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LX Fuel Sender & Gauge Reading


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#1 _crowe_

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Posted 04 May 2017 - 11:32 PM

Hi all,

 

I am having a few problems with and LX fuel sender and gauge not reading correctly. It basically reads just on empty when there is 25l in the tank still and just under 1/4 when there is 30l in the tank. I have pulled the sender out and tested it on the ohm meter following a couple of other posts about it. The sender is and older aftermarket job like you can get from rare spares etc.

 

It seems my sender is OK, it reads

 

10.5 Ohms Full

29.5 Ohms Middle of swing

74.0 Ohms Empty

 

I have moved the sender around whilst still connected to the gauge in the car with alligator clips, the gauge reads 1/2 tank when the sender is reading 21-22 ohms and this doesnt seem right. Does anyone know at what exact ohms the fuel gauge should read in the middle 1/2 tank? I initially thought it was the gauge and took it to a instrument reapir place that tested it on the bench, they said it wasnt too bad and told me 25 ohms should see the gauge exactly in the middle but other posts on here have indicated 32 ohms and I am getting 21-22 ohms which in my mind is very close to 10 ohms which is supposed to be full.

 

The gauge in the car does read full when the sender is all the way up and empty when all the way down but the middle ground is no good, attached is a photo of the sender position when the fuel gauge reads 1/2 tank. I havent measured it exactly but i reckon there would be close to 40l in the tank when it reads 1/2 on the gauge.

 

Can anyone shed some light or have any ohms readings on what the fuel gauge should read at 1/2, 3/4 and 1/4? Is there anything else i could be missing?

 

Thanks

 

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#2 IMORAL

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Posted 05 May 2017 - 07:41 AM

I've found no 2 senders will read the same. Unfortunately your best option would be bend the sender arm to where you are happy. The biggest thing is have the empty reading correct IMO.

 

I did all of the above then put it in the car. Filled it and it didn't read full. In the end I gave up as I wasn't spending another 200 on a custom sender. ( I run one that goes in from the top)



#3 VDO

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Posted 05 May 2017 - 10:11 AM

Your sender should provide between 23 - 25 ohms to the gauge for half full.

 

Don't use visual approximation of the float arm position for determining half way, as the sender resistor is linear and allows for the shape of the tank.


Edited by VDO, 05 May 2017 - 10:11 AM.


#4 hanra

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Posted 05 May 2017 - 03:14 PM

6F125A3A-03ED-4D01-9792-97345BFC51DF_zps

Looks like any where between 27.5 - 23.5 ohms is half.

#5 VDO

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Posted 05 May 2017 - 03:42 PM

That's the common theoretical figure , however most Instrument techs will use a slightly lower figure for half, like 23 ohms as in reality it doesn't show as half on a thermal gauge such as in the LH LX. 

 

Resistance of around the 25-26 ohms usually results in a thermal gauge showing a bit under half.

 

I find this every day and as a result I use the slightly lower resistance to get actual half on a bi-metal / thermal gauge.

 

I should point out however, that on a cross coil/magnetic movement or a moving coil gauge, 26-27 ohms is better for half.

 

Differences due to the scaling method of different gauge types.  Some are linear, some not so much.



#6 _crowe_

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Posted 05 May 2017 - 04:06 PM

Your sender should provide between 23 - 25 ohms to the gauge for half full.

 

Don't use visual approximation of the float arm position for determining half way, as the sender resistor is linear and allows for the shape of the tank.

 

Great, Thanks for the input, I have seen a couple of other posts about this now too (searching wrong area).

 

When my sender reads 21-22 ohms I am getting half on the gauge so given your info my gauge could be a tad out.

 

25ohms reads one needle thickness over the 1/4 tank line which is a big gap between it and 1/2 tank . The gauge basically travels from half to quarter over 5-6 ohms. The real problem is that the sender gives me a reading of 21-22 ohms (1/2 on the gauge) when there is 39-40l of fuel in the tank which is about 74% of the tank capacity. I dont know how to address this to get it more accurate.



#7 VDO

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Posted 05 May 2017 - 04:59 PM

Great, Thanks for the input, I have seen a couple of other posts about this now too (searching wrong area).

 

When my sender reads 21-22 ohms I am getting half on the gauge so given your info my gauge could be a tad out.

 

25ohms reads one needle thickness over the 1/4 tank line which is a big gap between it and 1/2 tank . The gauge basically travels from half to quarter over 5-6 ohms. The real problem is that the sender gives me a reading of 21-22 ohms (1/2 on the gauge) when there is 39-40l of fuel in the tank which is about 74% of the tank capacity. I dont know how to address this to get it more accurate.

The solution is to have the gauge calibrated to suit.  With a bi-metal gauge, 23 ohms should have it at half.

 

Now to open up a whole other can of worms for you. 

It would be worth checking the other gauges as if it's a voltage regulator issue, then all the gauges will be down a little. 

If that's the case, then it's the voltage regulator that will need calibrating (yes they can be calibrated) not so much the the gauges. 

The regulators get a little tired over the years and don't switch at the rate they should which means lower nominal voltage, therefore lower gauge readings.


