804A with ~52hz and AC undervolt?

[Leeeeroy J]

Hello All

I recently purchased a MEP-804A with 12 hours on a rebuild out of the Oregon National Guard depot. At first intermittently, and now constantly, the unit now only provides ~30-50VAC @ ~52hz on each leg, and the undervolt warning light is constantly on.

I can increase the frequency/voltage slightly by turning the Frequency knob to the maximum, but it never approaches 60hz. All of the settings are at the default (setting the unit to run at 50hz drops the output even lower), and the 10A service outlet shows the low output from the moment the unit is started, with the AC interrupter open and no load applied.

The unit is clean inside and out (no rodent damage), and is mechanically perfect.

I'll be working through the undervolt diagnostic procedure out of the TM over the next day or two. But I'd appreciate it if anyone who has seen this before could clue me in on what's wrong.

 

[Triple Jim]

I don't know that unit specifically, but I can tell you for sure that the engine is running below speed if the frequency is low, so you may have more than one problem.

 

[Suprman]

There are adjustments on the governor module. I think one of the tms has the procedure. I had a gen once that someone had turned all the adjustments all the way down.

 

[Guyfang]

Before you try and "fix" your speed problem, think about your improper voltage problem. Fix the under volt problem, your speed just might work right. Fooling with the injector pump is always your last resort.

 

[Suprman]

Governor module is blue box back wall of control panel.

 

[Guyfang]

1-12.9.2 The governor actuator is a linear electromechanical actuator which controls the output of the fuel injection
pump in response to the electrical input from the electronic governor control. The FREQUENCY adjust potentiometer,
located on the control panel and adjusted by the operator, provides a signal representing the desired engine speed/
generator frequency to the electronic governor control. A signal representative of the actual engine speed/generator
frequency is sent to the electronic governor control by the magnetic pickup. Any change in engine speed from that
selected by the operator, as sensed by the magnetic pickup, causes the electronic governor control to increase or
decrease the fuel pump output to maintain the desired speed. The load measuring unit senses changes in external
load demand and provides a change signal to the electronic governor control allowing the control to start its response
prior to any actual change in engine speed. The generator set frequency and power output are indicated by the
FREQUENCY meter (HERTZ) and the kilowattmeter (PERCENT POWER) on the control panel. The FREQUENCY
SELECT switch is used to set the generator for 50 or 60 Hz operating frequencies (50/60 Hz sets only).

Perhaps if you go tothe -24 TM, and start at PDF reader page number 56-58, and look at the diagrams, it will help you understand how the gen set works. Notice that the Electric governor, (A5) needs input from the AC side of the gen set. Also notice that the A5 needs an input from the Magnetic Pick Up, (MPU) The MPU tells the A5 what speed the engine is at, when to apply Excitation voltage, when to lock out the starter, all kinds of things. When you try and change the the Frequency, with the Frequency Control Potentiometor, (R2), what you see is NOT the engine speed being changed. The Frequency meter, (M2) is connected to the Frequency Transducer, (A7). The A7 gets a signal from TB1, an AC signal. What I am trying to explain is that the M2 will not give you a reading if there is no AC voltage. Your main problem is no AC voltage.

1. Test the S1 to find out if its sending 24 volts to the excitation system.
2. Look at, test, and adjust the MPU.

These are the first two steps to take in troubleshooting.

Oh, DO NOT switch between 50 and 60 volts with the selector switch, (S12) while the gen set is running!

 

[mcii]

The old adage in the Generator world is ? Voltage follows FREQUENCY. Best I remember and its dark outside rt now and late to boot, but I recall a 4 or 5 position DIP switch selector on the GOV board. Get the TM and start by checking the switch settings .. Can or does your experience help you tell if the set is really running at or near 1800 or 1500 rpm, does your vom read Hz ? if so, even if the AC voltage is low, if it will read Hz ..then in this case each Hz is equal to 30 rpm.. so somewhere in here you can begin to tell your real engine speed. my logic in you wanting an outboard ability to read Hz is to really verify true engine speed, the freq meter within the set is transducer driven and if that has failed the reading can be all over the meter scale and you will have little confidence in a direction to start from. Let us know how it unfolds. mac/mc

 

[mcii]

In reviewing some of the past here, Guy Fang did a treatis on this topic back on 07-08-17 that would be a good starting point as well. give that a review as well... best mac/mc

 

[Ray70]

This is surely a long shot, but how long are you holding the start switch after you start the machine? I've only worked on one 804 but have seen ( by others ) that you normally have to hold the switch in the start position an extremely long time ( as compared to the mechanical governor controlled units ) after it starts before the Hz and voltage come up correctly. releasing the switch too soon after it starts can result in low Hz readings and incorrect voltages.

 

[Guyfang]

Not a long shot at all. We have all seen/heard it often enough here in the forum.

 

[Leeeeroy J]

Thank you all for your responses. I'm finally making the time to fully troubleshoot the problem.
As a first effort, I swapped the voltage regulator for a new unit, which (surprise!) did not fix the issue.

I'd like to specifically thank Guyfang for his posts and collected technical manuals - the colorized schematics are particularly helpful for understanding what is going on.

I'll update again once I have the issue fixed.

 

[Leeeeroy J]

Aaand the problem was fixed by simply holding the main switch in "Start" position for a full 10 seconds. Thanks everyone!

 

[robertsears1]

When I crank either of my 804s, I hold the switch in start until the oil pressure stabilizes which probably takes around 10”.

Robert

 

[Guyfang]

When your volts and freq meter come up to rated reading, its good enough.

 

[Leeeeroy J]

Yes, that's what was throwing me off- I wasn't waiting long enough for the oil pressure to build and for the generator to "flash".

For anyone else wondering what I'm talking about, here's a youtube video that demonstrates the process:

https://m.youtube.com/watch?v=mQgnuJPV9Gs

 

[mrosalik]

Our 804A likes to start and stall with under-voltage light then immediately re-start and run. But one of the guys twisted the Main Knob off the switch! He did McGuyver it. Need a new one.

 

[Guyfang]

You guys want the info on the right S1 to order? Or do you have that?

 

[mrosalik]

I looked up a part number in one of many pdf files. Below is what I found. I think Green mountain has available but back ordered. Pricey.

Switch and Knob Master 88-21160


does this part number make sense?

 

[kloppk]

GMG parts are way over priced in most cases.
The one they offer for $298 is the blue plastic version that is prone to breaking. It's the Kraus & Naimer CHR10-C57400.
I'd recommend getting a 75902LV from Electroswitch. Last I knew you could order one from them for $97.45 + shipping. It's build like a tank and is a direct replacement.

 

[Guyfang]

OK, Here we go.

NSN: 5930-01-498-9903 Part Number: CHR10-57410 This is the first generation switch. Works, but not the best quality. Looks like yours was one of these. Cost $113.

NSN: 5930-01-531-2972 Part Number: 88-22827, 75901Lj, CHR10-C57410FE, DHR10 C57410EF Third version, Much better switch. Cost $36.58

NSN: 5930-01-368-5161 Part Number: C10-57410E This is the second generation switch. Works, but not the best. Cost $70.

All of the above fit, and will work. The Producer of the switch, (and here in Germany is considered a VERY good company) is Kraus & Naimer.

I would plug all the part numbers and NSN's into my search engine, If price is your concern. If a quality switch is what you want, CHR10-C57410FE, DHR10 C57410EF would be my first choice.