Fuel Gauge Ohm Values

Does anyone know the fuel gauge ohm values Sea Ray was using in 1993? The PO had the fuel sender replaced with an American Standard sender (Full = 33.5 ohms, Mid = 103 ohms, Empty = 240 ohms). I measured the ohm values at the sender. Full measured at 33 ohms. 60% Full measured at 90 ohms. My guess would be that 50% Full would be around 103 ohms, which is what it should be.

The gauge is reading Full at 33 ohms. The gauge is reading 12% Full (1/8) at 90 ohms (not 60% Full). It seems like the gauge is working but is using a different resistance standard. With the current situation, my gauge reading of Empty is equal to half a tank remaining and my gauge reading of Full is equal to a full tank.

Fuel senders wear out over time. As they wear down the resistance values change due to the wiper wearing away.

They are easy to replace. Just make sure the gauge you have matches the sender (resistance values)

These type gauges and senders have been used for years in just about everything.

https://www.westmarine.com/buy/moel...o27EPH3vHxayWhxVrOH932tar5RHWr5RoCCtoQAvD_BwE

Chip,
You can’t use the percent height of the sender float to predict the intermediate volumes unless your tank is a perfect square box (all 90 degree corners and straight sides)**. Additionally, because the sender float (and fuel pick up) do not go exactly to the bottom, the full to empty range will not match the nominal capacity of your tank. My 175 gallon tanks only have a useable volume of 165 gallons because the sender float and fuel pick up sit 1.25” above the tank floor.

The reason for this is that boat fuel tanks are often odd shapes. Mine fit against the port and starboard sides of the hull and the outboard sides conform to the curve and vertical angle of the hull shape. The non square shape means that the volume of say a 1” thick layer at the top will be different than a 1” layer at the bottom. That being the case, 50% of sender float height will not be equal to 50% of measured volume capacity.

The only way to truly test the resistance of the sender is to pull it out of the tank and measure the resistance with the sender all the way up and again all the way down. Or of course you can run the tank dry or pump out all the fuel.

Also, unless you have the ability to calibrate the gauge through mid point fillings where you start the process at empty, add some set volume (typically 25%) then calibrate, then add another 25% and calibrate, and repeat until full, the intermediate positions on the gauge will only be approximations.

** EDIT: I omitted that a perfect vertical cylinder, or barrel like configuration, has a direct correlation between vertical and volume. I'm pretty sure Sea Ray never used 55 gallon drums for fuel tanks, so no harm no foul.

I agree with all of the above. You're over thinking this one, Chip If you can peg the needle back and forth from the pink wire at the sender, then the wiring and the gauge are good. Diagnostics are done. Replace the sender.

The same can be true of the gage itself. If replacing the sender does not resolve, you may also need to replace the gage.

Chip S said:

Does anyone know the fuel gauge ohm values Sea Ray was using in 1993? The PO had the fuel sender replaced with an American Standard sender (Full = 33.5 ohms, Mid = 103 ohms, Empty = 240 ohms). I measured the ohm values at the sender. Full measured at 33 ohms. 60% Full measured at 90 ohms. My guess would be that 50% Full would be around 103 ohms, which is what it should be.

The gauge is reading Full at 33 ohms. The gauge is reading 12% Full (1/8) at 90 ohms (not 60% Full). It seems like the gauge is working but is using a different resistance standard. With the current situation, my gauge reading of Empty is equal to half a tank remaining and my gauge reading of Full is equal to a full tank.

Click to expand...

@Lazy Daze

As Lazy Daze says, keep diagnostics simple... Each step should prove or disprove a specific assumption.
Since the gauge is moving it appears the wiring is good.
But you could always be the victim of the "previous owner" so dont take anything for granted.

Normal US/SAE marine senders are 240 - 33 ohm (Empty-Full)
So open circuit the gauge should be at Empty, connect to ground = Full.
If the gauge does not do this you have a bad gauge or a non-standard gauge (VDO can use 10-90 E/F)
It's always easier to validate the gauge first then to pull the sender.

