Date:         Sat, 30 Aug 2014 08:50:02 -0700
Reply-To:     Brett Ne <brettn777@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Brett Ne <brettn777@GMAIL.COM>
Subject:      Re: Fridge on 12V
In-Reply-To:  <BLU436-SMTP178CF148740374E74F752CD90D80@phx.gbl>
Content-Type: text/plain; charset=UTF-8

Here is my response to a similar question posted four days ago:

###############################
I just finished testing my Dometic 215B yesterday after chasing down some
gremlins in my shore power 12v transformer.  The fridge started at 82
degrees in the afternoon and gradually cooled down.  By morning, it had
dropped down to 36.  By the following afternoon, the fridge had raised to
46 degrees while the outside temp was 90 and the cabin temp was 97.  This
was all done running off 12v with shore power plugged in.  For monitoring
fridge temperature, you really need a remote thermometer installed; the
human hand is poorly suited for measuring temperatures(other than a child's
fevered forehead) and every time you open the door, the cold air flows out
and is replaced by warm air, further taxing the meager cooling capacity of
the fridge.

These fridges take a while to cool down, so if your trip was only 2-3
hours, you won't see much cooling.  I usually try to run the fridge off
propane or shore power for several hours before a trip to pre-cool it. A
lot of people think that the 12v mode can't effectively cool the fridge
because they don't understand that the ammonia cycle fridge behaves
differently than a compressor fridge.  It takes at least 30 minutes of
heating before the complex cycle reaches a state where it will start
cooling the fins, and if heating is interrupted for more than a few
minutes, it has to start all over again. Most systems will cut power to the
fridge when the engine is shut off.  So, if you drive for 8 hours and stop
for gas, stop for lunch, stop at the store for a six-pack of camping
supplies, and stop for a quick tour of the string cheese museum, your
fridge may only be cooling things for 3-4 hours because of all the
interruptions in the ammonia cycle.  There is a nice write-up of this
available on the interwebs called "Reefer Madness" that is a must read.  I
have my fridge wired so that it will run off battery power even with the
engine off to avoid cooling interruptions.  If I'm going to stop for more
than 45 minutes or so, I switch it over to propane, to avoid deeply
discharging the aux battery.
###############################

Bottom line: cooling is okay on 12v, but not as strong as when using
propane, and you need to make sure that the cooling cycle isn't interrupted
periodically.  I believe that most Westies are setup with a simple relay
system that cuts power to the fridge when the engine is stopped.  The
heater element is a simple resistor & is easily tested.  Disconnect the two
leads
 going to the heater element and use a multimeter to measure the
resistance.  Find the amperage rating for your model(ex.- 9.8A @ 12.4v) and
calculate the theoretical resistance: ex.- Resistance = Voltage/Amperage =
12.4v/9.8A = 1.265 ohms.  The calculated and measured resistances should be
about the same.  Clean the connections to the heater element; copper
oxidizes over 30 years and can hinder the flow of current through the
connections.  Before reconnecting the heater element, measure the
resistance between the ground connection & ground and the positive
connection and the positive battery terminal(engine may have to be running
for this...don't let the terminal touch anything grounded).  Both of those
resistances should be pretty darn close to 0; if not, found out why.  If
you have a suitable ammeter, you could measure the current flow through the
heating element, but I think most of these fridges draw around 10 amps,
which is typically the limit on most cheapie multimeters & may damage it.

That's about it.  The potential problems boil down to:
Poor ground connection
Poor battery connection
Faulty heating element(resistance too high/low)
improper use(periodically interrupting the ammonia cycle)





--
Brett in Portland, OR
"Albert" '82 VanaFox I4 Riviera