How to make autometer fuel gauge read properly

[smoorenc]

Well like most of you I got the auto-meter fuel gauge made for Ford's pre 1989. I got the phantom series # ATM-5815 which has the compared values:

 

_____Stock Sender_____ Gauge

Empty __90 ohms______73 ohms

Full ____10 ohms______ 8 ohms

 

 

The problem was at the half way point on the stock sender the gauge would read just below 1/4 of a tank. I used the excel calculation sheet I got from here: http://mywebpages.comcast.net/pparaska/gagecalibration.htm which was used on Mike Kelly's gauge. According to the sheet I needed a 367 ohm resistor between the sending unit signal and ground. So I went to www.newark.com And ordered up some resistors. I couldn't get exactly 367 so I ordered an assortment and figured I would have to put them in series. These are the 2 that worked and I will explain how I had to put them in parrallel to get a resistance value of 173 ohms:

 

Part # 65K3789 $0.383 $0.38 350OHM

Manufacturer Part No: CW02B350R0JE70

Part# 01H0119 $0.34 $0.34 360OHM

Manufacturer Part No: AC03W360R0J

 

If you can find a 170/180 or 175 ohm 3watt resistor it would work easier than 2 in parallel.

I found one at www.newark.com 64K8918 180ohm for .99cent. You might want to try that first.

The only reason I used the 2 in parallel is I didn't have one resistor in the 170 ohm range. Remember for basic resistance

in seriries "add" the values. in parallel, divide the values.

 

I don't know why but nothing in the 300ohm range would work. So I just started experimenting with different resistors until it worked right. At 173 OHMS it seemed to work good.

 

Here is the original half way point on the gauge:

 

Setup with no resistors:

Full

 

Half

 

Empty

 

Putting the 2 resistors in parrallel and soldiering them together / heatshrink / adding leads:

 

Then I connected them to the ground post and the sending unit post

 

This is how the gauge is measuring now:

Full

 

Half

 

Empty

 

I did not come up with all of this info. I just put it together with pictures.

[naviathan]

Nice work. Well laid out. Now are you using the stock sender? I assume yes, but I just wanted to make sure it was clear.

[smoorenc]

Yes its the stock sending unit.

[TravRMK]

Nice write up!

[1 tuff z]

i knew if i waited that someone smarter than me would figure it out and i could then follow. thank you! my autometer gauge does the exact same thing, drives me nuts, now i know how to fix it.

[dsommer]

So how would it work for the GM guage? I stink at electronics/electrical work.

[Tony D]

That Autometer works BETTER than my STOCK gauge in the Wife's 260Z!

I have a red paint mark at around '3/8' tank where half actually is...

I replace the tach on that car because the Autometer was $40 cheaper than a stocker, and was more accurate above 2000 rpms. Below there, welll....

 

nice writeup.

[MaliceIntent]

Thanx for the write-up! ...came in very handy today. I used a single 180ohm resister...hopefully its close enough. I will find out tomorrow when I get gas

[khooper]

I used AutoMeter gauges in my Grand Sport, but I never had to work through this issue with the stock sender, as I used an aftermarket foam-filled Jaz fuel cell with a top-loaded AutoMeter enclosed sending unit made specifically for that application.

___________________________________________

tips: better use chamois cloth in cleaning your car

[janaka]

Next question which stock wire in the harness that plugs into the back of the stock gauge is the signal wire to tap into for the autometer guage?

Thanks guys.

[smoorenc]

The yellow one.

[janaka]

Its funny I used the 'search' function and was about to come back to post that I found it to be the yellow one from another post.

Thanks for beating me to it.

 

Jason

[EvilC]

Its funny I used the 'search' function and was about to come back to post that I found it to be the yellow one from another post.

Thanks for beating me to it.

 

Jason

 

Who are you telling, I printed this one off about a month ago so I can get my gauge to work.

[hdflh]

Did any of you also install the oil pressure guage? Is it necessary to change the sending unit to the one sent with the autometer guage? My sending unit does not have same threads as the stock unit. Thanks for any input.

[jeffp]

I am not totally sure how the gauge is wired, but if is wired this way:

 

One side of the gauge is wired to 12 switched with ignition switch.

second side of meter is wired through your resistors to the sender. The sender then makes the variable part of the resistor make the gauge move as the fuel level repositions the sender to a different resistance.

 

So if you took one of the resistors you have in parallel and sub in a variable resistor, say a 500 OHM resistor (ten turn) then you can vary the resistance of the variable resistor to move the gauge needle perfectly over the full tick on the gauge. Then you connect say a 10 OHM resistor in series with the two resistors (one fixed the other the variable resistor) you can by adjusting the resistor in series and the resistor in parallel get the gauge to read exactly on the gauge at what level the tank is. You get accurate detent of no fuel level, to full level.

