Validating fuel level (was: Rebuilding instrument clusters)

[Reprise]

So, over the last couple of years, I've been trying to figure out why the Sierra's fuel gauge won't register 'full'. No, not dedicated effort, just when I'd be working on the truck for some (other) purpose. I'll spare the details of what I've replaced, but it's pretty much everything you'd think of.

There has been improvement... when I first got the truck, the pump would shut off at roughly 1/2 tank, and I'd have to coax it to 3/4. Now, I can fill it, and it shuts off at 3/8ths to go (3 notches from 'F').

The other day, I filled it, drove it home, and decided to see how much more I could add. Roughly, you can fit a couple of extra gallons in most vehicles before it spills out of the filler neck (and you should not do this regularly, as it floods / ruins the carbon canister, etc.). That was what I found with the Sierra -- about 2.5 gals brought the level up to within 10 in. of the top of the neck / cap, as 'officially' measured (that means I strung a couple of 11" zip ties together to use as a measuring stick... lol) And I could see the fuel in the filler neck. So... I know the tank is 'full' (there's no blockage, etc., in the tank, as I had it down twice to replace / upgrade fuel pumps).

Looked at the gauge after putting the extra gas in, and it did not move higher. Still 3 notches away.

Since I've got a couple of issues with my cluster (biggest being a burned out / dim PRNDL), and I'm a sloppy solderer, I figure I'll send the cluster out and have it rebuilt and upgraded (LEDs and needles). I can get it done for under $200 USD shipped both ways, and the vendor I'm looking at says he uses GM-approved stepper motors (so, no Chinesium steppers). And he guarantees it for 'life' (he didn't specify whether his or mine, but I'm optimistic that it'll be 'one and done', with no issues).

So... after reading through that...
Who has rebuilt their cluster, or had it rebuilt by one of the many people who do this?
And... how was the result?

If you used a vendor, you're free to give out bouquets or brickbats, as you see fit.

If you've done it yourself, you're free to convince me to have a go at it myself, but as I said... I'm not great / patient with the soldering gun, and it's worth it (to me) to make the spend for this. AFAIK, all the other gauges are OK, and I still have the DIC display working.

 

[azswiss]

I redid the bulbs and fixed the dimmed PRNDL indicators on my '03 Suburban so I would expect my setup is similar/identical to yours. The needles are not keyed to the stepper shafts, they are friction fit only so they can be moved. Is it possible that the needle on the fuel gauge is not properly indexed on the stepper shaft? Has it ever dropped below Empty? If the needle has an offset (to the low range) then you should see the needle approach Empty if you used 5/8ths of a tank. Get it as close to Empty as you dare and then record how much it takes to fill the tank (based on the pump stopping, not the fuel gauge) and then compare that against the spec'd capacity; if they are different then it is an indexing problem. You will need to answer this question before you send the cluster out for repairs since the needle position for each gauge is recorded during disassembly and then replicated during reassembly. You would not want to replicate any mis-indexed needles.

If you were just doing the PRNDL or the bulbs I would say give it a try yourself as they are surface mount. The steppers, on the other hand, are through-hole so the desoldering & resoldering processes are more involved (equipment & technique).

A related item: the fix for dimmed compass & temp LED's on the rearview mirror is exactly the same as for the PRNDL LED's (touch up the solder connections on some surface mount resistors).

 

[Chickenhawk]

Had my steppers and burned out PRNDL fixed perfectly by a supplier here in Canada. I was surprised to learn he was only a few kilometers from where I live in Manitoba, and I was able to drop it off and pick it up directly at his front door. Mr Whizard is highly recommended.

 

[Reprise]

The needles are not keyed to the stepper shafts, they are friction fit only so they can be moved.

Whaaaa ? Noooooo....

Is it possible that the needle on the fuel gauge is not properly indexed on the stepper shaft? Has it ever dropped below Empty?

