Onan Problem

[RTyler]

I continue to have voltage problems with my 7.5 MDJE. First it was making about 165 volts ( at 1800 rpms ) and after checking the connections on the voltage regulator and wiring harness it now produces 120 volts but it's unstable occasionally bouncing from 110 to 135 volts. Last weekend I isolated the problem to the volatage regulator assembly in that when I hold it, isolated from the vibrations of the engine, the voltage is stable. As soon as I touch it to the top of the generator voltage becomes unstable again. I have removed the regulator to be sent to Flight Systems for a rebuild.

When I get it back I am thinking of mounting the regulator assembly box remotely from the generator, probably from a salon floor stringer so it does not have to endure the vibrations of the engine. I would just make a cover for the top of the engine systems control box that it normally covers. My question is, has anyone else done this, or does anyone see a problem with it.

Robert Tyler

 

[Nonchalant1]

How long will the wiring runs be? Need to check voltage and size wires accordingly.

I had that problem (15KW MDJF) except I didn't try holding it by hand, so I just got a new board ($280 I believe) from Onan/Cummins. The new board is slightly different, but replaces the old one exactly. I have had no problem with it for 3 years.

Doug

 

[captbuddy]

I mounted mine on the ceiliing in the engine room. Virbrations kept breaking the legs of the diodes on the board. Just mounted it upside down. Need to make a cover for it but haven't yet. Working fine for about 3 years. Also you can lossen up the rubber mounts on the box and turn them 180 degrees and they will not vibrate as much. Mine over the years had sagged down until the box was resting on the gen set and not on the gronmets.
captbuddy

 

[RTyler]

I'm thinking about mounting it directly above the generator to the underside of the salon floor framing, so the leads will be less than 12 inches. I suspect that something on the printed circuit board is cracked or broken and I just can't see it. Unfortunately the voltage regulator components are not available from Onan anymore.

Robert Tyler

 

[Bob Bradley]

I have quite a few MDJE parts available, including a voltage regulator. Here's a link to the Parts Ad:
http://www.samsmarine.com/forums/showthread.php?t=13147

 

[RTyler]

Bob:

That sounds like it might work. Is the voltage regulator you have the entire assembly in the box that sits on top of the engine control box, or are you talking about just the printed circuit board?

Robert

 

[TedZ]

Any $200 125 VAC AVR will work as a voltage regulator for this generator. They have minimal connections: voltage in and field voltage out. It isn't going to replace all of the safety shutdown relays but it will regulate the genny much better. There are universal AVR's that have pretty good ampacity...connect as instructed to the "F" terminals on our genny as i recall...do not have the schematic in front of me. As long genny excites properly - sounds like this one is - should work great. EBay has lots. Vibration not an issue as all solid state.

Ted

 

[RTyler]

Ted:

I had wondered about that. What safety shutdowns are in the Voltage Regulator other than the high voltage reset circuit breaker? I was going to call Flight Systems on Monday and see if they could put together an assembly that would replicate this VR but all solid state.

Robert

 

[TedZ]

Very few but some AVR will shut down field voltage if the unit fails. Some like Mecc Alte's original SR7 fail high and can do a lot of damage. Some monitor the frequency and shut down. Engine damage is protected in all of the relays, nothing to do with the VR. It is not easy to hurt those generators and a lot of safetys are not usually present. However, it is easy to hurt down-stream equipment but if you have been running 165 VAC in spurts, guess all is ok.

Flight safety is teriffic but you can't expect them to build an AVR from scratch when they are so cheap to buy. EBay has Marathon AVR's for $60!

Ted

 

[RTyler]

Ted:

I looked at the AVR you mentioned on Ebay and noticed that it said it was for generators with brushes. Now bear in mind I know very little about generators, but I believe mine is brushless. My VR looks very similar to what is termed as beginning Spec AA in the parts catalog, which corresponds to a brushless generator. I assume that makes a difference in whether the AVR on Ebay will work.

Robert

 

[Bob Bradley]

Bob:

That sounds like it might work. Is the voltage regulator you have the entire assembly in the box that sits on top of the engine control box, or are you talking about just the printed circuit board?

Robert

It is the complete box.

 

[TedZ]

The only thing the box on top of the engine needs to see is 115 VAC to complete the timing relay cycles for the engine safetys. That connection would not be disturbed. The "F" terminals connect to a new AVR, brush or brushless makes no difference, and after voltage adjustment should work fine. Remember i don't have a schematic in front of me...i may be missing something but it would be minor to make all systems work.

There are generators that do not control field voltage to control the voltage output but they are not as common as the kind we all have.

Ted

 

[TedZ]

Maybe we should go a little farther.

If anyone cares an AVR is a very simple device. It looks at the AC voltage and compares it to a voltage determined by a setting you control on the circuit board. It then raises or lowers the field voltage, like 5 - 15 VDC to make the output voltage correct. Some count the frequency after say 15 seconds to make sure the engine is up to speed. Many do not do this. Some worry about under voltage, i.e. the field voltage is maximized and output doesn't reach say 100 VAC, then they will do something (shutdown, alarm etc.). Conversely, overvoltage checks.

Any AVR can be checked with a $40 meter. You should read something stable and reasonable (3 to 12 VDC usually) on the F terminals with your meter on DC.

 

[RTyler]

Ted:

Thanks for the all the help with the voltage regulator. I went ahead and sent mine to Flight Systems last night for a rebuild. We'll see how it works out. The plan is still to mount it to the underside of the salon floor framing to minimize vibrations. Thanks again.

Robert