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Brain Teaser (Electrical)

Since acquiring our '93 U280 several weeks ago, I have spent every spare minute getting acquainted with our new coach.  It has been a STEEP learning curve, for sure, but also a rewarding endeavor.  Wednesday afternoon, while poking around in the engine compartment, I made a somewhat disturbing discovery.  Before I could investigate further, I ran out of daylight.  Wed. night a winter storm moved into our area, and temps will be below freezing until Sunday at the earliest.  Until I can get back outside, I can only hypothesize about what I will eventually find.  In the meantime, thanks to the VERY complete service record file that came with the coach, I do have a few pertinent facts to work with.  Based on that info, I would like to offer up a "brain teaser" to the electrical experts on the forum (you know who you are).  I will see if their "solution" matches what I found.  Here is the scenario:

My coach, according to the OEM wiring diagram in the owner's manual, was originally equipped with a "3-wire" alternator.  According to the diagram, the negative post went to ground, and the positive output post attached to the center (un-bridged) post on the original 4-post isolator.  The 3rd wire, which is labeled "59 RD - ALT EXCITER" was connected to the "ENG BATT" post on the isolator (the one on the right-hand end of the isolator in the wiring diagram).  So far, so good.  In 2001, with 122,872 miles on the coach (according to the maintenance records), the OEM alternator was replaced with a "new Leece-Nevelle" (sic).  Further digging through the records reveals a receipt from Brownlee's Truck Service in Ardmore, OK for installation of a part number 110-555 alternator.  I also found the original installation sheet tucked away in the folder (did I already say the maintenance record file is EXCELLENT!).  This sheet shows the alternator model number 8LHA2070VA (sales number 110-555, 12 volt, 130 amp)) high-lighted in yellow, so I am confident this is, in fact, what was installed.  The wiring instructions on the installation sheet clearly show this is a "2-wire" alternator - the negative post should be connected to ground and positive post should go to BATT(+). 

Now we come to my "brain teaser".  Suppose the installing "truck" mechanic, being typically well-meaning but ignorant of RV electrical systems, first hooked the existing large gauge ground wire to the NEG post on the alternator.  Well then, the other large gauge wire must go on the alternator POS post, right?  So he did that.  Good job, so far.  But wait - there is still one smaller gauge wire "left over" from the original alternator wiring harness.  What to do with that wire?  Hmmm.....  Suppose, for the purpose of our mental exercise, he decided to also attach this 3rd (smaller) wire to the POS output post on the alternator.  He then closed up the engine cover, and they sent the happy owner home with the coach.

My question:  What would be the result of this installation?  Remember, this happened over 12 years ago and the coach now has over 162,000 miles, so it has obviously been "driveable".  So what happened?  Did everything work normally?  Any unusual indications to the driver?  What do you "experts" predict would occur in this situation?

I will be very interested to hear your opinions.  Then, I will tell you about my "disturbing discovery" mentioned in the 1st paragraph.  Thanks for your attention!

Re: Brain Teaser (Electrical)

Reply #1 –
OK, here is my read:

Since the replacement alternator doesn't require an external  "sense wire", look for a small wire from large B+ terminal to the small sense terminal on the alternator.  If there is one, my initial thought is that you could cut that small wire and attach it to the small wire from Chassis battery lug of the battery isolator.

As far as what would happen if wired as you suggest Bubba may have done, I see two things:

First, you will be "loosing" around .7 VDC to the batteries, as the alternator is only seeing voltage on its side of the isolator.  So, if alternator is putting out 14.0 VDC, the battery side of the isolator would only see about 13.3 VDC.

Second, that small wire from B+ to chassis battery lug would be the "little wire that could NOT".  It would try to charge the chassis battery lug at the full 14.0 VDC, but most of that extra .7 would likely be lost to heat due to resistance in the small wire trying to carry a lot of current.

Easy to test:

With engine at high idle, check voltage at alternator lug of battery isolator/or at B+ of alternator-- will be the same.
Check voltage at chassis and house battery lugs of battery isolator. Should be about .7 VDC lower.

Now, the critical information-- what is reading at the battery lugs of the isolator.  If around 14.0, you are good to go.  If about .7 VDC lower, it would be very hard to fully charge the batteries and would suggest you dig into this further.

