Fuel tank sender issues

[Anrw]

I have a VDO (22401100024) dip tube type sender unit fitted which wasn't working when I bought the Haf. It reads full regardless of fuel level. The guage tests fine as does the wiring to it and the earth wire shows no reistance. The correct resistance range for the VDO sender is Less than 3 Ohms at full and 60 - 90 Ohms at empty. With no wires connected to the sender the guage reads full(with the ignition on) and when the sender is upside down (when not in the tank) with the float is at the top end (simulating full) it reads empty and when it is the right way round and the float is at the bottom (simulating empty) the guage reads full. My simple electrical knowledge suggest that if I reverse polarity of the two wires (sender body to trigger wire and trigger terminal to earth) the situation should resolve itself - sadly it still reads full!

When testing the guage with another swing arm/float sender from another negative earth vehicle the guage correctly reads empty with the float down and full with the float up.

Any ideas?

Nigel

[AustHaflinger]

Hi Nigel and welcome to the forum. Please include where you are as this is an international forum.

Of course the Haflinger did not have a fuel gauge as such only a low fuel (reserve light).

However I have basically the same system as you have installed in my Haflinger - never had an issue with it but I doubt I can be much help to you other than - download the wiring diagram etc for the VDO system and check against yours - your testing with a different style of sender and it working indicates to me your wiring is right, the gauge is working. If you have no luck checking against the specs for the sender, I would assume the sender is faulty and get a new one to match the resistance of the guage - sorry I cannot be of more help. As an aside, does your low fuel light still work - its uses a different sender.

[Anrw]

Hi many thanks for your response. I too had concluded that a new VDO sender might be the way forward. I wouild add however that I am getting a 4.1v feed down the trigger wire that doesn't make much sense to me? I hadn't realised that a fuel guage was not standard but strangely my tank (that looks original) has a clear flat space in the pressing which looks like a sender location. I do not have a connection to the low level sender - I assume the sender failed and it was replaced by a the guage arrangement.

[heinkeljb]

Some pictures of the offending pieces might help work out if things are original or replacement items. I had issue with my gauge after having had the tank out to fix a leak.
When I put things back together, the gauge did what yours is doing, or was it reading empty even when there was fuel(?), I can't tremember. In the end, I used various web sites to work out what the wiring should be and ended up adding new "seperate" pieces of wiring from the sender to the gauge and battery - it works at present on the new wiring. I might try getting the old connected at some point as it was working using that prior to me removing the tank for repair.

Someting like this:- https://www.youtube.com/watch?v=BE9Cr0fN2fg

Wire colour is irrelevant .

John

[Anrw]

John
Very many thanks for youir response. I am due a new VDO sender unit today from Merlin Motorsport and will try that. Failing success with that I will have the dash out again and photograph the guage wiring to see if it gives any clues and try a temporary wire from guage to sender. In my case the sender is earthed to the body locally and it may be that wired directly back to the earth post on the guage might be a way forward as this system relies on resistance readings and this might be critical. I'll report back when I have tested the new sender,

[Anrw]

The new VDO sender has arrived and gives the same OHM's reading as the existing one and displays exactly the same gauge readings - that is:
Ignition off the gauge reads empty
Ignition on with no sender connected reads full
Sender in simulated full position (float at top) reads empty
Sender in simulated empty position (float at bottom) reads full
The gauge has three terminals
1. The background light fed from the instruments light switch.
2. Sender wire (red in photograph). I have testerd this by bypassing it and the results are the same.
3. A feed from the ignition switch which continues on to feed the OP light and the charge light. This reads 12.6v

There is no earth lead to or vacant connection point on the gauge. If I fix an earth to the mounting threads there is no change to the readings. Whilst the earth for the gauge itself is via the sender I would have expected an earth for the illumination bulb? I assume then that the lamp earths through the gauge body or clamps - a bit flaky?

My only conclusion is the VDO sender and classic AC gauge operate under a different resistance range but on the other hand the previous owner said the gauge worked fine!

Very strange! Any further ideas?

[Anrw]

Two gauge photos

[heinkeljb]

Hi Nigel,

Try this link to a trouble shooting page for a two wire fuel gauge system. I am pretty sure that although the wire colours will be different, the connections would work if you get he correct wire going toteh correct place.

https://www.boatingmag.com/story/how-to ... uel-gauge/

Is you fuel send out of the tank? This would make trouble shooting easier, but if it is in the tank and then you could use a "WOODEN / PLASTIC STICK" (not metal in case you cause a spark)!!! to move he float arm via the petrol tank filler hole.

I remember mine gave me grief until I ran two new wires from the sender all the way to the dash and the gauge.

John

p.s. The two metal studs are where you make a ground connection, it is worth making a wire with ring tag on it to go from one of those terminals to a good ground. The bulb uses the metal body of the guage as a ground connection so only need the 12volt positive wire from the light switch to turn it on.

[Anrw]

John
Very many thanks for all your helpful and constructive suggestions. I have followed all the various wiring configurations and finally concluded that the AC gauge that I have is either not working or that is is not in a compatible resistance range for the VDO sender. I have therefore bought a new VDO gauge and it now works fine. Whilst the new gauge has simple graphics it does look a bit shiny/new and less period, but at least it works. Since I bought a new sender also I now have one surplus to requirements in my parts store! Problem solved and thanks

[heinkeljb]

Nigel,
Well done for solving the issue.
I know how hard it is to sort something out when you have limited knowledge and the thing is rare! Maybe you can open up the two gauges and swap the backgrounds? Usually, the gauges have the ring that holds the glass on "rolled" over and round a pip on the bodu of the gauge. Very careful and time consuming working a knife blade and then a small screwdrive in under and working your way around will allow you to open the guage (I have done this previously to replace the glass on such things).
You do run the risk of damaging the internals when you try to take the needle off to be able to swap the scale.

John