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Duvac 20a fuse


Verles
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Does any know why the fuse would blow at start up on a 2000 Monaco Diplomat Leece Neville 160a alternator.

Unable to find anything wrong and have tried everything I've been told and on 3rd alternator.

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Regarding alternators, due to recent experience, and a ton of help from here..., listed below is what I know:

  1. My alternator, 8LHP2170, is a DUVAC, for a 2005 Beaver Marquis (CAT C13)
  2. On my alternator, there were two primary posts, Positive & Negative, along with 3-smaller posts (which are NOT used for my situation)
  3. Albeit a lot more expensive, I was strongly advised by several members here to only purchase from AJ-Electric    sales@aj-elec.com    https://tinyurl.com/y35lqfw2   (Which I did.  They were AWESOME and... I'm glad that I did!)
  4. After replacement, my voltage directly off of alternator Positive terminal was a constant 14.2 VDC

 

As far as blowing a fuse goes, In My Opinion (IMO) - If it were me, I'd try/do:

  1. Turn your main "house switch" off prior to starting
  2. Look at your chassis batteries for possible bad and/or internally shorted cell(s)

 

FWIW (For What It's Worth), I asked AJ-Electric about them possibly rebuilding my failed alternator, to keep as a ready spare, and they said it wasn't worth it. (something else would probably fail sooner or later...)

 

Good luck!

 

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No I can not get the original alternator back, the house side is fully disconnected, and house and chassis batteries are less than 6 months old.

On the Leece Neville 2824 I have the +,-,key, tach, and duvac. I even ran a new wire for the duvac with fuse hooked to the chassis batteries, and even had the tach and key wires disconnected just the +,-, and duvac connected, no matter what the fuse keeps blowing.

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I did not have that problem so all I can say is theoretical... my first quess would be a bad regulator but that assumes that the Duvac source (positive) is connected to correct sense terminal and that the regulator sense wire inside is also connected to the same correct terminal. If you trip a 20A breaker/fuse, that's some serious current and there could be a short at the inside terminal wiring or the post bolt itself could be grounded. Easy to test and check by lifting the regulator. You say that you are on your third alternator, I suppose the regulator has not been reused but the posts could be switched around, shorted or mislabeled?

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Yep, looks like a short to the ground/ housing. Either in the regulator or the post is grounded. On mine, there were 3 wires inside, I think blue, black and third one to the duvac post. If you undo the 4 little regulator bolts, you can lift it and check the wires. Maybe ohm the duvac post against ground first, there's got to be a short somewhere. Would be much easier with the alternator out... was this alternator ever working?

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When I bought it 2-2 1/2 yrs ago everything worked, picked it up a month ago inverter charger dead,  batteries dead, and alternator dead and still fighting the alternator and possible have an generator issue that I haven't looked at yet. Shore power works great with the new inverter charger and still waiting on other devices for the inverter charger for a complete upgrade.

But decided to take it to a big truck shop and see if they could find what I'm not finding. 

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I'm sorry that I'm coming in on this post late.  Have you resolved your problem?  If not, could you be a bit more specific as to which fuse you are blowing?  I'm looking at a 2000 Diplomat Wiring diagram, and there are two 5-Amp fuses shown near the alternator; one in the Ignition (IGN) [what I think you are calling "Key"] and the other in the Stator (S) line which I think you are calling "Tach".  Also in the IGN line is a 10-Amp fuse, that I think is in the Front Run Bay.  I also see mention of the 20 Amp Fuse/Circuit Breaker.  I don't find that in the wiring diagram, but it could be much further upstream.

I believe you have said that you disconnected the the "Key" wire (which I believe is the Ignition) and the "Tach" which is the Stator (S) and you still blow the fuse.  Those are the two wires that have the 5-Amp fuses - so if they are disconnected, somewhere else you have a short, independent of the alternator, or neither of those fuses are the ones you are blowing.  

Can you provide more detail as to the fuse you are blowing?  I have some L/N troubleshooting and repair documents that might help, once it is positively confirmed that the problem lies in the alternator itself.

  -Rick N.

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It's the 20amp duvac fuse that is blowing, comes off the alternator also across from the ignition (key).

It use to be in commercial trucks along time ago and apparently the rv shops know nothing about them I've been to 3 of them and all I get is a dumb look.

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Ok Verles, I finally found the 20-Amp DUVAC fuse on the wiring diagrams.  These are some of the worst diagrams I've worked with.  Good news is the only thing that (according to the Monaco Wiring Diagrams) that this fuse go to is the DUVAC terminal on the alternator.  In case you are not aware, the main reason for the DUVAC is to monitor the voltage at the starting battery, when it is NOT directly connected to the Alternator + Terminal.  This is the case where you have some sort of "isolation circuitry" to keep the house battery and the chassis battery isolated, and that "circuitry" is in-between the Alternator + Terminal and the Chassis battery + terminal.  If you didn't have a DUVAC alternator, the alternator would "regulate" the voltage at the input to the "isolation circuitry" which is typically not at the same voltage as the Chassis Battery.  I am wondering if you are connecting it to the correct terminal.  You can NOT go by where the physical lug is on the back of the alternator unless you have the exact same manufacturer/model of alternator.  I am attaching a Leece-Neville (which is now Prestolite) Maintenance Instruction which discusses the DUVAC Alternator that was commonly used in your era Monaco coaches.  After review, if you have questions, let me know.  I'm pretty sure you have the wrong connection on the Alternator, or the Alternator/Voltage Regulator (which is usually built-into the Alternator) is bad.  But if you have gone through 3 Alternators, it's highly unlikely that all were bad.

 

  -Rick N.

Form4801.895.pdf

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