# 18 Volts in back!!!



## jammydodge (May 28, 2019)

This is a long one so please bear with me.....

1979 Dodge Sportsman
5.9 litre petrol v8 (I think... that's a separate issue)

So...... as its an old vehicle it has a dynamo rather than an alternator, which then has an external rectifier (hope that's the correct name, tried to do my homework)

Well, the rectifier gives 12.18 volts to the front battery so obviously not enough charge
but 18ish volts to the leisure batteries!!!!!! 
I can only assume the rectifier is not working correctly

My questions are:

For a quick bodge can i switch over the two outputs so the 18 goes to the front and put something like a volt stabiliser on it? and then have the 12.18 go to the leisure batteries? Will that charge the leisure batteries?

This is the rectifier...








And to fix it properly, does anyone know where to get a replacement part? I have tried rock autos but could only find this.... https://www.rockauto.com/en/moreinfo.php

Would this work?

Well I think that's about it

Many thanks in advance


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## Penquin (Oct 15, 2007)

18v going ANYWHERE would worry me, the normal ( in inverted commas) charging voltage is around 14.4v not 12.18 on modern vehicles.

From memory (therefore liable to be inaccurate) the rectifier/dynamo system voltage is partially engine rev dependent with the rectifier trying to smooth it out. Your 12.18v reading was with the engine running or not ?

It may be that the 18v apparently at the back is a reasonable CHARGING voltage, but not a resting voltage. There is no way that a 12v battery system can give 18v at rest AFAIK.

Sorry, not a full answer, but some more details about how those figures were obtained would probably help. Dynamos are very different beasts to the modern alternators so things in the wiring loom may also have major differences.


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## rayc (Jun 3, 2008)

Not being pedantic but to clarify. A rectifier converts AC to DC [ the opposite of an inverter] whilst a Regulator 'smooths it out' i.e. regulates it.


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## Penquin (Oct 15, 2007)

I think they were usually referred to as "dynamos" back in the days when I had and maintained a Morris 1000 Traveller, whose brand new dynamo (fitted in Exeter) failed at Quimper after crossing Ply,ouch to Roscoff........ it took the Austin-Morris dealer 2 days to locate a new one as all the current cars then had alternators.... That cost the breakdown insurance (AA) a few quid as well for emerg NYC accommodation at L'Orangerie at Quimper....

Oh, memories of such things and times (total brake fade as descended from the top to the Ardeche.....), the looks of wonder at a WOODEN CAR or so it appeared..... that was 1972 though with a '63 registered car....


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## rayc (Jun 3, 2008)

I wonder if the box that is in the photo is in fact a battery isolator i.e. it allows the generator/alternator to supply two batteries whilst keeping them isolated from each other?
https://itstillruns.com/charge-extra-battery-driving-car-5343827.html

If so then perhaps the OP could just confirm what readings he is measuring directly at the vehicle and hab battery terminals when the engine is running? 
It would appear to be a risky thing to do to swop connections on the box. One is battery negative [common to both batteries], one is the alternator/generator output and the other connects to the habitation battery positive. For it to work correctly it must be connected correctly. [Of course the alternator/generator output is already connected to the vehicle battery positive terminal].
If I am correct then the isolator does no regulating or anything electrical except to isolate the two battery positives and any discrepancies of voltages is not anything to do with it.


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## rayc (Jun 3, 2008)

Looks the same unit to me: A lot of the video is changing the alternator but it shows the isolator


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## tugboat (Sep 14, 2013)

Is there no possibility of upgrading to an alternator? I would have thought there would be something that would physically fit.


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## eurajohn (May 9, 2005)

An alternator generates ac power, a dynamo generates dc power.

Alternators fitted to vehicles will be self contained with the necessary components for the control and rectification components to convert the ac to a usable (to the vehicle) dc current.
Dynamos however require and have a separate "regulator" to control the already dc current.

Typically both devices will supply in the region of 14 volts, to the system it is attached, this of course refers to a 12 volt system and not a 24 volt one as found on many commercial vehicles.

How or where the 18 volts discussed is coming from would need some more basic information on the vehicle in question i.e. is the base vehicle 12 or 24 volt.

.


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## pete4x4 (Dec 20, 2006)

A dynamo can quite easily generate 18v unregulated so either the regulator to the leisure batteries is not working or there has never been one fitted.
The rockauto part is for an alternator and is a rectifier so will not work, as already mentioned dynamos are DC so need regulators. They used to be in big black boxes with LUCAS written on them well any circa 1960s car did.


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## raynipper (Aug 4, 2008)

Just in case Jammydodge.
I have a 1978 to 1979 Dodge Motorhome Chassis manual covering M300, M400, M500 and M600 engine chassis and they all show Alternators.

I am willing to sell the manual for £20 p&p from France. If your interested.

Ray.


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## tugboat (Sep 14, 2013)

eurajohn said:


> An alternator generates ac power, a dynamo generates dc power.
> 
> Alternators fitted to vehicles will be self contained with the necessary components for the control and rectification components to convert the ac to a usable (to the vehicle) dc current.
> Dynamos however require and have a separate "regulator" to control the already dc current.
> ...


Yes, I understand that John.

I meant upgrade the vehicle to an alternator set-up like any contemporary van.

To go travelling with an (sorry,no offence meant) old fashioned system, if there are problems there is less likelihood of finding someone to fix it or supply parts.


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## Pat-H (Oct 30, 2009)

Id agree a dynamo system has a regulator that ensures the dynamo output is only switched to the battery when its above the battery voltage (otherwise the battery will feed the dynamo as a motor!) and a second component that disengages the feed when the battery voltage reaches a certain level (typically 14 or so volts) The black box regulator normally has 2 coils in it to do these two functions.
Its possible the pre regulated output has been fed back to the second battery but it would then have to have its own regulator or you risk massively overcharging the second battery.
Dynamos are less effective at low revs so the move the an alternator would be a smart choice. In which case the stock regulator would need to be removed and bypassed and a splitter added to ensure the two batteries are isolated from each other.

When measuring a dynamo system you need the revs to be quite high to get a decent output tick over or a low rpm won't produce much output.


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