SharkAttack

My 70 big block fuel gauge needle 'bounces' within a range of where it should be reading while I'm driving. Followed the Willcox process and narrowed it down to the gauge. Ordered a new gauge (AC Delco), but compared to my existing gauges, the face markings are darker green and the needle is thicker and much brighter orange. Not sure I want to install it, may return it.

Anyone ever successfully remove the face of a new gauge and install the face from your original? How did you remove the needle, and how did you handle the 2 small black rivets?

I heard rebuilding them is intricate, but what's involved? Receipts from the prior owner's restoration show it was rebuilt about 10 years ago, guess they didn't do such a great job.

7T1vette

I'm not sure how you narrowed it down to the gauge....but assuming that you did and that you are correct in your diagnosis, the gauge must be having some sort of intermittant reading problem.

And the only way [that I know of] such a symptom could occur would be a fractured feed wire from the coil on the inside of the gauge. Once you have the gauge out, you could take a look inside and see if there is a fractured wire that is making intermittant contact. If so, just use an X-acto knife (very carefully) to remove the shellac insulation coating off the ends of that broken wire, then use some fllux and a pencil-type soldering iron to attach those wire together again.

If that really is your problem, it's a rather easy fix.

SharkAttack

Thanks 7T1, here's a little more detail on the symptom.

When the car is sitting, the gauge sits still. When I drive (hence fuel is moving around in the tank), it should read about 3/4 of a tank (rough measure) but instead it bounces back and forth between around 1/2 and full. The motion varies in how much it deviates from where the needle should be sitting (still). In other words, it may swing 1/8 of a tank higher, then 1/4 tank lower, then 1/4 higher, then 1/8 lower, etc. I did the Willcox test on the brown lead wire and it seemed to eliminate the tank float. Then I turned on the key and had my wife watch the fuel gauge as I used a plastic 'stick' to slowly move the float downward (to simulate the tank level lowering). I couldn't see the gauge from my viewpoint and asked her "Did it move slowly or bounce around"? She said it bounced around. Did this twice to confirm. All of this leads me to believe it's the gauge (and likely the fluid dampening, but I'm no expert on this gauge - yet). I haven't torn into the console yet or checked the gauge itself because I'm really wanting to get all potential parts to fix it while I'm in there (hopefully not have to leave the console apart while I wait for parts, etc.). Hope this helps.

7T1vette

There is NO reason why the gauge (itself) would behave in that manner. However, if the sending wire was sending erratic voltage signals, the gauge would react accordingly. I suspect either a bad sending unit....OR...an "iffy" ground connection back at the fuel tank.

If you could rig up an ohmmeter (and necessary extension wiring) to measure the resistance output of the sending unit, you could drive around a bit and see if that variability is in the signal line (with gauge OUT of the circuit). You could do that test with 1) ohmmeter ground 'tapped' into the existing ground wire and 2) with the ohmmeter ground hooked directly to a good electrical ground point. If you get variability with the existing ground connection, but not with the 'good' ground, then grounding is your problem.

SharkAttack

Thanks again, I'll see if I can try those next. FYI: The car has all new wiring harnesses (replaced 8-10 years ago during prior owner restoration). For the main ground to the frame (below battery box), I already removed that bolt and cleaned all surfaces well to ensure a good ground at that main point. Will also try to call the prior owner and see if he knows whether the gauge always did this or just recently started doing it.

mikem350

These gauges have internal dampening that slow the needles down from fluttering. The inputs from the sending units are always moving around and the gauge "evens out" their readings.

Including the amp and temp gauges too....

jnb5101

7T1 is correct. when the car is not moving and the gauge bounces when you push on the sender arm, it's the sender. Inside the sender there is a winding of fine resistance wire wrapped around a curved insulator. The sender has an arm with a small pad that moves along this coil and sends a variable voltage to the gauge. Since the tank is generally between 1/4 and 3/4 full, the resistance wire takes a beating in that area. And since the pad covers more that one loop of wire due to it's width, as the sender arm moves the voltage output jumps erratically. If you have a good voltmeter, connect it between the output stud and ground. Do the thing (gas tank thing !) with your wife and watch the voltage fluctuate.

