Challenging OBD II failure; 2 seconds missing...

Hi.
It's a 1997 Chevrolet Astro, 4.3L Vortec V6

When turning ignition just to on, the fuel pump runs for 2 seconds, as
should.
When turning off the running engine, the fuel pump does NOT keep
running for 2 seconds.

As part of the misbehavior, unplugging the fuel relay with engine
running, the fuel pump stops. It shouldn't.
Disconnecting the oil pressure switch (that feeds +12V to the fuel
pump), the fuel pump keeps running.
In other words, the fuel pump relay is active at all times with engine
running.

No error codes... replaced relay, done the basics. Car runs just fine
with no fuel problems, ~100 miles a day.

My profession is electronics troubleshooter for 30 years, and this time
don't see light with my own car ! (I do not know the innerworkings of
the ECU)

Any expert around with suggestions, please ?

Miguel

"Externet" > wrote in message
oups.com...
> Hi.
> It's a 1997 Chevrolet Astro, 4.3L Vortec V6
>
> When turning ignition just to on, the fuel pump runs for 2 seconds, as
> should.
> When turning off the running engine, the fuel pump does NOT keep
> running for 2 seconds.

Correct. That's how it is supposed to work.


>
> As part of the misbehavior, unplugging the fuel relay with engine
> running, the fuel pump stops. It shouldn't.

When you unplug the relay you remove power to the pump. How do you
suppose it will keep running ?


> Disconnecting the oil pressure switch (that feeds +12V to the fuel
> pump), the fuel pump keeps running.
> In other words, the fuel pump relay is active at all times with engine
> running.

Not familiar with the oil pressure switch being tied into the fuel pump
but I doubt it supplies power to the pump.

>
> No error codes... replaced relay, done the basics. Car runs just fine
> with no fuel problems, ~100 miles a day.
>
> My profession is electronics troubleshooter for 30 years, and this time
> don't see light with my own car ! (I do not know the innerworkings of
> the ECU)
>
> Any expert around with suggestions, please ?
>
> Miguel
>

"Mike" wrote

> > As part of the misbehavior, unplugging the fuel relay with engine
> > running, the fuel pump stops. It shouldn't.
>
> When you unplug the relay you remove power to the pump. How do you
> suppose it will keep running ?
>
>
> > Disconnecting the oil pressure switch (that feeds +12V to the fuel
> > pump), the fuel pump keeps running.
> > In other words, the fuel pump relay is active at all times with engine
> > running.
>
> Not familiar with the oil pressure switch being tied into the fuel pump
> but I doubt it supplies power to the pump.

No listing for a 99 Astro, but for a 99 S10/Blazer...
http://www.autozone.com/images/cds/g...3d8018c179.gif

GM does tend to do weird things like this with their wiring.

Example: A friend's older Monte Carlo has the oil light and choke heater both
controlled by a dual-terminal oil pressure switch. The 12v-powered oil light's wire
goes to switch terminal #1, then continues to the choke, where the tiny bit of
current grounds through the heater's element. The light is "turned off" by the oil
switch closing and applying another 12v (supplied to terminal #2 through a
separate circuit) to terminal #1, effectively short-circuiting the bulb, and sending
a full 12v (not restricted by the bulb) directly to the choke.

If the oil light's on, either there's no oil pressure, the switch is bad, or the switch
has come unplugged.

If the light *never* comes on, either the fuse is blown, the light is burned out, a wire
is broken, the switch is bad, the choke isn't plugged in, the choke element is bad,
or the choke isn't grounded.

An elegant way to combine functions, but a bitch to diagnose if something stops working.

"Externet" > wrote in message
oups.com...
> Hi.
> It's a 1997 Chevrolet Astro, 4.3L Vortec V6
>
> When turning ignition just to on, the fuel pump runs for 2 seconds, as
> should.
> When turning off the running engine, the fuel pump does NOT keep
> running for 2 seconds.
>
> As part of the misbehavior, unplugging the fuel relay with engine
> running, the fuel pump stops. It shouldn't.
> Disconnecting the oil pressure switch (that feeds +12V to the fuel
> pump), the fuel pump keeps running.
> In other words, the fuel pump relay is active at all times with engine
> running.
>
> No error codes... replaced relay, done the basics. Car runs just fine
> with no fuel problems, ~100 miles a day.
>
> My profession is electronics troubleshooter for 30 years, and this time
> don't see light with my own car ! (I do not know the innerworkings of
> the ECU)
>
> Any expert around with suggestions, please ?
>
> Miguel
>
It appears that your oil pressure switch fuel pump circuit may be
malfunctioning. It should keep the fuel pump running for about 2 seconds
after the ignition switch is turned off (on a running engine) because it
takes about that long for the oil pressure to bleed off and allow the
contacts to open.

