Has anyone put a 75 fuel tank into an Alfetta?

[Sam]

I have a Megasquirt EFI setup on my Alfetta.

I'm using a 75 fuel tank because they have baffles and an internal fuel Pump which works well for my EFI setup.

As you would expect the fuel Sender that fits in the 75 tank is not directly compatible with the Alfetta fuel Gauge.
I thought both units (75 and Alfetta) would work on similar principles, so I thought there must be a way to make the 75 sender work on the Alfetta.

I'm having problems so I pulled apart the Alfetta sender and fuel gauge to so see how they work.
Without going into details, I could see how they work and was able to verify my understandings with a multimeter.
Its just a pretty straight forward Variable Resister setup and makes plenty of sense.

On the other hand I cant seem to make sense of the 75 Sender. A multimeter across the earth and the other wires just shows no resistance at all. I cant figure out how it's suppose to work.

The 75 unit is fully enclosed and  I don't want to start pulling it apart if I can help it.

I was wondering if anyone knows how these work?
Or maybe someone has used a 75 tank on an Alfetta before and got over this problem?


any ideas?

thanks in advance

Sam

[Darryl]

To answer your question I pulled mine apart 

Pulling it apart isn't to hard. The top "cap" that locks into the fuel tank just clips to the tubular lower section.

Inside the tubular section is a float that is only about 25mm shorter than the tube. This float has a recess to contain/seat a spring at the bottom and another at the top.

There is an adjuster on the spring seat at the bottom.

The top spring recess in the float has a resistive track down one side, and a "empty" sensing contact on the other. At the bottom of the recess bot of these connect to a metal end cap that the spring sits on. A similar cap at the top of the spring connects to the centre contact on the termnals on the cap. The other 2 terminals on the cap are attached to a pair of wipers / contacts that slide up/down the resistive track on one side, and the empty sensor on the other. The resistive sensor is the "T" terminal, the empty sensor is "W".

The resistance between centre "-" and "T" should vary between about 50 and 300 ohms. It should be 0 ohms "-" to "W" when the tank is empty, open cct otherwise.

Hope this helps.

[Sam]

hi Darryl
you have helped a lot!!!

I bought this sensor from an Alfa wrecker down here in Melbourne.
I was told it came straight out of a car and it was checked and working fine, I had no reason to doubt them.

But after reading your message I'm starting to think they were wrong.

I took the following measurements with it removed from the tank and on my bench.

I checked resistance accross T & '-'  and its 0.8 ohms. Between W & '-' and its 1. (open loop?).

I put it into a bucket 1/2 full of water but the readings didn't change.

I'll take it back as soon as I can and I'll be taking the M/Meter with me next time

Thanks for your help

Sam

[Darryl]

I think they can suffer a bit buildup of fuel residue and/or corrosion and I'm not convinced you ever get a good stable reading/contact from them. A fuel gauge will tend to hold (really, decay slowly) the peak (least resistance) it sees and still work ok if the contacts are a bit intermittent.

Another detail - the resistive contact is actually made up as thin circuit board with a vertical resistive film strip *that the contacts don't touch* with a series of fine metal (plated copper I presume) horizontal tracks from it. Imagine rungs of a ladder where the ladder has one stringer (or whatever you call the side parts of a ladder) removed. The resistive film is the stringer. The contact "wiper"runs up and down the part where the rungs protrude. This would normally mean that when the contact was between rungs you would get an open circuit - but the wiper is bifurcated - 2 "fingers" one a tiny bit shorter than the other so they should each make contact in turn (ie one is on a rung, other is in a gap, mive 1/2 a rung and the opposite applies (its a ladder with a lot of rungs for people with strong toes...)

ASCII art attempt || is the track on the circuit board < is the contact..
   

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then as tank empties a fraction

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While I've got mine apart I'm going to try to clean fuel residue etc off. You or your parts vendor might need to do the same with yours.

EDIT:
Repairing 75 TS fuel sender.

I had checked the sender with multimeter before pulling apart and noted it was intermittent (even though it had been working when last in car) so thought it worth trying to improve. So I did this:

Pull top cap off (carfully prise over lugs on each side).

Remove top spring (just sitting there).

Try not to remove float as the bottom spring and the spring seat it sits on are a pita to get back in (I removed it and to replace used chopsticks..)
Note the float if slid up far enough (or removed) can rotate - make sure it is reinstalled in correct orientation (there is a guide in the outer tube that only extends part way up ot that a similarly partial length slot in the float fits around).

On the cap you will see the pair of contact wiper arms protruding like feelers (one for empty sense and one for level).

Clean the ends of these that make contact with the strips on the float.

At the bottom of the spring recess in the float is a metal spring seat.
The spring is being used as a "wire" from the centre contact down to this seat so you need to clean the crud off it to get it shiny/metal looking to get a good contact.
Do the same to the top spring seat on the cap between the feelers.
Clean the ends of the spring...

Carefully adjust the contact feelers if they look bent/uneven not "preloaded" enough to make reliable contact. But - be aware that you need to try to get the cap back on without munting these between top of float and cap instead of getting them to go down inside the spring/sensor recess in the float (guess why I needed to rebend/straighten mine).

Place top spring in recess in float.

With unit on its side (so float isn't bouncing on springs/falling due to its own weight slide float up (well horizontally really) so top is level with top of outer tube. Install cap (taking care to orient correctly) at an angle so one of the contact feelers goes where it should easily then pivot cap to get the other one in and slide cap onto outer tube.

I used a bit of everything to clean it - carby cleaner to start with then brake clean to remove residue, cotton buds on contact areas, plus a quick spray with electrical contact cleaner/lube at the end. I don't think there is anything terribly solvent sensitive to worry about (ie if it doesn't disolve the nylon body and float its ok).

And that worked, though I might have increased the contact tension too much - it sticks a bit - probably not a big issue once car is moving. I have a nice solid progressive reading other than the "sticktion" (needs a shake sometimes to move) testing in a container of kero.The outer tube has only a small hole for the fuel to enter/exit so it takes a while to respond to level changes (which is good when driving, annoying when testing).

[GTVeloce]

I rebuilt the sender I had as I wasn't confident of how well it was working. I have kept the standard tank in the alfetta GTV but welded the ring that holds the 75 fuel sender into the top. This way I can use the standard TS in-tank fuel pump and still have a working fuel gauge.

I am curious however, to discover if the tank out of a motronic fuel injected spider will fit as it appears to be quite similar in pictures and may not require the need for welding (always good when you are talking a petrol tank!). I have posted about this recently but with no luck.

The pics below highlight what Darryl was describing.

[Sam]

guys , thanks for your help, its been invaluable

I didn't get around to returning the sender, so I took your advice and cracked it open. (so to speak)

Yep it's a PITA to put back together!

I cleaned the residue off the swipe arms and I noticed one didn't look like it could be making goo contact so I bent it out more.

I tested it in a bucket of Water and I got between 300 ohms (empty) and <10 ohms when the bucket was full.
I dried it off very lightly with a hair dryer before installing it in the tank.

I put some fuel in the tank, but I didn't pay attention to how much I put in. I thought it  was close to 3/4 of a tank (there was some there before) but the gauge is showing a bit over 1/4 .  I hope the gauge is right and I'm wrong :-)

Anyway I'll go to the Service station tomorrow and fill er up. Hopefully it will show correctly on the gauge and I can move onto the final stages of the EFI build.

Sam