'98 1/2 ton silverado with 97,000 miles.

My fuel gauge needle just started vibrating like crazy. It reads accurate, just moves back and forth a lot. Sometimes it's right away and sometimes it's not until I've been driving for a while. Any ideas?

Any other instrument on the dash not reading quite right?

Negative, everything else looks fine.

Sending unit. Might as well replace the whole fuel pump module while you are in there.

Very, very, very, very, very, very......x 1,000,000,000,000 common problem!
It may be the sending unit...but it also may be the stepper motor in the guage...which I would think is more likely the case. Good luck finding one
Chances are a junyard guage would have the same issue....and as yet I haven't been able to find a source for new/after market stepper motors for the GMT400 guages.

Howard, were you able to find a manufacturer's name stamped on the outside of the "stepper motor"? If so, I might be able to locate some.

I put stepper motor in quotes because these things are not actually stepper motors in the GMT400 clusters as far as I know. The GMT800 ones are likely real stepper motors.

Hi Pat,
I've never actually pulled the guage apart. Mine doe sthe hummingbird thing with the occasional stumble but apart from that it's still quite accurate. Changing pumps and senders twice had no effect on it (didn't make it better or worse). I've read a few (well...probably hundreds) of accounts of the same issue and I believe there was a recall for any vehicles 'bought' in CA....due to fuel additives (maybe Ethanol?) causing the 'stepper' to fail.
I have no idea what the part number would be. Maybe someone else here has one apart they can look at for us????

fired up the escalade after about 7 months of sitting and same thing, was fine before.

I've got the steppers from the guage cluster that speeder gave me.
I'm not using them if anyone needs them/if you think they'll fit.

What year are they from Ed?
Can you find any part numbers on them that Pat can use?

hquick wrote:

I believe there was a recall for any vehicles 'bought' in CA....due to fuel additives (maybe Ethanol?) causing the 'stepper' to fail.

Ok, maybe this is a dumb question but how would "fuel additives (maybe Ethanol)" cause a stepper motor in the guage to fail?

I live in South Dakota and am a proponent of Ethanol so I'm interested in any and all "negative" issues with the product. Ethanol is becoming a very large part of our agricultural economy here in SD.

ToddH-2500HD wrote:

Ok, maybe this is a dumb question but how would "fuel additives (maybe Ethanol)" cause a stepper motor in the guage to fail?

I live in South Dakota and am a proponent of Ethanol so I'm interested in any and all "negative" issues with the product. Ethanol is becoming a very large part of our agricultural economy here in SD.

Ethanol wouldn't cause the actual stepper motor to fail (notice the quote marks) but as the sending unit is immersed in the fuel, ethanol can attack the metals in the sending unit, loosening parts making it move more freely. This will make the gauge bounce much like a failing stepper motor. Ethanol has an excellent track record of attacking materials that petrochemicals won't, like plastic, rubber and aluminum. To be E85 compatible, aluminum must be anodized or the part replaced with stainless steel, and rubber must be replaced with nitrile rubber to live in ethanol. Here's a nice little FAQ on ethanol on the Mercury Marine site.

From what I've read....the additives change the resistance characteristics of the fuel which effects the sender...which inturn damages the guage. This is why so many people with this issue have no luck when changing the pump/sender unit.

Ok, thanks for the expanded explanation guys. Hopefully you're not putting E85 in 1998 trucks, having issues, then blaming the fuel. Like Speeder said, Flex-Fuel vehicles (including EVERY component) must be engineered to handle the fuel options. If this issue is happening on Flex-Fuel stuff, and indeed the Ethanol is the cause, it's surely an engineering problem in the vehicle, not the fuel.

I'm just defending our local economy a little. We have 5 vehicles in our family, all GM, ranging from 1999 to 2004. 3.4L V6s, 3.8L V6s and an 8.1L V8. My 2002 truck with the 8.1L has 40,000 miles on it. But the V6s are 100,000 on the lowest up to 190,000 on the highest. We use E10 in them all almost exclusively and have never had a fuel related issue (including pumps) with any of them. Take it for what it's worth anyway...

My 87 isn't having problems with E10 either, other than lower mileage. Vehicles have been made E10 compatible for almost 30 years now, ever since the rash of warranty claims made when ethanol was first put into fuel without checking what the effect on the fueling system would be on the long term.

