Ohm reading for fuel sender?


The old and reliable.

Yard Art
Yard Art

Posts: 50

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

Posts: 9

Joined: Mon May 14, 2018 11:15 am

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

Posts: 50

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

Posts: 3367

Joined: Thu Nov 29, 2012 9:28 pm

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

Posts: 50

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.

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