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Old 10-09-2019, 08:49 PM   #1
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Dead chassis batteries, alternator problem?

Getting ready to head out for a few days. Went to start my rig and had dead batteries. Nothing at all. So I used the battery boost switch and it started right up. As soon as I took my finger off the boost switch the coach died. Tried again and looked at the alternator gauge while still holding the boost switch and the gauge didn't move at all. No charging showed on the battery. I let it run for about 3 or 4 minutes, when I took my finger off the boost switch, it kept running for about 60 seconds and then died. Tried it 2 more times and same thing happened both times. I measured the voltage at the battery and it showed 7.3 on both batteries.

Is this just a dead battery issue, or would it be the alternator? I thought the alternator would keep the engine running even if the batteries were dead.

I've got an '02 Holiday Rambler Endeavor 38PST. I don't want to just replace the batteries if they're just going to get drained again.
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Old 10-09-2019, 09:07 PM   #2
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Using a helper and volt meter, check the big wire on the alt with engine running...could be a bad regulator or worn out brushes....
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Old 10-10-2019, 07:51 AM   #3
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You could hang an external charger on it and see if it comes up. You're not going to rehab a near dead battery with the alternator in a few min. How old is the chassis battery? If more than 3 change it, especially if you live in the south. With a charged battery you can assess alternator output easily.
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Old 10-10-2019, 07:56 AM   #4
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Charge the chassis batteries up (if they will in fact accept a charge) and try again. You can use a portable battery charger, or jumper from the house system with cables if the coach is plugged to shore power. Once you have the batteries charged and the engine running, you can verify alternator output (or not).


If the coach was sitting for very long and the engine not run, there is no reason to suspect the alternator. The alternator can't do anything to help the batteries unless the engine is running, a and running for just seconds or even a few minutes isn't going to do much for a dead battery.
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Old 10-10-2019, 08:11 AM   #5
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If it doesn't charge while running, you have a charging system issue.

Starting it while holding the boost switch proved a low chassis battery.

Holding the boost switch for a while proved that the chassis batteries can accept a charge. That's why it ran for 30 seconds. If you held the boost switch it will keep running until they die.

The fact that it dies out after letting go if the boost switch proved no alternator output.

The contact between the commutator and brushes can corrode, while sitting, and that causes a no charge condition.

A bad connection between the alternator and battery can cause a no charge condition.

If A Dulvac system, a 5 amp fuse powering the sense wire on the voltage regulator, can cause a no charge condition.

A bad or missing connection of the sense wire, at the batteries, can cause a no charge condition.

If equipped with a battery isolator, a blown diode can cause a no charge condition.

More info and some testing is needed to narrow it down.
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Old 10-10-2019, 08:25 AM   #6
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The first thing you do is check and clean all batt. connections .
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Old 10-10-2019, 08:45 AM   #7
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Quote:
Originally Posted by twinboat View Post
If it doesn't charge while running, you have a charging system issue.

Starting it while holding the boost switch proved a low chassis battery.

Holding the boost switch for a while proved that the chassis batteries can accept a charge. That's why it ran for 30 seconds. If you held the boost switch it will keep running until they die.

The fact that it dies out after letting go if the boost switch proved no alternator output.

The contact between the commutator and brushes can corrode, while sitting, and that causes a no charge condition.

A bad connection between the alternator and battery can cause a no charge condition.

If A Dulvac system, a 5 amp fuse powering the sense wire on the voltage regulator, can cause a no charge condition.

A bad or missing connection of the sense wire, at the batteries, can cause a no charge condition.

If equipped with a battery isolator, a blown diode can cause a no charge condition.

More info and some testing is needed to narrow it down.
Thanks twinboat. I don't believe it has a battery isolator. Where would the 5 amp fuse be located and how would it be marked? I looked in the buss boxes and couldn't find anything to indicate something related to the alternator. How would I know which wire is the sense wire. I'll check all of the connections and get back with anything that I figure out.

Thanks
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Old 10-10-2019, 08:47 AM   #8
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Charge the chassis batteries up (if they will in fact accept a charge) and try again. You can use a portable battery charger, or jumper from the house system with cables if the coach is plugged to shore power. Once you have the batteries charged and the engine running, you can verify alternator output (or not).


If the coach was sitting for very long and the engine not run, there is no reason to suspect the alternator. The alternator can't do anything to help the batteries unless the engine is running, a and running for just seconds or even a few minutes isn't going to do much for a dead battery.
Thanks Gary. I thought the engine would continue to run once it was started even if the batteries were very low.
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Old 10-10-2019, 09:06 AM   #9
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Thanks twinboat. I don't believe it has a battery isolator. Where would the 5 amp fuse be located and how would it be marked? I looked in the buss boxes and couldn't find anything to indicate something related to the alternator. How would I know which wire is the sense wire. I'll check all of the connections and get back with anything that I figure out.

Thanks
I don't know the fuse location. It may be in the engine compartment labeled DuVac.
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Old 10-10-2019, 11:09 AM   #10
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First thing I would do is get engine started and check alternator output.
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Old 10-10-2019, 12:34 PM   #11
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Thanks Gary. I thought the engine would continue to run once it was started even if the batteries were very low.
That comment shows your age
Before ecm we could run the screw in and run until out of fuel when a generator or alternator failed
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Old 10-10-2019, 08:26 PM   #12
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Dead chassis batteries, alternator problem?

We had an alternator fail on our Jeep Grand Cherokee and I was able to limp into a repair shop on just the battery.

It only took about 20 minutes to swap the alternator but they wouldn't let us go until they charged the battery. The owner of the alternator shop said we would run the risk of burning out the new alternator if it was forced to recharge a dead battery (10 volts).
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Old 10-11-2019, 09:25 AM   #13
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"Dead" is not a very precise description of battery status. At 12.0v, a battery is about 50% charged. At 11.5v, it is no longer usable for some purposes and could be called "dead". By the time is reaches 10.5v it's really dead and unusable for anything except small lights. Much below that, it may not even light one small bulb.


What I'm getting at is that you can't make reliable diagnostic judgments based on the description of a battery as "dead". Some things may continue to work for low values of "dead", while others cannot function at all at that same level. For example, the starter motor requires a lot more battery capability than the dashboard lights. If you know you have low battery voltage, get that corrected first and then diagnose the rest of the systems.
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Old 10-13-2019, 04:05 PM   #14
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Turned out to be a blown fuse. I noticed that the tachometer wasn’t working either which made no sense. New 10 amp fuse, used battery boost to start it everything worked and batteries charged back up.
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