Date: Tue, 9 Jun 2009 18:36:56 -0700
Reply-To: mark drillock <mdrillock@COX.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: mark drillock <mdrillock@COX.NET>
Subject: Re: Help with 12v Dometic wiring
In-Reply-To: <030c01c9e966$0bb66a90$6801a8c0@PROSPERITY>
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I'm right in this case. The ignition key switch does nothing to the
circuit except when in the far start position it puts 12 volts on the
relay "ground" while cranking. This keeps the relay from activating
during starting. Some years use this "ground" for the fridge relay and
some don't. This is a redundant feature since the blue wire won't
trigger the relay anyway until the engine starts and the alt light goes out.
The resistance of the starter motor solenoid coil is pretty low and it
works fine as a ground for the small relay until 12 volts is applied to
it. Then it sits at closer to 12 volts at the input end, dropping to
zero volts at the true ground end. The relay ground leg is wired to the
starter coil input so the low resistance normally looks enough like
ground to it. The smaller relay coil has a much higher resistance so you
end up with 2 resistors in series. Basic electronic theory says the
voltage will divide between the 2 series "resistors" in proportion to
the individual resistances. Most of the voltage from the blue wire thus
appears across the small relay coil so it activates, unless the ignition
switch puts the starting circuit 12 volts on the shared point. With
near 12 volts at both small relay coil pins very little current flows
and thus the relay just sits there in the off position until the key is
released and the starter coil again looks enough like ground.
This also explains something that more than a few people have
experienced. If you add a "hard start relay" for the starter motor to a
van that has either a fridge relay or aux battery relay that is using
the starter as a "ground", the starter will re-engage as soon as the alt
led goes out even with the key released. That is because the small "hard
start relay" coil resistance closely matches the fridge or aux battery
relay coil resistance. So when the extra relay is added for the starter
it no longer provides the apparent "ground". Instead the 12 volts from
the blue wire gets divided nearly evenly between the 2 small relays and
6 volts is often enough to trigger both of them. This makes the starter
grind into the flywheel of the running engine.
Mark
Scott Daniel - Turbovans wrote:
> that's different than the way I figured it out ......
> and you could be right, and I wrong .....
> but here is how I understand and diagnose.and wire that relay under the
> driver's seat.
> Assuming we're talking about the same thing .....
> that relay is there so either 12v for fridge operation, or charging to a
> aux. battery, can occur ONLY if the alternator is charging.
>
> what I find is that the blue wire from the alternator warning light is
> on one side of the 'triggering' side of the relay.
> If it could just find it's way though the mini-coil inside the relay to
> a ground on the other side ..........that would do the job.
>
> I'm sure they had a good reason for doing it this way .....( instead of
> having a permanent ground the 'normal way." )
> This wire that we want grounded, ( to complete the circuit to energize
> the relay, to send 12v where we want it ) ........
> here's how that wire works to me, from my testing.
>
> when you turn on the key , that wire is ....I forget what, but it's not
> ground.
> However, after your turn the key to 'start' and then back to 'on'
> .................that wire becomes grounded.
> and once the alternator kicks in, the relay energizes.
>
> Mark says it's something to do with 'starter motor coil' .
> that could be so.......
> but in my figuring this out ............sure seems to me that it's a
> clever ignition switch that supplies ground to that wire only after
> moving the ignition switch from on, to start, then back to 'on', like
> when starting up the engine ..........I could be wrong, but
> 'functionally' .....this thinking or understanding of it 'works',
>
> Scott
> turbovans
>
>
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