Alternator questions for Cat C9,

[Bryan Beamon]

Background: Contessa 2007 with 211k miles and the original alternator and 12 year old chassis batteries. Recently noticed voltage dropped to 13.3 volt momentarily and then back up to 13.7 volt which is the standard voltage most of the time at 1400rpms. This voltage drop is unusual as I have never seen this low voltage with all accessories on before ie lights ,AC ,windshield wipers etc.
Also recently ran the generator while traveling and using the roof AC and noticed the same voltage drop.
Any ideas as to the cause?

[Steve Huber]

Bryan,
I had a similar problem with varying voltage. Found a loose connection in the rear run box on the ANL fuse panel.
Steve

[Bryan Beamon]

Thanks Steve , will check that out

[Eric Maclean Co-Admin]

Bran
If I read that right you stated the chassis batteries are 12 years old ?
Most chassis batteries have a life span of 3 to 5 years .
I would bet at some point they have been replaced either way it would be worth doing a load test on them to see what kind of shape they are in .
 As Steve pointed out a bad connection anywhere between the alternator and the batteries can cause the charge voltage to be erratic.
It appears the alternator output goes directly from the alternator to the 200 amp fuse Steve spoke of and from there back to the chassis battery disconnect before going straight on to the chassis batteries .
To check the system with the engine running and a good load on the alternator such as headlights ,air conditioning etc
Take a volt drop reading across each of the connections to determine which connection is at fault.
Don't forget the ground side of the equation as well
Remember 13.3 is still a reasonable charge voltage and your volt fluctuations could be the result of an intermittent heavy load such as the air conditioning compressor clutch cycling on and off

Good luck with your search
Eric

[Bryan Beamon]

Eric ,thanks for the input. The batteries are 12 years old  as that is the last time I replaced them and I will buy new batteries shortly.
I have not attempted to tighten the ANL cables as of yet, just hand checked. Do you recommend turning off the coach and chassis batteries disconnect before attempting to tighten the cables to avoid inappropriate contacts with other connections?
Your suggested test for a voltage drop it is not in my current experience level. I do have a good digital volt meter but have not done the voltage drop test before .
We have not been running the dash AC as our system needs recharging and flushed so no load on alternator.
Also curious as to why voltage  raises when we slow down the coach from 13.3 or.5 to 13.7 to.9?
As always appreciate you and Steve as valuable resources to keep us on the road

[Eric Maclean Co-Admin]

Bryan
It's not a bad idea to turn off the chassis main disconnect before working on the cables as there is a large current capability on those and a mis step and resulting short would be a bad thing.
Remember the engine starter cable is not turned off by the disconnect and will remain live with the disconnect turned off.

A volt drop test is relatively simple .
The concept assumes that in a loaded circuit the applied voltage is consumed by the load it self say a starter motor but in reality there are other things in the circuit which can consume some of the voltage through resistance things like solenoid contacts or cable connectors.
Say we apply 12 volts to a starter circuit and we connect one voltmeter lead to one side of the starter solenoid and the other lead to the other side.  with the solenoid closed or making contact there should be a zero volt reading on your meter as the contacts should be passing the current and voltage straight through but in reality there will be some resistance in the connections in the solenoid and with use these contact's get dirty causing higher and higher resistance until they finally they won't pass current any more
With the meter connected as the contact resistance increases the the voltage drop across the connect increases this voltage drop will show on your meter  0 volts would be a perfect connection and anything over 1 volt would be a poor connection.
REMEMBER the circuit has to be under load to make these tests.
If you took the starter circuit and measured the volt drop for each of the cable connections and the starter itself the total of all the volt drops should add up to the applied voltage.

Each of your battery cable connections will be the same if a connection has any resistance under load it will generate heat and the voltage will drop across it .

Here is a video link that may explain volt drop testing better than I could.

https://youtu.be/APLic9F7ZsA

Eric

[Bryan Beamon]

Eric thanks again as your explanation was great. I watched the video and with your preview it made it easy to understand. I now have some new knowledge on how to track faulty connections.

[Bryan Beamon]

It turn out to be as Steve had suggested a loose ANL cable in the rear run bay. Thanks again to Steve and Eric.

[Joel Ashley]