SharkAttack

Earlier today I hooked up my multi-tester to the sender and the ground (both on the sending unit itself) to isolate the sending unit and eliminate all other wiring variables. When I slowly moved the tank float up and down, the ohms reading jumped around, sometimes with only a small variance to where it should be reading but other times with a very wide variance. Did this twice to verify, so thanks to 7T1 and jnb. I'll be returning the gauge for a sending unit.

SharkAttack

Well, s%#*t. Replaced the tank sending unit and the gauge needle still bounces when driving (about the same as before). Plan to follow the Willcox troubleshooting steps this weekend (again) to see if it points to a ground issue or the gauge (dampening issue).

R6n350GT

Mine does exact same thing. I went straight to the gauge as I was trying to fix other things there. Using Wilcox guide the resistor off the gauge was 36ohms and on the gauge 86.5ohms. Will *** said around 40 and 90 so think mine is fine. I also put 12 volt on pin 2 and ground the other pins and saw needle flick to empty and then full so I believe gauge is fine.

Ill test my float and other wiring today


When you pulled yours apart do you recall seeing this wire ?
https://www.corvetteforum.com/forums/c3-tech-performance/3592504-check-engine-light.html

Im not sure if its actually the check engine light or ac light that goes at the top the grill .. Its on when ignition is off .. Thanks

SharkAttack

I haven't tested or removed the gauge yet, seems that it or a grounding issue is the cause of my gauge needle bouncing around when driving. It stays pretty still when parked with the engine running, which makes me think the gauge (not dampening when the fuel is sloshing around from movement).

mikem350

See my #6 response

R6n350GT

So are you saying if the gauge checks out with willcox, 12v, ohm and grounding tests that it could still be faulty ? mine is fine on idle and then bounces when driving.

0Willcox Corvette

Yes... A gauge that sways with the movement of the car has a gauge damping fluid issue. Yes, the gauge will still work correctly but if the fluid is gone the gauge sway excessively.

Willcox

R6n350GT

ok so not even worth checking the fuel float then.. i filled it right up and the float needle went to full but then as i drove it flickered.. so ok new gauge. OP i think u have same issue as me then.

Does your Oil pressure just sits at 40, and not move really, is that ok ?

SharkAttack

Well, that's just my luck. Figured it was the gauge at first, ordered one and was not happy with the face & needle (looked different than my current ones). Asked about how to possibly swap new gauge with old gauge face & needle while also describing the needle bounce symptom, then others said it's definitely the sending unit (not the gauge). Returned the gauge, replaced the sending unit, issue remains. Must be the gauge...

zwede

Didn't see this thread until today. Yes, it's the gauge. I went through this years ago. My solution was an autometer gauge and I swapped over the stock face and needle. It works fine now, the autometer gauge is properly dampened.

It's been 15 years sine I did this, can't remember details of the swap. That probably means it wasn't difficult as I would have remembered it if it was.

The needle is pressed on. I usually use two flat-bladed screwdrivers to push it off. Put some masking tape on the face to protect it.

StingrayLust

If you ever find out where you can buy those little black, gauge rivets, please post that information as I'm interested!

7T1vette

There is a ground wire for the sending unit back at the fuel tank. I believe that it is mounted to one of the frame rails. If the connection there is 'iffy' or if the ground wire is frayed, it could provide the "jumpy" signal to the gauge, also.

I have never seen such a problem because of the gauge. But if Willcox says it can happen, I'll accept that it can.

cardo0

If u want to rebuild your original gauge or gauges try Corvette Instrument Service: http://corvetteinstrumentservice.com/

Rodger rebuilt mine and he installed a fuel press gauge in the clock hole for me to.

Good luck.