The sender is the trim, or variable resistance based on the resistor on the sender. The parallel resistors will set the span of the gauge, in other words how much the gauge will swing when the sender is moved from empty to full, and the second variable resistor will determine where the gauge needle will rest when either full and empty.

This setup will calibrate the gauge to the sender exactly.

You don't need big resistors, a 1/4 watt resistor will do the trick. Remember this is very low current we are talking about here. The current must be very low so that there is no way if the contacts in the sender become disconnected that there will not be a spark of any kind. Sparks in the fuel tank are not good as you may already know LOL.

So if the sender resistor is linear throughout ist range, you should have a very accurate fuel gauge reading.

Myself I went with a DEFI temp gauge, and tach in my stock dash assembly. Both units work excellent and look as good. Now for the fuel gauge LOL.

[zredbaron]

Did any of you also install the oil pressure guage? Is it necessary to change the sending unit to the one sent with the autometer guage? My sending unit does not have same threads as the stock unit. Thanks for any input.

 

yes, autometer pressure gauges expect you to use their sender, which requires a pipe fitting adapter from metric to sae.

 

very painless install, autometer will sell everything you need.

[Skittle]

Do they have a fuel sending unit that will work for us s30 guys? Mine is bad and cant seem to find a viable aftermarket one.

[zredbaron]

Do they have a fuel sending unit that will work for us s30 guys? Mine is bad and cant seem to find a viable aftermarket one.

 

the fuel indication problem is actually VERY complex, one that i believe has no current solution unless autometer lets us dictate the guts of their gauges (i even called them at one point to make such a request and was shot down). i'm in the middle of a reply on this very thought.

 

to answer your question, though, yes, you can buy generic senders, and no it's not what you want to do.

 

the problem with using non oem senders, in any application, is that generic senders are linear, and therefore to be accurate, the tank must be consumed linearly (ie a rectangular tank). no production car has a square tank, so you would essentially guarantee yourself an erroneous display.

 

i was able to buy a new nissan sender about 5-6 yrs ago, no idea from where, though. good luck!

Edited April 4, 2010 by zredbaron

[Skittle]

Thnx MSA sells the ones for the 240's but us 75/76 guys are out of luck.

[zredbaron]

this is something that i would really, really LOVE to be wrong about! by all means, if someone has the above setup in their car, and can verify that it works, then i'll be quite excited to go out and duplicate it!

 

the quick summary is this:

short of creating your own sender, i don't believe an accurate fuel indication can be attained in our S30s with an oem fuel tank and an autometer fuel gauge.

 

the reason is that the autometer gauge, an american product, is designed to compensate for "dumb" fuel sending units. our sender, a japanese product, already does this compensation, sending a nearly perfectly linear signal, very much indicative of the actual fuel level. our oem gauge is "dumb" in that the needle's indication is directly proportional to the resistance it feels from the fuel sender's rheostat. the "smart" autometer gauge on the other hand, assumes the sender is dumb and therefore the needle's indication is NOT proportional to the resistance it feels from the fuel sender.

 

in short, the problem is that the complete fuel circuit is double compensated and therefore inaccurate.

 

this was verified through experimentation in an electrical engineering lab years ago in 2004 while i was still in college. we all agree that plugging the autometer gauge in "as is" isn't accurate. i came up with a similar solution (using resistors to alter the signal "felt" by the autometer gauge), but the best i could do was either a} have the level accurate at exactly two levels only (ie once at 3/4 and once at 1/4) or b} have it "pretty close to accurate" down to 1/4 tank, and after that, gauge indication was useless. i elected to have b} since my Z was a daily driver at the time, whereas a} would be more useful to me now that it's rarely driven (i'd rather know for sure when it's about to be empty, different resistances change at what points it is accurate).

 

this would be a really long technical writeup. i have no problem writing it up and sharing my very, very extensive excel spreadsheet for all to see/critique, etc. it's also going off of the assumption that autometer isn't offering a new, "dumb" version of their fuel gauges (i installed my gauges in '04). god, i'd love to pay the $80 for a new gauge and be done with it!

 

that said, not being a fan of fruitless labor and recognizing that it's certainly possible that i missed something and am just plain wrong, i'll wait for a reply to say "dude i have this in my car in it works" or "dude i have this in my car and you're right it doesn't work, what gives?" before publishing my dissertation on "why installing autometer fuel gauges in street-modded S30s is for cosmetic purposes only."

Edited April 4, 2010 by zredbaron