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Onan 7.5DKDFJ Low A/C Output


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  #1  
Old 04-07-2018, 04:32:53 PM
patsmobile1 patsmobile1 is offline
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Default Onan 7.5DKDFJ Low A/C Output

Long time reader, first post, no clue what I'm doing! 1994 Onan rv unit, 7.5dkdfj...ser#K933101186...spec66044D...options.E 006...Z510..Z616....diagrams..612-6442-08....612-6631-03.......During Hurricane Irma last yr, ran the unit in the rv for 2 days off and on before it stopped putting out ac power. I am a totally disabled veteran, and have ADD, PTSD, great combination with physical disabilities also! But in the last week I have felt well enough to try to get the unit going, before the next one hits us in central Florida. I diagnosed what I could with the hard to reach unit, in a Monaco Dynasty diesel pusher, no slide out, so I pulled the unit out, found the end head bearing toast. Tore the unit down, everything I could see and test looked good, except the Transformer regulator. After replacing the bearing, I reassembled the whole thing (Not back into rv) and started it up, had less than 8vac on both 110vac leads, ordered the sx460 and correct 10ohm resister and installed them today, now with voltage adjustment I get 25-28vac? Been an ASE master tech most of my life, always been very week with electrical. Tried flashing the field as the manual shows, no change, tried swapping the field wires as the sx460 instructs, no change, the one thing I did not test was the rotating rectifier? Any input is greatly appreciated, sorry for my lack of a better understanding of these units, first time going into one. Also left the circuit breaker in line, and believe I wired the 460 correctly, loop for 110 on 3-4 prong, loop for 60hz, T-1 and T-4 tied to prong 7-8, then field 1-2 to the F-1 and F-2 on the 460 with the 10ohm in line for the field to give me about 20ohms. Pat.
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Old 04-07-2018, 05:12:58 PM
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Default Re: Onan 7.5dkdfj low a/c output

I picked up two gifts in Vietnam Agent Orange and PTSD. So I have some idea what you are going through. I won't even get started on the VA.

I would check the rectifier. Then go back and check the rotor and stator. We do have some good people on here. So give people time to show up and study things. I will get my manuals out. When the bearing went out was there any damage done to two items. Sounds like they are ok but worth rechecking. Been awhile since I worked on that model. But as I remember it was pretty reliable.
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Old 04-07-2018, 05:17:52 PM
patsmobile1 patsmobile1 is offline
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Default Re: Onan 7.5dkdfj low a/c output

Heading out now to test the rectifier, wish I would've tested more than what I did, found the transformer and said gotcha! I know better, back at it, Thank's for your service, and reply!!
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Old 04-07-2018, 07:52:49 PM
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Default Re: Onan 7.5dkdfj low a/c output

I have seen the rotating rectifiers go bad on DKDs for some reason so that is a good place to start. It is a pain to test them though since you have to disconnect the leads going to them to test. The fact that you got an increase in the output voltage also leads in that direction. You could try one more test before digging the rectifier leads out . If you disconnect the F1 And F2 leads from the regulator and connect a 12 V battery to the field when the machine is running you should get significant output like 100V+. Basicly you are simulating a good voltage regulator output , if you get voltage that is a good indication that the rotating rectifiers , rotor coil, and stator are more or less good. If you don't get voltage one of them is likely bad and the most likely one is the rotating diodes. It is still possible to have maybe one of the diodes gone and get output voltage but usually the pattern is that you get a decreased voltage like 110V with one gone maybe 100V with two gone and so on , so 28V is quite a bit less than that. Give that a try and report back. Your resistance readings for the field sound good, flashing the field is pretty much a waste of time. One more thing you could try that is easy is to reverse the F1 and F2 leads at the terminal strip and running the unit before tearing anything apart. Cheers Dan
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Old 04-07-2018, 09:53:04 PM
patsmobile1 patsmobile1 is offline
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Default Re: Onan 7.5dkdfj low a/c output

I was in the shop already, wish I would have tried flashing while running before pulling the end bell housing and rectifiers. But I did try swapping f1 and f2 with no change in output. I got very erratic test results between the positive and negative rectifier, and both tested very different between the 3 diodes in each side, assuming some are bad, not sure how many would need to be bad to give me 28vac, before the sx460 I was only getting 7-8vac. The rotor and stater looked really good, never saw deathly waves of not so magical smoke, the tests I could do with my very expensive (No clue on how to use most of the functions) blue point meter were good, minus the transformer regulator, it tested way out of limits. Ordered the positive 358-0070 and negative 358-0069 rectifiers should be here Wednesday. I wish I felt confident in my wiring, I see no issue with the F1 and F2 hook up, and the loops are all in the right places for the 110, 60hz, the one I am a little hazy on is the gen leads, and exactly how they tie into the 460? I have T-1 and T-4 in a block going to load one way and leads the other way to 7-8 on the 460? Now somebody set me straight, does this work by the f1 and f2 being controlled by the avr, and the amount of dc volts it puts into T1 T4 determines the amount of ac volts put back out of the unit to load? Damn-it-to-hell I hate electrical!!!!

