Fuel gauge - strange readings, any ideas ??

[Sonic ZS]

Was working all day yesterday and when I arrived in the car park in the morning, my fuel gauge was only just off empty and the light was on (I'd been keeping it low purposely, ready for removing the senders).

So went to the pumps in the evening and put in 10 litres (just over 2 gallons) leaving the engine idling again to clear the ice

This time, the needle did move as I added the fuel. So now the car had approximately 3 or 4 gallons in the tank (2 gallons added plus whatever was already in there) - so enough to slosh around and make the floats rise and fall.

Driving home, I decided to activate the IPK diagnostics for the fuel tank (19, log off, 6) to see what it was displaying. On a smooth, straight road it was showing roughly 0180-0200, which I believe is about 19.0 litres and this ties in roughly with the amount added plus whatever was already in the tank.

Now, knowing the two senders are connected in series, cornering the car should see the displayed amount stay approximately the same, because as one float drops, the other rises and hence the overall series resistance will remain pretty constant. Are you with me so far....??

The interesting bit was that on a left hand bend (fuel slops across to the right hand saddle/pump side), the figures would go up into the 0300's. Whilst on a right hand bend (fuel moves to the left saddle/filter side) the readings dropped to almost zero.

Makes me wonder if the O/S sender is working, whilst the N/S is fixed at a low resistance for some reason (jammed, missing float, sender track shorted, etc) ????

NOTE: I can't find any confirmation that 'our' system displays individual values for the left and right senders, it appears to give just an overall figure for both units, which makes sense as they are connected in series. Do we know if BMW models had their senders connected in parallel ??

Will hopefully get into the tank again for a more detailed look tomorrow

[SD1too]

An interesting exercise Paul, thanks.

Originally Posted by Sonic ZS

Makes me wonder if the O/S sender is working, whilst the N/S is fixed at a low resistance for some reason ...

How about wiring just one side at a time to the IPK and repeating the cornering test?

Originally Posted by Sonic ZS

I can't find any confirmation that 'our' system displays individual values for the left and right senders ... which makes sense as they are connected in series.

Exactly; you have the confirmation!

Simon

[Sonic ZS]

That's a very good idea Simon, thank you, I hadn't thought of that....

I can feel a special diagnostic lead being made up, that plugs in to run the pump, and allows each sender to be switched in or out as required....

[T-Cut]

Originally Posted by Sonic ZS

Now, knowing the two senders are connected in series, cornering the car should see the displayed amount stay approximately the same, because as one float drops, the other rises and hence the overall series resistance will remain pretty constant. Are you with me so far....?? The interesting bit was that on a left hand bend (fuel slops across to the right hand saddle/pump side), the figures would go up into the 0300's. Whilst on a right hand bend (fuel moves to the left saddle/filter side) the readings dropped to almost zero.

Hello Paul, you're really getting your teeth into this! I wonder how much flow over the central hump there will be with 19 litres total? Shortly after filling up, most (and perhaps all) the 10 litres added will still be in the offside.The tank plumbing includes a venturi driven equalising system to maintain a balance when the contents fall below the central hump. But it will take a period of driving for equalisation to be reached. Unless your turns were pretty fast and long, I wouldn't be surprised if the two halves behaved like independent tanks. So, the readings could simply reflect what each float is doing independently. I seem to recall that the float arms are orientated left to right, which could explain the results. You can see the same effect by manually rocking the car.

Makes me wonder if the O/S sender is working, whilst the N/S is fixed at a low resistance for some reason (jammed, missing float, sender track shorted, etc) ????

The Diagnostic Routine is supposed to tell us that, but you can easily confirm it as I suggestsed earlier.[/quote]

NOTE: I can't find any confirmation that 'our' system displays individual values for the left and right senders, it appears to give just an overall figure for both units, which makes sense as they are connected in series.

That was my conclusion too.

Do we know if BMW models had their senders connected in parallel ??

There's never been much info about the mechanics of the BMW system. It's clear that the adopted diagnostic routine needed some work that MGR didn't get around to.

Will hopefully get into the tank again for a more detailed look tomorrow

It would be great if you could check out the float fault codings as set out under Section 6.2

TC

[Sonic ZS]

A few more hours spent on this today.......and finally think I've located the problem !!!

Decided to look next at the N/S sender unit and after vacuuming all the muck from the top (probably not advisable on a petrol), I carefully opened it up. Managed to get the unit part way out of the tank before deciding it would be easiest to un-clip both the float arm section and the plastic pick-up part. Looking at my spare sender on the bench (to see how things came apart), the pick-up section just un-clips from under the top grey round body whilst the float section detaches further down, again with a small clip, but being careful not to lose the metal spring. With these separated, you can lift the main body up & out of the way, allowing the sender/rheostat part to be lifted clear into the car. at this stage, I didn't detach the wiring so it had to be examined in the car:



Using a multimeter to test the resistance, I ran the float arm up and down it's arc, but as it moves very easily on it's own, it was difficult to get steady figures in any one position.



