Posted to Technical Tips Forum on 2/19/2009
20 Replies
Hi TTF,
I had a broken vehicle the other day; I thought I would
share some of the things I learned. This (as I'm sure you'll
agree) was a very easy diagnosis. What I found interesting
about it, is how much easier life is with newer vehicles in
some instances. The way you attack problems is the same as
before, but given all the scan data, codes, and
bi-directional controls at your figure tips, diagnosis
becomes much faster and less dirty. I realize that the
following case study is in regards to a newer vehicle. A
vehicle that most of you won't see until after the warranty
period, but it should be a good pre-cursor as to what's
coming down the line.
The vehicle is a 2009 Chevrolet Tahoe, with a 5.3L engine
and 20,000 Km on the clock. It was towed into the shop. The
complaint was "no-start". I rarely get any more info then
that. The service writer has already decided that it needs a
fuel pump :-)
The first thing I did was to try to start it. Guess what,
the battery is stone dead. Luckily it's the end of the day
anyways, so I put it on slow charge for an hour, and duck
out early.
The next morning (with the battery fully charged) I again
tried to start the vehicle. This time the vehicle cranks
over smoothly, but will not start. I grabbed my scan tool
and cleared all the "low voltage" related codes that
resulted from the dead battery. After doing a little bit of
reading I found out that this vehicle has a Fuel pump
control module (FPCM). It also has a fuel rail pressure
sensor. At this point I haven't opened the hood yet. With
the Tech 2 I went into the FPCM data list, then cranked over
the vehicle. This is what I see:
[2009 Chevrolet Tahoe, ECM/Inputs/Outputs scandata]
You'll notice that there is no fuel pressure, also the fuel
pump command says "off". Obviously that's why the vehicle
won't start. While I'm in the FPCM I selected special
functions [2009 Chevrolet Tahoe, Fuel Supply scandata]
and commanded the fuel pump to run:
[2009 Chevrolet Tahoe, Fuel Supply scandata]
As you can see it does build fuel pressure, the vehicle will
now start and run for a couple seconds, then die. Next I
check for codes in both the FPCM and the ECM:
[2009 Chevrolet Tahoe, ECM/Inputs/Outputs scandata]
[2009 Chevrolet Tahoe, ECM/Inputs/Outputs scandata]
Just so you know the other three codes in the ECM were
caused by me later on.
Looking at the setting criteria for DTC P025A, there is an
enable circuit that goes from the ECM to the FPCM. The ECM
sends 12V down the enable circuit wire (Dg/W) when the
ignition is in the on position. This circuit controls the
two second prime as well as normal engine running
conditions. The DTC sets when the serial data message from
the ECM, does not agree with the enable circuit. Basically
the FPCM is not seeing the enable circuit voltage. Now I
might have to get down and dirty and actually test
something. So I get one of the apprentices to raise the
vehicle on the hoist. The first thing I have to do is dig
out the FPCM as it's covered in snow:
[2009 Chevrolet Tahoe, Fuel Supply photo]
[2009 Chevrolet Tahoe, Fuel Supply photo]
Next I went into the ECM special functions and commanded the
Fuel pump relay to on (this function will run the enable
circuit).
[2009 Chevrolet Tahoe, Fuel Supply scandata]
[2009 Chevrolet Tahoe, Fuel Supply scandata]
I measured the voltage (on the enable circuit) at the FPCM
and saw this:
[2009 Chevrolet Tahoe, Fuel Supply waveform]
I now know that the enable circuit is good. I checked the
connector for water intrusion and it was dry. The only thing
left to do is to order an FPCM. I installed the new FPCM,
used SPS to program it, and the vehicle fired right up. I
fixed this vehicle in late December. I see that in January
GM came out with #PIT4784, which addresses this concern.
I thought I would share anyways,
Cale from British Columbia
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