Position of Fuel Gauge Needle - pics please

[songzunhuang]

So, one of the odd things left on my car is the position of the Fuel gauge needle. When I fill my car, it shows FULL for a long time and then only seems to start moving down when the tank is at 1/2 or something. A while back, I took the instrument apart and manually rotated the fuel gauge needle counter clockwise a bit hoping to get a more accurate reading. Well, I think I may have rotated it too much.

So I am reaching out to the community to see where other people's needle points. I thought I'd start with the position when the gauge is at rest. Here's where mine sits, in the red light "warning light" area.

Pointing in the warning light area when off.

Screen Shot 2016-12-12 at 12.51.26 PM.png (1.85 MiB) Viewed 5628 times

Should I rotate it clockwise to the red zone in the actual gauge area?

[sansouci]

Song,
When you start it, do you get the red warning light? BTW, mine falls slowly until half full and then drops off more rapidly thereafter. I had a problem once on the way to the airport: the red light came on and I thought I had 20 miles till empty. No such luck. More like 10. Oh well.
-Ken

[dwcains]

Perhaps this thread will help - note the "hidden" function of the OBC mentioned near the end of the thread:

http://www.bimmerforums.com/forum/showt ... as-reading

[songzunhuang]

Perhaps this thread will help - note the "hidden" function of the OBC mentioned near the end of the thread:

http://www.bimmerforums.com/forum/showt ... as-reading

Thanks for the info. I replaced my sender and think I have a bad solder joint in my gauge. I was having all kind of weirdness in my speedo, but after many repair attempts, the resoldering finally fixed it. I am noticing that tapping on the instrument cluster makes the fuel gauge jump around. This was the exact behavior I was having on my speedo right before the resoldering fixed it.

My 84 633 has the older style trip computer and doesn't have a 1000 button on it. I can get the remaining range function, but the last time I was stranded it was showing a good 30 miles range left. When I fill up it says I can get over 300-340 miles. I've never come close. The last time my reserve light came on I was at 276 miles. So, I don't trust it so much.

Reserve light comes on at 1/2 tank.

Reserve at half.png (1.59 MiB) Viewed 5606 times

Light on at 1/4 tank, more reasonable.

Reserve at quarter.png (1.3 MiB) Viewed 5606 times

Now, I noticed that when I fill my tank, normally at somewhere around 275 miles showing on the odometer, it takes 10-11 gallons to fill. The fuel gauge could be reading 1/4 or sometimes 1/2. Once my warning light came on when it was showing 1/3 tank. One other time I kept driving when the light came on and got stranded on a mountain freeway. I am very distrustful of the fuel gauge.

That being said, I spent $200 on a new sender and it's bugging me that I still can't get get a decent reading. So, this is the object of my obsession for now. BTW - I do not recall the warning light coming on when I start the car.

Also, my original question still stands. Where does your needle point when the ignition is turned off?

[dwcains]

Song:

Here's mine, with the ignition off:



Please note that mine isn't any more accurate than yours, and while I hate the expression "they all do that", it may apply in this case.

I've also got a spare E28 cluster I'm going to scavenge the gauges from so my E24 can have its correct Euro gauges back, and the position of the fuel needle is similar:

[songzunhuang]

Song:

Here's mine, with the ignition off:

Please note that mine isn't any more accurate than yours, and while I hate the expression "they all do that", it may apply in this case.

I've also got a spare E28 cluster I'm going to scavenge the gauges from so my E24 can have its correct Euro gauges back, and the position of the fuel needle is similar:

Ah, perfect. That's what I needed to see. I think I over rotated mine clockwise. Today as I was commuting, I noted that at 75 miles on the odometer with a new tank of gas, it's still showing full. I'm going to tweak it a bit when I have time. It must be better!

[Ralph in Socal]

Mine is in the same location as Dean. However I think there is a definite mechanical stop preventing it from going farther to the left. Location may not be all you need to gain accuracy.

Since my previous response to fuel gauge anomalies, I have been driving 2 other cars. Remember my previous response was with my daughters 86 53i5 auto and the fuel gauge woulld act exactly as yours. No movement for the first 150 miles from a full tank then sudden drops in gauge levels at an erratic rate.

The 83 635 euro provides what seems to be a much more accurate level readout. I can watch the needle slowly move as I am traveling. Same for a US 1988 528e that I have also been driving recently. Accuracy is much improved over the 86 535 but I have no idea how to explain why. My thinking is that if the 83 635 is accurate then there is a problem with the others if they have become erratic. The gauge setup on the 83 635 is not euro as I did swap out the tank with a US model so all tank side parts of the fuel gauge system is US while the instrument end is euro!! And it all works very well. I have not been adventurous enough to determine the reserve capacity of the car.

