2004 Suburban PCM/Fuel Level Problem

[TylerHagerdorn]

Hey Y'all.

My 04 Suburban is driving me nuts. I'm not sure where to go from here. Heres what's been done.

New rebuilt cluster, new Delphi fuel pump, used PCM matching numbers and from a truck with matching RPO codes.

I had a junkyard PCM I installed on Saturday, then we installed a new Delphi pump. Then went and filled the truck up. Gauge was reading dead empty. We hadn't changed wiring, but upon research the PCM was from a VIN T GAS truck, which the pump connector is different so the fuel level wires must have been opposite. Because as we were driving, the fuel level would go like it was using fuel normally.

I installed my original PCM again and it wouldn't read past half. When I know my fuel level was 3/4 tank. I had suspicion that the PCM might have been bad on that circuit. So I hunted down a PCM from a VIN Z Flex truck with matching rpo codes as mine.

Installed it yesterday, did the security relearn and it runs, but the gauge will still not read over half. I'm just completely lost right now. I have 19 plus gallons in my 31 gallon tank, and should be 3/4 tank or close to it. It's driving me nuts because I know it has to be something so simple.

Where do I go from here? Nothing was obstructing the level in the tank. I know according to my diagrams that the fuel level sensor goes from the pump to C152, then to the PCM. PCM connectors are good. I have not gone and put more fuel in it to see if it'll move up past half. But it's just strange to me that it refuses to move any higher than half.

Sorry if I've discussed this before. I know my other threads were a bit different. We have to be close to finding our what the true issue is.

 

[strutaeng]

I think I posted my thoughts on one of your previous threads.

Anyways, here's a wiring diagram for an 04 with dual tank setup, although I suspect the single tank should be similar.

The fuel pump/sender has basically a variable resistor on the sender portion. It's just s ground and the purple wire as shown. The purple wire goes directly to the PCM.

You either have a wiring integrity problem or a faulty fuel sender. I know you said you recently replaced the fuel pump, but that doesn't mean the sender is bad. Does the gauge drop any after fuel level drops? I've read on other websites guys have actually posted what the resistance should range based on fuel tank level.

These threads may have useful information for you:

Stock Fuel Level Sender - PerformanceTrucks.net Forums

GMT 800 & Older GM General Discussion - Stock Fuel Level Sender - What is the stock sender ohm range? I am making a custom fuel cell for my 2002 Silverado and I'll need a sender that will work with my stock gauge on my dash. I think it's 0 ohms empty, 90 ohms full but I might be wrong.

www.performancetrucks.net

It looks like there was a change from 99-03 vs 04+? At least on the trucks? Maybe the wrong part was installed? That would definitely cause your problem because of a mis-match in calibration for the senders.

99-03 vs 04+ Fuel Level Sensor - PerformanceTrucks.net Forums

FUEL SYSTEMS - 99-03 vs 04+ Fuel Level Sensor - So to finally put any confusion to rest about the specs on the senders/sensor for the fuel level on the factory returnless vs return systems, here it is from the GM service manual: 84781...

www.performancetrucks.net

I hope this helps.

Edit: just realized the screenshot didn't attach.

 

[TylerHagerdorn]

I've been working in parts and on cars for 14 plus years. I have never in my life out of hundreds I've sold seen a Delphi pump defective off the shelf. Cheaper pumps yes. But not Delphi.

Pump was just installed Saturday. I had a PCM from a GAS truck installed and the wiring is reversed as I've said. So dead empty was truly full with that PCM. And yes it did move like it should as fuel is consumed.

The PCM in there now is the correct one for a FLEX truck. However it still will not go above half on the gauge. And I know there's a bit more than half in it.

I have the service manual page for ohm values on the fuel level sensor...I got 0 ohms no matter how I tried to read it. I followed my meters manual on how to hook it up. And got zero. So either I did something wrong or something is wrong.

Now voltage I got. Voltage at each side of C152 was 1.84v. According to the chart for my 31 gallon tank, 1.9v is half tank and 1/4 is 2.3v. So I'm under the 1.9v for half tank, so the 1.84v would be the correct value to my knowledge of what's still in the tank. I've used 12 gallons according to my DIC, so there's about 19 gallons in the tank still. Which 18 gallons is half. So it should be a tad above half. Which is correct.

So either there's something going on, or I have wiring backward that I don't know about because none of the wiring to the tank has been touched or corroded or anything. If it's reading backwards, then the gauge would be correct in being a touch under half.

 

[TylerHagerdorn]

Alright so I just went and filled it up, and it won't go up past full where it should be. And when I got home from my short drive it went down a bit more between full and 3/4. I'll try and attach a picture of the gauge right when I filled it up. It won't go up further than that. I know that the PCM sends the fuel level on the class 2 data line to the cluster.

 

[Fless]

What make and model pump was installed?

What does a scan tool report as the fuel level %?

