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Ask Hank F. Your Own Question

Hank F.
Hank F., RV tech - emphasis on gensets
Category: RV
Satisfied Customers: 13927
Experience:  RV tech since the beginning of the millenium.
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onan 7.5HDKAJ spec A diesel generator..fault code..connector..injector

Resolved Question:

I have an Onan 7.5HDKAJ spec A diesel generator that won't crank. It throws the 37 fault code "Service Required." The fuel pump will operate and there is good fuel up to connector just below injector lines. 12.3 volts on chassis batteries. Engine turns by pulling on fan blades. Oil is full, water is full. Is there a reset button or any other ideas? Thanks, Curt  PS the fuse under the radiator cap area is fine.
Submitted: 3 years ago.
Category: RV
Expert:  Hank F. replied 3 years ago.
Thank you for choosing Just Answer for the solution to your problem.
My name is XXXXX XXXXX I am going to assist you with this.

There are 3 fuses behind the access cover where the radiator cap is.
Did you check all 3?

What is the battery voltage at the genset while the start button is pressed?
Customer: replied 3 years ago.
Hank, I only saw one fuse so I'll check tonight after work. Haven't checked voltage during cranking but will if all fuses are good.
Customer: replied 3 years ago.
Hank: Forget what I said this morning, the onan does not crank. Just blinks the 3 + 7 lights. And pulling on on the stop end the fuel pump will run.
Expert:  Hank F. replied 3 years ago.
I understand that the genset will not crank.

But I still need to know the battery voltage while the start button is pressed - it can tell us if we have an electrical issue somewhere.

Code 37 will not prevent the engine from cranking.
Customer: replied 3 years ago.
Will check tonight. Curt
Expert:  Hank F. replied 3 years ago.
No problem.
Customer: replied 3 years ago.

I checked the three fuses - all good. The battery voltage is about 12.3 resting, over 14 when plugged in and on the inverter. This is at the genset. It makes no difference if the starter switch is depressed because nothing happens when depressed.


I jumpered the starter solenoid and it cranks over just fine (should have thought of that sooner). If I had a schematic I'd know which way to go, but can't find one anywhere. Right now I'm baffled.


Can you tell me where the majority of controls circuits are and how to get to them? We do occassionally have pack rat problems, but I can't see any damaged wires but will look inside the set once I know how to get to them. Thanks!

Expert:  Hank F. replied 3 years ago.
On this set, the solenoid control power comes directly from the inverter.

Check for 12V at the small terminal of the solenoid while the start switch is pressed.
If there is no 12V, either the wire or connection between the inverter is bad, or the inverter itself is bad.

If there is 12V, the solenoid is bad.

I have uploaded a copy of the schematic for you.
Click here to download it.
Customer: replied 3 years ago.

Thanks for the schematic. One last question and we're done - is the controller/inverter in the area below the local start/stop switch? Thanks, Curt


Expert:  Hank F. replied 3 years ago.
Yes, it is.
It is very difficult to get out (major disassembly required) - and it is very expensive.
The last one I had to replace was about $1600.
Hank F., RV tech - emphasis on gensets
Category: RV
Satisfied Customers: 13927
Experience: RV tech since the beginning of the millenium.
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