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Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 15010
Experience:  Certified on Onan and Generac generators
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My General 20kW generator Model 58750 (about 6 years old)

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My General 20kW generator Model 58750 (about 6 years old) apparently suffered a surge and all three 5 amp fuses blew. I replaced them and the display looks normal even when switched to automatic. When I do a manual start it starts but does not come up to full speed before shutting down with an overspend alarm. I tried manual start 3-4 times with same result. Can you help

What is the serial number of the genset?

Customer: replied 9 months ago.


The first thing you need to do is to connect a frequency meter to the output leads of the genset, and test the output frequency.

What frequency reading do you get?

Customer: replied 9 months ago.
The generator is my son's and will be a few days before I can get back to it. I did monitor voltage as it was starting and it only reached 55V and only ran about 3 seconds. Definitely not coming up to speed.
Also found that the battery is not charging. My understanding is that charge comes from 12V terminal on load she'd module in transfer switch. With 12V lead disconnected from battery, that terminal only has 0.4V. I'm thinking that module was damaged in the surge.

I need to know the frequency reading.

Customer: replied 9 months ago.
Sorry for the delay. I did several test runs, measuring generator frequency. Readings I got in Hz were: 41, 46, 47, 46. Each time it shut down indicating "overspeed" after just 2-3 seconds.

Right above the oil cooler is a stud with a wire #18 attached to it.

Set your meter to ACV.

Connect one meter lead to a good engine ground.

Backprobe the other meter lead into wire 18.

Crank the genset.

How much voltage do you see during cranking, and how high does the voltage go for the 2 or 3 seconds that the engine will run?

Customer: replied 9 months ago.
As I mentioned, this is my son generator and some 20mi away. I expect to be there Friday morning. Can I call you late Friday. I'll get the reading you ask for before I call.

I will be unavailable for phone calls for the rest of the week.

Customer: replied 9 months ago.
I get 5 volts on wire #18, the terminal you instructed when I start the generator. It still shuts down after 2-3 seconds.

Since the unit is only getting to 45 Hz, but is shutting down on Over Speed, and you have good voltage at wire 18, then the controller has been compromised in the power surge. You are going to need a new controller.

Customer: replied 9 months ago.
Where can I get the best price on the controller? Do you have the part number?

The part number is *****

I do not know the cheapest place to get it, as I have to get my parts directly from Generac.

Customer: replied 9 months ago.
I see an 0H6680D. Is it the same?

Yes. The D is just an upgraded version of the B.

Customer: replied 9 months ago.
I've ordered a board. Do I need to give you feedback that it worked?
Customer: replied 9 months ago.
I ordered the controller. Do I need to let you know when I have it operational?

Yes, you need to rate my performance for me to get credit for my time.

You can rate now if you like, or wait until you change the controller.

Customer: replied 9 months ago.
I received my part #0H6680D and the plugs do not come close to working. I seems my part is a #0H6680A. Does that make sense? Plugs are completely different and there is an extra white plug coming from the new board that mine does not have. Guess I need to see if I can return it. I attached a picture of the back of both control boards

The 0H6680A is no longer available. The new number is *****

But the 0H6680D is an upgrade from the B version.

I did forget that when switching to the D version, you must also purchase an upgrade kit that has the new wiring harness.

Customer: replied 9 months ago.
Do you have a part number for upgrade kit? Can you supply it?

The part number for the kit is 0H6680DSRV.

We do not supply parts. You would need to get it through a Generac dealer.

Did this answer your question?

Customer: replied 8 months ago.
I ordered the upgrade kit. Should receive this week. Will let you know.


Customer: replied 8 months ago.
Still problems. I purchased Adapter Harness 0K6680TSRV, p/n 0K1756. It plugs into the new control board ok but the other end does not match th generator plugs.
Wire numbers on the harness match wire numbers in generator wiring. I suppose there is no reason I couldn't cut the plugs off and but splice the wires or does it take a different harness?

It should not require a different harness.

As long as you get all the wire numbers matched correctly, there is not reason you should not be able to splice it.

Customer: replied 8 months ago.
After 3-4 hours of tedious wiring identification, soldering and heat shrinking, I got th new controller board installed. It worked an the generator is operational.
Do you know if anyone takes the old controller and reconditions it?

There is no one that I know of that rebuilds those boards - not even Generac.

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