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Old 05-26-2016, 11:39 AM   #1
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Alternator Fail Light and tach

Hi All,
I am new to being a member, so forgive me if I ask this in the wrong place. My alternator fail light comes on at startup. Tach does not work either. I used a voltmeter to check it all out. The batteries sit at ~12.3 until the coach is aired up, then usually within a few minutes the alt fail light goes out, tach starts working. Voltmeter then reads 13.8-13.9 at the batteries, both house and chassis, under load and without high load.
I have tried to figure out what could be causing a delay in the alternator charge getting past the isolator for that long.
Any thoughts out there? Is there a better or more appropriate place to ask this on the site?
Does that sound like the isolator or one of the many solenoids that I have, with no idea what they are?

1998 Monaco Executive DP

Thanks, Brian
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Old 05-26-2016, 02:20 PM   #2
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Normally the battery isolator would be between the chassis and house batteries, the chassis batteries should be directly hooked to the alternator. Perhaps your voltage regulator isn't coming on until the battery isolator connects the two battery banks together and showing need.
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Old 05-26-2016, 02:32 PM   #3
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Many units before 2003 or so have diode isolators that split the charge. Model years after that, used alternators directly connect to the start batteries and solenoids to isolate the house.


With that, there could be a heavy draw right after startup ( Grid Heaters ), or you could have a slipping alternator belt.


Many diesel engine tachs runs off the alternator so both things are related.
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Old 05-26-2016, 04:49 PM   #4
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Replies!

Thanks for the thoughts.
Belt is good on the alternator. I went over most of the basic stuff, connections clean and such. I had my son start the rig while I witnessed first hand at the back end. I checked batteries off and running, fail light on and off. It could be related to the heavy draw on start up, but I am not sure if the air compressor qualifies as heavy or not.
On another note, the alt fail light came on and tach failed in the rain twice. I wasn't running much besides headlights and wipers at the time, maybe dash fans.
Cummins guy here in NC said the alternator was good. I am unsure if the regulator is internal to the alternator or not. Is there anything else that would have a delay that has gotten out of hand?

Thanks again, Brian
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Old 05-26-2016, 05:02 PM   #5
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The rain and alt fail light, points to the belt or bad tensioner. Make sure the tensioner is not seized.

The air compressor is gear driven so no electrical draw there.
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Old 05-28-2016, 10:36 AM   #6
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Get some clip leads and clip your voltmeter directly to the alternator and check voltage there.

Could be a combination of poor connections that may cause the issue.

If alternator checks good then problem is loss of drive or connection.

Check belt too.

My old gas station training was to attempt to turn alternator by hand on stopped engine by applying force on the alternator fan.

If it turned the belt was loose.

If no fan then use a socket wrench and use shaft bolt.

It should not turn easily and if it does belt is slipping.

1 hp is about 770 watts.

Watts is amps times volts.

So a 100 amp alternator is 1200 watts so well over 1 hp so lots of grip needed.
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Old 05-28-2016, 10:59 AM   #7
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Brian ; Welcome to iRV2.

What you haven't mentioned is your engine type and outside air temp , and if the problem happens at every start up or only when cold.
The cycle of the intake air heater on a diesel engine can draw enough power to stop the alternator charge from reaching the batteries; the intake heaters can cycle if the sensors get triggered on a cold rainy day too , usually at idle or coasting down hill.
Due to alternator options on my chassis there is a multiple connector about a foot behind the alternator to attach the different "excite wire" multiple connectors to the different brands of alternators, bad connection there gave me fits , till I found it.

The " Owners Forums " and " Monaco" section there would have been a slightly better spot to post , but your getting ideas here , so this is working.

You can add your coach info to your signature line so you don't have to type that info into every question.
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Old 05-28-2016, 01:41 PM   #8
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Alternator is turning at start up. Was 88 degrees today, same start up issue. Batteries are charged, start the engine and volts drop to 12.4 and sometime after the compressor fills the air system suddenly the alt fail light goes out, and the volts at the battery jump to 13.8, everything runs. 1998 Monaco Executive, M-11 Cummins without glow plug or preheat. Otherwise runs like King of the road!
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Old 05-28-2016, 01:49 PM   #9
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Thanks for the reply. The alternator turns at startup. It was 80 degrees today and same issue. So, same thing today - alt fail light on, dash volt gauge reads about 12, actual voltmeter on battery reads 12.3-12.4 and tach not working. Sometime after the air system reaches max and does first purge, things go to normal. Sometimes the delay is longer, but I dont recall ever having full function until after the air is at capacity. Monaco Executive M-11 Cummins
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Old 05-28-2016, 02:08 PM   #10
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At startup, have you ever given the throttle a bit if a tap to rev up the engine ?

Some alternators need a certain RPM to start outputting.

The only thing that changes is the compressor unloading, possibly causing a slight increase in RPM.
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Old 05-28-2016, 05:56 PM   #11
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Yes, I have put up some rpm 's to see if that affected anything, but no. Its a mystery, one of the ghosts in the machine I have seen posted on other coaches! I would think it is something fairly straightforward in the wiring diagram, except it has done the same thing in the rain while driving as well. The belt is tight, and the alternator spins at startup.
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