: Fuel level sender modification
kengsx 05-26-2003, 09:51 AM Stock fuel tank and fuel level sender, and an aftermarket fuel gauge. Is there any way to adjust the fuel gauge unit's resistance to match the gauge?
Standard value for the DSM is 3 ohms for full, increasing to 110 ohms when empty (pg 8-203 in volume 2 of the import service manual). The guage has 6 sender options, only 2 are viable because they go in the correct direction (GM and VDO decrease resistance, and Ford and Stewart Warner/Sun increase).
Ford: 10 ohms (F) to 73 ohms (E)
Stewart: 33 ohms (F) to 240 ohms (E)
The apparent problems would be the gauge indicating the wrong value unless I can add some sort of resistor or change my fuel sender. I checked autometer's sight just to see what their gauges were rated to since I've seen people here with those gauges, and none of their's matched either.
I'm much more mechanical than electrical. Any help is appreciated.
Ken Young
James92TSi 05-26-2003, 12:40 PM Looks like you'd have to use the Stewart setting, and add a potentiometer to the line so you can fine-tune the resistance to get Empty to read correctly (would be set to approximately 30 ohms to get the Empty reading correct). It wouldn't work perfectly, since a potentiometer is basically an adjustable resistor... you could use it to skew the readings by 30 ohms across the board, which would make the stock sender work at 33 to 140 ohms (instead of 3 to 110)... but the gauge is looking for 33 to 240. You can see that the gauge would read Empty correctly (at 33 ohms), but it would read 58% with the tank full (140/240). Unfortunately, I don't know of any way around that, short of replacing the stock sending unit... maybe someone else has some ideas.
kengsx 05-26-2003, 03:33 PM I'm going to swag something here, but is there a way to "double" the resistance and then add more with the pot? Doubling the sender output across the board would yield: 6-220 ohms. Then use the potentiometer to add 20 ohms: 26-240. That would put you where you want to be plus give you a 0% earlier for a nice safety factor.:confused:
Ken
brads 05-26-2003, 05:49 PM The fuel level sender appears to simply be a variable resistance to ground, so you just need to adjust the spec of the DSM one to match one.
I picked the Ford one, because its easier to work with due to the smaller range. The DSM has a resistance of 3-110 ohms, or a 108 ohm range. The Ford goes from 10-73, or a 64 ohm range. You want to scale that 108 resistance to get a 64 ohm result. Add a resistor in parallel to accomplish this, here are the calcs:
1/R1 + 1/R2 = 1/Rtotal
1/108 + 1/x = 1/64
Solving this you get x=157 ohms
Now doing the math using this 157 ohm value, you can find the value at empty and full.
For the resistance at E:
1/3 + 1/157 =1/x
x= 2.95 ohms
For the resistance at F:
1/110 + 1/157
x=64.7 ohms
for a total range of around 3-65 ohms. To get that to the 10-73 ohms, you simply add a 7 ohm resistor in series. That gives you 10-72. Close enough to be in the ballpark.
So add a 157 ohm resistor in parallel to the sending unit, and a 7 ohm in series with its output, and it should work fine.
Brad
kengsx 05-26-2003, 06:52 PM It's all coming back to me, series is R1+R2+...=Rtotal, parallel is the inverse of each like you said it. That sir was nothing short of magnificent.
Thank you. Now to actually do it.:) Im still wondering if anyone with an autometer setup had the same issue.
Ken Young
brads 05-26-2003, 09:19 PM I forgot to mention his will only work if both of the sensors have the same output slope. Its matched up at both ends, but whether or not it matches in the middle is unclear.
Brad
eyebrowski 05-29-2003, 09:03 PM prallel is actually 1/R1+1/R2=1/RTotal
series is R1+R2=RTotal
Originally posted by kengsx
It's all coming back to me, series is R1+R2+...=Rtotal, parallel is the inverse of each like you said it. That sir was nothing short of magnificent.
Thank you. Now to actually do it.:) Im still wondering if anyone with an autometer setup had the same issue.
Ken Young
eyebrowski 05-29-2003, 09:08 PM double post...sorry
kengsx 05-30-2003, 12:08 AM I think I'm just going to graft on a different sender.
series is R1+R2+...=Rtotal, parallel is the inverse of each like you said it
That's what I said. We're all saying the same thing.:) I just don't want to overcomplicate things with adding resistors when it might not be so robust.
Ken
Defiant 05-30-2003, 10:09 AM rats, there's a VFAQ someplace for this, and now I can't find it.:rudolph:
kengsx 05-30-2003, 10:11 AM I just talked to the company I bought the gauge from, and they just updated their line to include our cars' sender range. They will exchange it for free.
This is by far the easiest solution, however the question of how to do it still remains. Before he realized they had the update, he was suggesting they make me a custom program for it.
Ken
eyebrowski 05-30-2003, 06:23 PM sorry, didnt read the last part of your post...
Originally posted by kengsx
I think I'm just going to graft on a different sender.
That's what I said. We're all saying the same thing.:) I just don't want to overcomplicate things with adding resistors when it might not be so robust.
Ken
AudioGod 06-03-2003, 10:18 PM Originally posted by Defiant
rats, there's a VFAQ someplace for this, and now I can't find it.:rudolph:
I've seen this link for adjusting the gauge. Not sure if this was the same kind of thing you were talking about.
Gas Gauge Fix (http://www.geocities.com/CapeCanaveral/Cockpit/6144/DSM_Home/2G_Fix_It_Center/flakey_gas_guage.html)
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