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Troubles with an AMBAC CU671C governor!


This is an old Libby Welding Generator Model# 20-924-77, built for the Federal Government. It is a...

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Old 11-14-2011, 03:17 PM
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Default Troubles with an AMBAC CU671C governor!

This is an old Libby Welding Generator Model# 20-924-77, built for the Federal Government. It is a 20KW, 120\240, 100 amp unit. It has a built in ASCO trasnsfer switch, and powered by a White D2300 diesel engine with a Stanadyne DBGFC-133-1LR injection pump. This set has a 32 volt start and control system.
It is installed at a remote farmhouse that experiences long and frequent winter power outages.
The generator had performed normally for several years, when it stopped exercising. It was some time before I was able to get to the farm and find an open MPU. I ordered a 250 ohm MPU and sent it to the farm where the farm mechanic installed it. The engine would then crank with no response from the governor. Some months later I went back to see what was going on, my first thought was that the MPU was improperly gapped, but it appeared to be correct with a 3.7 volt output at 1800 RPM. I checked all of the inputs to the governor and all appears to be normal. The AGB100 actuator makes a slight jump when the power to the governor is turned on. I replaced the MPU with another I had in my junk box that has 2K ohms resistance, it makes nearly 30 volts at 1800 RPM. There was still no response from the governor at startup. After much further fiddling, I found that if I moved the governor lever slightly into the run position, the governor would take over and operate normally. It would appear that the governor controller is not seeing (or recognizing) a sufficent signal to start the run sequence. I was able to get the generator to operate by screwing in the minimum fuel screw on the pump lever, then the engine would crank, start and the governor takes over and operates somewhat normally. This creates another problem when the generator runs at light load as the governor cannot slow the engine enough to maintain 60hz. The linkage is free of binding and is symmetrical throughout the movement range.
I am unable to find much information on the internet to pinpoint the proplem, it maybe just my own ignorance with the internet?
I have it in my mind that the controller is failing, from what little information I can find, that it should have worked fine with the 3.7 volt 250 ohm MPU. And possibly using the 30 volt MPU has only partially brought the controller back to life. The generator is 350 miles away, so doing any quick tests are impossible.
Anyone have any ideas?
Regards
Mike
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Old 11-14-2011, 07:16 PM
25Eagle34 25Eagle34 is offline
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Default Re: Troubles with an AMBAC CU671C governor!

Do you have the model and/or National Stock Number/ Federal Stock Number off the name plate of the gen set and I will see if I have a manual for same.
Don in Auburn, CA.
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Old 11-15-2011, 05:05 PM
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Default Re: Troubles with an AMBAC CU671C governor!

The model # is 20-924-77. I have not found a Federal Stock Number.
Regards
Mike
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Old 11-16-2011, 07:55 PM
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Default Re: Troubles with an AMBAC CU671C governor!

Golly!

Just some random thoughts on this that may or may not be of any use.

Right voltage to the controller?

Any other inputs such as an idle switch somewhere?

Can you make it work with the 250 ohm MPU? Bump the throttle make it come to life?

Has the controller been off? Any chance the controller linkage might have been disturbed/geometry different?

Not all that familiar with that pump but you may be able to disable the electronic governor and use the built in centrifugal/flyball/mechanical one. Military made them in utility and precise models, precise having an electronic governor. In that application you'd probably be OK with anything between 59 to 61 Hz.

A momentary solenoid to nudge the linkage and get it to start at which point it should operate normally.

Replace the Ambac with Woodward.

Will ponder/sleep on this one.
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Old 11-17-2011, 04:45 AM
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Default Re: Troubles with an AMBAC CU671C governor!

Thanks Gunny

Here is what I do know:

The controller voltage is correct at 32 volts (36 volts on the charger), the controller will manage 40 volts.

No idle switch on the unit.

The controller will not respond with the 250 ohm MPU. Controller is supposed to function at a minimum of one volt. Even bumping or pinning the throttle will not work.

The actuator linkage has been undistrubed utill I ran the low fuel screw in to make the engine start on it's own.

I tried to use the mechanical governor in the pump, it appears to be a dumb governor with just overspeed. It will not maintain a governed speed, it runs like the throttle lever is connected directly to the metering valve.

I like the idea of a bump solenoid, it just might make it work??
The current unloaded frequency is 64-65 hz and scares me just a bit.

This generator came out of an FAA site. It may have been built to thier specs?

Here is a link to a picture of the controller. http://lite.surplusman.com/Detail.ph...id=2366&imgs=1

Thanks Gunny
Still looking of the needle in a stack of needles!
Regards
Mike
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Old 11-17-2011, 07:18 PM
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Default Re: Troubles with an AMBAC CU671C governor!

Fuel pressure? Seem to remember that those military machines use a very small day tank with gravity fed to the injection pump. Might see what happens with 4 psi or so with an electric pump.

Consider looking for a replacement injection pump with a mechanical governor, probably find one for around $350 or so with some looking and patience.

Just a couple of thoughts.
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Old 11-21-2011, 04:30 PM
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Default Re: Troubles with an AMBAC CU671C governor!

Gunny
I made another trip over this last weekend to visit the genny. It does not have a day tank, but it does have an electric pump. Did not have a fuel pressure gauge but hotwiring the pump did prime the new fuel filters, so it does pump something. It exercised last Wednesday and has run for several small outages without any problems, I'm going to stop worrying about it for now and see if I can find a controller with a start fuel feature. They are available surplus and going for about $300.
Thanks everyone for the suggestions.
Regards
Mike
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Old 04-18-2012, 11:43 AM
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Question Re: Troubles with an AMBAC CU671C governor!

We have a farm in East-central Ohio (Amish country). We have a Warner-Swasey Federal aviation single phase 25kva genset for backup power that uses the same speed control unit - cu671c.

The unit was working fine until there was an electrical short in the block heater which apparently damaged the governor. I can start the continental engine and the generator works but the governor does not.

I don't know if the problem might be with the actuator, the governor, or possibly the connections to the governor.

Is there a way to test the cu671c connections to see if they have the correct voltages? The mil-spec connector goes to a connect strip so I can easily check each connection.
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Old 04-20-2012, 01:42 PM
turbodiesel09 turbodiesel09 is offline
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Default Re: Troubles with an AMBAC CU671C governor!

Google ambac Actuators You can find a lot of information on Different ones. I am not sure which one you have. I was reading, a lot of the time when you are checking a MPU you should get a 1-2 volt A/C (some times D/C) volt's at crank up. Woodward I think now own's Ambac I could be wrong. They own almost all the other governor companies. I hope this helps you out. You can go to there web site woodward.com and see if you can find a spec sheet. I know you can do this for some of the Barber Colemans. Ed
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Old 04-21-2012, 11:13 AM
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Default Re: Troubles with an AMBAC CU671C governor!

I made a Mistake Ambac is still in Busness I yahoo them last night and there are a list of controls and actuator's, I also found some spec sheet's that tell you what ohm and volt's to check your system's. When I get more time I will check them out again.
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Old 04-22-2012, 10:21 AM
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Default Re: Troubles with an AMBAC CU671C governor!

Wow Thank you!
I found the web site, AND found a trouble shooting guide at http://www.ambac.net/images/CU671C_Series.pdf
That will help a lot.
And they do have both the control unit and the actuators available for sale. It's amazing that they are still in use after 50 years.
It's also nice to know that there is one company that hasn't been taken over by Woodward ;-)
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