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Old 15-10-2012, 03:43 PM
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Default Erratic fuel gauge

Have been searching for info on this but can't find a suitable reference. Problem is that the gauge shows at full most of the time and only briefly shows the correct fuel level. Luckily, the low level warning light is working. I did clean the nut some time ago in the cluster and the gauge worked ok for a while after that. There is only one sender unit in this as its a 316i so maybe this is kaput. I checked that there is power coming to the sender unit. All other gauges are working. Not a major issue but would like to get it sorted. I either need a gauge or a sender unit but which?
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  #2  
Old 15-10-2012, 05:35 PM
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i'd say its the nut again to be honest, make sure to clean all the connections with a bit of sandpaper
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Old 15-10-2012, 06:18 PM
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Will do, thanks age. Bit of a pain taking out the cluster---was hoping it might be the sender unit but not sure how one would test this.
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Old 15-10-2012, 06:56 PM
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if you are prepared to take the sender unit out of the fuel tank
and have a multimeter it can be checked.
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Old 15-10-2012, 07:10 PM
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I have taken the sender unit out and cleaned the connections on it xworks. I also do have a multimeter. Can one open this dip tube type unit as I think the float is inside it and could be sticking maybe? The gauge flickers to full though as if its a broken connection.
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Old 15-10-2012, 08:06 PM
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Before removing the sender you can do a resistance check to see if it's the
the source of your trouble.

Depending upon the type of sender you have, the 3 terminals may or
may not be marked. The marking are (g), (w) & (31). You are only
interested in two of them (g) and (31).
(terminal (w) is for the low fuel warning light).

If your fuel level sender hasn't got the terminals marked then pull back
the rubber boot on the sender connector to reveal the 3 wires conecting
to the sender.
There should be a brown/green, a brown/grey and a brown wire.
You are looking to identify the 2 terminals the brown/green and the
brown wire connect to.

Once you have identified the two terminals on the sender unit, set your
multimeter to do a resistance check (200 ohm position) and connect
the two multimeter probes to the two sender terminals.

A full fuel tank will read very close to 0 ohms (maybe 2 or 3 ohms).
An empty fuel tank will read close to 60 ohms.
And depending on how full the tank is the reading should fall somewhere
between 0 & 60 ohms.

If you get a reading anywhere between 0 and 60 and your fuel gauge is
still reading full then it's unlikely the sender unit is the problem (unless
your fuel tank actually is full to the brim).
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Old 16-10-2012, 02:29 AM
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Thanks for the clear write up on that. Would like to at least eliminate the sender unit as its an expensive enough part to buy if not necessary.
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Old 16-10-2012, 09:37 AM
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This should be 'stickied'
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Old 16-10-2012, 10:54 PM
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Quote:
Originally Posted by LaoisE30 View Post
This should be 'stickied'
yep
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Old 16-10-2012, 11:35 PM
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Cool

I don't know about the single sender ones but I've had trouble with the fuel gauge being erratic on two of my e30's turned out to be the wiring going into the sender on the left hand side of the tank extended the wires a little and haven't had a bother since!
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Old 17-10-2012, 06:27 PM
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Wouldn't have thought so here happy as the wires and connections look good. When I pull the connecter off the sender unit, the gauge then shows empty As a follow up to xworks instructions, it looks like the sender unit is ok. Resistance is reading at 55.6 with the tank empty and 04 with the tank full with the multimeter terminals connected to the brown/green and brown connections on the sender unit assuming I am reading this right. It does look like attention now moves to the gauge.
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  #12  
Old 17-10-2012, 08:31 PM
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It is starting to look like the fault may be in the gauge itself or in the dash
cluster. But to narrow it down a little further there is another test you can
do with the multimeter.

This test is to check the wire leaving the dash cluster and traveling all the
way down to the fuel tank sender unit is ok (brown/green).

Before you start you'll need to know the resistance again at the two terminals
of the fuel tank sender you've just checked. If the tank is still full
it can be better to use up a bit of fuel to drop the level back to half full or
anywhere other than full.

As you've seen from your checks the sender reads about 4 ohms with a
full tank of juice and 56ish ohms with the tank near empty. For the next
test ideally you want the sender to be reading 15 ohms or more (so
anywhere from three quarters full to nearly empty.)

The reason for this is if the brown/green wire had been rubbing against the bodywork
somewhere along it's run and was now touching bare metal, then when you go to test
the resistance along this wire you will measure 0 ohms. As a full tank of fuel also reads
close to 0 ohms then you can never be sure if the wire is reaching the fuel tank sender ok
or shorting to earth along the way.
At least if the fuel level has dropped down a bit the sender should be reading 15 ohms or more,
and then if you check the brown/green wire and find 0 ohms you can now be sure theres a problem
with the wire.

Once you've got your ohm measurment at the tank sender pins
(for arguments sake we'll say 30ohms),reattach the connector plug to the
fuel tank sender and make sure it's nice and secure.
Your now going to check the other end of the brown/green wire up at the dash
where you should find the exact same reading (30ohms).

To find the brown/green wire up at the dash you'll need to get access to
the rear of the dash cluster. Once you've done this you'll see 2 connector
plugs plugging into the rear of the dash cluster, a white and a blue plug
(there may also be a yellow and green plug if your car has an on board computer
or cruise control).

The plug you are interested in is the white plug (C2) as it contains the
brown/green wire leaving the cluster and heading for the fuel tank sender.
You are looking to identify terminal number 4 of this plug.
The picture below should help identify it. Looking head on at the
connector it's 4 up from the bottom on the left hand side...........



Unfortunately the holes in the connector are too small for the probe of
your multimeter to fit in to, so, you'll need a small strand of wire or a paper
clip to push into this hole to make contact with the pin inside. (gently)

When you have the wire/clip in place, set your multimeter to the
200ohm setting again, touch one probe off the piece of wire sticking out of
pin 4 and touch the other probe off the battery earth terminal or a decent
earth point on the car (ignition barrel is usually a decent earth).

You should get exactly the same ohm reading as you got when testing
the two pin's back at the fuel tank sender (+ or - 1ohm).

If you do, the wire from the dash cluster to the tank sender unit is sound
and the next step would be to swap in a different fuel gauge to the cluster.

(If your working at the back of the dash cluster while lying the cluster forward
on top of the steering coloum shrouds, it can help to place a rag/cloth over the
shrouds first to stop the clear perspex at the front of the dash cluster picking up
irratating scratches)
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Last edited by xworks; 26-10-2012 at 03:35 PM.
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  #13  
Old 20-10-2012, 07:04 PM
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Thanks for easy to follow instructions and online tutorial as well as saving me the price of a sender unit. Resistance at the sender unit and gauge connecter are similar as you suggested. Had driven about 100 miles yesterday and seem to have used up about 4 Ohms worth of petrol. The fuel gauge has been working since it was filled but I also cleaned/tightened the nut on the back of the cluster when I had it out. Many thanks for your help and I can probably set up now as a sort of certified tech.
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  #14  
Old 26-10-2012, 09:57 AM
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Can this be Sticky'ed? It is an excellent write-up
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  #15  
Old 13-07-2013, 04:14 PM
Jack Mahlatsi Jack Mahlatsi is offline
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Mine gives me 74.8 and the tank is somewhere at 1/4. Is this correct?
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