House Batteries not charging while driving

Hey guys...  This is my latest project.  I suspect this has been an issue for a while because the previous (original) owner went through three sets of house batteries in 4 years.

They charge fine off shore power, generator and the solar system I installed.  I leave the battery disconnect switch on full time.

Someone suggested to check out the solenoid and circuit breaker on the fender under the coolant tank. They both seem to be ok... battery voltage is on both sides of the solenoid that jumps to 14+ VDC with the engine running on both sides.  The CB seems to be ok, but it is very rusted already, so I have a new one coming and will replace it.

Does anyone have a understanding of how this part of the charging system works and can explain it or sketch a quick diagram?  I know the house batteries are supposed to get charged directly off the alternator with the engine running, but I don't know how that happens or what/where to check.  I want to get this solved. Thanks a lot!

Wiring diagram:

It is that breaker, it's a common failure.

Hey Don... thanks for the diagram.

It is interesting that the picture does not show the 12VDC going from the alternator to the batteries. 

Would the power run from the alternator,  through the inverter to get to them?   That doesn't make sense.

Is the charging off the alternator leg missing from this diagram?

Thanks!  Kip

It's good to know that 40A breaker is the weak link.  The new one should be here tomorrow.

Thanks again, Don.

K.

[quote author=CaptainKip link=topic=4510.msg40111#msg40111 date=1627993072
Someone suggested to check out the solenoid and circuit breaker on the fender under the coolant tank. They both seem to be ok... battery voltage is on both sides of the solenoid that jumps to 14+ VDC with the engine running on both sides.  The CB seems to be ok, but it is very rusted already, so I have a new one coming and will replace it.
Does anyone have a understanding of how this part of the charging system works and can explain it or sketch a quick diagram?
[/quote]

Kip, that diagram that Don provided is electrically correct, but might be a little confusing to follow.  The actual circuit routing is very straightforward--at least on my 2019 2552.

The alternator input to the solenoid comes directly from the main truck battery buss in the under hood Ford fuse panel.  If you look, you should see one heavy wire to the solenoid runs directly to your fuse panel by the radiator--that's where it gets it's alternator voltage (when the engine is running).  The solenoid pulls in whenever the key is ON, and feeds the alternator voltage from the other side of the solenoid directly to the 40A circuit breaker--you should be able to see that connection.  The alternator output then runs directly from the output side of the circuit breaker through the firewall to the main positive connection on your house batteries--on my unit it's easy to see that same size wire tying in to my positive battery terminal.

So the actual connections for the charge circuit are pretty easy to follow.  If you have verified that you get 14+VDC on both sides of the solenoid with the engine running, your feed to the solenoid and solenoid pull-in are apparently good.  You should then also get 14+VDC on both sides of the circuit breaker, and 14+VDC on your positive terminal of your house batteries (minus a couple tenths of a volt or so at the batteries for voltage drop through the wire).  As Don says, based on your tests so far, the most likely culprit is that circuit breaker--but if you'll check both sides of the breaker with your voltmeter, you should be able to see a measurable voltage difference that proves that.

Mike

Thanks Mike...  your explanation was what I was after.

As you can see, the CB is pretty sketchy looking.

The new one should be here tomorrow.  I'll pop it in and report back!

Thanks again...

Kip

Mike...  I was just re-reading your post.

It sounds as if I should only see voltage on the other side of the solenoid when the key is on and the connection is made.

As far as I can remember, I have power running through the solenoid all the time...  12.something with the key off and 14 something with the engine running.

Having battery voltage on both sides of the solenoid at rest is not correct, right?

I'll check it again tonight, but that is what I remember.

K.

Having battery voltage on both sides of the solenoid at rest is not correct, right?

Kip, it is correct.

Electrically, the main posts on that solenoid are both connected to 12 volts (with the engine off).  The input post is connected to the main Ford battery buss, so it sees the 12 volts of the Ford battery.  The output post of the solenoid is connected to the 12 volts of the house batteries by a direct circuit through the circuit breaker (that assumes the breaker contacts are not totally open).  If all is good, with the engine off, you will basically see 12 volts on every main terminal of the circuit---BUT it will be coming from two sources: the Ford engine battery on the input terminal of the solenoid, and the house batteries on the solenoid output and both breaker terminals.

IF everything is good, once you start the engine, the input terminal of the solenoid goes to 14+ volts (from the alternator feed), but--because the solenoid closed when you turned the key switch on--everything else goes to 14+ volts as well (both circuit breaker terminals and the connection at the house battery post); that's how the house batteries get charged.

So, if you have a bad breaker, how can you be seeing the 12 volts on the output side of the solenoid with the key off??  IF the circuit breaker is completely open--failed entirely--it would not conduct any voltage/current at all, so it would totally interrupt the connection to the house batteries and you would not have the 12 volts on the output of the solenoid with engine off.  But when a set of contacts in a circuit go bad or get corroded over time, whether it be a circuit breaker, a switch, a solenoid, or anything else with mechanical contacts, they will frequently conduct the tiny amount of current needed to show voltage on a meter, but will not pass the heavier current in the circuit.  That's why, when checking voltages in a circuit, it should always be under load to get an accurate reading.  It is very likely that your breaker is conducting the tiny current load that your meter draws when you take a reading, but will not pass the much heavier charging current to your house battery circuit.

Mike

Wow...  thanks for the class, Mike!  It makes perfect sense..

Let's hope the new CB does the trick!

Hey Don... thanks for the diagram.

It is interesting that the picture does not show the 12VDC going from the alternator to the batteries. 

Would the power run from the alternator,  through the inverter to get to them?   That doesn't make sense.

Is the charging off the alternator leg missing from this diagram?

Thanks!  Kip

Actually, the diagram does show it.  At the far right the "from alt" gets connected by the solenoid, through the 40amp connector to the inverter and the house batteries.

That 40 amp breaker is the one on the driver's side wheel well under the radiator overflow tank.   It is a common failure.  Make sure you install the new one with the same orientation of the silver and copper leads as the original.  It's important.

I had the same problem and posted about it in late July.   We had just returned from two weeks off grid with new coach batteries and couldn't get any charge from the alternator.   The guys on here were most helpful in describing the circuits and the two most likely culprits  (circuit breaker and solenoid). 

I ran the voltage tests and the meter showed OK but I replaced the circuit breaker anyway in case it was weak (apparently a common problem).    That was it.  Problem solved.   

There are some very helpful people on this forum with great technical, mechanical and engineering skills and I appreciate all the help I have gotten on issues over the years. 
.   

Thanks very much for the encouragement...   I changed out the 40A  CB and that solved my problem!

Thanks, everyone!

Thanks very much for the encouragement...   I changed out the 40A  CB and that solved my problem!

Thanks, everyone!

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