Date: Sat, 17 Oct 2009 17:18:20 -0700
Reply-To: neil n <musomuso@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: neil n <musomuso@GMAIL.COM>
Subject: Re: Jetta swap: Check Engine Light Install Electrical Questions
In-Reply-To: <02db01ca4f80$6c729fe0$6401a8c0@PROSPERITY>
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On Sat, Oct 17, 2009 at 4:20 PM, Scott Daniel - Turbovans
<scottdaniel@turbovans.com> wrote:
> re
> "No continuity is shown between new wire from
>>
>> ECU and vehicle ground ign. on or off. "
>
> if you actually mean what you have written there, you don't understand the
> circuit.
> The ecu SUPPLIES ground to the 'ground side' of the CEL.
>
> Switching can take place in either the ground side, or power side of any DC
> circuit - this is an example of switching in the ground side.
> The LED for the CEL has power on one side. As long as polarity is observed,
> if the other side 'sees' ground........
> the light will be turned on.
> The ecu is what SUPPLIES this ground signal.
>
> at no time is there any connection between that ecu pin and chassis ground.
> One could say you are trying to make this too hard.
> re
> Should I test continuity between new wire "harness ground"?
>
> no no no no no .
> look at it this way.
> pretend there is a tiny switch ( there is, but it's solid state ) inside the
> ECU.
> the body of the ECU is grounded to the van's body.
> if anything , you'd check that the ecu's ground has continuity with the body
> of the van, which is sure has to have, or it couldn't work.
> So when that little switch 'closes' ........
> it PROVIDES a ground path for the LED, turning it on.
> Cant' say it any simpler than that.
Really glad you wrote Scott.
FWIW, the OEM Jetta CEL is incandescent.
I already knew that the ECU supplies ground, when powered up. But,
from what you've pointed out, I'm a little clearer on the difference
between a ground that the ECU supplies, and a ground supplied via
frame of vehicle. I just wasn't seeing the ECU as having an internal
switch (a solid state as you point out) that supplies a connection to
ground. With that in mind.....
Inside the ECU, I bet there's a diode, or equal, in the path that
supplies ground (when appropriate) to the CEL. If so, that would make
sense. For lack of a better way of saying it, power through this
ground would only flow one way, thus checking ground continuity to
frame would not work. (I won't check that again! <smile>)
Though not directly related, the Jetta diagrams do specify phrases
similar to "Motronic Harness Ground" and "engine block ground". But in
my mind, it seemed that they all ended up connecting to the frame.
But, what you point out directs me towards thinking more clearly of
how the ECU internally supplies ground.
> that wire........which in my opinion you should never check that it is
> conencted to ground, EXCEPT , by observing that it turns on an LED (
> incandesent will work too, but it expects to see an LED ) only is ground,
> when the ECU switches it to ground inside the ECU.
>
> if it supplies a ground, that turns on the LED, you're done. Like if you
> turn on the key, the LED is on, you fire up the engine ( with no codes
> present ) and it goes out, 'YD' - you're done.
>
> or.........when codes are pressent to turn on the LED ..it should be one.
> But don't do anything with or to that wire other than connect it to an LED
> properly, with correct polarity.
Ok. Yup. CEL works as it should. I'll leave it be.
> re
> but then why do I see
>>
>> 12.xx volts with ign. off?
>
> don't do that !
> this is akin to asking ..........., well, can't think of a good enough
> example..............but by doing that .you are looking for the wrong thing
> at the wrong time in the wrong place with the wrong method.
>
> we don't know what the engineers had in mind or what they did.
> All we can do is check that things work like we understand them to work.
> like test for what we know should be there, and 'don't make things up.'
> I don't think it says anywhere in any troubleshooting or Bently book to look
> for voltage on the ECU CEL pin with the key off.
> Then why do that ?????????????????????
Initially checked new CEL wire I installed as I wanted to confirm that
12V present when key turned to ign. on position. Which it did. But in
testing, first thing I checked was to see if circuit was off with ign.
off. Seeing 12V confused me.
> mercury must be retrograde of something ! Yikes !
> and I think Batt + power on the 'other side' of the LED is the real right
> way, not ign power. As I said, all soobies are that way.
> I can well imagine that it's not clear in Bentley for your year engine
> though.
>
Really appreciate info on which + source to connect CEL to. I see why.
The switch is on the ground side of circuit from ECU. I wired +
connection to 15 as I wanted to make sure there wouldn't be any
battery drain (given that I found 12V when ign. off) if I wired + to
30 (B+)
Again, really appreciate you taking time to reply Scott.
Given that I had no easy way (other than physically/electrically
following PCB paths) to make sure I wasn't missing any components that
*might* be involved on the + side of the CEL, and that there was no
CEL wire to start with, installing a permanent CEL *appeared* to be
more involved than "normal". Turns out it was pretty straight ahead!
Neil.
--
Neil Nicholson '81 VanaJetta 2.0 "Jaco"
http://tubaneil.googlepages.com/
http://groups.google.com/group/vanagons-with-vw-inline-4-cylinder-gas-engines