Step by step determination of what type you may have:
https://www.vdomarinegauges.com/vdokb/determine-sender-resistance-for-a-gauge-2009-2/

Use these for $12 and you can test just about any gauge.
2pcs RV24YN20S Single Turn Carbon Film Rotary Taper Potentiometer + 2pcs A03 knob + 2pcs dials (B501 500 ohm)
by SEA-GULL
Learn more: https://www.amazon.com/dp/B074KBX9RZ/ref=cm_sw_em_r_mt_dp_ehD-FbY8NPWXK?_encoding=UTF8&psc=1

As stated above, the fuel gauge in my 1993 Sea Ray 200 OV is out of calibration. The PO replaced the sender and now when the gauge reads empty the tank is really half full. When the gauge reads half full the tank is really 3/4 full. It is a 32 gallon tank, so each 1/4 mark should equal 8 gallons. Each 1/4 mark is now equal to a 4 gallon reduction from full (32 gallons).

I checked the values at the new sender and the ohm value when full is 33.5 (gauge reading full) and when half full is 103 ohms (gauge reading empty). In order to calibrate the gauge I need to convert the 240 ohm value to 103 ohms while keeping the 33.5 ohm value the same. This isn't possible, but I can convert the 240 ohm value to 103 ohms with a minimal change in the 33.5 ohm value by adding a resistor in parallel to the sender resistor.

WARNING: MATH AHEAD

The total resistance of two resistors connected in parallel is equal to:

1/R (total) = 1/R1 + 1/R2

Using 1/103 = 1/240 + 1/R2 and solving for R2, the answer is 180. Adding a 180 ohm resistor in parallel to the sender resistor will result in the fuel gauge seeing a resistance of 103 ohms when the tank is empty. This should result in the fuel gauge reading empty when the tank is empty.

Using the equation 1/R (total) = 1/33.5 + 1/180 the value for R (total) is equal to 28.3 ohms. The difference from 33.5 is minimal, and will result in the fuel gauge being pegged on full for a short period of time until the resistance increases to 33.5 ohms. Many cars exhibit this type of behavior, and I think it is an acceptable trade off to be able to see my fuel status below the half full level.

I purchased a 180 ohm, 2 watt metal oxide resistor. Given the intended use, I did not think a 0.5 watt carbon based resistor would be rugged enough.

The fuel tank currently has approximately 24 gallons of fuel in it (32 gallon fuel tank). The fuel gauge should be reading approximately 3/4 full, but is instead reading approximately 1/2 full.



I attached one end of the resistor to the sender post of the fuel tank sender and the other end of the resistor to the ground screw of the fuel tank sender creating a parallel resistor to the resistor in the fuel tank sender.



After attaching the resistor the fuel gauge reading changed from approximately 1/2 full to approximately 3/4 full.



The reading after the addition of the resistor is slightly above 3/4 full, but it is close. This upcoming season I will compare the fuel gauge readings before I fill the tank to how much fuel I actually add to the tank. If I don't feel the fuel gauge readings are close enough to the actual value, I may replace the resistor with a resistor of a different value. I won't know for sure until I have a chance to actually run the boat and test it.

Hopefully the above information will help anyone that is having calibration issues with their fuel gauge. The resistor can be added to the back of the fuel gauge (sender post to ground post) instead of the fuel sender if that is easier, but the access to my fuel tank sender was much better than the back of my fuel gauge.

Fuel gauges dont just “ go out of calibration”
I truly dont believe doing an electrical kludge repair is safe on a sensor inside a gas tank.

By design the voltage and current in that sensor is limited to “intrinsically” safe levels that can not generate enough energy to spark or ignite.

if the resistance of the sender checked out from low to high then there is a fault inside the gauge.

Replace the gauge.

I am serious about the hazard you are risking. I am an ASE Certified Master tech and former USCG MK1

If most are actually reading what ChipS is posting - I would be surprised.

A) He knows the sending gauge was replaced by PO
B) He knows what the resistance range is for E/F

He was trying to find out what the correct range was, to which hugespat57 provided - thank you.

