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914World.com _ 914World Garage _ fuel gauge sender

Posted by: 80cap Dec 5 2016, 01:46 PM

Before I install my fuel tank is there a way to bench test the fuel level sending unit?
Thanks Brian

Posted by: Dave_Darling Dec 5 2016, 02:18 PM

There should be three (possibly four) electrical contacts in the plug for the sender. One will be a ground, one will be for the gauge, and one will be for the warning light. The ground contact should have a "three parallel lines" ground logo; the wire from the light should be labeled "WK", and the wire from the gauge should be labeled "G".

The ground connection would have a brown wire that plugs into it, the warning light a black wire, and the gauge signal a green wire.

So, with the sender out of the fuel tank and held upright:
- Resistance between WK and ground should be about zero
- Resistance between G and ground should be about 70-100 ohms

With the sender turned upside down:
- Resistance between WK and ground should be infinite (no connection)
- Resistance between G and ground should be ~1-10 ohms

The float inside the sender should slide up and down relatively freely when the sender is turned upside down and rightside up.

--DD

Posted by: 80cap Dec 5 2016, 03:07 PM

Thanks Dave!
Well like most things so far on my restoration it is shot.
Just ordered a new VDO sender on the inter web.
Brian headbang.gif

Posted by: 914Bryan Dec 6 2016, 10:03 AM

Does anyone have pictures of this?

Posted by: jim_hoyland Dec 6 2016, 09:49 PM

agree.gif Just went through this. The inside of the sender has a float that "rides" on two wire strands, a third wire, usually 25 gauge copper, connects the top and bottom of the sender. This wire frequently breaks; don't know why. If it is broken, the low-fuel light will not function. It is easily replaced and only needs a drop of solder at each end to repair or replace

Posted by: Hans Jan Jan 25 2017, 09:59 PM

Just ran out of gas this evening. The reserve light was on (I thought it to be a malfunction of the light), but the gauge was pointing to 1/2 full.

After filling up (approx. 13 Gallon) the gauge points to full. So I guess the gauge gets stuck at 50%.

How can this be corrected?

PS: This is the only thread I could find with the search,

Br,
Hans jan

Posted by: Dave_Darling Jan 25 2017, 10:29 PM

The warning light contacts are on the same float as the gauge sender stuff, so it is unlikely that the float is getting stuck.

It could be that there is a short or problem in the resistance wire inside the sender, or it could be that the gauge is wonky.

We tell people to always trust the light over the needle on the gauge.

--DD

Posted by: Hans Jan Jan 25 2017, 10:52 PM

QUOTE(Dave_Darling @ Jan 25 2017, 10:29 PM) *

The warning light contacts are on the same float as the gauge sender stuff, so it is unlikely that the float is getting stuck.

It could be that there is a short or problem in the resistance wire inside the sender, or it could be that the gauge is wonky.

We tell people to always trust the light over the needle on the gauge.

--DD


Thanks Dave,
I will start looking for a new/used gauge.

Br,
Hans Jan

Posted by: bdstone914 Jan 26 2017, 09:42 AM

QUOTE(Hans Jan @ Jan 25 2017, 10:52 PM) *

QUOTE(Dave_Darling @ Jan 25 2017, 10:29 PM) *

The warning light contacts are on the same float as the gauge sender stuff, so it is unlikely that the float is getting stuck.

It could be that there is a short or problem in the resistance wire inside the sender, or it could be that the gauge is wonky.

We tell people to always trust the light over the needle on the gauge.

--DD


Thanks Dave,
I will start looking for a new/used gauge.

Br,
Hans Jan


I have several fuel tank gauges. Cheap. What year is the car ? Silver dot center? The connectors on the fuel gauge are different. You can just replace the fuel gauge portion.
You can test the gauge by grounding the sending unit wire and it should peg the gauge.
Bruce

Posted by: Dave_Darling Jan 26 2017, 02:23 PM

QUOTE(Hans Jan @ Jan 25 2017, 08:52 PM) *

I will start looking for a new/used gauge.


I suggest checking the resistance of the sender both full and empty, before spending money on replacing the gauge.

--DD

Posted by: Hans Jan Jan 26 2017, 06:51 PM

Have not yet figured out how to read the resistance accros the sending unit (there are three wires).
Used the little time I had today to remove the unit and verify that it shows 50% on an empty tank (see pictures).
Attached Image

Posted by: Hans Jan Jan 26 2017, 06:51 PM

See pic


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Posted by: Dave_Darling Jan 26 2017, 10:50 PM

One of the three pins is ground, one is the signal for the light, and one is the sender for the gauge. See my earlier post about what sorts of readings you would expect from the latter to to the ground pin.

