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 Post subject: linear fuel sending unit
PostPosted: Mon Jan 20, 2020 10:52 am 
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TBI Slant 6
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Joined: Mon Sep 23, 2013 6:52 pm
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Location: Annapolis, Maryland
Car Model*: 1974 Dart Swinger 225
Has anyone found a solution for the linear resistance designed into the Spectra Premium fuel sending unit? This 2006 post discusses it but does not have a solution.

The problem is this. The Spectra Premium sending unit increases resistance at a steady rate (the resistance is "linear") as the fuel level drops, whereas the factory unit increases resistance very slowly at first, then more rapidly as the tank approaches empty. With the Spectra unit installed, the gauge reads full when the tank is full, and reads empty when the tank is empty, but reads very low when the tank is partly full. In other words, as you drive, the gauge quickly drops down toward E, but then hangs out just above E, creeping almost imperceptibly closer to E as you continue driving.

As SlantSixDan suggested in the 2006 post, I called Spectra and explained the problem. Here's a paraphrase of the relevant part of the conversation:

Quote:
SPECTRA REP: What's the part number?
ME: FG69A.
SPECTRA REP: And what's the problem?
ME: It doesn't make the gauge read correctly. The resistance is not correct at all fuel tank levels.
SPECTRA REP: It works to the factory specs: 10 ohms at full and 80 at empty.
ME: It reads correctly at both extremes, but too low in between.
SPECTRA REP: We have been doing this for many years and we haven't had any reports of this problem.
ME: Here's your report, I'm reporting it now.
SPECTRA REP: What's the application?
ME: 1974 Dodge Dart.
SPECTRA REP: OK, I'll post a report to the quality team.


I followed up the phone call with an email saying:
Quote:
Part no. FG69A should have its resistance increase *nonlinearly* from 10 ohms (tank full) to 73 ohms (tank empty). To work properly with the factory gauge, the resistance should increase slowly at first, then more rapidly as the tank approaches empty. As designed, the resistance increases linearly. That causes the gauge the read half when the tank is 3/4 full, and near empty when the tank is half full.


In the photos below, I've opened up the box containing the sliding contact and the resistive wire. The variable resistance comes from the total length of wire between the sliding contact (that moves with the float) and the case, which is electrical ground. Each time the contact touches a new turn of the wire, an additional length of wire is added to the circuit and so the resistance increases. You can see that all turns of the resistive wire are of equal length, so as the sliding contact moves along, the resistance between the contact and ground increases by the same amount each time a new turn is contacted. Also, the turns are equally spaced, so every inch of contact movement adds the same number of turns to the resistor.

It occurred to me that I could adjust the response by stretching some of the turns apart while pushing others together, but the wire is wrapped pretty tight and I don't think I can do that without breaking it. The wire may actually be glued or clamped in place.

Image
Image


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PostPosted: Mon Jan 20, 2020 11:29 am 
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Supercharged
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I might have the same problem.

I will check into this.


Thanks for sharing.


Greg

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PostPosted: Mon Jan 20, 2020 1:36 pm 
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TBI Slant 6
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Location: Annapolis, Maryland
Car Model*: 1974 Dart Swinger 225
I just picked up a salvage sending unit out of a 1969 Dart. Here's the resistor part taken out of it. You can see how the non-linearity is built in.
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PostPosted: Mon Jan 20, 2020 1:42 pm 
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Supercharged
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I will experiment with adding / modifying with some resistance wire.


Greg

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PostPosted: Mon Jan 20, 2020 6:22 pm 
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So.... I have one of these currently in the Hpak duster, and installed one way back when in the '67 Beater Valiant....
Both didn't read right when I installed them the first time...on the Duster I ended up pulling and using pliers to bend the
arm correctly after a few fill ups...

By the time the Beater Valiant had come to the house (I had to replace it's sending unit as the "resistance washboard" actually became friable and fell
out of the circuit causing no reading on the gauge), I had a Duster gas tank that had a bad sending unit ring that wouldn't seal...
so I cut a window in the top of it, installed the sending unit and a set of jumpers so I could manipulate the arm in the tank and watch the
gauge on the dash... oddly, the gauge didn't register...then I found a break in the purple/violet wire in the wire chase under the drivers door...
cut the wire, jumped it and got to bending and checking the dash gauge... then rewired the length of wire with the proper guage
and that worked well... I then changed that sender over to the 1973 Mint Duster and found the dash gauge was toast, I ended up
using an aftermarket Autometer gauge on it.. and found the reading was very off...after replacing the cracked and corroded sending
unit clip and a length of wire that was 'green' under the cracked insulation it read, but I found using an ohmmeter that the gauge
read a range that was outside the range supplied by the sender, so it would read longer on one end until the arm dropped.

