The infamous fuel gauge shake

[98_2DR]

Ok my fuel gauge is doing the well known shake rattle and roll dance, (actually just the hummingbird shake) but I have a brand new fuel pump and replaced the wire harness that was included in the box, still had issue, replaced gauge cluster and issue got worse. I have a buddy who had a wrecked z71 truck with same cluster and he let me borrow it and the gauge was steady for a while then started shaking. This happens after about 15-20 mile before that its solid and steady. The owner of the borrowed cluster said it did that to him also, sooooo I'm pretty convinced its the motor behind the gauge like all the other posts have suggested. I have searched high and low for a replacement but can't find a new one... Does someone have a part number or know where I could find a new gauge motor? eBay and amazon has failed me already looked there...

 

[Zipper369]

I'm not sure right off hand, but when I get to work I'll see if I can find a number for you. I work at CarQuest, but I might be able to find a number that you can cross-reference or something.

 

[Zipper369]

So, unfortunately I could not find just the motor. The only thing I could find was the entire gauge cluster, which is way overboard. I've never looked at the back side of my gauge cluster before, but does anyone know if the motors are easily visible? If so, then you may be able to search for a similar stepper motor.

 

[Tex99Tahoe]

I kinda looked into this awhile back, and seem to recall that the servo-stepper-motor type guages came out later, like 2000, so we've got the older analog type guage. Other posts on other forums mentioned the driver circuit for these puts out a jittery signal, and GM's solution was to pack the needle bushing with high viscosity silicone grease. One poster mentioned his procedure using a syringe to vacuum, then fill the bushing with instrument grease, kinda like the way the windshield chip repair kit works.

My fuel guage was jittering about 1/16", got another cluster, jitter on this one is less, enough to ignore, but you know how these things can just eat at you. Just refreshed all the grounds, found a couple really corroded, including the ground lug by the tank, but it didn't change the jitteryness any...

 

[Zipper369]

...Other posts on other forums mentioned the driver circuit for these puts out a jittery signal...

Hmm, that makes me wonder. I know a bit about electronics and a "jittery" signal would make me want to assume that it's a PWM (Pulse-Width Modulation) signal. If that's the case, I'd try and throw a capacitor-resistor pair from the signal wire to ground. That would help smooth out the signal current. I can't recommend you try that just yet, because I don't want you to fry something.

My 99 Tahoe suffers from the same hummingbird shake, so I'll test around with mine first. If I find a good cap-resist combination, I'll post it on here. Now I just wish I had an oscilloscope... Oh well, I guess my digital multimeter will have to do.

Before I start randomly tracing wires, does anyone know where that particular signal wire runs? That way I'd have a good idea where to start looking. Yes, I'm too lazy to rip out the entire dash just to find a single wire, lol.

 

[Tex99Tahoe]

1999 Tahoe fuel guage schematics

Took some pics from my `99 manual. The wire that drives the fuel guage comes out of the VCM, Vehicle Control Module, the master brainbox under the hood, driver's side, above the brake control module, and has 4 connectors. Fuel guage line, circuit 30, purple, comes out of VCM connector C1, pin 9. Might be easier to play with it there.

BTW, the lines 'A' and 'B' that are at the top and bottom of the guage are 'switched 12v' and 'Ground', the ground goes to the lug on the panel pipe/bracket near the parking brake handle.

 

[Tex99Tahoe]

1999 Tahoe VCM connectors

Here's the VCM pinouts:

 

[Tex99Tahoe]

I suspect that the sensor input signal goes in to an A/D converter, is digitally processed by the VCM for some reason, then sent out a D/A converter. I suppose a filter could be put in this line, but it would have to be non-resistive, as the guage is driven by current. Perhaps a low-impedance LC filter?

 

[lesterl]

Just try a cap from guage feed to ground, + on the signal - on the ground. Looks like a standard gauge that has been around for many years.....

 

[Tex99Tahoe]

Just try a cap from guage feed to ground, + on the signal - on the ground. Looks like a standard gauge that has been around for many years.....

That's right. It's the old x-y Sine/Cosine type meter, used for 40-50 years. The signal line is very easy to access from the panel backside, including its power and ground. The meter pins are just shoved onto openings that have the flex circuit connections. Since the jitteryness is low frequency (visible, within audio range), it may take a rather large value cap to damp it, maybe start with a 470uf...