Generac quietpact 75d Diesel Generator voltage dropping under load

I've searched the forum and many others trying to find answers to this topic, but most are just a little different or don't quite fit the bill.  We bought our coach a few months ago (01 Rexhall Aerbus), and when I checked it out with the mechanic, the generator ran fine, including with both A/C's running and microwave.

It's a -- Generac Quietpact 75d  7500watt

About 3 days after purchase, the A/C wouldn't start up well on the Genset, but on shore power it all worked fine.  So I started to inspect the generator and opened the junction box right on the firewall where the main leads from genset come out and connect to the RV leads.  I put my leads on and tested power with NO load, and I have 125 VAC at 62hz.  It will run like this for hours, and I can run the tv, and other small things just fine, but as soon as I apply a larger load, like microwave or A/C compressors, the voltage immediately drops down to around 60 VAC at about 20hz, however, there is absolutely no change in engine speed.  It doesn't even hesitate for a second.

I have panel access to the control board.  I have increased governor speed a tiny bit and VAC and HZ went up respectively, so the governer seems fine.  Wanted to get advice to see if the slip rings or voltage regulator or whatever else can cause this behavior so I know where to look next.  BTW, getting to the slip rings/brushes is gonna be quite difficult.  Also the belt and tensioner will be hard as well, so I want adive to see if this is something on the control board or regulator cause those things are easy to access. Thanks

Here's a link to the manual for your generator. maybe that will help.

https://generator-parts.com/manuals/generac-rv/0F4996.pdf

Thanks!  I have a little older copy of it, so this helps to have, but I don't see anything different than what I have or have tried.  I followed the diagnosis flow path, but it doesn't have much.  I read the onan manual and it is way more detailed in diagnosis, but both of them really only address "generator stalls/dies when load is applied" and for that it says to adjust the governor, which I did a tiny bit, but it's already at the proper no load volts/frequency. 

It sounds to me like the governor is not detecting and reacting to load changes. The governor has to open the throttle when the load increases in order to maintain rpms and this voltage and frequency. If it does not, you get the results you observed. Sometimes the problem is the governor itself and sometimes it is fuel delivery. If the governor opens the throttle but insufficient fuel flows, the engine dies anyway. Something as simple as a clogged filter or a crack in a fuel line can do it.

Oh, okay.  I thout because it was "constant speed" it wasnt supposed to increase with load, but that makes sense then.  I saw on another similar post that a guy had a "stuck" governor assembly, but I just figured he had a variable speed or something different than mine.

I already have the pane off so I can get right at the governor.  I will grease it and excercise it tomorrow morning and check results, and start down the road of fuel delivery if that doesn't work. Thank you sir, Ill post back in the a.m.

It IS constant speed, but governor is responsible for making it that way.  The generator engine slows down as the electrical load increases, requiring more horsepower to run the alternator. The governor senses the falling rpms and opens the throttle to get back to that "constant speed".

Chances are it is fuel delivery issue rather than the governor itself. Good luck with the hunt for the problem!.

Okay, I'm about to be home to look at the governor again, but I want to just make sure we are on the same page before I go down this road.  When I apply the load and the voltage and frequency dips, I do not hear any change in the engine RPM, as if the governor IS doing it's job and applying more torque to keep the rpms constant even though there is load.  However, I have human ears, so maybe that sort of decrease in engine speed is something I might not hear?  Just want to make sure you knew that part and still think it's probably fuel or engine speed not compensating correctly.  Thanks again for your help!  I'll take a picture of the front of the unit as well for you guys with the panel off so you can see the guts, including governor and such.  Be back soon.

You should hear the engine noise increase as it burns more fuel to get the horsepower to drive the alternator. I can't imagine there is no audible reaction when the load increases more than 1200 watts (a/c compressor starts).  The rpms should start to fall and then the governor opens the throttle to bring them back up. The diesel gulps more air and fuel to do that, so the exhaust noise increases. Maybe it's just hard to hear the changes, but the frequency and voltage readings are essentially a tachometer.

I suppose it is possible the alternator simply isn't putting out any more current despite the increased amp draw, so there is no additional load on the diesel engine, so no audible change.  I believe this is a direct power generation system, i.e. an alternator produces the 120v power directly (not an inverter-type). Instead of consuming more horsepower, the alternator just drops its output and the load on the engine stays the same. That would explain why you hear nothing different from the engine. I think that is possible, but am not enough of a genset expert to say so for sure.

Let us know what you figure out.

If the volts and the hrz drop like that, engine speed is likely the problem . Can you try and throttle it by hand? I had a 7000 watt generac an when I loaded it the engine would slow a bit and pick right up. The hrz is the sign that the engine speed is too slow. Don't use grease on the governor, that will attract dirt and cause more problems. I would try cleaning it with WD40.

Thanks guys.  I got to mess with it yesterday some more.

Gary: you are correct, according to the manual, it says it's an alternator type, not inverter. 