Just as an FYI, deep six the soft plastic float and get a good brass one for a 1965-1973 Mustang, and change the cheap plastic sock out,
it works fine on a lo-po daily driver, but will not flow right on a hi-po engine under demand.

Not sure if these will work right on the Pre-67 units since I haven't installed one there.

Good luck, and happy bending...


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PostPosted: Mon Jan 20, 2020 7:40 pm 
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Supercharged
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Thanks DI.

What is the problem with the plastic float?

Greg

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69 Dart http://www.youtube.com/hyperpack
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PostPosted: Mon Jan 20, 2020 9:32 pm 
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Supercharged

Joined: Thu May 12, 2005 11:50 pm
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Location: So California
Car Model*: 64 Plymouth Valiant
There's a solution, it doesn't involve the sending unit.


I'll have to find the thread on FABO

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64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

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PostPosted: Tue Jan 21, 2020 4:42 am 
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Here are a couple of interface units. I just read/saw something on the net/forum, about a home built unit, but can't find it now.

https://www.tanksinc.com/index.cfm/page ... prd748.htm

http://www.yogisinc.com/index.cfm/page/ ... at3725.htm

https://www.ebay.com/itm/Marshall-Progr ... 0#viTabs_0

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PostPosted: Tue Jan 21, 2020 4:58 am 
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Supercharged

Joined: Thu May 12, 2005 11:50 pm
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Location: So California
Car Model*: 64 Plymouth Valiant
This is slightly cheaper metermatch (was from the FABO thread)

https://www.technoversions.com/MeterMatchHome.html

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64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

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PostPosted: Tue Jan 21, 2020 5:56 am 
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TBI Slant 6
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Joined: Mon Sep 23, 2013 6:52 pm
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Location: Annapolis, Maryland
Car Model*: 1974 Dart Swinger 225
So there are some options, then.

1. The Metermatch unit is $84 shipped. Add $46 for the Spectra Premium sending unit (Rockauto price today), and you're up to $130.

2. Also somewhere on FABO there's a discussion of a John Wolf Company (https://antiqueinstrument.com) that will rebuild your factory sending unit. I called them this morning; for $155 plus shipping costs, they sandblast your old unit, check & repair the resistor, add a new float and gasket, and send it back with a one-year warranty.

3. I found a NOS sending unit for $195 on the web.


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PostPosted: Tue Jan 21, 2020 10:13 am 
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Yeah, I'm real sure dudeguy from Spectra will "post a report to the quality team" right away and they'll, ah, urgently get on that. :roll:

Lookit here and here; maybe those might be of interest? Prices are in Australian dollars; as of right this minute A$100 = US$69.

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PostPosted: Tue Jan 21, 2020 10:41 am 
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TBI Slant 6
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Joined: Mon Sep 23, 2013 6:52 pm
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Location: Annapolis, Maryland
Car Model*: 1974 Dart Swinger 225
SlantSixDan wrote:
Yeah, I'm real sure dudeguy from Spectra will "post a report to the quality team" right away and they'll, ah, urgently get on that. :roll:

Well, they are Canadian, eh. :-)

Quote:
Lookit here and here; maybe those might be of interest? Prices are in Australian dollars; as of right this minute A$100 = US$69.

Great find! Bit of a markup on shipping, though. :shock:

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PostPosted: Tue Jan 21, 2020 11:10 am 
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LOLROFL. I don't guess they've set up their website for shipping outside Australia. Might want to send them an email about it.

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PostPosted: Tue Jan 21, 2020 12:43 pm 
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Supercharged
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Location: IRWIN PA
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Awesome options fellas.

Getitsolved!

Greg

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69 Dart http://www.youtube.com/hyperpack
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PostPosted: Tue Jan 21, 2020 5:28 pm 
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Location: Salem, OR
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Quote:
What is the problem with the plastic float?



It's kinda cheap and the one in the duster sunk after a couple of years...but the brass replacement
hasn't needed replacing now for 15 years....

I kinda wonder if the plastic is vulnerable to ethanol and methanol....but Spectra probably
doesn't have the quality control results on these or the MSDS on the material to see
how well it resists...


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