Setup of an AutoMeter fuel bridge

[timk225]

I finally got tired of the Year One fuel sender reading being way off, my 1973 Dusters' fuel gauge going from F to E in 125 miles with 75 to go in the tank, and having to think about it.

I finally got around to getting an auto meter fuel bridge calibrator and a #2652 Z series fuel gauge.

The installation itself is no big deal, I already did it with my Z series 5000 rpm tach and oil pressure gauges. But calibrating this fuel bridge to the sender and gauge has me thinking. Their procedure could be improved. Biggest problem is you start with the tank FULL and go every 1/4 tank until you get to Empty. If you started with Empty and just put in 4 gallons for each 1/4 tank setting (in the case of a Duster gas tank), until you got to Full, it'd be a lot easier.

So I was thinking how I could not put 16 gallons in the tank and take out 4 at a time. My Duster is sitting now with the tank empty, I guessed it just right and got it home right on E, and then with the car backed onto a slightly downhill piece of ground, it stalled out from the gas flowing to the back of the tank.

My big plan is to get my multimeter and take resistance readings right at the tank, then fill it with 4 gallons, take a reading, 4 more gallons, take a reading, etc.

Then get a 0-100 ohm potentiometer and connect it to the wire and ground leading to the fuel bridge, and calibrate it in order, full to empty. I think that'll get it pretty close. As I go through each step, adjust the potentiometer to give the same reading the sender unit did at each fuel level.

If anyone downloaded the pdf of the calibration procedure, I am doing it on the 4B section for a custom fuel sender in the 0-1000 ohm range, since I'm using a replacement BG914 sender from Year One. https://www.yearone.com/Product/chrysler-a-body/bg914

I'll be paying attention to grounds and wire connections, don't want extra resistance throwing it off.

 

[67Dart273]

I don't know why that would not work.

Bridge circuits are just plain inherently more accurate, assuming the components use are decent quality. They are less liable to supply voltage changes.

 

[DionR]

I finally got tired of the Year One fuel sender reading being way off, my 1973 Dusters' fuel gauge going from F to E in 125 miles with 75 to go in the tank, and having to think about it.

I finally got around to getting an auto meter fuel bridge calibrator and a #2652 Z series fuel gauge.

The installation itself is no big deal, I already did it with my Z series 5000 rpm tach and oil pressure gauges. But calibrating this fuel bridge to the sender and gauge has me thinking. Their procedure could be improved. Biggest problem is you start with the tank FULL and go every 1/4 tank until you get to Empty. If you started with Empty and just put in 4 gallons for each 1/4 tank setting (in the case of a Duster gas tank), until you got to Full, it'd be a lot easier.

So I was thinking how I could not put 16 gallons in the tank and take out 4 at a time. My Duster is sitting now with the tank empty, I guessed it just right and got it home right on E, and then with the car backed onto a slightly downhill piece of ground, it stalled out from the gas flowing to the back of the tank.

My big plan is to get my multimeter and take resistance readings right at the tank, then fill it with 4 gallons, take a reading, 4 more gallons, take a reading, etc.

Then get a 0-100 ohm potentiometer and connect it to the wire and ground leading to the fuel bridge, and calibrate it in order, full to empty. I think that'll get it pretty close. As I go through each step, adjust the potentiometer to give the same reading the sender unit did at each fuel level.

If anyone downloaded the pdf of the calibration procedure, I am doing it on the 4B section for a custom fuel sender in the 0-1000 ohm range, since I'm using a replacement BG914 sender from Year One. https://www.yearone.com/Product/chrysler-a-body/bg914

I'll be paying attention to grounds and wire connections, don't want extra resistance throwing it off.

Here is how I mapped my sending unit resistances.

Another gauge corrector--cheaper than MeterMatch, 33.00

Read the whole post, there are other posts with useful info too.

I can't find where I said how I set mine up, if I did. Been several years now so having to work hard to remember. But I am pretty sure I took another sender I had laying around and wired that in so I could move the float up and down to get the output values I wanted. So I effectively did it dry with a sender on the floor of the car and the sender in the tank removed from the circuit. I remember finding the points where the dry sender matched the resistance I was looking for and then drawing a line so I could match it later. Did that on a bench and then just used the lines when I had it wired into the car.

The best option would be to build some kind of a resistor board that for sure gave the correct resistances at the set points.

 

[timk225]

I did my gas fill and measure the resistance thing today. Here is the resistance map I got for my BG914 fuel pickup / sensor from Year One.

I got lucky coming home the other day and the car ran out of gas as I was putting it in its parking spot. It would not start without some pedal pumping, then it'd die. Knocking lightly on the bottom of the tank makes the fuel float rattle. So I'm sure it was empty. All the way empty. Car won't run empty. My handyman was here and I even took the step of borrowing his digital level and measuring the rocker panel at several points to make sure the car was sitting level. It was 0.5 degree uphill, close enough.

I took my 5 gallon gas cans and got precisely 4 gallons in each, then carefully filled the tank from 0-16 gallons. Here's the numbers I got on my multimeter:

Empty ------------------ 76.6 ohms
1/4 tank / 4 gallons -- 61.5 ohms
1/2 tank / 8 gallons -- 40.4 ohms
3/4 tank / 12 gallons- 20.5 ohms
Full tank / 16 gallons- 7.6 ohms

I have a 0-100 ohm potentiometer coming from Amazon, I'll use that to replicate the resistance numbers to set up the fuel bridge.

 

[timk225]

An additional piece of trivia. The BG914 fuel sender has a thread size of M5-0.8 . I thought it was a 10-32, but it wasn't. I looked up the specs on those thread sizes and they are very close, but the M5 is 0.006" larger diameter, and the thread pitch is less than 0.002" difference. But that's enough.

I had ordered a Dorman 85845 sending unit connector, but it was too small to slide onto the bolt, as the OEM sender connector did.

I had a roll of automotive high heat compatible 14 gauge wire with a lot left over, so I pulled the fold down rear seat out of the car yesterday and ran the wire along the OEM path. I also soldered on an eyelet connector for the sender connection. The crimp style connector wasn't good enough, I'm getting all nerdy with the electrical conductivity and reliability, it had to be soldered. It'll have a little lock washer and the M5-0.8 nut holding it on.