No. When I shut it off, it parks at E, like it's supposed to. I don't have an issue with it parking, say, at 1/4 tank or so. It's gotten better over time... at first it was just over 1/2 tank when it would shut off. After several rounds of part replacement, it would go to about just under 3/4 full before stopping, and lately, I've been able to get it to just over 3/4 before stopping. Last couple of fills, I've even gotten it to within 1 notch of full -- but then it settles down and I 'lose' two notches a couple of minutes after I finish the fill. Just some 'fyi' info.

I've driven it low, but not all the way to 'E'. I do display the 'range' / DTE on the DIC most of the time, so I keep aware. And DTE seems (?) to be in sync with the gauge. When it's filled, I think it reads about 238mi of range. I'd expect closer to (or over) 300mi if the tank were truly full. When I added the extra 2.5 gal, the gauge didn't move farther up, and the DIC still showed 238 or so, IIRC (not absolutely positive on this, but I can check tomorrow). Plus I'd need to start / run / drive it a bit to see if the range indicator creeps up a little bit as a result.

Get it as close to Empty as you dare and then record how much it takes to fill the tank (based on the pump stopping, not the fuel gauge) and then compare that against the spec'd capacity; if they are different then it is an indexing problem. You will need to answer this question before you send the cluster out for repairs since the needle position for each gauge is recorded during disassembly and then replicated during reassembly. You would not want to replicate any mis-indexed needles.

CCSB of that gen got a 26gal tank, with no 'dual' or 'larger' option. It's never taken anything close to the full 26; I'd consider it 'no longer a problem' if I ever got 23gal or more into it. Almost every vehicle I've owned over the last 20-30 years always fills shorter than the stated capacity. And it's usually by about 1-2gal. I only fill to first click probably 95% of the time in my vehicles; sometimes I'll round up to the next dollar, but that doesn't add very much more.

In the case of the truck, I tend to fill it at about 1/2 tank or so, because I drive it so little, and the gas that's in there has aged a bit. Looking over a recent fill-up for the truck, I got about 16gal in it -- and I'm pretty sure the tank was at 3/8 full (between 1/4 and 1/2). If it's at 5/8th full, it only takes 7-8 gal before it shuts off. Actually, that doesn't sound logical, as I review this before I post, but I do remember from the service manual that the gauge isn't 100% linear, either.

I'm not absolutely 100% positive on the above, bc my gas purchases aren't sorted out by vehicle. I use Gas Buddy, so it keeps track of all of them, but I have to go by memory of which <vehicle> / <gallons> / <date>, and I have three to fill up. But I think I'm pretty correct on the amounts I can get into the truck / levels.

When I first read your reply, I thought "I hate him... " <kidding, of course>, because I thought I'd finally figured out that the gauge was the culprit, since I filled the tank almost to the tippy-top, and didn't see it move farther up. Of course, I'm glad I posted this now, because I would've just happily sent a couple hundo and my cluster, and maybe not fixed the gauge problem (which would've bummed me out, even if the PRNDL, etc., were fixed). I'm used to the PRNDL being out; if it never was fixed, I'd live. But not precisely knowing (ha!) how much fuel I have causes me anxiety, and I try and limit that as much as I can. Thankfully, it reads pessimistic, not optimistic.

I have a couple of 5gal gas containers, and will carry them in the bed when I take the camper out, and would carry at least one no matter what (if nothing else, the gas will go into my generator, if I don't use it for the truck).

Anyway, that's some more info on the subject; maybe it'll spark an idea or provide missing info.
In about a week, I can run the gas out of the truck, try and run it down closer to 'E', fill it to first click and report back on how much I got into it (and compare to the gauge, pre-fill).

 

[azswiss]

When I shut it off, it parks at E, like it's supposed to.

If it parks at E at shutoff then the lower needle bound is correct while only the upper bound is shifted down; the mis-index theory doesn't work (now I hate me too!!) but two possibilities remain. The first is a bad stepper. The second possibility is a bent fuel level float arm (i.e. the cluster accurately represents the data from the fuel-level sending unit but the reading is off due to the shifted position of the float). Regardless of the cause, I would have the same anxiety not knowing how much gas I really had left. Curious to hear what you learn with the fill test.

 

[TollKeeper]

I found a guy on Craigslist that rebuilt the cluster in my Rendezvous for 100 bucks. New GM stepper motors, new bulbs, and cleaned. Got it done in about 1-2 hours.

