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Fuel Gauge Reading Changes With Engine Speed
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HaggardMelon
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PostPosted: Wed May 12, 2021 12:39 pm    Post subject: Fuel Gauge Reading Changes With Engine Speed Reply with quote

Hey Everyone,

So I'm having a weird fuel gauge problem. I bought a VDO Cockpit Series 2 1/16" gauge (V301020) as well as the recommending matching sending unit (V221012). They are both supposed to run in the 73-10 Ohms range.

If I fill the tank all the way up, it usually shows around 7/8 of a tank. The real trouble shows up when it gets closer to half tank (on the gauge, not necessarily in the actual tank). If the engine is idling around 800 - 900 rpm, the gauge will sometimes fall all the way down to empty. If I rev it up a bit it jumps back up to where it was in the middle. Sometimes, it will just fluctuate around at idle moving up and down and random intervals.

I'm getting at least around 12v or 13v at idle and none of my other gauges seem to have any issues.

It's important to note the wiring situation here. When I put the gauges in, I added my own little secondary fuse block.

I have a 10 gauge wire running directly from the battery up to an inline fuse, then to a relay (which gets switched on with the key). The relay sends 12v to the fuse block. All the gauges get their own dedicated power wire coming from the fuse block. They all ground through the fuse block as well which has a direct wire going back to the battery ground terminal.

I wouldn't think this arrangement would experience any issues as it's a pretty simple routing setup. Any advice on why the slight increase in voltage resulting from higher engine speed is needed for the gauge to work? I'm not even sure how accurate it is because I'm not sure if I can trust it. I end up putting gas in it every other time I go out. It usually will only take a few gallons too.

It's kind of a bummer because I was looking forward to having a reliable fuel gauge. I couldn't really get the original gauge in the speedo to work right.

Thanks!
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Starbucket
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PostPosted: Wed May 12, 2021 3:05 pm    Post subject: Re: Fuel Gauge Reading Changes With Engine Speed Reply with quote

I would pull the sending unit and check the OHMs from E to F as it might be your problem if the readings get erratic around 1/2 tank. A inline resister might be required but you would need to contact the Mfg. to find out what one.
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ashman40
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PostPosted: Wed May 12, 2021 10:20 pm    Post subject: Re: Fuel Gauge Reading Changes With Engine Speed Reply with quote

Does your tank sending unit have a dedicated ground wire of does it ground thru the tank itself? If it does not have a ground wire, try adding a jumper between one of the hold down screws and a good ground point (ideally the same ground as the fuel gauge). Make sure at the sending unit the ground wire make good electrical contact with the metal part of the unit. Some sending units have insulating washers below the hold down screws. If you place the ground wire on top of this washer it will ground the tank but may not ground the sending unit. Place the wire under the washer so it is in contact with the body of the sending unit.
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PostPosted: Wed May 12, 2021 11:16 pm    Post subject: Re: Fuel Gauge Reading Changes With Engine Speed Reply with quote

Sounds like a weak battery or dirty connections. You should start by cleaning electrical terminals. Start at the battery and then just keep going. You need to physically remove the wire ends and clean them. On the grounds. Clean to bare metal and coat with dielectric grease. Then reinstall. I bet the voltage, through the system, becomes more stable.
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bomberbob
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PostPosted: Thu May 13, 2021 5:05 am    Post subject: Re: Fuel Gauge Reading Changes With Engine Speed Reply with quote

Can I slip in here with a question? I am putting in exactly what the OP is, 10 AWG wire, from the battery to a relay, switched so all this current isn't going through the cheap aftermarket switch. I have not fused this wire, totally forgot about it. Where is the best place to put the fuse on the 10 AWG? Right at the battery? I would think so, that way if the wire rubs through anywhere along the wire right up to the battery, the fuse protects the rest of the wire. If I put it in the trunk area with the rest of the electricals, any short between the battery and the fuse will just get red hot and burn. No fuse protection. I am powering my oil pressure gauge, cylinder temp gauge, radio, backup lights, and probably the halo's on my LED headlights off this auxiliary fuse box. I am also going to relay my headlights so all that current is not going through the cheap aftermarket headlight switch.
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ashman40
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PostPosted: Thu May 13, 2021 5:32 am    Post subject: Re: Fuel Gauge Reading Changes With Engine Speed Reply with quote

bomberbob wrote:
Where is the best place to put the fuse on the 10 AWG? Right at the battery? I would think so, that way if the wire rubs through anywhere along the wire right up to the battery, the fuse protects the rest of the wire. If I put it in the trunk area with the rest of the electricals, any short between the battery and the fuse will just get red hot and burn. No fuse protection.

I think you meant "the wire" will get red hot?
Your thinking above is correct. Place the fuse as close to the battery (or power source) as possible.


bomberbob wrote:
I am powering my oil pressure gauge, cylinder temp gauge, radio, backup lights, and probably the halo's on my LED headlights off this auxiliary fuse box.

