Voltage drop while driving

[Rick Mayer]

We have 1993 Grand Marquis Beaver with drives like a dream except for the voltgage coming off the altenator, no load around 12.5v, overhead panel shows 12.2v for coach with +12 amps and 13.2v with + 6 amp on chassis batteries.  As soon as you start loading the altenator up, AC fan, head lights etc, the volt meter continues to drop below 12v.  Just put on the secong new altenator in two years (160 amp), replaced both chassis batteries, ( one had a bad cell)  found a frayed wire shorting against the engine block. But still have same issue.  As a side note,the inverter voltage is only reading 105V with the engine running, with shore power or generator operating, it reflects the ac line voltage. We have had the Coach for almost four years and are still learning, do I have an invertor that is sucking all the power from the altenator or is it something else?  Thanks for your help  Rick Mayer  605-321-0966

[Edward Buker]

Rick,

Probably the most useful tool here would be a clamp on DC Ammeter. One that has a jaw opening of .7 inches will allow you to get on most wires in some tight areas. You zero the meter with a button before you clamp on and then clamp on and read the current. This will allow you to see how much draw the alternator has on it and where the current is going. You could have batteries drawing excessive current, a bad inverter, a bad alternator, a bad isolator, or a wiring issue. Between a voltmeter and a unit like this one in the link you can take the measurements that will give you your answer. The voltage is low coming out of the alternator as measured at your panel with what seems to be with minimal current draw. You did not mention the house batteries condition that feed the inverter, they are suspect also. You should also check the AC current load coming out of the inverter. That voltage is also low, and your panel calibration may be suspect given the generator voltage is also low. You can confirm those voltages with a voltmeter.

http://www.amazon.com/Extech-MA120-Ampere-Current-Detector/dp/B000BEZV5O/ref=sr_1_9?ie=UTF8&qid=1371248644&sr=8-9&keywords=dc+clamp+ammeter

Good luck on your hunt.

Later Ed

[Bill Sprague]

I have a Craftsman clamp on meter I like.  Click:  http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P?PDP_REDIRECT=false&s_tnt=39869:4:0.  Be sure you get one that does both AC and DC.

FWIW, consider that house batteries that have discharged from use or storage put an over-sized load on an alternator that was intended to only replace the current use in a start up and run a bunch of lights on a truck.  Getting 4 to 6 large deep cycle batteries up to snuff will heat up the alternator to the point that early retirement become frequent and necessary.  Use the Onan.

[Joel Ashley]

I have to bolster Bill's comment in that probably the most common alternator failure is due to owners starting their coach up after dry-camping without using their Onan generator to charge batteries back up first.  Even a new 160 amp alternator won't hold up long against the stress and heat of building back the charge on that many drained batteries.  I hate to say you may have to try returning your last alternator under warranty, but run Ed's tests to zero in on the problem.

Here is the clamp meter I've used for years, a Craftsman 82062.  It wasn't an arm and a leg to buy, but I wouldn't be surprised if either Ed's or Bill's models are improved over mine, especially where Ed's is more dedicated to clamp metering.

Joel

[Bill Sprague]

..... but I wouldn't be surprised if either Ed's or Bill's models are improved over mine, especially where Ed's is more dedicated to clamp metering.

I'm crushed that you think I have a weaker than Ed's!  :-)  It sure walks and talks like a clamp meter!

[Joel Ashley]

That's OK, Bill.  We'll still vote for ya.  

I just have a feeling, especially given Ed's electric experience, that a stand alone clamp ammeter might offer better performance in that dept. than my combo unit.

Now, if you actually have a meter that walks and talks, that might get mine and even Ed's attention   .

Joel

[Edward Buker]

Actually this is the meter that I own. I was troubleshooting a friends boat with a variety of electrical problems, so I needed a smaller probe to get into the harnesses that were very tight and hard to reach. This is a good meter which includes DC/AC volts and amps but it has a max current measurement that is limited to 100amps. That is sufficient except for maybe what a large starter might draw. Having the ability to measure DC amps by clamping onto a wire has come in handy quite a few times. All these meters are probably very good ones including Bill's :-)  Don't leave home without it....

http://www.amazon.com/GTC-CM100-Current-Clamp-Meter/dp/B001TCWL1E/ref=sr_1_5?ie=UTF8&qid=1371434431&sr=8-5&keywords=dc+clamp+ammeter

Later Ed