Date: Tue, 25 Sep 2007 23:17:08 -0700
Reply-To: Michael Elliott <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Michael Elliott <camping.elliott@GMAIL.COM>
Subject: Re: The two throttle valve switches, redux
In-Reply-To: <249593.75275.qm@web82714.mail.mud.yahoo.com>
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On 9/25/2007 10:56 PM David Kao wrote:
> --- Michael Elliott <camping.elliott@GMAIL.COM> wrote:
>
>> On 9/25/2007 9:44 PM David Kao wrote:
>>
>>> --- Michael Elliott <camping.elliott@GMAIL.COM> wrote:
>>>
>>>> Wait. I got this wrong. Lemme try again.
>>>>
>>>> On the earlier 1.9L's there are two switches on the throttle valve. One
>>>> closes when the foot is off the gas pedal, and the other closes when the
>>>> pedal is floored. They are wired in parallel and both connected through
>>>> one single wire to Pin 4 of the ECU per Bentley's 97.55. Both switches
>>>> ground Pin 4, the ECU doesn't have any way to know which switch did it.
>>>>
>>>> Now if the engine is at 1500 rpm or faster and you take your foot off the
>>>> gas, the idle switch closes, grounding ECU Pin 4. Bentley's says that
>>>> under these conditions, the ECU shuts off fuel to the injectors (24.33).
>>>>
>>>> But if you /floor/ the pedal, the full throttle enrichment switch closes,
>>>> which ALSO grounds ECU Pin 4. Bentley's tells us that when this happens
>>>> the ECU will enrich the mixture (24.34).
>>>>
>>>> So taking your foot off the gas or flooring the pedal both ground Pin 4
>>>> ... so, uh, how does the ECU know whether to shut off the fuel or to
>>>> enrich the mixture?
>>>>
>>>> --
>>>> Mike "Rocket J Squirrel" Elliott
>>>> 71 Type 2: the Wonderbus
>>>> 84 Westfalia: Mellow Yellow ("The Electrical Banana")
>>>> 74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
>>>> KG6RCR
>>>>
>>> My guess is the ECU reads the signal from AFM. The signal from the AFM is probably
>>> different enough between floored and foot off.
>>>
>> (To better follow this exciting thread, bottom posting makes more sense.)
>>
>> Maybe so.
>>
>> Say you're cruising along with the engine at 3,000 rpm. You stomp on the
>> gas. This pulls Pin 4 of the ECU down and the AFM says "throttle is open."
>> Based on this, the ECU enriches the mixture.
>>
>> Or say you're cruising along with the engine at 3,000 rpm. You take your
>> foot off the gas. This also pulls Pin 4 of the ECU down, but the AFM says
>> "throttle is closed." Based on this, the ECU turns off the fuel supply.
>>
>> There's nothing in the Bentley's or the Fuel Injection Manual about the
>> ECU using Pin 4 + AFM to decide which action to take. I wonder how we
>> could test this hypothesis?
>>
>> If it turns out to be correct, then it doesn't explain why the O2 monitor
>> in my van indicated an over-the-top rich mixture last week when I was
>> going down long grades with foot off the pedal (Pin 4 of ECU pulled down
>> by idle switch and throttle presumably closed). Those two conditions
>> should have caused the ECU to cut the fuel.
>>
>> --
>> Mike "Rocket J Squirrel" Elliott
>> 71 Type 2: the Wonderbus
>> 84 Westfalia: Mellow Yellow ("The Electrical Banana")
>> 74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
>> KG6RCR
>>
>
> Hum... are your injectors leaking gas? Foot off and high RPM means very high
> vacuum. If injectors are leaking it explains why. and it may very well explain
> why your gas mileage is lower than average. The engine may be on the rich side
> whenever you let go the pedal. I never had injector problems on my 83.5 and 84
> though.
>
No indication that my injectors are anything but hunky dory. If I take my
foot off the gas while, for example, exiting a freeway, the O2 monitor
goes very lean for a bit. It's on downgrades where the engine runs very
rich with foot off gas. The AFM + voltage on Pin 4 idea doesn't explain
that, but Joel Walker proposed an alternate idea: that the ECU uses rpm
along with Pin 4's voltage to make the decision. We know that the ECU
decides that a low Pin 4 + rpm < 1500 means kill the fuel.
So to follow Joel's idea, it would go like this (same scenarios as used
above):
Say you're cruising along with the engine at 3,000 rpm. You stomp on the
gas. This pulls Pin 4 of the ECU down and the rpm-o-meter says "engine
fast." Based on this, the ECU enriches the mixture.
Or say you're cruising along with the engine at 3,000 rpm. You take your
foot off the gas, closing the throttle. This also pulls Pin 4 of the ECU
down, but the rpm-o-meter says "engine slow." Based on this, the ECU turns
off the fuel supply.
While the AFM + Pin 4 idea does not explain why the O2 monitor in my van
indicated an over-the-top rich mixture last week when I was going down
long grades with foot off the pedal, Joel's rpm + Pin 4 idea does: With
Pin 4 of the ECU pulled down by the idle switch AND with the engine rpm
forced high by the incline, the ECU would think that the throttle was wide
open, not in the idle position, and would enrich the mixture.
--
Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
KG6RCR
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