 

[Reprise]

but two possibilities remain. The first is a bad stepper. The second possibility is a bent fuel level float arm (i.e. the cluster accurately represents the data from the fuel-level sending unit but the reading is off due to the shifted position of the float).

Well, I was hating you... now we're BFFs

I can 100% guarantee it's not the float arm... the two pumps I put in had the exact same angles bent into the wire as the original I took out (yes, I checked). The 'original' was actually a Carter replacement, but since the float arm was the same on all of them, I'm saying that's not the problem.

So... sounds like my logic was good, and you're in agreement that the stepper is likely the cause, after all. Life is good.

I'll still do the testing like we discussed... but I'm confident that I'm gonna be putting this issue in the rear view mirror soon. Next update from me after I refill the Sierra's tank.

 

[azswiss]

Out of curiosity I went thru the Tech 2 Pathing reference document and, sure enough, if you have access to a Tech 2, there is an option to perform an IPC Gauge Sweep Test.



YT video showing the IPC Gauge Test (note: using an Autel MaxiSys but the idea is the same):
GM gauge cluster bench testing with a scanner after rebuild. Ipc instrument panel bench test

 

[TJBaker57]

Show of hands,,,, who knew I would show this being done with a Veepeak OBD2 adapter and a serial terminal app??

 

[azswiss]

Nice (& no surprise BTW)! What are the header & command details?

 

[Reprise]

Ok. I said I'd update at tank fill, but I have some new info that may be pertinent.

First... some photographic evidence. This was taken immediately after start up today, ambient about 80F



So here, we see the fuel gauge a couple notches off 'F', and DTE as 235 miles.

But immediately before I started it, at key on, I noted a couple of things...
- The gauge was actually touching the bold hash at 'F'. It wasn't past the mark, like a fill-up would normally show, but the needle was resting just short of flush.
- The range displayed about 278 miles, IIRC

I wish I'd taken a picture of it, as immediately after I started the engine, 'range' showed up at '258', and then a second later, showed the '235' you see here. At the same time, the gauge also lost a tick, and displayed its customary 3 ticks from the full mark.

Obviously, this was thermal / vapor expansion, given the ambient temp, and the truck being parked on an asphalt driveway.

One more piece of info... I drove the truck about 10 miles today. I'd expect to see the gauge and DTE decrease, if only a little. Yet, they remained the same (the range stabilized at 237-238, and maintained that throughout the drive).
What does that tell me? That the tank was, indeed, 'full' (technically, a little overfull, given that the fuel was most of the way up the filler neck, as I mentioned earlier.

Tomorrow, I'm driving a bit south of Chicago proper and back. That's about 140mi or so, both ways. At that point, I can drive the rest out when I get back and do a fill-up later tomorrow. Will post back after that point, with gallons to 'first click' and any other pertinent info.

No need to reply unless you have something 'earthshattering' that I need to look out for tmrw.

PS: If I multiply 26g tank x 14mpg, expected range of a full tank should (?) be around 364 miles.

Given the 278 I saw when it was just touching the 'F' mark... that's a difference of 86 miles, or about 6.1 gal (imperial). There's definitely some fudge factor in those numbers. More tomorrow (night), after I fill up.

Ambient tomorrow is supposed to be similar to today, and the tank should be 90% highway miles (probably about 18mpg, IIRC). I do expect DTE to increase over the tank, as a result. And I'm using A/C both days, with the auto HVAC set to 74F.

 

[TJBaker57]

Nice (& no surprise BTW)! What are the header & command details?

That is all in the youtube description for the video.

Header I used was A88310
Secondary ID is 12
data byte is hex value between 0x00 and 0xFF where 00 is of course zero percent and FF is 100 percent.

In the video I sent values of 0x80, 0x40, 0xC0, 0xFF, and 0x00 though I cannot say if that was the particular order.

I would imagine a header of A883F1 would also work though I cannot say I remember having tried that.

And if the PCM detects a change in actual tank level it will transmit a message that will reset the gauge to that detected value.