The ECM has a timer that will de-energize the fuel pump relay about 2
seconds after the ignition is turned to run or crank, if the engine does
not start. The fuel pump relay should and does remain energized by the ECM
as long as the engine is running because it gets a reference signal from the
ignition module which is used to control the relay. Once the ECM quits
receiving the ignition signal it turns off the fuel pump relay immediately.
It appears that the oil pressure switch circuit is used as a back up to
provide continued fuel pump operation in the event the fuel pump relay would
fail, and it is NOT used to shut down the engine in case of an oil pressure
failure (like some might think). However, the ECM has a circuit to monitor
weather the power to the fuel pump is being supplied by the relay or the oil
pressure switch, and should set a DTC in memory if it detects a failure in
the circuit. Have you scanned for stored DTC's?
Also, you could test the oil pressure switch with an ohm meter to see if the
contacts close when you have oil pressure. If the oil pressure switch
contacts do close, then the problem is an open circuit from the oil pressure
switch to the fuel pump or from the ignition switch circuit to the oil
pressure switch.
--
Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green

"Mike" > wrote in message
...
>
> "Externet" > wrote in message
> oups.com...
>> Hi.
>> It's a 1997 Chevrolet Astro, 4.3L Vortec V6
>>
>> When turning ignition just to on, the fuel pump runs for 2 seconds, as
>> should.
>> When turning off the running engine, the fuel pump does NOT keep
>> running for 2 seconds.
>
> Correct. That's how it is supposed to work.

Wrong... The oil pressure switch will keep the fuel pump running until all
the oil pressure bleeds off.
Kevin

>
>
>>
>> As part of the misbehavior, unplugging the fuel relay with engine
>> running, the fuel pump stops. It shouldn't.
>
> When you unplug the relay you remove power to the pump. How do you
> suppose it will keep running ?

The oil pressure switch provides an alternate power source for the fuel pump
as long as there is oil pressure and the ignition switch is on.
Kevin

>
>
>> Disconnecting the oil pressure switch (that feeds +12V to the fuel
>> pump), the fuel pump keeps running.
>> In other words, the fuel pump relay is active at all times with engine
>> running.
>
> Not familiar with the oil pressure switch being tied into the fuel pump
> but I doubt it supplies power to the pump.



Obviously, you are not familiar with how this system works. The oil pressure
switch does indeed supply power to the fuel pump when the engine is running.
Kevin

>
>>
>> No error codes... replaced relay, done the basics. Car runs just fine
>> with no fuel problems, ~100 miles a day.
>>
>> My profession is electronics troubleshooter for 30 years, and this time
>> don't see light with my own car ! (I do not know the innerworkings of
>> the ECU)
>>
>> Any expert around with suggestions, please ?
>>
>> Miguel
>>
>
>

Hello Kevin M.

What a difference knowledge makes !!!
The missing 2 seconds at shut off come from the residual oil pressure
and not from the ECU "like some might think" WOW!
I was one of them.
There is something worse than lack of knowledge : Learning it wrong !

You have solved the mistery, the oil switch IS open, it's been like
that for a while, and kept disconnected since I found it was not
powering the fuel pump...

...."the ECM has a circuit to monitor weather the power to the fuel pump
is being supplied by the relay or the oil pressure switch, and should
set a DTC in memory if it detects a failure in the circuit. Have you
scanned for stored DTC's?"...

That's something else learned today ! Yes, scanned, no DTC's. -- What
DTC number should it be ?

Then, the oil switch powering the fuel pump is a safety myth as on a
rollover sucking air will not turn off the fuel ? Or the ECU sensing
circuitry will then act ?

You are great, all makes perfect logic sense ! Thanks,
Miguel

"Kevin" > wrote in message
news:_jJvg.66767$9c6.30906@dukeread11...
>
> "Mike" > wrote in message
> ...
>>
>> "Externet" > wrote in message
>> oups.com...
>>> Hi.
>>> It's a 1997 Chevrolet Astro, 4.3L Vortec V6
>>>
>>> When turning ignition just to on, the fuel pump runs for 2 seconds, as
>>> should.
>>> When turning off the running engine, the fuel pump does NOT keep
>>> running for 2 seconds.
>>
>> Correct. That's how it is supposed to work.
>
> Wrong... The oil pressure switch will keep the fuel pump running until all
> the oil pressure bleeds off.
> Kevin
>
>>
>>
>>>
>>> As part of the misbehavior, unplugging the fuel relay with engine
>>> running, the fuel pump stops. It shouldn't.
>>
>> When you unplug the relay you remove power to the pump. How do you
>> suppose it will keep running ?
>
> The oil pressure switch provides an alternate power source for the fuel
> pump as long as there is oil pressure and the ignition switch is on.
> Kevin
>
>>
>>
>>> Disconnecting the oil pressure switch (that feeds +12V to the fuel
>>> pump), the fuel pump keeps running.
>>> In other words, the fuel pump relay is active at all times with engine
>>> running.
>>
>> Not familiar with the oil pressure switch being tied into the fuel pump
>> but I doubt it supplies power to the pump.
>
>
>
> Obviously, you are not familiar with how this system works. The oil
> pressure switch does indeed supply power to the fuel pump when the engine
> is running.
> Kevin
>