So I decided to un-clip the metal arm from the plastic piece that holds the rheostat slider. Once the arm is out, you can temporarily re-insert it approx 180 degrees around, back into the slider pivot hole to align the slider again, but the weight of the metal arm and float now won't affect the readings i.e. once you move the slider, it'll stay exactly where you put it until you move it again so as to get a steady resistance figure (hope that makes sense ? )

Having previously tested the spare unit, I knew the resistance should range from about 5 to 190 ohms in reasonably linear graduations. As I started to gently move the slider up the rheostat, the figures I was expecting to see would be something like:

5, 12, 17, 22, 27, 33, 36......evenly up to......178, 182, 188, 193 ohms.

However, at approx one third of the way up my rheostat I saw:

67, 71, 95, 112, 120, 278, 5420, 5420, 345, 131, 90, 96, 102, 110, 118...

So after checking it twice in both directions, it was clear there was a section of very high resistance that shouldn't be there (look closely and you can just see the metal float arm is pivoting the slider but now isn't actually clipped to it):



So, I decided to swap this rheostat for my spare one - which is another job and a half....how on earth did Rover ever assemble all the pipe work, floats arms and cabling into the tank ??? Especially the two cable ties that clip the small black wiring tube to the fuel tube over the saddle bump...???

Anyhow, after about an hour of swearing and hands stinking of diesel, the new rheostat was in and connected and everything appears to work as it should once again

On the bench, I took a pic of the failing unit and enlarged it, and you can just see where the silver colouring on the tracks has worn away (not sure why this pic is so small ??):



Will be watching this for a week or two now, to see if the erratic gauge movements have gone ? But I'm guessing the previous owner must have normally had half a tank or less of fuel, and this has caused the extra wear on the lower section of the sender. Possibly a good argument, now our cars are getting older, to keep the tanks fairly full to help reduce the amount of movement of the floats ?

Also, could Rick-sta or DMGRS look into where the original circuit boards came from and see if there are any spares sitting around anywhere ? (Remember, the N/S and O/S boards are slightly different, and the wiring is a different length too ).

If anyone needs any other info, please ask. And I'll update in a few weeks once I've given the system a decent test

[COLVERT]

When I read, at the start of your post, when you said the arm moved very easily I immediately though --that sounds like something is worn.--( ie. the rheostat isn't being touched by anything. )

I hope your fix stays fixed for you now as you need a break from diesel fumes before you catch alight.

PS. Stay away from naked flames for a while.---

[T-Cut]

Another productive session from Paul. I wonder how a worn rheostat resistance strip causes the gauge to wander up/down over a relatively short period? is the worn area at the low end? If so you'd expect the wonky readings only at low tank contant, is that what happens?

Does this worn (abnormally high resistance) rheostat show up as code '2.6.2' in the diagnostic mode section 6.2?

Another bit of useful info would be a confirmation of the direction the float arms point (front/back or left/right).

TC

[Sonic ZS]

T-Cut, the worn section was approximately one third up the rheostat strip, so between about a quarter and the half full position. This ties in roughly with the odd readings, but I guess any slight movement of the fuel might cause the sender slider to briefly contact the 'dead patch', sending a spurious high resistance signal off to the gauge ?

What it doesn't explain is the time I pretty much filled the tank up (with the engine running...) but had no movement at all of the gauge needle until I switched off and on again ??

However, I was just glad to finally find something today that was definitely wrong, and also have the spare part to fix it !!

In diagnostic mode, this morning (with the faulty rheostat), it was showing initially 1_6.2 (all ok ?) then later it changed 2_6.2 (faulty sender ?), but this evening it's changed back to 1_6.2 and stayed there for about 35 miles.

I will fit the two new rubber seals in due course, if everything proves to be working correctly again, so will try to remember to look at float direction then

[SD1too]

Absolutely first class work Paul!

Given that this problem seems to predominately (if not exclusively) affect diesels, I am particularly interested in your hypothesis here:

Originally Posted by Sonic ZS

... I'm guessing the previous owner must have normally had half a tank or less of fuel, and this has caused the extra wear on the lower section of the sender.

Could this be connected with preventing the fuel level falling to a point where the in-tank pump gives trouble (so I remember reading some time ago)?

Simon

[Sonic ZS]

Hi Simon

I was simply thinking that because the wear was situated on the lower section of track, the slider must have rubbed here more than anywhere else, and that would equate to a tank that's commonly only a quarter to half full.

However, a full (or near full) tank will see little movement of the sender arms, as the floats will generally be forced up to their highest position - so less wear at the top of the track.

But I guess the natural movement of fuel from braking, acceleration and cornering will always give more float movement lower down the scale, as more empty space appears in the tank so the fuel can slop about.

Not honestly sure if diesels are suffering worse - after all, diesel is a lubricant, unlike petrol, so there should, in theory, be less wear ?

Could it simply be that 75/ZT diesels tend to do higher mileages and hence this wear is starting to show as many are at 150,000 miles or more now ??

Would be interesting to get mileages & fuel type from any member experiencing a gauge problem