[T2] fridge relay nightmare !

[Christophe Guilbert]

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