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Discussion Starter #1
I can't figure out how to make my gas gauge work.

My 71 Spider has the benefit of a 'new improved'/homemade wiring system. :mad:

Until I can save up enough for an Auto Italia wiring loom, I'm going through it and making up my own 'new, new improved'/homemade wiring system.:D

Engine, Headlight, turnsignals, backup lights - I got em all: BUT

I don't have a working gas gauge. Well, to be more precise, I DO have a working gas gauge and a brand in-tank new sending unit, as well as an apparently working old in-tank sending unit. However, I can't get the gauge to work.

Rather than playing with electricity and gasoline at the same time, I have made a little test rack that holds the gas gauge and the sending unit, and I've hooked up my battery charger (fused @ 5 amps and set on low charge) and tried to figure out the electrical wiring by looking that the excellent wiring diagram that Papajam provided me.

On my test rig, the low fuel light works properly - when the float on the sender is lifted, it will go out. However, the gauge either does nothing or slams up to over max - in effect the rheostat on the sending unit acts as an on/off switch rather than increasing/decreasing the current flow enough to lift the needle in a graduated manner.

The way I BELIEVE that the gauge/sending unit should be wired:
There's a red lug, a pink(violet?) lug and a grounding lug on the back of the gauge.


I believe that the red lug should be connected to the fuse box; that the pink/violet wire should run to the sending unit and the ground lug on the gauge housing should be grounded. I then also think that the sending unit should be grounded. This circuit results in the gauge slamming to max or doing nothing. Additionally it make no allowance for the low fuel light in the circuit.

Now: I have two wires that run from the dash to the sending unit on the tank - a red wire and a pink violet wire. I've checked the continuity on them and they're unbroken. Obviously the ends at the gauge should go onto the matching lugs there.

I have two sending units - the one in the car ( I don't think it's original as it has the fuel pipe on it that's capped) which has 3 (three!) electrical lugs on it. It also has a grounding lug on the top of the unit. The 'new in box' I bought on ebay only has 2 (two :confused:) lugs and no grounding lug on it.

Further I note on the wiring diagram that the fuel pump (it's electric on the 71) should somehow be part of this circuit but it doesn't seem to be on my car.

Soooooo Help Please! How should this thing work? It's got me baffled.

Confusing me I have the original red wire
 

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1966-2013
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IIRC, and barring the pump or low fuel warning lamp, you have key activated power going into the sending unit, through the winding in the float to get variable resistance to voltage flow~through, then out from there directly to the gauge head where it goes through said gauge making the pointer move relevant to the voltage being passed from the sending unit, then out to ground.

Dunno how much of a match it is wire color~wise, (sender is the same 69~94), but there's a photo of the sending unit on a 80's spider up in the FAQ in that subsection that shows what wire goes to what terminal on the sender.
 

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Are the sending units and the gauge the same make? ie Jaeger or Veglia Borletti

The 3rd lug on the sending unit is for an in-tank fuel pump. Your 71 preceded these pumps by about 8 years.
 

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Lokki,
Maybe a couple ideas to test here:
- try using an ohmeter between the spade you believe is for the gas gauge (as opposed to the empty light), and make sure that the rheostat actually shows a gradual change in resistance. Do the same test for the spade you think is for the empty light and make sure you get an on-off behaviour. Measure between the spade and the body of the sender.
- try switching the wires between empty light and needle -> the behaviour of the needle you describe just makes me think you've hooked up the wire for the needle to the spade for the empty light ??
Although for sure, as papajam notes, you should be sure your parts match one another, and it's probably better to focus on the earlier two-spade sender.
I have some resistance readings for my sender in my notebook ('74) if you need to compare. Don't remember right now which manufacturer my stuff is but that's easy to check.
I'm sure the wires in my case are different colours so that won't help.
Cheers
Neil
 

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The resistance readings shown here I think are from a Veglia unit (off a Euro carbed car for sure).
 

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I dug out my notes for the resistances, FWIW:
Full approx 5-8 ohms
Empty 344 ohms.
I didn't take an intermediate reading like in papajam's pictures.
This is for a sender that works with a '74 vintage Jaeger GTV instrument cluster.
Cheers
Neil
 

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Lokki
Let's start with the basic principal of operation for any ALFA fuel gauge I've come across in the last 50 years:

a) One terminal at the gauge is connected to key controlled battery power via the fuse panel.

b) All terminals at the gauge are isolated from earth/ground.

c) The sending unit at the tank must have a well established connection to earth/ground.

d) The color of the wires between the tank sending unit and the gauge is moot but one should follow original wiring colors if possible. The integrity and continuity of these wires must be confirmed.

e) The operation of the system is similar with any manufacturer - the gauge is "live" at one end of its coil or winding ... the sending unit is earthed/ grounded and as the float arm changes position, the resistance to ground changes. This variance is transmitted to the gauge causing the needle to move. The exact amount of change in resistance can be different between manufacturers - the principal of operation is the same.

Your "extra" connection at the sending unit is (as previously mentioned) due to substitution of a later model unit.
 

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Lokki,
Maybe a couple ideas to test here:

- try switching the wires between empty light and needle -> the behaviour of the needle you describe just makes me think you've hooked up the wire for the needle to the spade for the empty light ??
Although for sure, as papajam notes, you should be sure your parts match one another, and it's probably better to focus on the earlier two-spade sender.

Neil
+1 on Neil's comment. I replaced my sender unit recently, and happened to wire the sender unit incorrectly: I plugged the low fuel light connector to the rheostat and vice versa. I observed similar behavior - fuel gauge either full or empty, while the low fuel light seemed to work OK.

I'm not saying this is the cause of your situation, but you might want to check this first - it's a quick check after all.

Good luck!

enrique
 

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My 74 has only one wire, that goes to the guage, which has 12 volts on one terminal. the resistance of the sender goes to ground and increases the current through the guage. it has no wires to the battery.
cliff
 

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Discussion Starter #10
Wow - Thank you guys! I had to run off on an unexpected business trip for the last few days, but I'm taking tomorrow off as my reward - I'll provide a full report of my advertures!

Thanks again

Lokki
 

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Discussion Starter #11
Success! Thanks to all. As it turned out, I had a mixture of things going on which combined to make this quite a puzzle as isolating any single problem didn't resolve things. After I went to absolute basics I got it figured out.

I now know, finally how much gas there isn't in the tank :D.

As it turns out, I had to rerun new wiring from the gauge to the sending unit for both the fuel level and the low fuel light. This is a problem unique to my car as is the fact that the gas gauge that I bought on eBay had the power in/sending unit color rings reversed (Red was where Violet should have been and vice-versa) Again a unique circumstance.

However there were a couple of other things that I learned which may be of use to future Alfisti who are having gas gauge problems.

1. The 2 lug sending unit and the 3 lug sending unit work differently, electronically. This means that a gauge designed for a 3 lug unit won't work with a 2 lug sending unit. (This was a problem in my case since I purchased an eBay gas gauge and a NIB never used 2 lug sending unit). Electronically, they didn't work together.

2. You MUST ground at the sending unit AND at the lug at the back of the gauge as well. The gauge has a 2-electromagnet set up that moves the needle based on the variance in the charge in each electromagnet. If your gauge quits working after you change the light in the gauge, check the ground at the back of the gauge.

3. The sending unit is a pretty simple piece of gear, and probably won't need to be replace in most cases unless something happens to the plastic float. The sending unit can be easily disassembled to clean and slightly bend the contacts and get an old one working.

4. A volt meter is your friend.

Again thanks to all who contributed here... much appreciated:)
 
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