Date: Tue, 9 Jun 2009 19:00:21 -0700
Reply-To: Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM>
Subject: Re: Help with 12v Dometic wiring
Content-Type: text/plain; format=flowed; charset="UTF-8"; reply-type=response
well that last scenario sounds nasty.
ok, tell me this ..........
if the starter trigger wire fell off the solenoid ,
and you roll started the van ......and the engine fired and the alt.
started charging,
would the fridge relay then not be able to energize ?
( due to lack of esoteric connection to the fridge relay from the starter )
Scott
----- Original Message -----
From: "mark drillock" <mdrillock@COX.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Tuesday, June 09, 2009 6:36 PM
Subject: Re: Help with 12v Dometic wiring
> 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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