PS... If you monitor the address 0x83 you should see what the PCM is directing the gauge to display. To see the entire message turn on header display amd for easier reading turn on spaces between bytes.

ATTP2
ATS1
ATH1
ATMR83

 

[Reprise]

Following up a little later than anticipated... because I had to make two trips to actually empty the tank.

(side note: If you're expecting a Tech2 test result in this post... stop reading now, as I've not done it yet. Soon... )

Also, I'm probably going to rename this thread to reflect its true nature (testing)

So I made the trip just south of Chicago and back, as I said... and instead of having about 1/4 tank left, I had just over half. That was because, by overfilling the tank, I made an easy 30 miles with the fuel gauge still pegged on (what it represents as) 'full'.
It was at that point that I finally thought to reset Trip odo #1, so the pic you see with the elapsed mileage is easily 30-40 miles short of actual (e.g.; I drove more like 350-360 with the gas I added, vs. the 321 displayed here)




A couple of days later, I decided to finish the tank off, and wanted a highway trip with as little stop-start driving as possible. So I headed toward Madison, WI via I-94.

As far as the gauge being accurate at the end of the tank... as far as I can tell, it is.

Like clockwork, at 30mi DTE, the low fuel light and chime came on, and the DIC notified me as well.



This brings up a sore point I have with GM of vehicles of this generation (the Envoy does the exact same thing)...
Once the DIC switches to 'Range: Low' (instead of 'Range 30mi'... you can cycle the display until your thumb falls off, and it will NOT display an updated number until fuel is added to the tank. Right as you're needing a countdown to '0', GM hides that info. Bad GM. I have another story about what happened once with the Envoy with a trailer on the back. Anywho...

I had looked up area gas pricing with Gas Buddy while I was at a rest stop beforehand, and was able to navigate to a local Shell without incident. Right as I pulled up to the station, the tank reached 30mi to empty and the truck performed its light and sound show. Couldn't have planned it better. Yay me.

Now for the fillup...
First click came in about 20.4 gal. I was only able to get another 1.3 gal in the tank before it overflowed -- so, 21.7 to fill it up. After cleaning my windshield and the spilled gas off the side of the truck, I hopped in, turned the key to 'on', and this is what I saw...



At first glance, it looks like 'problem solved', huh? As soon as I started the truck and drove out of the station, the gauge jumped back from here to its traditional 'full' indicator (a little less than 7/8 full). The DTE also dropped accordingly.

Just like the last tank that I added fuel to by hand, it took an easy 40-50 miles for the needle to move enough where I could notice it. Made it home without further incident.


So... what did we learn?

- The gauge *can* display 'Full', properly. For a couple of seconds, at least. And it works normally once it gets below 7/8 of a full tank.

- I can completely fill the tank (it's up to me how much to put in after the pump clicks off).

- Given what I know of the gauge's behavior, I should be able to trust it as-is (so, no 'real' need to send the cluster for repair, except for the PRNDL repair (it would be nice to discern 'D' from '3' when I need to downshift for a grade descent, for example), but since all of the other gauges / lamps work perfectly... why bother?

And now you know why I'm going to rename the thread in a day or two... because (at least for me) there will be no cluster rebuilding / upgrading after all -- for now.

Once I dig out the Tech2 from its hiding place, I'll do a gauge sweep test and post the result, since a couple of people brought it up, and I do have the gear to do it. But for now, given what I know, I'm inclined to leave the cluster as-is, and work on other things.
There's nothing more for me to repair or optimize on this truck, before I start my next big project with it. But before that, it's time to finally...FINALLY... go camping.

Thanks for reading!

 

[Mooseman]

Just a word on filling, or overfilling... don't do it. Once it clicks off, stop. Otherwise you can damage the charcoal canister and fill it with raw fuel. My Avalanche had the problem of overfilling on its own due to a broken filler valve and I saw the fuel in the canister caused by it.

 

[Reprise]

Agree 100%; I'm usually a 'first clicker', myself, and I mentioned the charcoal canister in the first post of the thread.
Such a simple thing, to fill the tank... unless you can't.


::Changed thread title to reflect actual content::