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Old 06-01-2015, 03:22 PM   #1
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1987 Motor Home 325 model alternator - HELP!

I have tried to launch my (new to me) 1987 235 MH.

Mechanic assured that I was getting good voltage out of alternator but 2 hours down the road the unit stalled(backfiring too) and had no power to restart.

Determined by local mechanics that that alternator has failed and I was running on battery power alone. The alt that was on board was a 140 volt heavy duty marine unit and could not be found anywhere with the proper plug-in.

They hooked up, what I was told, a custom built 140 volt unit(that had not voltage regulator). Recharged that batteries and told it was good to go.

Three hours down the road the same thing happened and the O'Reilly boys showed that the alternator "voltage regulator" had failed and now the weakest battery was spent.

It this time it was pointed out to me the "kill-switch" unit on the rig had been in the "OFF" position. Could this have caused the conditions I have noted? Can the rig be driven if the kill-switch is engaged?

I am now going to have the old alt rebuilt and have them run a LONG continuity test on it before I attempt to drive it the rest of the 1,000 miles back home.
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Old 06-01-2015, 03:40 PM   #2
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also check all ground wires. They sometimes overheat and burn through..I had that problem a few times until I figured out which wires were not 100% connected..
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Old 06-01-2015, 09:28 PM   #3
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1986 34.5' Airstream 345
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I'm a green horn keep that in mind.
I think the switch should be on.
I started my 34.5 today for the second time in ten years no charge with switch off turn it on and it was charging about 13 volts with nothing on but the engine running and only had two batteries hooked up. Best I can tell you.
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Old 06-01-2015, 09:29 PM   #4
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Thanks Alan,

Will pass that along to the mechanic
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Old 06-02-2015, 12:21 PM   #5
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1977 31' Sovereign
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Quote:
Originally Posted by realpank View Post
.....alternator has failed.....
If you can't find a replacement alternator, you may hafta get yours rebuilt at an old fashioned auto (speedo) electric shop. If that's the case, you'll need to schedule some "downtime."

FYI - Even so-called new alternators aren't really new any more. They're "remanufactured" in some off-shore sweat shop. Just so you know.....

Tom
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Old 06-02-2015, 03:37 PM   #6
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Quote:
Originally Posted by realpank View Post
I have tried to launch my (new to me) 1987 235 MH.

Mechanic assured that I was getting good voltage out of alternator but 2 hours down the road the unit stalled(backfiring too) and had no power to restart.

Determined by local mechanics that that alternator has failed and I was running on battery power alone. The alt that was on board was a 140 volt heavy duty marine unit and could not be found anywhere with the proper plug-in.

They hooked up, what I was told, a custom built 140 volt unit(that had not voltage regulator). Recharged that batteries and told it was good to go.

Three hours down the road the same thing happened and the O'Reilly boys showed that the alternator "voltage regulator" had failed and now the weakest battery was spent.

It this time it was pointed out to me the "kill-switch" unit on the rig had been in the "OFF" position. Could this have caused the conditions I have noted? Can the rig be driven if the kill-switch is engaged?

I am now going to have the old alt rebuilt and have them run a LONG continuity test on it before I attempt to drive it the rest of the 1,000 miles back home.
What 'kill switch'? Some of these units had two switches one of which was in series with the engine battery and the other in series with the coach battery. The switches actually worked heavy duty solenoids that would totally disconnect the batteries. Other units had one switch which was wired to the heavy duty appliances and would remove power from them for storage purposes.
If you have the storage switch it has nothing to do with the engine alternator charging the battery.

The alternator usually does not burn out its wires or fail due to mechanical bearings. In fact if the bearings go bad you can usually hear them.
The primary failure mode is the power diodes which are part of the regulator.

Another possibility is the 'dual battery diode isolator' unit. Some of these motorhomes use this to charge both the engine and coach batteries from the alternator. These have two heavy duty diodes which couple the two batteries together so they can both be charged from the alternator. This unit can prevent charging of your engine battery. The output of the alternator will measure good but the voltage is not getting to the battery.

The first thing you should have checked is the primary grounds from the battery and the alternator where they couple on the chassis.
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Old 06-03-2015, 10:58 PM   #7
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Thank you Howard and Tom,

I am having alternator rebuilt and will pass along the ground info. Is there any way to tell if that is set up correctly?

Are these "dual battery diode isolator' units hard to find? Any idea where it might be located on this unit or where to buy one?

Exactly the kind of information I was hoping to garner! I've dealt with two mechanics on this issue(two breakdowns) that surely have not understood the set-up of this RV. Have found a large truck mechanic, hope I've found someone that can fix it. With this advice I'm much closer to making sure all the system works.

Cheers,
Chris
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Old 06-04-2015, 03:50 AM   #8
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They are usually mounted on the outside fire wall and accessible behind the front hood. You will have 3 heavy duty wires from the two battery banks and also one from the alternator output. A fourth control wire may be included also from the alternator to a separate terminal on the isolator. This provides a signal to control the regulator to optimize the charge voltage for the engine battery. Here is some info:

DB Electrical - 200 AMP DUAL BATTERY MULTI ISOLATOR W EXCITER NEW 48162

Here are schematics, pictures etc.

https://www.google.com/search?q=dual...w=931&bih=1045

Good Luck
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Old 06-04-2015, 10:26 AM   #9
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Thanks again Howard,

Yes, I have seen it on the drivers side.

Continuity check on this will verify it's viability?
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