Ohm reading for fuel sender?


The old and reliable.

Yard Art
Yard Art

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Joined: Wed Aug 02, 2017 11:17 am

Post Mon Jun 25, 2018 11:00 am

Ohm reading for fuel sender?

Does anyone know what the Ohm reading the fuel gauge is looking for on a KB-7? I thought my old one was a 0 (empty) to 90 (full), but the one just ordered like that from Tanks inc. pegs the gauge in the empty position and lowers the gauge in the full position. Half way on the float shows half way on the gauge. Any suggestions on this? I am running a 12v to 6v converter unit to power the gauge.

The previous thread on sending units does not seem to address the Ohm reading. Thanks for any help on this.

Rookie
Rookie

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Post Tue Jun 26, 2018 8:33 am

Re: Ohm reading for fuel sender?

I recently acquired my 1947 KB-2. It had been converted to 12 volts by the previous owner and the fuel gauge fluctuated all over the place. I got another sending unit with the truck. The previous owner had passed away, so I had no information that sender. I tested it with an analog DVM and got the same readings as in your post (0 ohms empty and 90 full). The reason it is important to use a analog multi meter is you can move the arm of the sender slowly through it's range and if there are any bad (open) spots in the rheostat of the sender, the needle of the meter will drop down instead of gradually going from 0 to 90 ohms. The spare sender was good.
I hooked the spare sender to the fuel gauge using test leads. I moved the arm of the sender through it's range very slowly and the gauge moved properly except it didn't go all the way to empty. Since my truck had been converted to 12 volts and had reducers on the fuel and oil pressure gauges, I measured the "reduced" voltage at the gauges. It was 8 volts.
Since most electric gauges in cars I have encountered run on a lower regulated voltage than the charging system, I decided to try regulating the gauges to 5 volts. I did this by desoldering all of the components off the little circuit board and putting an LM7805 regulator in there place. This is a cheap and easy to find (ebay) device.
After doing that my fuel gauge worked correctly with the new sender.

Hope this helps,
John Seymour
So Cal

Yard Art
Yard Art

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Joined: Wed Aug 02, 2017 11:17 am

Post Thu Jun 28, 2018 7:14 am

Re: Ohm reading for fuel sender?

Thanks John,
I didn't know an analog meter would make a difference. Sounds like you had a similar situation. I talked to the sending unit manufacturer and they suggested wiring a potentiometer to the gauge and dialing it up so the gauge reads empty, quarter, half, etc. and taking ohm readings at each stage to determine exactly what was needed to work the gauge. Then send back my unit for one that works in my application, as they have many to choose from.

I couldn't help but wonder if converting from a positive ground to a negative ground system would have any affect.

Golden Jubilee
Golden Jubilee

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Location: Lyman, IA

Post Thu Jun 28, 2018 7:43 am

Re: Ohm reading for fuel sender?

Polarity make no difference on the King Seeley "thermal" type gauges. There are some gauges it does make a difference but not the ones in question here.

Yard Art
Yard Art

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Joined: Wed Aug 02, 2017 11:17 am

Post Tue Jul 03, 2018 2:12 pm

Re: Ohm reading for fuel sender?

So I ordered a $12 potentiometer from Amazon. A 25 watt, 0-200 ohm unit. I did some testing with it, set it at 35 ohms, wired to gauge, it read half. Dialed it to 10 ohms, wired to gauge, it read full. Dialed up to about 80 ohms, it read empty. 20 showed 3/4 full and 60 showed about 1/4 full. So I ordered a 73 ohm (empty) to 10 ohm (full) sending unit. Apparently they were used in the early Fords and the Chrysler products.

Yard Art
Yard Art

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Post Sat Sep 08, 2018 1:38 pm

Re: Ohm reading for fuel sender?

How did this work out for you, Brian? Getting my fuel guage working is #1 on my winter project list for the KB2. Did we ever get to a part number for one we can order that will work on 6 volt? Thanks.

Yard Art
Yard Art

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Post Tue Sep 11, 2018 12:15 pm

Re: Ohm reading for fuel sender?

sender.JPG

Hi Worthy,
I did get it to work ok, not great. If I am above half a tank, the gauge reads about 1/4 higher than it is. From about half to empty, it's pretty close. I am unsure why it acts like this given my previous testing. The tank is a new 15 gallon tank I mounted on the frame like a saddle tank, so I am dealing with a pretty small tank. I used a sender from ISSPRO. I was happy with the product and the support. isspro.com
They do 6 volt, 12 volt, custom built units, etc.

Rusty Driver
Rusty Driver

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Post Wed Sep 12, 2018 7:09 am

Re: Ohm reading for fuel sender?

Any chance this might work for you?

Speedway Fuel Level Gauge Sending Unit Interface Module

I used it on mine to get the sending unit dialed in because the SW wings fuel gauge doeant come in a config to work with an S-10 sender.
"A good scientist is a person with original ideas. A good engineer is a person who makes a design that works with as few original ideas as possible. There are no prima donnas in engineering"
Freemnan Dyson

AZD

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Golden Jubilee
Golden Jubilee

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Location: SLC, UTAH

Post Wed Sep 12, 2018 8:21 am

Re: Ohm reading for fuel sender?

That’s a pretty neat little gadget, thanks for pointing it out. Funny thing, I was thinking of building something like this on my own if my fuel sender was bad, but it turned out to be good, even after 26 years of floating in varnish.

Basically I was thinking of buying the cheapest, most common sender for VDO gauges or whatever. I would then use the sender as an input to a nonlinear DC current amplifier which would drive the gauge needle (which is a thermal device by the way, not a magnetic one, as in a D’Arsonval gauge, i.e. “analog voltmeter”). Looks like Speedway got there first. Oh well…

Golden Jubilee
Golden Jubilee

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Joined: Thu Nov 29, 2012 9:28 pm

Location: Lyman, IA

Post Wed Sep 12, 2018 9:15 am

Re: Ohm reading for fuel sender?

I think most of the problems people have are the gauges are thermal and non linier. It is hard for the layman to understand. I don't fully understand them but it is the reason for the pulse unit in the senders (older K/S) or the pulsed supply (newer K/S). Fitting a flat resistance sender without a pulse doesn't work well. The gauges fail to respond accurately. Nor does fitting a IC "5 volt" regulator work to take the place of the pulsed OEM unit, which didn't supply a steady current.
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