 

[TylerHagerdorn]

What make and model pump was installed?

What does a scan tool report as the fuel level %?

I put it above, but it's a Delphi. All I use for GM. Never had one issue in all the 14 years I've been working on cars and working in parts. Never have I seen a Delphi pump fail. Not sure the one in my truck was bad either. It was a Delphi too. Part number is FG0811. For the 2004 VIN Z Flex trucks.



I mentioned in another one of your comments I believe, I don't own a scan tool that does anything other than basic code scan and basic live data. Fuel tank is completely full now.

According to the voltage I got earlier when I had just above a half tank, it was 1.84v. Half tank per service manual is 1.9v. So the voltage I got earlier was correct for the fuel level that was in it. I have not checked voltage since I topped it off but I'm sure it's good.

Fuel Level is sent to the gauge via class 2 data from the PCM. Comes directly from the pump to connector C152 under the fuse box, then from there to the PCM. And then it's sent from the PCM via class 2 to the cluster. There is a pin on the cluster connector for the fuel gauge. The cluster connector has been replaced and a better used one was swapped in.

I'm wondering if I got a faulty cluster yet again...the original one I got from ISS was no different though on the fuel level. I already sent it back but I tested it yesterday to see. So I don't know if I have something else going on that's preventing it from showing the correct fuel level.

There was a class 2 data wire going into connector C3 of the BCM that had been not hooked up and they had the wipers connected to that wire for some reason. I fixed that and got the wipers wired properly. And at first when I connected that class 2 wire back up, I had hardly any interior functions. But I tried it again and everything functions. So I truly don't know what the issue is now. I've got to figure it out because it's driving me crazy. It has to be so simple.

Obviously it seems the fuel level sensor is working as it should, per the voltage reading I got per the service manual. So it's something else. I'll look back on my printout I have of the BCM connector and cluster again and see.

But no I don't have the ability to read fuel level pid from a scan tool

 

[Fless]

Well, I thought you had said that about the scanner but since the info seems to be in several places it's hard to track. Rhetorical question: why not keep the same thread going so there's context?

Wonder if the auto parts store would have a code reader you could use for a second in their parking lot to check your codes (wink, wink); maybe it would display the fuel level percent. You're chasing your tail if you don't know what the computer is telling the gauge.

 

[TylerHagerdorn]

Well, I thought you had said that about the scanner but since the info seems to be in several places it's hard to track. Rhetorical question: why not keep the same thread going so there's context?

Wonder if the auto parts store would have a code reader you could use for a second in their parking lot to check your codes (wink, wink); maybe it would display the fuel level percent. You're chasing your tail if you don't know what the computer is telling the gauge.

Voltage reading at the wire going to the PCM is what I'm checking. Direct from the service manual. Obviously there's an issue with the gauge reading.

Tank is dead full. Voltage I just got was 2.51v.
According to the service manual, that is "Low Fuel Indicator On" or "E"...which, the gas truck PCM I had in it read dead empty with the low fuel light on for a full tank...I disconnected the battery when I swapped the PCM for the correct one...but it seems that the computer is still reading empty for full, but the gauge is seeing just under full now. With the gas truck PCM the gauge read empty for full and had the same 2.5 volts and that was all reading properly...so I'm not sure what the deal is now.

PCM is sensing empty for full...gauge is reading just under full...something isn't right. It runs phenomenal every time. So is a fuse blown or missing a leg? What do I check next. Basically we need to get the PCM to read right...it's a good flex PCM in my truck same rpo codes and everything...so it shouldn't affect it like the gas PCM did.

 

[TylerHagerdorn]

Well, I thought you had said that about the scanner but since the info seems to be in several places it's hard to track. Rhetorical question: why not keep the same thread going so there's context?

Wonder if the auto parts store would have a code reader you could use for a second in their parking lot to check your codes (wink, wink); maybe it would display the fuel level percent. You're chasing your tail if you don't know what the computer is telling the gauge.

Also, I've read that the PCM relearns after about 150 miles of driving...we drove about 180 miles on the gas truck PCM. And I've only driven about 15 miles since putting the correct PCM in. So, is that part of the problem?

Some people were saying you can have the key on and remove the two ecm/PCM fuses and wait and put them back in and it'll go back to the base software.

 

[TylerHagerdorn]

What make and model pump was installed?

What does a scan tool report as the fuel level %?

Fless,

I'm outside currently testing ohm reading and voltage reading. Remember the tank is full. I let it click off and stopped. I know overfilling is bad.

Here's my ohm reading. 238 ohms
Voltage reading 2.46v.

I checked this directly at the PCM and at connector C152 under the fuse box. Same reading at both.

The fuel pump connector at the tank was clean and good before we put the new pump in.

So what we have here is the PCM is saying the fuel level is empty, while my cluster is saying it's just under full...so where do I check from here?