--DD

Posted by: Hans Jan Jan 27 2017, 03:10 PM

After removing I find the following readings:
Upside down (like a full tank) 4 Ohm.
Straight down (like empty tank) 34 Ohm.

Compared to figures needed (from eatlier post) it seems that the empty tank situation gives only halve the resistance needed (hence gauge shows halve full).

Looks like I do not need a new gauge after all, but a new level sensor instead.

Thanks for the help you'all.

Ps: some one is offering a NOS level sensor (mine is VDO). Is this compatible?



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Attached Image

Posted by: fixer34 Feb 12 2017, 02:14 PM

QUOTE(Dave_Darling @ Jan 25 2017, 10:29 PM) *

The warning light contacts are on the same float as the gauge sender stuff, so it is unlikely that the float is getting stuck.

It could be that there is a short or problem in the resistance wire inside the sender, or it could be that the gauge is wonky.

We tell people to always trust the light over the needle on the gauge.

--DD


In the 'ask me how I know' department, I will concur with DD's statement about trusting the light more then the gauge. Similar to Hans, gauge showed 1/4 full, but warning light was on. Sputtered to a stop about 2 miles from home, had to call wife to bring the gas can. Guess I'm pulling the sensor and doing a resistance check.

Posted by: Dave_Darling Feb 12 2017, 09:15 PM

We had similar trouble when we bought the wife's SC. She called me on the way home (I was leading the two-car caravan) asking what the light in the gas gauge meant. I said she needed gas. She responded that the gauge showed a half-tank, and I said "We are getting more gas now."

Yup, the tank was just about empty.

Needed a new gauge module.

--DD

Posted by: 914Sixer Feb 12 2017, 09:57 PM

Take the sending unit and let it soak in a some lacquer thinner about half the length of the tube in a tall container. Make sure it seeps in the side hole. Let it fill up. Put your finger over the hole and shake the thinner in side the sender. Repeat several times. Drain thinner out. Turn up side down and listen for float movement. Repeat process until float moves freely.

Posted by: forrestkhaag Feb 13 2017, 11:52 AM

For what it is worth, I had to open my sender tube like a can of tuna.. Then very very gently clean each wire in the sender tube with lacquer thinner on a q-tip. Then I sprayed the whole shooting match with WD-40.

Once I had the float sliding up and down freely, I re-crimped the top "can" edge and it solved the low level light, the gauge accuracy, and for the past 18 months or so, everything works fine. fyi

beerchug.gif

Posted by: Red76 Nov 3 2017, 02:11 PM

The real thin wires on the outside edges of the float are.043 resistance wire. A spool of that 100 ft long is about $5 on ebay. Those wires are usually the first to break when you are cleaning things up. Just pay attention to the routing of them and how they attach to the float . If you dont get it back together right there will be no signal to the guage. Very easy to repair one of these units. 2 beers and a spool of wire. A lot cheaper than a new one!

Posted by: Costa05 Nov 30 2020, 11:46 PM

QUOTE(forrestkhaag @ Feb 13 2017, 12:52 PM) *

For what it is worth, I had to open my sender tube like a can of tuna.. Then very very gently clean each wire in the sender tube with lacquer thinner on a q-tip. Then I sprayed the whole shooting match with WD-40.

Once I had the float sliding up and down freely, I re-crimped the top "can" edge and it solved the low level light, the gauge accuracy, and for the past 18 months or so, everything works fine. fyi

beerchug.gif


Was getting a bench test 75 ohm reading full or empty. Popped it open via tiny nut on the bottom, cleaned the micro thin wires with Acetone and scotchbrite. Retest is 67 ohm empty, 7 full. Thanks @forresthaag !! Took 15 minutes. beerchug.gif

Posted by: neilbardsley Apr 13 2021, 01:39 PM

I'm having a problem with my fuel sender/gauge. The needle does go below half way. The light works to say the tank is empty.

I've had the sender out. I disassembled and cleaned.

The resistance is between 0 and 70 ohms. Which is I believe correct.

I put it back in but it's still not going below halfway.

Any ideas?

Posted by: Retroracer Apr 13 2021, 02:25 PM

So that would point to either wiring or gauge being incorrect. 0 - 70 ohms is about right for the sender. Couple of things to check:

1) with the sender disconnected, take off the sender wire at the gauge. Measure the resistance to GND on the wiring. There should be none measurable (high impedance), BUT if the wiring has a resistive short to GND, then that would throw the "empty" reading off.

2) make sure the +12V feed to the gauge is fused, switched 12V - not via anything random or resistive (like a backlight bulb). It is always surprising how creative PO's can get with wiring "fixes"....

3) make sure the combo gauge has a GND connection to chassis (so it isn't trying to GND itself through the sender)

4) if the sender is disconnected from the gauge, with power on, does it read "empty"? If you then short the sender terminal to GND does it read "full"? If not your gauge is broken...

