Alternator Output is .76 Volts

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Alternator Output is .76 Volts

October 03, 2016, 06:29:01 pm

The alternator is not charging the engine battery, is putting out .76 VDC and it's exicitation/sense voltage is zero. The attached photo shows a loose terminal that looks like it came off a circuit breaker, which has 13.6 VDC on both sides of it, near the isolator.

Questions:

1. Does anyone know what this wire does? or Where it is supposed to go?
2. Could it have been disconnected on purpose? Of so, why?

I just purchased this 1997 U270 three weeks ago, so I am a newbie at diagnosing it's issues. The isolator has 13.6 VDC on two legs. The rig is on shore power now. The alternator and isolator connections are dirty but tight. I tried reconnecting the terminal, thinking the nut just backed out and slipped off and got a small spark. I am too presumptive and impatient. The loose terminal shown has zero volts on it.

Thanks for your help.

Re: Alternator Output is .76 Volts

Reply #1 – October 03, 2016, 06:41:28 pm

The wire should have a number on it after you peel back the black chafe protection. If you have the wire schematics, you can trace it that way. Or perhaps someone here could look for you if schematics are not available.

Re: Alternator Output is .76 Volts

Reply #2 – October 03, 2016, 07:10:37 pm

Tim,

First thing to do is find out why zero on excite/ignition hot and sense wire. Without them, you are not asking the alternator to do anything.

You can spend a little time tracing them, but don't get carried away. Too easy to run a new excite from any ignition hot source in the engine room and new sense wire from any chassis battery side of isolator. Just make sure to isolate both ends of the old wires before driving.

Re: Alternator Output is .76 Volts

Reply #3 – October 03, 2016, 11:29:56 pm

One of those wires goes to the AC fan on side panel opposite the cooling fans. It could be that one and maybe removed if AC is not working???
Do as Brett said and run a new sense wire to Alt' then run engine and check middle lug on Isolater to see if you then have 13+ volts.
JohnH

Re: Alternator Output is .76 Volts

Reply #4 – October 03, 2016, 11:30:37 pm

Lucky I found the schematic diagrams for the 1997 U270, attached. The wires will be traced tomorrow. I have a feeling that I caused the problem by screwing in the breaker panel in front of the navigator's chair. It was loose and fell down, so I raised and secured it. However, in the process I warped the breaker panel and perhaps disturbed a connection or cracked the Ignition Exciter breaker. Time will tell.

As far has the loose wire in the first photo, it is connected to the sense terminal of the alternator and the remote engine start stop switch. I am not sure why it would draw a lot of current.

Re: Alternator Output is .76 Volts

Reply #5 – October 04, 2016, 06:39:44 am

Some wiring diagrams related to the alternator.

Re: Alternator Output is .76 Volts

Reply #6 – October 04, 2016, 09:57:56 am

Tim, the exciter wire does not go thru a breaker it is from a 12v ( with ign on) source direct to the terminal on Alt.
JohnH

Re: Alternator Output is .76 Volts

Reply #7 – October 04, 2016, 12:38:31 pm

I think these years were known for having the alternator excite wire going to the breaker that runs the A/C condensor fan and the remote start relay. It's shown on the lower left quadrant of img 4705 labeled "Isolator board". There have been threads about this in the past and how it's caused problems.

I think following years, FT took the sense (and maybe excitation?) to a more direct location. I think the alternator on my '98 built in sept '97 is self excited because it doesn't charge until I rev to about 1400rpm or so after starting.

Re: Alternator Output is .76 Volts

Reply #8 – October 04, 2016, 12:52:17 pm

My 96 does the same as yours Krush. build 4810.

Re: Alternator Output is .76 Volts

Reply #9 – October 05, 2016, 05:44:15 pm

I deserve a public flogging. The ignition +12 volt source breaker was blown and would not automatically reset. It's no wonder, the alternator was drawing 16 amps from the IGN terminal. This test partially melted the insulation on my ammeter test wires. Upon further tests, the IGN and SENSE terminals are practically shorted together inside the alternator with a resistance of about 2 ohms between them. I have the alternator off the rig, Unipoint model number ALT-0050 and am looking to replace it with something reliable.

Any advice?

Re: Alternator Output is .76 Volts

Reply #10 – October 05, 2016, 08:05:29 pm

I think that a better choice is have a dedicated breaker/fuse near the start battery bank and to run new dedicated voltage sense wire from the alternator directly to the start battery bank area. Sharing a breaker on the isolator panel is not a good idea, and picking up the start battery voltage at the isolator panel can be problematic.

Re: Alternator Output is .76 Volts

Reply #11 – October 06, 2016, 11:16:46 pm

Here are the facts regarding the $170 remanufactured Wilson alternator I just installed. I purchased it from NAPA, since it was identical to the previous owner's unit, which worked fine on a recent 2700 mile trip:

1. Wrong Alternator installed (ALT-0050)
The previous technician installed a one wire alternator and connected the AC output of the alternator (yes, some truck alternators output Alternating Current for some truck accessories) to the DC Ignition input, causing damage / an open circuit in the IGN circuit breaker. Fortunately, Foretravel included several spare breakers, including the correct 6 Amp Ignition circuit breaker.

2. No Excitation
The alternator I installed today needs momentary +12VDC excitation to produce voltage, otherwise it measures .76 Volts DC!
The current Foretravel circuitry has a diode-based battery isolator, so the alternator does NOT receive 12VDC directly. I could jury-rig a solution by installing a momentary push button switch to excite the alternator.

