Fuel tank sender question.

[Jack21]

Fuel tank sender is OE CR. It's supposed to be GM 0 - 90 ohm sender. Fuel gauge is Autometer Z series, GM 0 - 90 ohm.

With the tank empty I gave it its first 5 gallons, and the gauge read 1/2 full. Driving, the fuel needle bounces all over. With 5 more gallons, the gauge reads full, but still bounces around when the car is driven. When I fill it up, the needle stays pegged on full and doesn't bounce until half the tank is used.

What's going on with the tank sender? The fix?

[Double Venom]

Jack,
Using the CR's tank and sending unit I assume?

Pretend someone else told you this. Drop the tank, pull the sending unit out. Take some alcohol, the wood type and clean the resistor. (Petek - right terminology?). Pulling the sending unit out AND putting it back in can be a little tricky. I suspect that the float arm could have been bent a little when it was installed. This would indicate the sticking needle problem, a dirty resistor indicates erroneous readings.

DV -- now I'm back to my corner

[BLACKJACK]

HELLO, JACK,

I DUNNO - - YOU SUPOSE MAYBE IT'S THOSE D.C. TANK TRAPS THAT HIS HONOR THE MAYOR REFERS TO AS DEM LI'L POT HOLES ???

YOU GOTTA EXPECT A LITTLE BOUNCING WHEN YOU'RE AXLE DEEP WHERE THE ASPHALT USED TO BE.

Y'ALL HAVE A REALLY GREAT DAY

BLACKJACK

[petek]

DV,

Yup it's a resistor, if you want to get technical it's a potentiometer. If you want to get real technical (and sufficiently nerdy) it's a rheostat. It’s a wire wound resistor, one end tied to ground, the other end floating, and with a wiper attached to the float. Since it has two ends it could be used as a potentiometer, but since it’s only using the wiper with reference to the ground, it’s a rheostat … now aren’t you sorry you asked!

The fuel gauge on my CR (using Autometer and CR sender) also bounces all over the place. Seems to be getting a little better with time. If it doesn’t smooth out I’ll probably experiment with a low-pass filter (nerdy technical term for a capacitor) at the gauge (simply because I’m too lazy to pull the tank).

[Whaler]

Hi Pete

thats why any extra resistance in this loop is so bad to true auge readings.Crimp ons and or other crappy connections should be helped out by soldering them as well as crimping. The more electrically secure you can make the connection the better

Tim

[John Wais]

When mine is empty, the tank only takes 13.5 gallons.
I'm told its a 17 gallon tank
Meter also bounces all over the place and always has.
It lately occasionally gets stuck all the way clockwise - loose connection I suspect.
Will craw under it this week
John W.

[Double Venom]

Petek,

Oh I love it when you talk dirty! Potentiometer! I knew that. Had two on my boat for the outdrives. Just my normal brain fade on a late Saturday night!

John, hmmm, it IS a 17 gallon tank. Don't have any major dents in the tank do you?

Petek, I need to find out from you how to wire in a series of toggle switches to ignite the ignition. IE; flip toggle B,C,D, to start, any other combination fails to start. Can we use the switches for other purposes, or do they have to be proprietary to the ignition?

Ohh, asking questions is fun isn't it!

DV -- 300 Cobras are NOT enough! Are you registered?

[Double Venom]

PS... to the gauge bouncing around problem. I just thought of another reason, and probably the biggest cuplprit. If you used the ground wire only, that comes in the harness, leave it grounded, BUT, make up another short ground wire, hook to the mounting plate then right to the frame! This will take care of almost all the sending unit problems!

You can do this while the tank is in place.

DV

[petek]

DV,
re: "combination lock" toggles

No problem at all. You can have the toggles even control other things *IF* you don't mind those other things maybe being on when the starter is engaged.

I'll try to post a picture here on this thread later this evening.

[Jack21]

This sounds like a common problem. First, getting the tank out is such a hassle that I'd rather not fool with band-aid fixes that may work, and may not. Sounds like a replacement sender would be in order. I think AM may have them.

There's a Rock Valley that makes fuel tanks and accessories for everything. They have a full page ad in virtually every issue of Street Roder. A-C Cobra is even listed. Do any of you have a spare or presently not in service tank pickup and sending unit to use for an example for Rock Valley to take measurements from?

Next problem is an electrical damping circuit for the needle oscillation. A low pass filter (as it were) for a very large subwoofer. Say, .1 - 1 Hz. Petek, any ideas on this?

[Double Venom]

Jack,
Jegs, etc, sell replacement units, but I seriously doubt you need one. Just hook up an OHM guage to it when you pull it out and give the arm a couple of swings. I've even taken the little gadget, arm , contact, what ever it's called and bent it a little to put more pressure on it to the rheostat.

Don't be suprised if it is nothing more than an inadequate ground!

DV

[petek]

Jack,
Try the ground first (although I do have a separate ground to my sender already...)

I doubt that you'd need much capacitance at all as I'm guessing the noise is actually up in the 100's or perhaps 1000's Hz range. It's just that the meter isn't damped so the meter is capable of sensing the jumps from the sender. (Again, a guess on my part as I haven't taken the meter apart.) For a quick SWAG, try a 10uf, 50 V cap from ground to the sensor lead.

DV... haven't forgotten about you... give me a few minutes... picture coming...

[John Wais]

DV:
No dents!
Jacked up the car today and let things quiet down.
As soon as I "wiggled" the car, the meter started to bounce big time.
Crawed under, and "wiggled" both the signal and ground wire with no reslults.
I think its got to be the sender and/or possibly it could be too sensitive to fluid movement.
Since sometimes mine goes full clockwise and locks up, I need to fix mine at some time.
If I have to drop the tank, I will want to replace it with a new one.
Do you have the Mfg. and part number ??
John W.

[petek]

DV,
Ok if I've done this correctly a schematic will appear below this line.


This is just the general idea. It could be made significantly more complex. In this simple one the first toggle switch (leftmost) must be off. The second and third toggles must be on. The fourth toggle off. And then the starter button depressed. Note that I've used only half of a dpdt toggle. The other half of the toggle could be used for controlling other electrical items.

If you wanted to get very fancy, you could make all of the toggle switches in the car part of the combination lock as long as the engine wasn't running. Then, after the engine was running, the toggle switches would revert to their normal function (you'd want to flip all of the toggles off after getting the car started). This fancy method would require relays at each toggle and a simple circuit to measure the rpm of the engine (fundamentally a simple integrator circuit made up of a transistor, a couple of resistors, and a capacitor). Obviously out of scope for a simple reply here on the forum

I did have a thought of using the high beam button on the steering column stalk as the starter button ... but the impulse passed

Back to the simple circuit... of course you'd want to lable the switches to confuse folks. Name the first switch "fuel pump" (since it has to be off for the car to start), the second "oil heater", the third "brake lights", and the last one "ignition module".

Hope all of this helps...

[Double Venom]

John,

You need to match the sending unit to your guages. Again, most speed shops have them in stock. S&W to S&W, VDO to VDO, etc.

Petek, that is so cool! Thanks a ton fella. We'll start on it today!

DV -- You al' come now ya-hear! 300 Cobras are NOT enough!