Ya, it is cold out, so mental gymnastics is the order of the day. You EE's jump in!

Brett

Re: Brain Teaser (Electrical)

Reply #2 –
IF someone altered things you might be as lucky as I was in searching around the wire bundle as I did for the missing 12V hot with ignition on.  The truck mechanic used a jumper wire from the hot post to tell the alternator that it is ok to charge.  It worked but created a loop bypassing the boost switch thereby keeping my coach and chassis batteries connected all the time.  After replacing with the correct alternator, searching the wire bundle, I found a wire that had a piece of electrical tape around it.  When I checked, that wire was only hot with the ignition on so he obviously cut the extra wire off and taped it up.  Truck shops don't seem to understand the set-up with a battery isolator present.

Re: Brain Teaser (Electrical)

Reply #3 –
Yes, look carefully at the alternator/alternator wiring diagram.  Is it an excite terminal (i.e. ignition hot) or sense terminal (tells alternator the voltage on the battery side of the battery isolator)?

Re: Brain Teaser (Electrical)

Reply #4 –
Additional information:  This is a diagram of the rear of the 8LHA2070VA replacement alternator described in my "scenario".  I have verified that the alternator presently installed in my coach looks exactly like this.  This diagram shows a small wire (one attached to each post) coming from inside the case (probably from the internal voltage regulator).  One of these is probably the small wire Brett is talking about (in the 1st sentence of his initial reply above).

Re: Brain Teaser (Electrical)

Reply #5 –
Dwayne:  The 3-wire alternator originally installed in my coach did not require a "hot with ignition on" wire.  It was apparently a "self-excited" design.

Brett:  The factory OEM wiring diagram is somewhat confusing.  As I said, the 3rd wire in the wiring diagram is labeled "ALT EXCITER", which would lead you to think it is the "hot with ignition on" wire mentioned by Dwayne.  HOWEVER, the wiring diagram clearly shows this 3rd wire connected to the "ENG BATT" (start batteries) post on the isolator, which of course would be "always hot".  I think, on the wiring diagram, the 3rd wire should be labeled "ALT SENSE".  Like I said, it's confusing.

Re: Brain Teaser (Electrical)

Reply #6 –
Yes, from the diagram of the alternator, that small wire from B+ into the regulator section of the alternator would be a sense wire, not an ignition hot excite wire.  I say that because the B+ terminal in most applications (those without a battery isolator) are directly connected to the battery, therefore always hot.

As I see it, all you would need to do after verifying that voltage at the battery side of the isolator is less than ideal is to cut that small wire and connect it to  the sense wire from chassis battery terminal of the isolator.

Re: Brain Teaser (Electrical)

Reply #7 –
So, is Brett Wolfe the only one who has a guess what happened after "Bubba" custom wired the incorrect replacement alternator in our new-to-us coach?  I'll give you a hint: Brett hit very close to the mark with his comment about "the little wire that could NOT".  I will give the "EE's" time to respond (if they care), and then tomorrow I will reveal my "disturbing discovery" mentioned in my initial post.  Stand by for the exciting conclusion!

Re: Brain Teaser (Electrical)

Reply #8 –
Interesting to note that some alternators are self excited and some require external excitation.  I replaced an external exciter with a self exciter and just left the excitation wire taped off and in the wire harness for future use just in case.  It is my understanding that the older alternators req'd the external wire and the newer ones are self excited.  I also understand that you could connect the external wire to the alternator field and it would be OK too.  I would check with tech man at the alternator supply house and get their input.  Have a great day  ----  Fritz

Re: Brain Teaser (Electrical)

Reply #9 –
The winner is:  Brett Wolfe (Best answer to my Brain Teaser)

So, as promised, here is what actually happened when the replacement alternator was improperly wired.  The OEM "sense" wire, which Bubba attached to the alternator POS output terminal, heated up due to excess current flow (as predicted by Brett), eventually melted some of the insulation, and finally broke apart.  I do not know when this happened.  Could have been 10 years ago, or last week.  Hard to tell.  At any rate, this is what I noticed when poking around the engine compartment.  I saw the exposed ends of the broken wire, which is covered with the black plastic ribbed protector, and is part of a wire bundle near the alternator.  The break occurred about 10" from where the wire terminates on the alternator.  I pulled the broken wire ends out of the wire protector until I got to un-melted insulation.  Found out about 12" of wire is damaged.  That is where I left it, since I ran out of daylight that day.