Hi 92:  I am able to advance the governor by hand, but only a little bit, as it seems to stop after about a  1/8 inch.  I did use a WD40 competitor product, can't remember the name right now. The engine does respond, and I see a direct increase in no-load voltage and hz.  I even ramped it up to 129V @ 67hz with the adjustment screw, then went to apply the microwave load and again, it just drops right out to 40-50 volts, 20-30hz, and the engine speed does NOT seem to change one bit. 

I'm not sure, but should the governor be able to move quite a bit beyond just a little bit with that slight increase in Volts/hz, like should it be able to really move up, or is it normal how I described it?  I've also read a forum response where someone thought the belt might be slipping, but from what I can see (and that's quite minimal being that it's not on a slide, and the main chassis frame rail is right up against it), the tensioner pully seems to be pushing down on the belt, and I don't see/hear any squealing or slipping.  I did get much closer this time when my wife applied the load, and it seems like I hear a Popping or clicking sound coming from the alternator and it seems to disappear when the load is released, but I've read that can be normal, so I don't know.  It's like an electrical pop sound, that's quite random every few seconds. 

So I'd like to start testing Ohms across the various leads from the windings and brushes/slips, but that will be really hard to access, and I don't know if that's even necessary cause the user manual says nothing about that for this type of issue, although it doesn't really address this anyway, like I said, it only addresses "engine starts, but dies after a few seconds/minutes"

I don't know enough about testing an alternator to help you much. You already know that it fails to maintain frequency and voltage under load; the only question is whether this is caused by low rpms or an internal failure. I think you need to somehow assure that the alternator rpms are correct and remain correct as the load comes on. If the rpms stay up but the alternator output fails, it seems to me that says the alternator itself is bad. If the rpms dn't stay up, then the governor is bad or the engine isn't responding to the governor. Is there some way to put a tachometer on the alternator? Or the engine pulley that drives it?

I think there is some sort of feedback loop to the "exciter" that regulates alternator internal operation and thus controls its output. I know car engine alternators have that and lacks of it causes the alternator to produce a fixed level of output.  Wonder if that feedback wire broke off somehow?  Just guessing - that's too deep for my limited alternator knowledge.

I've just realized I never posted the outcome of my problem on this thread and I wish more people did that, so although it's old, hopefully it will help somebody who might experience the same symptoms.

It turns out, the reason for the immediate drop in Voltage was because my belt was slipping on the alternator pulley as soon as the alternator built resistance to spinning with the load applied.  I was able to quickly confirm this because a gentlemen in the spot next to ours told me about using "belt conditioner & anti slip" spray, so I went and bought a can from Auto Zone and wow, it stopped slipping and my voltage drop was only a few volts when I turned on the microwave to test.  This only lasted a couple days until the tackiness of the spray went away, so I knew I had to either tighten the belt tension or replace the belt altogether.  I had no way of accessing the tension pulley, or replacing the belt without dropping the unit out of the coach, because part of the left side panel is tight up against the chassis frame rail, so I ended up using a 3inch hole saw, and EXTREMELY carefully cut a hole thru the side panel (1/8" thick sheet metal) where the tension pulley is located, and made myself an access hole to stick my ratchet extension thru.  The pulley is just like any automotive tension-er, except instead of being spring loaded, it has a bolt to set it into position, so you loosen that bolt, then use a 3/8 drive socket ratchet to turn the pulley tighter, while you then tighten the set bolt to lock it into position.  After I adjusted the pulley, the belt stopped slipping altogether, so then I just purchased a rubber hole plug/grommet online and plugged it up until the next time I need to adjust.

Thanks for the follow up..And good job finding and addressing the problem.

Thanks for letting us know. It obviously was not a governor problem, the engine speed didn't change but the frequency and voltage dropped off. That puzzled me until you said the belt was slipping, I thought it was direct drive.

Yes, thank you very much for letting us know. You may have just answered several other people with similar problems, and with a practical solution at that!

I drew up and attached a quick diagram of where I drilled the 3 inch hole.  It's obviously not to scale or measurement,  but hopefully should give a general "idea" of my solution so I have access in the future.  I always like when I find helpful forum threads that people have included instructions with pictures. 

) )

At some point in the future...Some person will find this quite helpful. Thanks for the followup.

One day a chill will run up your back.....It will be that person saying THANKS.

Rv Rob -
Thanks for diagram. I was going to use it. Unfortunately I can?t get to location due to frame. I?m having issues with Glenn showing 120 at red 120 at blk, but when transfer switch opens Volta drop. I?m thinking same challenge your we?re having now that I?ve eliminated other possibilities.
Tomorrow I?m going for the spray to see if that helps.
WhT exactly is the spray I?m looking for?

RV_rob said:

I drew up and attached a quick diagram of where I drilled the 3 inch hole. It's obviously not to scale or measurement, but hopefully should give a general "idea" of my solution so I have access in the future. I always like when I find helpful forum threads that people have included instructions with pictures.

Click to expand...

thank you very much for this information ,I was searching all over for someone who deal with the same problem ,and I found it here,
I have exactly the same problem with my generator, and already spent $250 for the modular, which working just fine.
my generator after full load dropping voltage and make a clicking sound, so now I know what I need to do,thank you very much for sharing info here, it’s very helpful.