Well the continuing saga of motorhome ownership. Now it looks like my alternator has quit. I got a warning light on the dash and the computer readout said the voltage was too low. My voltage on my engine batteries is lower with my engine running than it is with the engine off. I have a cat c7 7.2 liter engine in a 2004 phaeton. I talked to the local cat shop that does my engine work and was told that the voltage to the batteries with the engine running should be between 13.5 and 14.5. It is definitely lower than that. It is in the 12.6 range or so. The voltage actually drops after the engine is running. The shop told me to check all of my connections. I know where the alternator is and can get to it through the bedroom floor. I can and will in the next day or 2 check all of the connections on the alternator itself. My question is after the wires leave the alternator where do they go. I assume over to the right rear of the motorhome where the engine batteries are. Are there any connection I need to check before I get to the battery compartment? Most likely my alternator is bad but I might get lucky and find a loose connection somewhere. Any suggestions or directions would be helpful.ThanksKen

The connections "at" the battery is the main area for a dirty, corroded or loose connection and they will cause poor charging and eventually a dead battery.The only sure way is remove the battery cable at the battery clean the terminal post and inside the cable connector with a special brush you can buy at any auto parts store. Cost less than $10.Sounds like a bad alternator not sure if they can check it on the RV like they do on autos just depends on how hard it is to get to but they do check them for free.

You could read the voltage at the alt, when you get the cover off. You list the voltage it should be.

If you can find a GOOD local Starter/Alternator Shop they can check it and repair it for a reasonable cost. Normally a lot less than new replacement.Alternators are fairly simple in operation and repair.

I would imagine that the alternator wire goes to a battery isolator if it also charges the house batteries. If it does you may want to start checking there. You can disconnect from the isolator and reconnect direct to the batteries then recheck the voltage. Make sure you check for a good ground. It is important for you to know your charging system and know how the alt. is excited and then the path of the output. Many people have thought the Alt. was bad because no output not always so. There are many good articles on the web if you find your config. then troubleshoot and get it right the 1st time.

May sound dumb, but make sure the belt hasn't broken.Doug

Alternator Quit ( I think) | Good Sam Community

13 Replies

bob_b1
Explorer
Jun 27, 2014
I had an issue with a small wire corroding off of the alternator. It was small red wire that ran from the alternator's positive lug to the internator regulator. It is a self-exciting regulator. With the wire off the regulator, their was nothing to tell it to start allowing voltage to flow. Since the wire was under the alternator, I never saw the issue until I outlayed a fortune on a new alternator. In fact I thought that the new alternator was bad too and ordered another one. It was during my second attempt that I notice a wire just hanging down. I connected it and it was all good. The original alternator was already sent back as a core by the time I found my mistake. Oh well.

FYI: some manufacturers have a fused circuit for the wire that I was talking about, so check the fuse box to see if there is one.
n4hwl
Explorer
Jun 27, 2014
backhoe mike

I assume from what you are saying it is difficult to clean side radiators? I did not know that

Ken
backhoemike
Explorer
Jun 27, 2014
pros and cons to both, mine is a side radiator and the road salt had it rotten and I never knew it, altenator was easy to get too. I would rather have a rear radiator like you so I could wash and clean it regular
n4hwl
Explorer
Jun 27, 2014
Well I got another alternator installed today and things are working fine. I was not able to get the alternator off and took it to the local cat shop. These rear radiators sure make it hard to get to things and work on things. If I get another motorhome some day I would really like it to be a side radiator. Anyway thanks for the help

Ken
wolfe10
Explorer
Jun 25, 2014
1. With engine at high idle (1100 RPM), check voltage at the B+ terminal of the alternator. Should be 13.7-14.5.

2. If not, indeed check connections. You may have an external SENSE WIRE (common on rigs with diode-based isolator). You will have the B+ and ground connections-- at battery, likely at isolator and at the alternator.

3. If diode-based isolator (aluminum fins about 3" by 6") check voltage at the center lug as well as the two outer lugs with engine at fast idle.

Let us know what you find.
Tinstar
Explorer
Jun 24, 2014
Since your getting around 12.6 volts and less with the engine running, it indicates the alternator isn't putting out and it probably isn't a bad connection. Depending on what you find when you test further, pulling the alternator is really an easy job. There are only a couple of wires that go to the back of it. I had the same problems you are having. I pulled the alternator and took it to a local shop. They fixed it in less than an hour and it only cost me about $30. It was some kind of resistor and the alternator didn't need to be rebuilt. If possible, I would always take my own rebuilt alternator over another that I might buy over-the-counter.
Sprig
Explorer
Jun 24, 2014
n4hwl wrote:
... It is definitely lower than that. It is in the 12.6 range or so. The voltage actually drops after the engine is running...

Post like this always leave me scratching my head.

It is the "range" of 12.6 volts ... and what range? Is there some meter that says "range 12.6"?

Then, no mention of the voltage reading with the engine running.

And, no mention of WHERE the voltage reading was taken.

To the OP take one ACCURATE meter reading at the battery and NOTE the EXACT reading on a good meter. Then, start up engine again, and take 2 EXACT meter readings; one at the battery as before, and the second at the output terminal of the alternator.

With these two readings, you can BEGIN to determine what your problem is.

If you can't do the above, take it a shop that can.
wwest
Explorer
Jun 24, 2014
Remove the isolator connection to be sure that's not were your charge power is going.
ddndoug
Explorer
Jun 24, 2014
May sound dumb, but make sure the belt hasn't broken.

Doug
Dog_Trainer
Explorer
Jun 24, 2014
I would imagine that the alternator wire goes to a battery isolator if it also charges the house batteries. If it does you may want to start checking there. You can disconnect from the isolator and reconnect direct to the batteries then recheck the voltage. Make sure you check for a good ground. It is important for you to know your charging system and know how the alt. is excited and then the path of the output. Many people have thought the Alt. was bad because no output not always so. There are many good articles on the web if you find your config. then troubleshoot and get it right the 1st time.