Flex Fuel Composition Sensor

[HunterC]

Hello all,
Got an 04’ Tahoe throwing P0178, P0327, and P0332. Trying to focus on the P0178 for now. At the connector for the fuel composition sensor, I’ve got 14.7 volts off the pink wire, 2.2 volts off the white wire, and 0 on the black (ground) wire. All voltages seem off to me, I’d think 14.7 would be closer to 12, and it seems my 2.2 volt reading should be a 5 volt reading based off of what I’ve read. I was curious what my voltage was on the white wire at the computer, and it is also 2.2 volts. Could my 2.2 volt reading indicate a bad pcm? Everything I’ve read suggests I should be getting 5 volts from the pcm. Also, what gives with the 14.7 volt reading? Is this normal or too high? Any help would be great, hesitant to buy a $250 pcm or a $500 sensor without being sure what’s working and what isn’t. Thanks

 

[Fless]

Welcome to TYF from Colorado -- I have a similar Tahoe to yours (see my signature).

www.charm.li has lots of schematics and good diagnostic info. For example, here's a link to the connector pinout for the Flex Fuel connector:

Flexible Fuel Sensor — 2004 Chevrolet Tahoe 2WD V8-5.3L VIN Z Flex Fuel Service Manual | Operation CHARM

Detailed repair manual for the 2004 Chevrolet Tahoe 2WD V8-5.3L VIN Z Flex Fuel.

charm.li

Use that link to navigate around the site for info on your SUV. Notice that I looked at the 2wd section, so you may want to go to the 4wd section if you have it. I wouldn't expect the FF sensor wiring to be different, though.

A couple of questions:
The pink wire should be battery voltage -- it seems like the engine was running or the battery may have been on a charger when this was measured?
How much alcohol % would you expect to see? In other words, what fuel do you run? E10 or E15, or some mix of E85? Various mixes will have the sensor reporting different voltages.
Were those measurements done in-circuit, or was the sensor disconnected? (I assume in-circuit)

And the P0178 seems to relate to the frequency of the FF sensor; that can be read with a good bidirectional scanner.

 

[HunterC]

Welcome to TYF from Colorado -- I have a similar Tahoe to yours (see my signature).

www.charm.li has lots of schematics and good diagnostic info. For example, here's a link to the connector pinout for the Flex Fuel connector:

Flexible Fuel Sensor — 2004 Chevrolet Tahoe 2WD V8-5.3L VIN Z Flex Fuel Service Manual | Operation CHARM

Detailed repair manual for the 2004 Chevrolet Tahoe 2WD V8-5.3L VIN Z Flex Fuel.

charm.li

Use that link to navigate around the site for info on your SUV. Notice that I looked at the 2wd section, so you may want to go to the 4wd section if you have it. I wouldn't expect the FF sensor wiring to be different, though.

A couple of questions:
The pink wire should be battery voltage -- it seems like the engine was running or the battery may have been on a charger when this was measured?
How much alcohol % would you expect to see? In other words, what fuel do you run? E10 or E15, or some mix of E85? Various mixes will have the sensor reporting different voltages.
Were those measurements done in-circuit, or was the sensor disconnected? (I assume in-circuit)

And the P0178 seems to relate to the frequency of the FF sensor; that can be read with a good bidirectional scanner.

I agree, the pink wire should be reading 12 volts, somehow I’m reading 14 volts. The battery was not on a charger but the engine was running, perhaps if I just have the key in the ignition I’ll read 12 volts. Not sure, have to test that.

Historically running octane 87 with presumably 10% alcohol has never given me issues, in fact until I scanned the engine code I had been blissfully unaware the Tahoe was a flex.

The measurements were done both in and out-of-circuit. At the connector it was done out of circuit and read 2.2 volts. At the computer ( couldn’t find a good position to back probe so I stripped a small piece of the insulation on the white wire just off the computer ) the voltage reading of 2.19 volts was in-circuit.

I do not have access to any fancy scanners unfortunately. I hoped to resolve this with fairly basic tools.