C) Using the target resistance value, and the known value - he determined the appropriate resistor value to place in parallel. Awesome work!
D) The parallel resistor is NOT inside the tank - its outside, connected between the gauge sense wire and ground.

Yes, he could have purchased a new (correct) sending unit, but instead of spending $30-$75, he probably spent $0.05.

If he is not happy with how the gauge reads now, he knows the next step - buy the correct sending unit.

hughespat57 said:

I am serious about the hazard you are risking. I am an ASE Certified Master tech and former USCG MK1

Click to expand...

I have no desire to create an unsafe situation. I do not think I have for the reasons given below, but if I am wrong I will obviously remove the resistor. FYI, this approach is commonly done with antique cars.

Fuel gauges dont just “ go out of calibration”
The fuel gauge is the original 1993 Sea Ray gauge. Resistance values used in 1993 were not consistent with the current American Standard resistance values being used for the new fuel sender which was installed. Both the gauge and sender are working correctly. The issue is that the gauge and sender are using different resistance value ranges.

I truly dont believe doing an electrical kludge repair is safe on a sensor inside a gas tank.
There was no modification to the sensor inside the gas tank. The resistance values of the resistor inside the gas tank are identical with and without the added 180 ohm resistor. The sender inside the gas tank is seeing the exact same thing as before the 180 ohm resistor was added.

By design the voltage and current in that sensor is limited to “intrinsically” safe levels that can not generate enough energy to spark or ignite.
Voltage will remain the same. The current inside the fuel tank will remain the same, with the additional current going through the 180 ohm resistor which is outside of the fuel tank.

if the resistance of the sender checked out from low to high then there is a fault inside the gauge.
The gauge is working correctly, but is using a different resistor value range since it was manufactured before the American Standard resistance range was developed.

Replace the gauge.
Replacing the gauge with a fuel gauge using American Standard values would solve the problem. The new gauge would not match the rest of the gauges in the panel, and I would need to remove the helm assembly to get to it. Adding the 180 ohm resistor maintains the stock appearance of the helm panel and was much easier.

Adding the 180 ohm resistor does not affect the resistance values of the fuel tank sender. Adding the 180 ohm resistor changes the resistance that the fuel gauge is seeing. I do not see how this creates an unsafe situation. As stated above, this approach is routinely used with antique cars. I am willing to be educated if I am wrong, but I don't see anything so far that would indicate that adding the 180 ohm resistor creates an unsafe situation.

Chip, you mentioned that the previous owner replaced the sender. Looking at your picture, it appears to be the original sender.

How were you checking the "%" of sender arm movement? The sender does not look to have been removed - typically you'll see a clean spot around it where the owner wiped away dirt/grime to avoid letting that crap fall into the tank.

Unless I missed it, I didn't see where you measured the ohm reading at empty on the sender?

I'm pretty sure that the normal range of senders has been 33 to 240 for much longer than 1993. If that's not true, can you point me to that info? I'd like to learn/hear more about that for future reference.

Lazy Daze said:

Chip, you mentioned that the previous owner replaced the sender. Looking at your picture, it appears to be the original sender.

How were you checking the "%" of sender arm movement? The sender does not look to have been removed - typically you'll see a clean spot around it where the owner wiped away dirt/grime to avoid letting that crap fall into the tank.

I'm pretty sure that the normal range of senders has been 33 to 240 for much longer than 1993. If that's not true, can you point me to that info? I'd like to learn/hear more about that for future reference.

Click to expand...

I have the receipt from the PO where the fuel sender was replaced on May 7, 2018.

I remember seeing previously that the American Standard values were implemented in 1995. The IS0085d document I found indicates that the issue date was October 2002 (see attached). Either way is after the 1993 manufacture date of my boat.

I edited my above post to correct an incorrect statement. The original statement said that the current through the fuel tank would be less. That was incorrect. The current through the fuel tank after installation of the 180 ohm resistor would be the same as before the installation of the resistor.

Voltage = (current) x (resistance)

The voltage will remain constant. To make the math easier, I'm going to set the voltage at 12.