---------- Post added at 09:19:21 PM ---------- Previous post was at 09:08:07 PM ----------

I did write down all the things I tested, left out in the shop, will enter them tomorrow, some are very out of limit, and others are very close, will mean more to folks with experience with these units. Good night all, and thank you!

---------- Post added at 09:53:04 PM ---------- Previous post was at 09:19:21 PM ----------

A question for those that know generators well, when a unit such as mine did, and the single bearing goes out, is there certain failures that happen when the rotor and stater make contact? It was very minimal, very light scratch on the surface, no smoke or burnt wires, just curious, wish I would have done better preventive maintenance, had it a few years, took great mechanical care, but nothing as far as the generator end, as easy as the bearing is to change, It won't happen again!
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Old 04-07-2018, 10:25:18 PM
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Default Re: Onan 7.5dkdfj low a/c output

Most of the time it will be just some minor scrapes. But it can be bad enough to rip the whole thing apart.
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Old 04-08-2018, 12:03:02 PM
patsmobile1 patsmobile1 is offline
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Default Re: Onan 7.5dkdfj low a/c output

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Originally Posted by Billy J Shafer View Post
Most of the time it will be just some minor scrapes. But it can be bad enough to rip the whole thing apart.
I hear you, hope that's not the case here. Inspected every winding very closely, found nothing, the scrape is just that a scrape, can't grab your nail dragged over it? The tests that I could do with a ohmmeter were all good, but I don't have a powered unit, the way they say to apply voltage and check for ground's? We'll see. When it was still in the rv before pulling it out, I started it one last time, noticed a light in the circuit very dim, then out, so it was trying to make voltage till the end. Of course that voltage probably is what fried the transformer/rectifier, and hopefully nothing else!
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Old 04-08-2018, 03:24:23 PM
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Default Re: Onan 7.5DKDFJ Low A/C Output

Rectifier results.....CR1= 3.3 swap polarity 1.2 CR2 1.0 and 1.0 CR3 1.397 and no reading? CR4-5-6 all no reading one way, with polarity swap got CR4 1.694 CR5 1.584 CR6 1.416. From what I read should have high resistance in one direction, and low in opposite direction. So 4 of 6 are good? Or 2 of 6? Test H resistance between leads f1 and f2 was 12.4...regulating transformer test values...H1 and H2 1.0...H3 and H4 1.3....x1 and x4 1.9...x1 and x3 1.7....x1 and x2 .9....Test L Rotor 3.3ohms book shows 2.82 for dkd/mdkd? looking for the rest. Have to save this unit somehow, I love the set, working with a broken body, and laughable disability funded bank account! Reading through this blog, seeing some of the issues you folks have fixed, diagnosed, and solved, gives me hope! I was looking at bolting a Chinese gen head up, as a last resort, but the funky non SAE onan bellhousing on the 3cyl kubota makes that a doable, but expensive option. Thank's for the input, and help guys! I posted this in the wrong place a bit ago, oops.
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Old 04-08-2018, 04:56:17 PM
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Default Re: Onan 7.5dkdfj low a/c output

Quote:
Originally Posted by BigBlockChev View Post
...You could try one more test before digging the rectifier leads out . If you disconnect the F1 And F2 leads from the regulator and connect a 12 V battery to the field when the machine is running you should get significant output like 100V...
Yes - This is a good test...

Also, Lots written on the SX460 here:

https://www.smokstak.com/forum/showthread.php?t=139868

Maybe you missed the jumper needed on the SX460 at 1 and 2?



Also, maybe obvious, but the extra 10 ohm resistor goes on the field wiring (not the AC input to the SX460).

Last edited by Jim McIntyre; 04-08-2018 at 08:45:27 PM.
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Old 04-09-2018, 11:55:29 AM
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Default Re: Onan 7.5dkdfj low a/c output

Jim this is exactly the ADD type crap that I deny doing or having, yet I went back and read the instructions again, twice, slowly, and damn-it, "Remove link to connect remote voltage trimmer", even typing it, I still just see remove link? Might as well wait for the new rectifier's on Wed. seeing as they are not 100% anyway. Thank you for making me double check.
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