So, what to do about it?  My first thought was that I would need a new alternator with the proper 3-wire configuration.  However, after doing a lot of reading online (I'm currently stuck inside due to below freezing temps outside), I find myself in agreement with Brett.  I think it would be worthwhile to try to modify the existing alternator to function in a "remote sense" installation.  I found several posts on the Leece-Neville website blog where the factory techs recommended just such a mod.  As Brett also guessed, it is simply a matter of removing the little red wire (that comes out of the internal voltage regulator) from the alternator POS output post.  Then you connect this little wire to the existing OEM sense wire on the vehicle.  Assuming the other end of the sense wire is connected properly to the ENG BATT post on the isolator, this will allow the alternator to "see" the true start battery voltage.  Everything should then function correctly.

My plan (as soon as it warms up outside):  First, I will fully expose the existing OEM sense wire all the way from the alternator down to the isolator.  I will check for any other "hidden" damaged areas on this wire or on adjacent wires in the wire bundle.  Depending on what I find, I will either repair the break in the OEM wire, or replace the entire wire.  Next, I will ensure that the sense wire is correctly attached to the ENG BATT post on the isolator, and that the large gauge wire from the alternator POS output is correctly attached to the center (unbridged) post on the isolator.  Next, leaving the "sense" wire disconnected, I will fire up the engine and (once warmed up) set it at high idle.  With the alternator connected in 2-wire mode, I will measure the voltage (on the isolator) at the center ALT post, and at the outer ENG BATT post.  I should see whatever voltage the alternator is putting out on the center post, and a lower voltage on the outer post.  The difference will be the loss due to internal resistance in the isolator.  If the voltage showing on the ENG BATT post is 14.0 volts or better, then as Brett says, I am "good to go".  That means the alternator is putting out enough juice to overcome the resistance of the isolator and fully charge the start battery(s).  This is NOT a likely scenario.  Probably I will find the battery is only getting 13.7 volts (or less).  In this case, I will try attaching the sense wire to the alternator (as detailed above) and see what happens.  If the (isolator) ENG BATT post voltage increases to 14.0 or better, I'm done.  If not, then I'll be shopping for a new alternator.

If any forum electrical experts see a problem with this approach, please speak up!  One other thing.  If I do decide to utilize the "sense" wire, I am planning to follow the recommendation of one of the forum members (sorry, don't remember where I read it) and install a inline fuse holder in the wire.  This will be positioned as close as possible to the end of the wire that attaches to the ENG BATT post on the isolator.  This will hopefully eliminate the potential fire hazard of a "dead short" anywhere in this wire.  The idea of electrical wires in the engine compartment (or anywhere else) getting hot enough to melt insulation and break apart makes me VERY nervous!  This is why I said (in my initial post) that my discovery of the broken wire was "Disturbing".  I will do my best to avoid this situation in the future!

Re: Brain Teaser (Electrical)

Reply #10 –
Quote from: cajk –
One other thing.  If I do decide to utilize the "sense" wire, I am planning to follow the recommendation of one of the forum members (sorry, don't remember where I read it) and install a inline fuse holder in the wire. 

That would be WA_desert_rat after a sense-wire near-fire issue

 Update on the sense wire near fire (split from What did you do to your coach

Re: Brain Teaser (Electrical)

Reply #11 –
Chuck,

Just wire it up as you describe.  I would not run the engine until all wiring is complete. That means cutting the little "sense" wire at the alternator and connecting it to the OE sense wire (or new one of that wire is damaged) from the chassis battery lug of the battery isolator.

Yes, I would like to see around 14 VDC at the battery lugs of the isolator, but would not throw $$ at it if you read 13.5+ VDC.

 Actually, I would be surprised if you don't read the same voltage at the battery lugs of the isolator after the change as you see right now at the B+ terminal of the alternator at the same engine RPM.

Brett

Re: Brain Teaser (Electrical)

Reply #12 –
Chuck, Looks like you are being educated on FT type alternators as a good many of us have been.

