Automotive Fuel Gauge Technologies In Use?

The typical automotive fuel gauge has a float in the fuel tank turning a pot that's in series with an ammeter that's the gauge. The gauge is a dual coil moving iron type which is a bit unusual compared to a typical meter used in electronics. There's a good description of the system here.


If you measure the voltage on the wire between the sender and gauge, the system is essentially a voltage divider and you can measure a DC voltage proportional to fuel level.

This works most places where it's been tried, but one place it hasn't worked is a 2016 Toyota Corolla. Instead of a swing of 4 - 6 volts across the fuel range, we are seeing readings of less than a volt from an empry tank to a full tank. I'm not quite sure what's going on to account for this small range of voltage swing.

I'm wondering if the Toyota uses a different technology for reading the fuel gauge. I have read that ethanol additives in gasoline are wrecking havoc with fuel gauge senders, and some systems are energizing the system only for brief intervals while the gauge is being read. It's the same system but instead of being powered ontinuously, it's powered only when a reading is taken. Can anyone confirm this is the case on the Toyota or how wide-spread the use of this type of system is? Any info about other systems in use?

Thanks for any info you can share.

JonSea said:

it's powered only when a reading is taken.

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I can't confirm that, but it seems likely. The old-school gauge was quite current-hungry. With todays's electronics it would be a simple matter to reduce energy demands dramatically by passing a low constant-current through the sender periodically and measuring the voltage with an MCU A/D input. The MCU can store the result and update an analogue or digital gauge occasionally. AFAIK the Toyota Corolla uses stepper-motors in the instrument cluster. Not sure if one would be used for the fuel gauge though.

Ironically, Wikipedia talks specifically about the Toyota 2016 (FYI, that is rarely the first place I ever look. It popped up in a Google search).

These resistance sensors are also showing an increased failure rate with the incremental additions of alcohol in automotive gasoline fuel. Alcohol increases the corrosion rate at the potentiometer, as it is capable of carrying current like water. Potentiometer applications for alcohol fuel use a pulse-and-hold methodology, with a periodic signal being sent to determine fuel level decreasing the corrosion potential. Therefore, demand for another safer, non-contact method for fuel level is desired.

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Source: https://en.wikipedia.org/wiki/Fuel_gauge

If the signal is periodic, maybe you need to use a scope to get the correct voltages.

I read the Wikipedia article before, but I don't see where it says the pulse method applied to the Toyota. There's a picture of a Toyota gauge, but the text doesn't mention it.

True, but a scope on it would remove doubt.

jpanhalt said:

True, but a scope on it would remove doubt.

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True, but I'm helping a friend long distance with limited knowledge and test gear.

I'm wondering how common these systems are and if it's something we need to plan for in the "general solution."

Didn't realize it was long distance. Admittedly, I was a bit surprised with the Corolla part. I figured you more the Lexus or Mercedes SLC type.

There are plenty of Toyota fuel gauge problems and troubleshooting discussions on Google. I am sure you have seen them too. I have never owned a Toyota, so I don't have any shop manuals for that brand that I can reference for help. Generally speaking, I consider getting the shop manuals a relatively small and worthwhile investment for any car that I have owned -- except for my first VW 1300.

John

I only troubleshot ONE fuel gauge issue and that was in a 1968 vehicle and I wasn't very old back then and I may have read about the one in an 82 toyota, I do own a more modern Toyota (2000), but it's been healthy.

The 1968 and 82 had a "regulator". I think the 82 had fluid in the guage so it sloshing leaves it alone. You can tel, because when yu turn the car on or off, it takes a while for the guage to move. Steppers are used in a 2000 Impala, so they might stay where they are.

I know the '68 was pulsed every few seconds. The 1982 added an additional low fuel thermister.

Your odometer is probably your best fuel gauge in older cars (and parenthetically -- Hobbs engine time -- in airplanes too).

jpanhalt said:

Your odometer is probably your best fuel gauge in older cars (and parenthetically -- Hobbs engine time -- in airplanes too).

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I installed an **broken link removed** in my 1967 Cessna. It uses a flow-sensor in the fuel line going into the carb which contains a magnet in the turbine wheel and a hall-effect sensor. The instrument (Pic based) reads instantaneous fuel flow in gallons per hour, gallons consumed, gallons remaining, time to empty, gallons required to reach a GPS waypoint, etc. Takes the guess work out of fuel management. The oem float-based fuel gauges are next to worthless.

Turns out that the Toyota does in fact have a pulsed system as opposed to a continuously on system. I can't say what the on duration is, or the frequency but it turns out it's easy enough to deal with. If we take rapid measurements over a period of a few seconds and keep the periodic highest levels, we have a voltage level proportional to the fuel level. Very nice as this works for either a pulsed system or a continuously on system.

Can't you just use an edge-triggered IOC to take your reading for the pulsed system? It would be a small programming change for the continuous system.

JonSea said:

I'm wondering if the Toyota uses a different technology for reading the fuel gauge. I have read that ethanol additives in gasoline are wrecking havoc with fuel gauge senders, and some systems are energizing the system only for brief intervals while the gauge is being read. It's the same system but instead of being powered ontinuously, it's powered only when a reading is taken. Can anyone confirm this is the case on the Toyota or how wide-spread the use of this type of system is? Any info about other systems in use?

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My son drives a Toyota yaris... This only reads the fuel amount periodically.. We know this because if it reads when your going downhill you gain an extra bar on the gauge, and will be some time before it restores the gauge....

Most fuel gauge systems have a regulator.. It used to be 7 volt via a bi-metal vibrator but I'm sure they are in the 21th century now..

jpanhalt said:

Can't you just use an edge-triggered IOC to take your reading for the pulsed system? It would be a small programming change for the continuous system.

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I could, but the existing hardware will work fine with a little extra coding. Nothing has to happen very fast in this system so taking a few seconds for a reading isn't a problem.

If you take the highest reading during a few seconds, won't that capture the highs due to sloshing (assuming the actual on times may be several times a second)?

If you capture the value during every measurement period (or every other, etc.), then the averages should tend to cancel sloshing.

What I remember for the Chrysler. was it was on the order of seconds. I think the problem turned out to be the dash movement. I think it was a constant voltage pulse through the sender and the meter read the current. I know I put a lamp in place of the movement until we got a replacement.

Like here: **broken link removed**

So, bi-metalic like similar to a turn signal.