Onan Generator Problem

[mstailey]

Good day to all,
Have an Onan generator 7.5 MDJE series and last night when running on the hook opened up the fridge and the light was flickering. Both A/C compressors were running as well. I looked at the voltage meter and saw it swinging between 100 - 125V. Also note the load showed 3200 watts. My first thought was to shut down the A/C pronto as to not burn out the compressor motors. Noted the engine on the gen was not loping or hunting and was running at a steady state (acutal RPM unknown). I just spent a ton of money having the injector pump and injectors rebuilt and the fuel shut off solonid replaced.

When first powering up and starting the first then second A/C units I'd hear the gen rpm's lower a bit then recover to a constant state.

Any ideas on where I need to start or what to look at?

Thanks in advance.

 

[Avenger]

If your RPM is stable it's not a fuel sytem or governor problem. If you have a frequency meter you can see if your RPM is correct and stable. Voltage issues are in the "back end" maybe tired brushes (if this model has them) bad voltage reg. or wiring issues. The "back end" is not my area of expertise on generators, I'm sure some of our other members can steer you right if your freq. checks out okay. Good Luck!

 

[67hat34c]

on the older units with brushes, the contacts on the armature are insulated with Mica. the mica may need to be cleaned out with the use of a hack saw blade. The manuel should have this procedure in it. not aware that these units have a voltage regulator as they run steady rpm and the rpm's dictate the frequency and voltage. brushes and contacts would be a good place to start that is if yours has them.

Shop manuels are typicaly available for these, they are not that complicated so you should be able to ring out the problem and fix it.

 

[TedZ]

Hi,

I wanted to try to help...sorry this is the first chance i've had with internet service.

There are three things that cause varying voltage commonly. One is the stabilizing resistor whose sole function is to "snub" the exciter voltage for proper voltage regulation, two is the rectifiers (diodes that form a bridge) on the exciter board, and three loose, broken or bad connection on the nine or so wires that connect to the gen end from the control box. The generator itself is bullet proof and has as much copper as a 10 KW today.

The stabalizing resistors get dirty, blown or bad contact. The diodes blow and need to be checked by unwiring one end. Wiring needs to be inspected.

Comminutor segments and brushes might be bad but usually before they give up the ghost you hear a lot of noise on say a radio. Could be them however if the varying voltage is random. If in sort of a varying pattern, i would not chase those first.

Hope this helps. Good luck.

Ted

PS Read the manual on how the regulation on a revolving field generator works. Can't hurt and it can help trouble shooting a lot.

 

[Nonchalant1]

mstailey.........Thanks for this post. Maybe it'll help me too. I have the same exact problem on my 1978 Onan 15kw. The engine runs great, but the voltage sometimes just goes up and down between 100 and 125 and makes the lights flicker and probabaly other things don't run as well. This is particularly true when it first starts up or is being run at under 25% capacity, which it frequently is. (Yes, I've heard it's "good" to run them using more of the capacity). About half the time it's running, it flickers, the other half it's steady as a rock.

Please post what you find and how you fixed it. I've been considering just calling Onan to come fix it, since I'm OK at fixing mechanical things, but I don't know a resistor from a diode.

TedZ....I wish I knew how to check those things. I have the parts manual and service manual, so I'll go read up.

Thanks,

Doug Shuman

 

[TedZ]

A decent digital volt meter with diode checking feature and frequency measuring. $200 about, but well worth it.

Then check things and check them off the list. You'll find it.

The book also gives resistance values for fields etc. which will eliminate those as potential problems. The reason i don't think it is serious as regulation on older generators is a common problem which requires minor component replacement and/or a little tweaking on variable resistors.

Ted

 

[Nick]

I have a similar problem with low voltage output on the onan. Is there a shop manual available for the 7.5 onan '79 vintage. I'll pay the copying costs if someone has a copy available. I have the operating manual; there isn't much detail in it.

Could bad coils cause the low voltage at +- 60V?


Thanks

 

[eze2bme]

I have an Owner's manual and Parts Manual for my 1979 Onan 7.5 KW MDJE. It is for the Model AD. (You can call your Onan dealer and give him your serial number and he can tell you if it is the AD model.) Send me your mailing address and I can make you a copy. You may email me at "eze2bme@aol.com" and please put the word "Hatteras" in the subject line so that I don't delete the email thinking it to be spam.

 

[mstailey]

eze2bme -
Thanks for the offer. I will get that email off to you right now. The manual I have is old, tattered and missing pages and Onan (cummins) has been of no help locating another one.

Again, thank you.

 

[Nick]

eze2beme,

Thanks!

I have the operating manual and parts list I believe you sent me some time ago. Is there a shop manual with more detail for the MDJE model?

Nick

 

[eze2bme]

Nick

If there is a Shop Manual I don't have it ... sorry!

 

[TedZ]

I
Could bad coils cause the low voltage at +- 60V?

I'm not sure i understand the question. Are you looking at a sine wave with peaks at plus and minus 60 VAC? Where are you measuring this voltage?

A bad stator equals zero output. One bad field...don't know what that would do. Probable zero voltage also but i'm not sure.

The exciters on this generator are voltage regulators. They get the generator cooking and limit field current all in one...as i recall...maybe there is field voltage regulation also...can't remember but if you need to know i can look at the schematic.

Ted

 

[34Hatt]

I have one for Onan DKC, DKD, MDKC, MDKD 8k 1984

 

[Nick]

The output generator voltage is measured at 60 V at the main panel.

What controls the voltage? Is there a regulator circuit? The schematic would help.

Thanks

Nick

 

[captbuddy]

Nick I have a copy of the repair manual a friend let me copy his. It's on the boat. Doesn't look like I'll be back at the boat until Labor day weekend. If I can remember it I could bring it back and make you a copy. Let me know by PM.
captbuddy

 

[Nick]

Thanks!

Check your PM

 

[mstailey]

Thanks to all for the tips. Eze2bme, thanks for the manual. After basic checking per manual but no diodes and such (TedZ - this is beyond my scope to handle) the generator is within "spec" meaning 1800 (1804 to be exact) RPM, 122.2 volts. What I did notice is maybe the problem is me. When I first fire the generator I get it loaded up with A/C, fridge, lights etc. Last night when I fired the generator I did no load and waited about 3-4 minutes then hit a lamp. Flickered just a bit. Then added A/C followed by the fridge and for good measure threw in the hot water heater. Voltage guage stabilized and the light flickering stoped. Ran for 2 hours with no problem. To see if it was a fluke I did it again this morning - same drill - same results. Could be this old genny (1846 hrs) just needs a bit of warm up time?

Anyway, will run it this way untill I'm either told this is bad or it breaks.

Thanks again.