Hope this helps,

- Tony

PS. The obvious question: is your petrol tank half full? ;-)

Posted by: neilbardsley Apr 14 2021, 12:50 AM

QUOTE(Retroracer @ Apr 13 2021, 02:25 PM) *

So that would point to either wiring or gauge being incorrect. 0 - 70 ohms is about right for the sender. Couple of things to check:

1) with the sender disconnected, take off the sender wire at the gauge. Measure the resistance to GND on the wiring. There should be none measurable (high impedance), BUT if the wiring has a resistive short to GND, then that would throw the "empty" reading off.

2) make sure the +12V feed to the gauge is fused, switched 12V - not via anything random or resistive (like a backlight bulb). It is always surprising how creative PO's can get with wiring "fixes"....

3) make sure the combo gauge has a GND connection to chassis (so it isn't trying to GND itself through the sender)

4) if the sender is disconnected from the gauge, with power on, does it read "empty"? If you then short the sender terminal to GND does it read "full"? If not your gauge is broken...

Hope this helps,

- Tony

PS. The obvious question: is your petrol tank half full? ;-)


Thank you. I will try those. The tank isn't half full smile.gif

Posted by: Cairo94507 Apr 14 2021, 07:17 AM

I just replaced my fuel sender. It was sticking at 1/2 tank, 1/4 tank, 3/4 tank, you get the idea. Ordered one from Gaudin along with a new gasket and installed it the other day. Works like new now. NOS fuel sender was about $329. beerchug.gif

Posted by: neilbardsley Apr 16 2021, 01:21 PM

Ok. It's not the wiring. 1 ohms resistance on the wire from the gauge to the sender. Now it's reading 1/4 full if I turn the sender upside down. My son has a 1st Class Degree in EEE (first time he has been useful) so had a 100ohm resistor, which we measured at 80. We put this in between the gauge and ground wires nearest to the sender and the gauge empty! Therefore I need another 10-20oms from the sender?

I forgot to mention I had the sender apart and cleaned it. It's 4-69 ohms so maybe I need to clean again??

I did also buy a new vdo fuel sender. It doesn't have a separate circuit for empty and I haven't been able to get more than 5 ohms off it so I'm not keen to use it yet.

Posted by: euro911 Apr 16 2021, 02:07 PM

Here's a good thread on testing and refurbishing the sender unit:

http://914world.com/bbs2/index.php?showtopic=323753

Posted by: neilbardsley Apr 16 2021, 02:51 PM

QUOTE(euro911 @ Apr 16 2021, 02:07 PM) *

Here's a good thread on testing and refurbishing the sender unit:

http://914world.com/bbs2/index.php?showtopic=323753


Thank you. I had mine apart like that. The float moves freely and I cleaned some of the contact points but was reluctant to touch the wires as I've read they break easily. Im trying to talk my son into doing a resistance multiplier circuit

Posted by: Retroracer Apr 16 2021, 04:50 PM

Neil - so it does sound like a sender issue. Presumably you tried shorting the sender terminal to GND on the gauge to make sure you get "FULL" indicated? 4 ohms or so on the sender sounds OK within error limits, I'd just make sure that 1) the float is not getting snagged, 2) the wires/contacts are clean and 3) and you measure 4ohms or less consistently every time the float is at the top....

- Tony

PS. I have a similar qualification to your son. Agreed: It does come in handy sometimes...! smile.gif

Posted by: neilbardsley Apr 16 2021, 11:53 PM

Thank you. Yes connecting directly to ground moved the gauge to full. The float does run smoothly.

I'm teasing my son. Most of his course was programming and maths. For his final year project he converted a medical laser from using analogue controls to a LCD display and control.

Posted by: seatosummit Jul 20 2021, 10:55 PM

QUOTE(bdstone914 @ Jan 26 2017, 08:42 AM) *

QUOTE(Hans Jan @ Jan 25 2017, 10:52 PM) *

QUOTE(Dave_Darling @ Jan 25 2017, 10:29 PM) *

The warning light contacts are on the same float as the gauge sender stuff, so it is unlikely that the float is getting stuck.

It could be that there is a short or problem in the resistance wire inside the sender, or it could be that the gauge is wonky.

We tell people to always trust the light over the needle on the gauge.

--DD


Thanks Dave,
I will start looking for a new/used gauge.

Br,
Hans Jan


I have several fuel tank gauges. Cheap. What year is the car ? Silver dot center? The connectors on the fuel gauge are different. You can just replace the fuel gauge portion.
You can test the gauge by grounding the sending unit wire and it should peg the gauge.
Bruce



Hey bdstone914-

Do you still have fuel level gauges available? If so, I would love to buy one.

Please PM me.

Cheers,
Austin

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