One Warning.  When you install a NEW or repaired IGNITION ON SENSE wire be careful to tie it up so that it cannot short to ground. After I REWIRED a Leece Neville to operate in the FT way I let the sense wire lay across a coolant pipe, when the insulation aged and it shorted it shut me down in a very bad place.
Gary B

Re: Brain Teaser (Electrical)

Reply #13 –
An Ignition on/excite wire is very different from a sense wire. 

Looks to me like the alternator in the schematic does not have an excite wire/terminal, and to "make" an external sense wire, Chuck will have to cut the small wire from alternator B+ that goes into the back of the alternator and connect it to the sense wire, and through it to the chassis battery terminal of the battery isolator.

Re: Brain Teaser (Electrical)

Reply #14 –
I ran a fused wire directly from start battery positive terminal to alternator for voltage sense purpose, bypassing isolator.

Re: Brain Teaser (Electrical)

Reply #15 –
Michelle:  Thanks for the link!  You are right on top of things, as usual.

Brett:  OK, I will get the "sense" wire hooked up first, then crank up the engine.  I just thought there might be some bizarre chance that the alternator could be putting out more than 14 volts, and therefore fully charging the batteries even in "2-wire" mode.  Not much chance of that, I guess, since the internal regulator on this particular alternator is non-adjustable.

Gary:  I will protect the "sense" wire with a inline fuse.  I'm with you - shorts are bad!

As Brett says, two different things.  Throughout discussions of this topic, on this forum and on countless other forums, there is ALWAYS confusion about "sense" wire versus "ignition on/excite" wire.  My factory wiring diagram clearly shows this coach never had, or needed, a "ignition on/excite" wire.  There would be no advantage to adding one at this point, since the currently installed alternator (like most new alternators) is "self-exciting".

Barry:  That is certainly another option.  However, I like the "neatness" factor of having everything tied together at the isolator.  I think I'll stick with the OEM design in this case, as long as I can make it function correctly.

Re: Brain Teaser (Electrical)

Reply #16 –
There should be VERY little voltage difference between chassis battery lug of the battery isolator and the chassis battery itself, as you have a large gauge wire between the two.  Said another way, there should be no effective difference-- either connection for the sense wire would be fine.

On our sailboat, with its high-output alternator and only 8 gauge wire from alternator to battery I did have to run the sense wire direct to the battery to get full voltage there.

Brett

Re: Brain Teaser (Electrical)

Reply #17 –
Barry said in a previous post:
"I ran a fused wire directly from start battery positive terminal to alternator for voltage sense purpose, bypassing isolator."

I understand: "running a fused wire directly from the start battery positive terminal to alternator".
I disagree with doing this because while it seems to fix the problem caused by dirty wiring or connectors, it does not fix the dirty wiring or connectors.
Backgound: On my 1996 U320, the sense wire from the regulator in the alternator went to an A/C solenoid contact post which also had a wire going to the "start battery post of the isolator". I do not understand why Foretravel did this and I moved this sense wire to an isolated bolt to which all positive battery wires are connected.

Barry, I know you have indepth understanding of the Foretravel wiring and I am trying to understand what you meant by "bypassing isolator". Did you really mean: "bypassing the wiring harness"?




Re: Brain Teaser (Electrical)

Reply #18 –
Wyatt, using the word 'bypass' just meant we went directly to battery, rather than using the isolator's start battery terminal. We also originally had the sense wire to the A/C relay, which often get corroded and cause charging problems. When we replaced all the relays, breakers and solenoids on our isolator panel, I relocated the alternator voltage sense wire. Probably Foretravel double used the existing breaker on the A/C panel to protect the alternator voltage sense wire.

We use a variable voltage regulator in our Leece Neville alternator. Since the alternator does not use temperature compensated voltages, we lower the output voltage in summer to not over volt charge battery banks. We really don't count on the alternator to charge batteries at maximum voltages. To us, the alternator's main job is to supply electricity for a running coach, like keeping inverter, engine, lights, dash heater motor and other 12-volt loads.

Re: Brain Teaser (Electrical)

Reply #19 –
If you don't have Leece Neville's phone number you should write it down and keep it with you. This will come in useful for anyone reading this who has a Leece Neville alternator. They're not exactly the most reliable alternator I've ever had, just saying...

Leece Neville (Prestolite):  (866) 288-9853