With all those things hanging off this one wire you will need a larger sized fuse (30A+?). Consider installing a resettable circuit breaker instead of a fuse. These are rated for a certain amperage. When that is reached they trip and open the circuit to protect the wiring. Do a Google search for "wire gauge amp ratings" and look for a chart on how much current a given wire gauge can carry at a certain length. Base your circuit breaker size on the chart recommendation. Often a shoot for a slightly lower amp rating for the fuse/circuit breaker so the current load never reaches 100% of the wire capacity.
One advantage of a circuit breaker is there is usually a "Test" button which shorts the breaker to make sure it trips. This is useful when you want to power off the circuit but do not want to hassle with disconnecting the battery cable. On the last few Beetles I've owned I've installed a circuit breaker on the wire coming off the battery positive terminal running to the VR or front of the car. Tripping this breaker quickly disconnects the battery without any tools. Resetting the breaker is almost as quick.

Here is an example of a 12v circuit breaker for autos from 50A to 300A.
https://www.amazon.com/ANJOSHI-Circuit-50A-300A-Protection-Inverter/dp/B07K4ZK4VP/
There are also auto-resetting circuit breakers but then you won't be able to use it as a disconnect switch as they don't have a test button to open the circuit.
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bomberbob
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PostPosted: Thu May 13, 2021 7:07 am    Post subject: Re: Fuel Gauge Reading Changes With Engine Speed Reply with quote

Yes I meant the wire would get red hot, did not come out that way. Not sure why I did not think of a circuit breaker, used them for decades in aircraft applications, I did not know they had these panel mount style. Looked up the current rating for the wire, ordered a 30 amp unit to match.
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HaggardMelon
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PostPosted: Thu May 13, 2021 9:31 am    Post subject: Re: Fuel Gauge Reading Changes With Engine Speed Reply with quote

Bomberbob, I'm glad you mentioned this. I feel kind of silly for having one of my inline fuses in the trunk. It makes more sense to have it back by the battery. I'll probably change that asap.

Another thing I didn't mention regarding my fuel gauge situation that may be relevant is, I have one of those quick disconnect battery ground terminals with the knob you turn to shut off current to the whole car.

Could that be causing an issue with the fuel gauge? As I said before, nothing else seems to have any issues other than this one gauge. Maybe someone else has experience with that being an issue?
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Starbucket
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PostPosted: Thu May 13, 2021 12:03 pm    Post subject: Re: Fuel Gauge Reading Changes With Engine Speed Reply with quote

Is the ground from the gage and the ground from the sending unit grounded together? This will complete the electrical circuit between the two units without outside influence.
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HaggardMelon
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PostPosted: Wed Jun 09, 2021 6:36 pm    Post subject: Re: Fuel Gauge Reading Changes With Engine Speed Reply with quote

Just wanted to throw an update out on this. I ran a ground wire from one of the mounting screws of the sender to a ground post in the fuse box. This seems to have cleared up all the erraticism. However, I think it's still showing around ¾ when it should be full. Hopefully, I can just bend the arm a bit on the sender to get a more accurate reading. Thanks again everyone for advice.
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ashman40
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PostPosted: Wed Jun 09, 2021 9:40 pm    Post subject: Re: Fuel Gauge Reading Changes With Engine Speed Reply with quote

jwuori1635 wrote:
Just wanted to throw an update out on this. I ran a ground wire from one of the mounting screws of the sender to a ground post in the fuse box. This seems to have cleared up all the erraticism.

The issue with ground points is that ground potential differs at different points around the car. The gauge assumes its ground is the same as the ground used by the sending unit. For maximum accuracy ground both the gauge and the sending unit to the same ground point. This way both measure the voltage change as a common difference from the same ground.
You can actually measure this by placing your voltmeter at two different ground points. A good digital MM will show a small voltage. This is the difference in ground potential between the two points. It means there will be a small difference in voltage measurements by the amount shown.


jwuori1635 wrote:
However, I think it's still showing around ¾ when it should be full. Hopefully, I can just bend the arm a bit on the sender to get a more accurate reading.

Many gauges have an adjustment/calibration screw. It may be down inside a hole in the back of the gauge. This allows you to adjust the gauge reading when used with different sending units or differences in ground potential or voltage.
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'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}
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HaggardMelon
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PostPosted: Sat Jun 12, 2021 2:03 pm    Post subject: Re: Fuel Gauge Reading Changes With Engine Speed Reply with quote

Ashman, thanks for the explanation on differing ground points. I hadn't really thought about how that could possibly confuse the gauge. That makes a lot of sense. They are both currently grounded in my secondary fuse box, but I don't think they are on the same post in there. Ultimately, the whole fuse box grounds directly back to the battery. Do you think it matters if they're on the same post in there? I'll probably do it anyway since it will only take a second though.

Also, I'll check for a calibration screw. Although I don't think I saw anything about that in the documentation for the gauge when I was putting it in. That would be really nice if there is one.

Thanks!
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