOT COST EFFECTIVE TO REPAIR, THE CONTROL BOARDS ARE AVAILABLE, SEVERAL HUNDRED $$, AND MAY BE ALL YOU NEED.

Please turn off your caps lock key. If someone types in all caps and it is over 2 lines long, I skip it. (I didn't read this post.) And I am not alone.

THE PREVIOUS POST PRETTY MUCH COVERS ALL TH POSSIBLE PROBLEMS FOR YOUR SYMPTOMS. THE PRIME BUTTON IS ACTUALLY A PRE-HEAT (GLOW PLUG) SWITCH THAT ALSO RUNS THE ELECTRIC FUEL PUMP TO PRIME THE FUEL SYSTEM. THIS SWITCH WHEN ENERGIZED BYPASSES THE CONTROL BOARD AND APPLYS POWER DIRECTLY TO FUEL PUMP AND RUN SOLENOID. WHEN CRANKING, AS GENERATOR COMES UP TO SPEED AND A.C. VOLTAGE IS PRODUCED, THE CONTROL BOARD TAKES OVER, AND SUPPLIES POWER TO FUEL PUMP AND RUN SOLENOID. THEN YOU RELEASE START SWITCH, AND IT CONTINUES TO RUN, iF THE GEN SHUTS OFF WHEN YOU RELEASE THE SWITCH, THEN YOU LIKELY HAVE A BAD CONTROL BOARD, OR NO A.C. POWER BEING PRODUCED FROM GENERATOR. lIKELY CAUSES OF NO A.C. POWER ARE FIELD BRUSH WORN OUT OR STUCK, NOT CONTACTING ROTOR SLIP RINGS, BAD VOLTAGE REGULATOR, OR (WORST CASE)OPEN OR SHORTED WINDINGS IN GEN ROTOR OR STATOR. YOU CAN"T JUST JUMPER THE GLOW/PLUG/PRIME SWITCH, FOR VERY LONG, OR YOU WILL OVERHEAT GLOW PLUGS AND DESTROY THEM.
THIS IS A VERY DIFFICULT SYSTEM TO DIAGNOSE, AND AS O.P. SAID PARTS ARE GETTING TO BE SOMEWHAT OF A CHALLENGE. IF GEN END HAS GONE BAD (ROTOR OR STATOR) SET IS PROBABLY NOT COST EFFECTIVE TO REPAIR, THE CONTROL BOARDS ARE AVAILABLE, SEVERAL HUNDRED $$, AND MAY BE ALL YOU NEED.

Since the generator runs while the prime button is pushed, this pretty much rules out a fuel problem. The following is info for a 75D model:

Engine starts then shuts down when Start/Stop switch is released.
I assume that the Prime button ties in with Start/Stop

Engine oil level is low.
Defective Low Oil Pressure Switch.
Defective Coolant Temperature Switch.
Defective Control Module circuit board.
Coolant Level is Low.
Defective Low Coolant Level Switch.
Defective Start/Stop switch.
No A.C. output.
Defective engine control board.

If no A.C output:
Reset main line circuit breaker.
Generator internal failure.

Generac discontinued manufacturing RV generators several years ago, so servicing and obtaining parts will be a challenge.

At my job, when one of the diesels doesn’t run right, Someone usually yells out “It’s probably the fuel filter”, and then someone else yells “Either that or the carburettor”
Bunch of comedians at my job.

The PRIME button also operates a fuel pump. Now one of two ways

one: A separate pump (often in-tank) that lifts fuel to the main pump on the generator.

2 The main pump but bypassing a relay on the Generator's control board.

Now with in-tank electric (or even in line electric) pumps becoming more and more popular over cam-operated pumps of old. The control board, sensors and that relay are all suspects.

My first thought is fuel filter, with a diesel it's often the fuel filter.