TDi Fuel sender weirdness

tobee1k

tobee1k

Member
I have a late 2002 75bhp TDi. For a while now, I've had issues with the fuel gauge - when I filled the tank, the needle would go to the top and would accurately come down until it reached the half-full mark, it would then just sit there. On Saturday, I finally got round to doing something with it. To cut a long story short, despite trying everything suggested on posts on this forum (cleaning contacts), I couldn't get anywhere.

A stroke of good fortune - a similar age TDi was in the local scrap yard. I nipped over there, and swapped my faulty sender with the scrap one. The fuel level after all my fiddling about, and the drive to the scrap yard, was below half and I was very pleased to see the gauge show the correct level - the new sender would appear to be working.

I drove about 100 miles to climb some hills on Sunday and kept an eye on the fuel gauge which seemed to behave itself - slowly moving towards empty at about the correct rate. On the way home, I stopped and filled up with diesel - alas, the gauge only went up to the three quarter mark and has not budged from there over the 40 miles or so that I've done since.

I have a theory as to what is happening and I'm hoping some smart person can tell me if I'm right. Is it possible that I've got the sender sitting in the wrong position? Looking at the shape of the fuel tank, I wonder if the little arm with the float (?) is pushing up against the inside of the tank somewhere and is not able to extend fully to indicate a full tank? I saw a little arrow on the plastic top of the sender but couldn't see what that was supposed to line up with, is there a corresponding mark on the floor of the car that I missed?
 
You should find another arrow in the area surrounding the sender. The two arrows should point toward one another.

RAB

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Ah that's a thought - I don't know. The car I got it from was missing its number plates and had no bonnet, so I couldn't see the grill. It was a TDI with silver TDI badge (as opposed to red), this makes it a 75bhp car, right? So it should be pre-November 2003 (I believe this is when the 90bhp came along). Looking at the two sender units, they appeared to be identical. Do you know when the tank sizes changed?
 
OK thanks guys, I'll have a check when I get home. Does kind of sound like I've got a 34l sender in a 42l tank though!!
 
tobee1k said:
OK thanks guys, I'll have a check when I get home. Does kind of sound like I've got a 34l sender in a 42l tank though!!
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To be certain, you could revisit the car in the scrap yard, and read its VIN under the little storage cubby behind the handbrake and compare the relevant bits to Rab's link.

Andy
 
Andrew said:
To be certain, you could revisit the car in the scrap yard, and read its VIN under the little storage cubby behind the handbrake and compare the relevant bits to Rab's link.

Andy
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Or just look at the exhaust (if its still on the car) straight ish = small tank, tight bend = big tank (to get round the bigger tank).

Note this thread seems to imply that the senders work in opposite ways, so it would be very obvious if it was the wrong one:

Fuel tank size

Guys, Did a search but had no joy. What years had the smaller tank and which had the larger tank? Is this the same for petrol and TDi models? Cheers, S
www.a2oc.net
 
And just to add more confusion the tank sensor reading the opposite way around did NOT happen at the same time as the larger 42 litre tank
There was about 4 months of small tank with send ready like all the larger tanks worked
I’m not sure if the sensor unit for the 4 month period is the same as fitted to the larger tanks
There must be a part number on the sender unit
Cheers
Paul


Sent from my iPhone using Tapatalk
 
Hi guys, didn't get time to look at part numbers last night but from what you're saying Paul, I think I've maybe got a sender or tank from that 4 month period. The sender isn't reversed - i.e. it's not showing empty when the tank's full, it's just that a full tank only registers as 3/4 full. Actually maybe seems like I've got a 34l tank and the replacement sender is for the 42l. If that is the case, and it's not an impossible job, I might go back to the scrap yard and get the tank too, having a larger tank would be handy. I'll confirm with part numbers though. Thanks for all the input.
 
You will need the exhaust as well as the tank and I would think changing the tank is not a nice easy job. Look oh Ely need new mounting straps and fixings which is starting to sound like an expensive job for a few extra litres of fuel especially when you are getting around 60mpg

Paul


Sent from my iPhone using Tapatalk
 
tobee1k said:
It was a TDI with silver TDI badge (as opposed to red), this makes it a 75bhp car, right? So it should be pre-November 2003 (I believe this is when the 90bhp came along).
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If it had a silver 'I' on the back, then it's a TDI75 (assuming the badge is factory original). However, be aware that the TDI90 did not replace the TDI75, as the quote above implies. The TDI90 ran in parallel with the TDI75 for a few years. Although all TDI90s were made with the larger fuel tank and the later fuel level sensor, these changes were totally independent to the introduction of the TDI90.

tobee1k said:
Does kind of sound like I've got a 34l sender in a 42l tank though!!
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There are two types of fuel level sender, and the correct one of your car is determined by the instrument cluster, not the fuel tank. Both senders will work in both sizes of fuel tank, but if you've got the wrong one for your car then the fuel gauge will read backwards.
It is a common misconception that the later sender was issued with the later, larger fuel tank, but this isn't the case. The fuel sender and instrument clusters were updated in one stage, and the fuel tank volume was increased at a later stage.

tobee1k said:
The sender isn't reversed - i.e. it's not showing empty when the tank's full, it's just that a full tank only registers as 3/4 full.
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This verifies that the sender is the correct one for your instrument cluster. However, given the continuing issue, either the sender is faulty/dirty/clogged/corroded/etc, or there's an issue with the wiring between the sender and the instrument cluster, or there's a fault with the instrument cluster.

tobee1k said:
I might go back to the scrap yard and get the tank too, having a larger tank would be handy.
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I think you've already looked into this and realised that it's a massive undertaking for very little gain. :)

Cheers,

Tom
 
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My money would be on corrosion in the plug or bad wiring
Two different sender units both working the correct way and both giving different readings suggests a plug/wiring fault to me
Assuming the sender unit is installed correctly. Arrow to arrow that is

Paul


Sent from my iPhone using Tapatalk
 
My money would be on corrosion in the plug or bad wiring
Two different sender units both working the correct way and both giving different readings suggests a plug/wiring fault to me
Assuming the sender unit is installed correctly. Arrow to arrow that is

Paul


Sent from my iPhone using Tapatalk
 
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