Changing the resistance of a gas tank sender for aftermarket gauge? 1

[damianofhouston]

The stock sender sends a signal of 90 Ohm when empty, and 10 Ohm when full. The aftermarket gauge registers 73 Ohm Empty, and 8-12 Ohm full. The "Empty" measurement being accurate is more important to me.

Is there a way to add resistors in series (e.g. in-line with the signal wire going to the gauge) to change the value from 90 to 73 Ohm?

I suppose if there is not a way to make it work in series, I can also drop the gas tank and solder a wire from the sender to the signal wire. This will bypass the sender's variable resistor and I can put whatever resistor I need in the jumper wire.

73 is ~81% of 90.
8 is 80% of 10.

How do I decrease* resistance by 20%?

This is for a 12v automotive system.

 

[patprimmer]

Why not just put a paint out the E on the gauge face and put it in a new position to match where the needle sits at 73 ohms.

Otherwise you need an extra resistor in series, but that also strongly impacts on the full position.

If my memory of my high school physics still serves me 17 ohms in series will give you 90 and 27 ohms to the gauge in the empty and full positions.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[damianofhouston]

Yes the more I read up on resistors, the more I see that I cannot get them to work in percentages, only fixed amounts that will affect the top and bottom of the gauge equally.

There is another gauge that reads 90 Ohm FULL and 0 Ohm empty. I think I will just use that one, though I will probably get confused looks from my passengers when I tell them I need to stop for gas.

 

[IRstuff]

Why would that be? No one I know bothers to look at someone else's gauge...

TTFN

FAQ731-376

 

[evelrod]

Why not use a correct sender for the gauge? Senders made in varying resistance values for different gauges by several mfgrs...such as Fuel Safe and ATL for their cells.

http://www.atlinc.com/2010_Race_Catalog/page23.html

Similar setups to the ATL in my Mini...sender just to the left of the filler neck....and the same type unit on the Al tank of my street roadster...one is adjustable but the other is not. They both read correctly with one Autometer and on VDO type fuel gauge!

Rod

 

[evelrod]

Sorry, wrong attachment.


Rod

 

[waross]

Try a resistor of 386 Ohms in parallel with the sender. That should give you 73 Ohms at empty. It will read about 9.74 Ohms full. Within your 8-12 Ohm range.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[thruthefence]

Correct me, anyone, but I don't think the gas tank probe "sends a signal" anywhere. It's a resistance to ground (Really a rheostat) read by an Ohmmeter (the fuel qty indicator). I would think you could wire a cheap rheostat in series with the fuel probe and indicator, and adjust the 'empty' side however you want. Not much current involved here, so I wouldn't worry about heat build up.

 

[LionelHutz]

What Bill posted. That resistor size would have worked perfectly. Might require a few resistors to get that value but it's simple enough to do.

Not sure why someone posts on a forum, waits 30 minutes and then gives up and goes ahead with something wrong....

People often only read a forum once per day so it could take 24hrs or more to get an answer.

 

[evelrod]

30 minutes? Looks like a LOT less than that...I'll bet that it only took a couple minutes to realize that the correct way was also the cheapest and easiest way. Anyway, buying the correct gauge/sender is still going to be much simpler than wiring up a bunch of resistors in series and a lot less Mickey Mouse.

Rod

 

[thruthefence]

evelrod, did I ever tell you how I mounted the Ignition module from my Datsun 810 (on a LONG pigtail) in front of the air conditioner vent? Dang thing used to get hot & quit on me!

still like that when I sold it!

 

[patprimmer]

I still think the easiest way if it is temporary is to fill it, then mark the position of the needle with an F on the face with a permanent marker.

Then run till empty and then mark the position of the needle with an E on the face with a permanent marker.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[LionelHutz]

Pat - The gauge will be a nice shiny new Autometer or similar gauge that will not be replaced when the sensor is replaced. Having a permanent marker "E" and "F" on the face will ugly it up and be next to impossible to fix when the sensor is changed.

Finding/creating a 386ohm resistor and connecting it in parallel to the tank sensor is a simple task which makes the system work properly.

This appears to be just another 1-off hobby post anyways. We likely should have just red-flagged it.

 

[patprimmer]

OK use some slivers cut of a pressure sensitive label and stick it on the face. A little more trouble but not much and not permanent if that matters.

Also you are correct this certainly does not look like work related and the OP certainly does not seem like an engineer.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[waross]

Hi Pat;
I like your solution. Just as good as mine.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[patprimmer]

I was trying to keep it REAL simple and dead easy.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[LionelHutz]

Pat, as far as I'm concerned if someone posting on these forums can't figure out how to connect a resistor to impliment what would be the best "engineered" solution then they can go somewhere else for their fix.

Besides, with that gauge and sender combo the gauge will read empty with somewhere between 1/8 and 1/4 tank of fuel left so that fix doesn't work.

 

[Polatron]

A quick solution to your gauge problem. install a 20 ohm pot in line with your gauge and recalibrate the gauge with the pot use the center and one of the outer terminals taps of the pot, it should give you calibration control on the empty side. Make sure the tank is empty

 

[MRSSPOCK]

I'm 100% with waross's solution. Having said that, if the OP isn't fit to wire a few resistors in parallel, then they shouldn't be anywhere near a petrol tank and an electrical system. Here's a question I would have regarding adding resistors to an existing circuit. Without measuring current or voltage, how does one know which wattage of resistor to add? People have suggested adding resistors, but nobody has mentioned wattage. I'm asking that because I had to replace one recently within an ignition module, but couldn't determine what wattage the old one was. I only had the physical size of the resistor to go by, but I still wasn't sure. In my situation, measuring current and voltage right inside the module during use would have been difficult, since it would involve riding the motorcylce and trying to read maximum values. Easier just to buy a replacement module for £25, but I was curious how to know a reisitor's wattage in such a situation. (i.e. when the old one has puffed it).

 

[waross]

In most automotive applications the voltage won't go much above 15 volts. Use that as a starting point an possibly downsize your resistor wattage for specific circuits where you can be sure that the voltage will be less.

Bill
--------------------
"Why not the best?"
Jimmy Carter