Edited by VDO, 05 May 2017 - 05:00 PM.


#8 _crowe_

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Posted 08 May 2017 - 05:07 PM

Good news, i have sorted it out finally. Bad news....i dont know exactly what the issue is.

 

So I havent driven the car for ages trying to sort this issue out, last few good weekends beofre winter sets in so I reinstalled gauge and sender, chucked 30L fuel in it and no surprise was still reading less than quarter tank. Went for family drive and returned home at night so had to turn my headlights on.........lo and behold i noticed that the fuel gauge now reads 1/2 tank. Bizarre!!

 

Im not sure what the issue is but the gauge reads correct when the headlights are on, it jumped from under a quarter to half a tank when the headlights went on, I replicated this 4 or 5 times to check that I wasnt seeing things. Once the headlights have been on i can turn them off and the guage still reads correct but once i turn the key off and cycle power the gauge goes back to reading incorrect until headlights are turned back on. The gauge will correct with either park lights or headlights switched on.

 

Anyone seen electrical issue like this before or advise where would be the best place to start looking? would it be in the gauge cluster and be linked to the gauge backlight?

 

Thanks



#9 _Darylle_

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Posted 08 May 2017 - 05:17 PM

This being my third LX and they all read crap, I have always thought if I ever remove the sender I would bend the float arm a tad and see what changes.



#10 VDO

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Posted 08 May 2017 - 06:23 PM

Firstly, the lighting and instrument circuits are completely separate.

 

The voltage regulator in the gauges could be running below par and that would be giving you the low readings.

 

However when you turn your lights on, the alternator ups it's game and delivers more to the electrical system to compensate.

 

By doing so, the main voltage is probably increasing to a level that the gauge voltage regulator is able to deliver the full 5 volts to the gauges.  It's the same result as if you up the rpm of the engine, for a minute or so, you'll probably see the gauges rise also.

 

What concerns me is that the gauges voltage regulator needs such an increase in input voltage to function correctly.  It could well be that the gauges voltage regulator is fine, but the output of the alternator/regulator/battery is too low most of the time and only comes up to a correct level when the lights are turned on etc.

 

You might want to put a voltmeter on the battery and test the main regulator output levels.

 

Battery level with engine off:  12.5 volts

Battery level with engine idling:  13.8 volts

Battery level with idle up to 2500 rpm:  14.2 volts.

 

If you find this isn't the case, then you might want to replace the alternator's regulator.

 

If the levels are around what's shown above, then the gauges voltage regulator needs a look at.


Edited by VDO, 08 May 2017 - 06:23 PM.


#11 hanra

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Posted 08 May 2017 - 09:10 PM

I found that with my LM7805 solid state regulator that I have on my 3 gauges, when the lights were on all 3 gauges read high. I was concerned the first time it happened as I thought it was overheating.

The LM7805 uses the same ground circuit as the dash lighting. Something wasn't happy.

I ran a dedicated earth for the regulator back to dash frame.

All good.

#12 _crowe_

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Posted 09 May 2017 - 03:12 PM

I found that with my LM7805 solid state regulator that I have on my 3 gauges, when the lights were on all 3 gauges read high. I was concerned the first time it happened as I thought it was overheating.

The LM7805 uses the same ground circuit as the dash lighting. Something wasn't happy.

I ran a dedicated earth for the regulator back to dash frame.

All good.

Sounds good, I can try that. I was so enthralled with the fuel gauge working i didnt even take note of the other gauges. I will check to see what they do too. So in your case they were reading high and then corrected itself to read lower with the separate earth? It could well be the regulator. It has been replaced before but could still be the issue.

 

The battery is brand new and this issue happens whether the car is started and running or switched off with the headlights on. I might measure a few different voltages when the headlights are on and off see if anything changes dramtically...car running and not running. Thanks for the help guys



#13 hanra

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Posted 09 May 2017 - 03:54 PM

I had calibrated all my gauges using the heatsunk LM7805. They all read well under normal conditions, however when the lights were turned on, all the gauges read high.

I supplied a dedicated earth to the LM7805 rather than use the existing ground wire at the back of the cluster. Once I did this, the problem was resolved.

Is your fuel tank painted at all? Does your fuel sender have a good reliable earth at the tank?

#14 TerrA LX

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Posted 11 May 2017 - 05:29 PM

^ are you saying a new earth to the gauge chassis fixed your high reads?



#15 hanra

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Posted 11 May 2017 - 06:58 PM

A dedicated earth from the instrument cluster voltage regulator to the chassis/dash solved it.

The existing instrument cluster earth for the lighting circuit remained as is.

I measured for continuity of the earth circuit at the regulator initially and it was a dead short to the chassis.

Edited by hanra, 11 May 2017 - 06:58 PM.


#16 _76lxJAS_

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Posted 11 May 2017 - 11:27 PM

hey Brad do u have any links to this LM7805 so i know what im looking for??



#17 hanra

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Posted 12 May 2017 - 06:16 AM

It's just a 3 pin regulator with a heat sink. These jaycar jobs are pretty handy though.

https://www.jaycar.c...taSheetMain.pdf




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