The only real problems with ethanol fuel is it makes acetaldehyde emissions, which is a carcinogen. It also increases the emissions of greenhouse gases in its production, far more than the gasoline it will replace when made with corn. If made with waste materials, emissions are far lower. Plus, using corn is using food, which we shouldn't be burning to get to the mall when people are starving in the world.

Then there's the increased use in fuel to go the same distance, but if E85 were commonly available engines can be optimized for alky by running a higher compression this might not actually be a problem. With each atmosphere of compression you get an extra 30HP, and about a 5-8 percent increase in fuel economy with gasoline, so I'd expect that a smaller engine optimized to E85 would get the same power and fuel economy as a larger gasoline optimized engine. An alky engine can run at 12-13 to 1 compression, where gasoline likes 9 to 1. The only thing I'm not sure about is what effect an atmosphere increase would have on alky.

The oil pressure guage in my '01 is doing the dance too.

Apparently CA were adding something to their fuel 10 years ago...which no other state was. That's te reason for the recall/tsb in CA only. Whether that was Ethanol...or something different...I have no idea. Point is....everybody is quick to say the sender is stuffed....yet myself and ALOT of others have changed the whole pump/sender unit numerous times with absolutely no change in the 'whack fuel guage' situation.
Only ones I've heard who have fixed this issue...have changed out the guage. Now..problem with that is...on Ebay you can get the 'stepper' motors for every other model...so it seems, for around $10.00. It seems it's impossible to find them for the 96-98 era trucks/SUV's.
Hopefully Pat and Ed can find out some more on this???

hquick wrote:

Apparently CA were adding something to their fuel 10 years ago...which no other state was. That's te reason for the recall/tsb in CA only. Whether that was Ethanol...or something different...I have no idea. Point is....everybody is quick to say the sender is stuffed....yet myself and ALOT of others have changed the whole pump/sender unit numerous times with absolutely no change in the 'whack fuel guage' situation.
Only ones I've heard who have fixed this issue...have changed out the guage. Now..problem with that is...on Ebay you can get the 'stepper' motors for every other model...so it seems, for around $10.00. It seems it's impossible to find them for the 96-98 era trucks/SUV's.
Hopefully Pat and Ed can find out some more on this???

'95 ones should work too, the gauge cluster from my '96 was stamped "95 silverado" and my '99 esky cluster was stamped "97 silverado" on the circuit area, go figure. I had a really bad esky fuel gauge that would spin 360* back and forth fighting with the oil pressure needle. it was kinda funny, so I swapped the stepper from the '96 and its been working good...seems sensitive to fuel sloshing around, which I believe is the sender, but there is no 'hummingbird' or violent shaking going on anymore with the different stepper. this is exactally what my bad esky fuel gauge would do. except with the setup of the cluster, it would hit the oil pressure gauge alot, this is a VERY extreme case of a bad stepper, so you'res doesn't need to be doing this to be a bad motor:
http://www.youtube.com/watch?v=POTVkNUCzy8&feature=related
imagine it at night with blue LED backlighting, it looked like a lightsaber fight between the fuel and oil pressure needles.

hquick wrote:

yet myself and ALOT of others have changed the whole pump/sender unit numerous times with absolutely no change in the 'whack fuel guage' situation.

So, what's needed then is a set troubleshooting procedure for a quaking gauge. Sending unit assemblies are way too expensive to just be plucking and chucking over a jumping needle. Anyone still have their defective parts? If so, we'd need something like an ohm reading on a good and bad sending unit. Not sure what else would need to be checked, and I have no idea how you'd test a stepper motor. Anything that is turned up would need to be usable by anyone though, not just electronics experts with oscilloscopes at their disposal. It would be nice if you full size drivers could come up with a way of checking the sending unit like unplugging the fueling harness under the vehicle, take an ohm reading between these two wires, and say whether it's good or bad.

If someone can send me a failed cluster, I will find out what happened to it and try to locate a source for the replacement parts.

My guess at this point is the 98+ needle "motors" have a mechanical dampener in them that's failed. The PCM drives the fuel level gauge in these trucks. It's likely that the PCM generates the signal to the gauge using a PWM architecture. If they implemented this in software with a low frequency pulse due to a lack of processing power (likely), the needle would vibrate really quickly with a failed dampener. The fix would either be a replacement "motor" or a filtering circuit to smooth the signal. Or a 411 swap.