Spray contact cleaner??? Good Luck Song.

Ralph

[Brucey]

FWIW if you have a choice of believing the low fuel light or the gauge, believe the light; it is on a simple switch, so ought to be less likely to go wrong.

Although running the tank dry is a bad idea in that it risks sucking all kinds of crud into the fuel pump, I don't think it is otherwise a bad idea to test how far you can go once the light is on.

BTW IIRC the gauge reading is a balance between the current drawn through

a) the fuel level sensor (which pushes the needle to the right) and
b) a second current that is drawn to ground (and provides most of the return force for the needle).

( I believe this is indicated in the circuit diagrams BTW) If the gauge over-reads, I think the most likely cause is the current b) isn't high enough and this is usually a PCB fault, bad ground, faulty connection in the cluster.

Since the fuel gauge, SI display, and temp gauge all plug into the main board on prongs, and these prongs always look kind of iffy when you take a cluster apart, that'd be the first place to look for a duff connection IMHO.

cheers

[baders]

Song. It is easy to make a simple tester like below for testing all resistance based sensors. I made this when I was having problems with fuel gauge.

[songzunhuang]

Thanks guys, this is all great information.

Bruce, your explanation for how this gauge works makes me believe that my observations about bad solder connections (similar to my speedo), may have merit. When I have more time, I'm going to do the same thing to the fuel gauge PCB as I did for the speedo.

Bader, I purchased a new sender and measured my old one. They both check out fine so I'm looking elsewhere.

Ralph, I've cleaned all the spots I can think of. I'll do it again when I take the cluster apart for the millionth time.

I took this shot just to show the annoying behavior. You can see that it's still "Full" after 102 miles. What are the odds of that?

I don't think the tank is really full after I've done 102 miles.

Guage level.png (957.83 KiB) Viewed 5538 times

[jps635]

I have an aftermarket fuel gauge and even on a custom set up it behaves exactly the same. Shows full for the first 140km then drops at a uniform rate rather rapidly. I use the trip meter on the odo to fill every 400-450km

[panthercity]

The published tank size is 70 Liters or 18.4 gallons but my experience is that there is about 1 gallon of unusable fuel in the bottom. Recently I drove my '85 (Euro) until it was sputtering when going uphill due to fuel starvation. I happened to be close to a gas station and was able to fill her up without resorting to the spare fuel bottle I carry in the trunk. I was able to get 17.46 gallons in the tank on this occasion. Perhaps if I had been on a flat road I could have burned the remaining gallon in the tank but what I learned was that in practice I shouldn't expect to be able to use more than 17.5 gallons. Of course picking up debris from the bottom of the tank is never a good idea.

The warning light seems to reliably come on with 2~3 gallons remaining in the tank so I figure when the light comes one I've got about 40 miles left which normally leaves me plenty of road to find a filling station.

[songzunhuang]

The published tank size is 70 Liters or 18.4 gallons but my experience is that there is about 1 gallon of unusable fuel in the bottom. Recently I drove my '85 (Euro) until it was sputtering when going uphill due to fuel starvation. I happened to be close to a gas station and was able to fill her up without resorting to the spare fuel bottle I carry in the trunk. I was able to get 17.46 gallons in the tank on this occasion. Perhaps if I had been on a flat road I could have burned the remaining gallon in the tank but what I learned was that in practice I shouldn't expect to be able to use more than 17.5 gallons. Of course picking up debris from the bottom of the tank is never a good idea.

The warning light seems to reliably come on with 2~3 gallons remaining in the tank so I figure when the light comes one I've got about 40 miles left which normally leaves me plenty of road to find a filling station.

I guess for the USA it was different because everything I read says 16.8 gallons. I have grown so untrusting of the gauge that I normally look to fill the car when it reads 1/4 tank. This normally happens at about 275 miles on the odometer. Then it takes 11-12 gallons to fill it. I end up stopping for fuel way too often. My daily commute is about 50 miles round trip, so I can get through a week before fill up. The good news is that this is a steady state now so at least I know what to expect.

I suspect that there is something going on the the PC board of the gauge. On warmer days, the gauge seems pretty accurate. The there are times I know it's completely wrong. I wonder if it's suffering from the solder joint issues that had plagued my speedometer. Just a theory.

[panthercity]

Maybe the Euro cars have larger gas tanks? The euro gas gage is labeled 70 (liters) at full.

[dwcains]

The Euro's do have a larger tank. The US cars have a smaller tank because the larger US bumper supports take up more space inside the body.