3. No remote sense
All batteries will not be fully charged since voltage sensing is at the alternator output not at a battery. There is a .6 volt drop across the isolator, so the alternator regulator will not make up the shortfall. The charge voltage will only be 13.8VDC at the batteries.

4. Shall I order the correct DUVAC alternator, with the right connections?
This model appears to have the sense and IGN terminals: New 160A Alternator Duvac RV motorhome 2824LC 90772 A001090772 A0012824LC | eBay

5) Cost, long term
If the alternator does not charge the batteries fully, their life may be shortened during boondocking or long runs without shore power, thus costing more frequent replacement of the batteries and/or running the genny.

Re: Alternator Output is .76 Volts

Reply #12 – October 07, 2016, 07:45:45 am

Tim,

I would go with option 4 and one of the smart isolators that are now on the market. You might even look at a manual operated switch that takes the place of the isolator if you want a manual system.

According to the spec. sheet that alternator will work for this application and it has the correct mounting.

Pamela & Mike

Re: Alternator Output is .76 Volts

Reply #13 – October 07, 2016, 08:08:12 am

The U270 came with a 130 amp alternator and a160 amp isolator, if you upgrade to a 160 alternator you will have to upgrade the isolator at the same time.

Re: Alternator Output is .76 Volts

Reply #14 – October 07, 2016, 08:50:05 am

The alternator that you are looking at is a Leece-Neville KNOCK OFF, the add says not made by Leece-Neville.

Re: Alternator Output is .76 Volts

Reply #15 – October 07, 2016, 08:57:23 am

Tim,

The choices are to get an alternator (or wire the current one) for an external sense wire connected to the chassis battery side of the diode-based isolator.

OR

Dump the diode-based isolator for either a manual switch such as a marine Perko ON-OFF switch with alternator output and chassis battery to one lug and house battery to the other. Switch can be mounted at the foot of the bed.

OR

Replace diode-based isolator with a relay-type isolator with no voltage drop.

Re: Alternator Output is .76 Volts

Reply #16 – October 07, 2016, 12:27:01 pm

Quote from: wolfe10 – October 07, 2016, 08:57:23 am

Dump the diode-based isolator for either a manual switch such as a marine Perko ON-OFF switch with alternator output and chassis battery to one lug and house battery to the other. Switch can be mounted at the foot of the bed.

Re: Alternator Output is .76 Volts

Reply #17 – October 07, 2016, 12:29:29 pm

SOLVED!
I unfastened the sense lug from the $170 Wilson remanufactured alternator and connected it to the Foretravel standard sense terminal. Viola! The alternator voltage increased to 15.23 Volts and both engine and house battery voltages were 14.36 Volts. The quiescent (engine off) current demand from the sense line is 3 milliamps, so it will not drain a battery. See the photo for the voltage measurements.

Perhaps in the future, after adding solar, lithium batteries and an improved isolator, I will look back and laugh about this awkward but workable lead-acid solution. Thanks to the forum, led by wolfe10, for providing an intelligent sounding board.

Re: Alternator Output is .76 Volts

Reply #18 – October 07, 2016, 01:01:09 pm

Tim, is that isolator voltage at eng idle or at 1000 RPMs?

Re: Alternator Output is .76 Volts

Reply #19 – October 08, 2016, 04:48:17 am

The cruise control held the engine at 1000 RPM.

Re: Alternator Output is .76 Volts

Reply #20 – October 20, 2016, 11:42:01 am

Here is a photo of the old, dangerously-wired alternator showing the the ingition and sense output lines connected to the alternator's AC (Alternating Current) output. Output to output equals SMOKE! This alternator was probably ordered and wired in a big hurry because a self-respecting shop would have ordered a direct replacement DUVAC type alternator.

The photo is blurry, but it shows the above two connections to the AC output of the alternator. Originally I thought they were the sense and ignition inputs, but all three terminals on top of the alternator are the same: AC output. These connections are used for certain fleet trucks.

Re: Alternator Output is .76 Volts

Reply #21 – October 21, 2016, 11:41:10 am

I've never seen an alternator like that, but it gives me more ideas I pondered before. That's 3 phase variable frequency output. I wonder if that could be wired into a VFD to make 60hz. Voltage could be cranked up to reduce wire size and save the headache of the inverter/huge alternator combo that some do to run roof A/C while driving down the road.

Re: Alternator Output is .76 Volts

Reply #22 – October 24, 2016, 05:16:07 pm

I have been following this thread because I to lost my Alternator. I had it rebuilt and I reinstalled it. The White wire appeared have got to close to the alternator got hot and melted. I rewired and could not get the alternator excited. I found a 15 amp fuse in the dash that was blown. The wire is white and as soon as I replaced the fuse the alternator was putting out almost 15 volts. The batterys are down and I believe thats the reason for the slightly higher voltage. I will recheck when batteries are recharged and the alternator should be at about 14.6 volts. The a other problem now is my frig wont run on 12volts. Could there be a fuse blown if so where should I look? I'm told the frig needs 13.5 volts to run on 12 volts. Any comments would be appreciated.

Re: Alternator Output is .76 Volts

Reply #23 – October 24, 2016, 05:35:17 pm

No, the refrigerator PC board, igniter and gas valve are happy with 12.0 VDC or above.

And, you really don't care what voltage is at the alternator. It is ONLY on the battery side of the BATTERY ISOLATOR that matters. That is where the sense wire is connected-- chassis battery side of isolator. Because approx. .7 VDC is lost across a diode-based isolator, alternator output is boosted by that amount.

Re: Alternator Output is .76 Volts

Reply #24 – October 24, 2016, 05:50:56 pm

Thanks for the quick response Brett, What should that voltage be at engine idle.

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