With the tank full the resistance inside the tank is 33.5 ohms. This will result in a current of 0.358 amps.

The current going through the 180 ohm resistor (external to the fuel tank) is 0.067 amps.

Since the fuel gauge is seeing the total current (0.425 amps), the fuel gauge is seeing a resistance of 28.3 ohms. The fuel tank sender, however, is still seeing 12 volts and 0.358 amps. There is no change inside the fuel tank itself. All changes are occurring in the electrical circuit outside of the fuel tank.

Thanks for the link on the values - I'll take a look at that. But, man, I could sware I've changed senders in boats older than yours with a 33/240 sender.

2018? Something is fishy there. There's no way that's a new sender.

I take it that you haven't pulled the sender out, then? Try that (remove the resistor)... check resitance by manually moving the sender arm.

But... didn't you also say that at a sender reading of 33ohms that the gauge was properly reading full? That would indicate a 33/240 sender.

FYI, it's very common/normal to see the gauge on E when there's still a decent amount of fuel left. This is because of the shape of the tank and/or because the float doesn't go all the way to the bottom of the tank.

Lazy Daze said:

Thanks for the link on the values - I'll take a look at that. But, man, I could sware I've changed senders in boats older than yours with a 33/240 sender.

2018? Something is fishy there. There's no way that's a new sender.

But... didn't you also say that at a sender reading of 33ohms that the gauge was properly reading full? That would indicate a 33/240 sender.

Click to expand...

Please let me know if you find any additional information on when the American Standard values were implemented. Having that information would be very useful.

I agree the sender looks old. The PO brought the boat to be repaired because the fuel gauge wasn't working. The marina where he took it charged him $140 to replace the fuel sender. It also looks like a new ground wire was run. My measurements of resistance values at the fuel sender indicate that it is a 33/240 sender. A definitive answer on when the American Standard values were implemented would clarify if the sender could possibly be the original sender. If the American Standard values were implemented after 1993 then it can not be the original sender.

I googled some, but wasn't very successful in finding that info. The only thing I can say is that from experience (assuming my memory is working), 33/240 has been in use at least as long as your '93.

I looked at the sender pic again... nothing that I see points to it being new or even newish.

But... to fully test a sender, you need to remove it. I have a feeling, while I'll give you huge props for the effort you've been putting into this, that you've been doing a lot of extra homework for not. Pull the sender and check it out - that way we're dealing with factual information.

Sorry been offline catching up...
I agree with @Lazy Daze that you need to pull the sender.

This is not just for you @Chip S but because others may also read this with similar issues.
The Previous Owner supposedly replaced the sender. So you started with an unknown. Did they use the "right parts"?
Yes they did. Your tests show that the sender reads 33.5 ohms full tank. That indicates the present sender is an SAE standard sender. (however it does not yet prove it was installed correctly, and this is likely key).

The confusion about other types of senders does not apply to boats. Period, no reasonable arguments possible.
I provided the link because you believed the PO may have installed a wrong sender. And that is always possible, but you disproved that.

Background,
Automotive manufacturers do not need to follow SAE standards, most do today as related to safety for liability reasons, but any other aspect they are free to do as they like. When an OEM is making their own dash instrument clusters they can use whatever sensors they choose. They are making 100,000+/- model units per year. No boat manufacturer comes near those unit numbers. Thus boat, heavy equipment and truck manufacturers typically used "standard" round 2" gauges. It was simply more cost effective.

So back to the sender and gauge.
Most boats and older boats with a need to replace the sender are typically going to by a "universal" sender.
These allow you to adjust the sensor height in the tank (half the distance from top to bottom) and the length of the float arm. The float arm length is best set to 1 inch off the bottom, which will also be 1" below the top. No tank pickup should be sucking right off the bottom, it will pick any dirt or water in the tank. And no tank is ever to be filled 100%, allow at least 5% for expansion.



p.s.
If the sender checks out good, and is properly adjusted, then the gauge should swing E - F, if not check the wiring to the gauge and/or change the gauge.