Date: Sun, 2 Aug 2015 15:46:26 -0700
Reply-To: Christophe Guilbert <cguilbert@PICASSO.UCSF.EDU>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Christophe Guilbert <cguilbert@PICASSO.UCSF.EDU>
Subject: fridge relay nightmare !
In-Reply-To: <CAGVjP+tgh3QBAXJNKXH9dAir+0_P7i-u-vrbKHqwozKH8Ox7zQ@mail.gmail.com>
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Hi guys,
I am adding a secondary battery and 6 various gauges , I already started
an ambitious project to rewire the camper and dashboard accessories
dramatically. For that I am re-purposing the fridge relay which protect
the electronics when the starter kick in.
I spent hours trying to understand it but I still don't have a clear
view how it works especially when my voltmeter does not show what is
expected and my Bentley does not cover it.
Here is my understanding of how the two wires triggering the relay is
working, please correct me if I am wrong.
- Term 86, blue wire , goes to ignition switch , is over +10V when the
alternator is running which mean that it is over +10V when cranking the
starter and of course over 10V when the engine is running. I found
conflicting information about this blue wire, some people said in should
be +1.5V (from the instrument cluster alt warning circuit) when the key
is in the switch on position (just before the starter crank) and the
engine stopped; some other people said we won't have + voltage. In my
case I have 0.07V with no key and 0.2V at the switch on position and
around 1V at the cranking stage than 0.2V again with the engine running.
This is Grey area I really would like someone to explain me.
- Term 85 , red/black wire, goes to ground starter solenoid, is +12V
ONLY when starter is cranking , it is ground otherwise ( before running
the starter and once the engine is running).
Term 87a (water pump and LED panel) is connected to Term 87 (constant
+12V input).
Term 30 goes to the fridge
When the relay is not energized , 87 -> 87a , when the relay is
energized, 87 -> 30 but 87 still connected to 87a
On my relay currant flows (electron wise) from term 85 (-, red/black
wire) to term 86 (+, blue wire) and not the other way around , the relay
must have a diode.
So here are the sequences of event,
1) When no key, 85 is 0v , 86 is 0v, term 87 being connected by default
to 87a , 87a (water pump and LED panel) receive +12V from 87 , term 30
(fridge) is 0V not working
2) When putting key and switch to the on position while engine stopped ,
86 is supposed to be +1.5V and 85 is 0V (grounded), same effect as 1)
anyway because 87 and 87a are connected regardless of the relay state
and 1.5V different between 85 and 86 is not enough to trigger the relay
and close 87 with 30 (fridge)
3) Engine crank, starter working, the alternator and battery is running,
86 is +10V. 85 (which was grounded in step 1) and 2)) become +12V and
counterbalance the +10V of 86. Since there's a diode preventing flow
from 85 to 86, the voltage different between 85 and 86 is 0V and the
relay in not energized , terminal 30 (fridge) is not closed with 87. The
fridge is not working during the cranking process.
4) After the cranking and engine running , 86 re-become +0V and 85 is
+10V, the relay is energized and the fridge is working trough term 30
Correct me if I am wrong but the trick here to isolate the fridge during
the cranking seem to be the fact that the blue wire term 85 is +10 V <
+12V term 86 when the starter work. I supposed that the 1.5V you see in
step 2 was "removed" by the instrument cluster for the purpose of having 10V
I you read that far without too much headache , thank you so much ! ;-)
So why all this , well I like to idea of isolated my instrument during
the cranking process and this is a nice way of doing so, I also wanted
to be sure I understood correctly because I don't have +10V in my blue
wire right now , maybe it is due to the fact that my instrument cluster
is unplugged , I need to find out.
Thanks
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