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New age no-start
Posted to Technical Tips Forum on 2/19/2009 20 Replies


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

Files Referenced:

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car Vehicle Data

2009 Chevrolet Tahoe 5.3L

Engine5.3 L
Trans6-speed Automatic