Thanks for the explanation Kevin. I have never run across a setup like
that before. Is the oil pressure switch powering the fuel pump supposed to
be a backup circuit for the fuel pump relay ?

"Mike" > wrote in message
...
>
> "Kevin" > wrote in message
> news:_jJvg.66767$9c6.30906@dukeread11...
>>
>> "Mike" > wrote in message
>> ...
>>>
>>> "Externet" > wrote in message
>>> oups.com...
>>>> Hi.
>>>> It's a 1997 Chevrolet Astro, 4.3L Vortec V6
>>>>
>>>> When turning ignition just to on, the fuel pump runs for 2 seconds, as
>>>> should.
>>>> When turning off the running engine, the fuel pump does NOT keep
>>>> running for 2 seconds.
>>>
>>> Correct. That's how it is supposed to work.
>>
>> Wrong... The oil pressure switch will keep the fuel pump running until
>> all the oil pressure bleeds off.
>> Kevin
>>
>>>
>>>
>>>>
>>>> As part of the misbehavior, unplugging the fuel relay with engine
>>>> running, the fuel pump stops. It shouldn't.
>>>
>>> When you unplug the relay you remove power to the pump. How do you
>>> suppose it will keep running ?
>>
>> The oil pressure switch provides an alternate power source for the fuel
>> pump as long as there is oil pressure and the ignition switch is on.
>> Kevin
>>
>>>
>>>
>>>> Disconnecting the oil pressure switch (that feeds +12V to the fuel
>>>> pump), the fuel pump keeps running.
>>>> In other words, the fuel pump relay is active at all times with engine
>>>> running.
>>>
>>> Not familiar with the oil pressure switch being tied into the fuel
>>> pump but I doubt it supplies power to the pump.
>>
>>
>>
>> Obviously, you are not familiar with how this system works. The oil
>> pressure switch does indeed supply power to the fuel pump when the engine
>> is running.
>> Kevin
>>
>
> Thanks for the explanation Kevin. I have never run across a setup like
> that before. Is the oil pressure switch powering the fuel pump supposed to
> be a backup circuit for the fuel pump relay ?
>

Exactly! And surprise surprise. Most people think it is there to kill the
engine incase of a sudden loss of oil pressure, but that's not the case.

Kevin

"Externet" > wrote in message
oups.com...
> Hello Kevin M.
>
> What a difference knowledge makes !!!
> The missing 2 seconds at shut off come from the residual oil pressure
> and not from the ECU "like some might think" WOW!
> I was one of them.
> There is something worse than lack of knowledge : Learning it wrong !
>
> You have solved the mistery, the oil switch IS open, it's been like
> that for a while, and kept disconnected since I found it was not
> powering the fuel pump...
>
> ..."the ECM has a circuit to monitor weather the power to the fuel pump
> is being supplied by the relay or the oil pressure switch, and should
> set a DTC in memory if it detects a failure in the circuit. Have you
> scanned for stored DTC's?"...
>
> That's something else learned today ! Yes, scanned, no DTC's. -- What
> DTC number should it be ?

You scaned for DTC,'s but did you also go into the stored code section of
the ECM to pull up memory codes that are stored but never displayed because
they are not critical emisions related codes, and do not turn on the MIL or
show up in a regular scan. Not all scan tools have the capability of
retrieving pending and stored codes. I'm not shure what the code would be
for that particular problem, but it would probably a manufacturer specific
code and would not display on a generic scan tool anyway.

Kevin



Kevin


>
> Then, the oil switch powering the fuel pump is a safety myth as on a
> rollover sucking air will not turn off the fuel ? Or the ECU sensing
> circuitry will then act ?
>
> You are great, all makes perfect logic sense ! Thanks,
> Miguel
>

Hi Kevin.
Yes, my scanner is a cheap generic one.

For the records, the symptom of this open oil pressure switch failure
is the engine not starting instantly at the first try as always was,
but starting at the second crank ! Go figure !

Is there in the web a site to learn some of these hiding OBDII
behaviors ?

Miguel