Ferrari Mailing List: Re: fuel gauge on 308

[Rick Lindsay (richardolindsay]

Thanks Francis,

For completeness, and because a few of us also own LBCs (Little British Cars), here's the rest of the story, hinted at earlier.

>> 1. The fuel gauge in a 308 is a galvanometer... (BTW, most

>> British car gauges work by a completely different principle.)

The Smiths gauges in LBCs are not galanometers. Rather, they are electrically heated bi-metal strips with gauge hands attached. When a given amount of voltage is applied to the gauge, the heater (a coil of wire wrapped around the strip) warms and causes the strip to bend. The end of the strip moves the indicator hand across the face of the gauge and; Voila! A gauge indication!

Anyone unfamiar with the bi-metal strip is referred to this explanation.

http://en.m.wikipedia.org/wiki/Bimetallic_strip

So what alters the voltage, and therefore the gauge indication? That is the job of the 'sender' and it can be in about any configuration. Temperature senders, oil or coolant for example, use a material with a negative thermal coefficient. That is, it conducts more at higher temperature. That is atypical but quite useful to us. So a temperature sender in a car conducts more electricity when hot, allowing the heater on the bi-metal strip to get hotter and the gauge indicate higher! The fuel gauge sender has already been discussed. 

One last topic: Stabilization. Without voltage regulation, a weak battery would cause abnormally low gauge readings, since the supply voltage would be low. Alternatively, an energetic alternator might provide excessive voltage resulting in high gauge readings. Both of these problems are negated by using a voltage regulator. These little devices hold the AVERAGE supply voltage at about 10 volts.

I write 'average' voltage because Lucas voltage stabilizers do not produce a constant 10 volts. Rather, they supply the same voltage as input but pulse on and off so that the AVERAGE over time is 10 volts. The 'duty cycle' changes with the input voltage. More time 'on' when the supply voltage is low. Less time on when its high. Quite a clever idea for a tiny little box bolted behind the dashboard! Since Smiths gauges move slowly, because of the time required to affect the bimetal strip, the regulator pulses are not observed on the gauge.

A common error when troubleshooting Smiths instruments, senders and the voltage regulator, occurs when using a digital VOM (Volt-Ohm Meter). Digital VOMs also pulse or sample the incoming voltage at some rate. And the sampling is not synchronized with the voltage stabilizer! The result is numbers all over the scale, sometimes reading when the voltage is on and sometimes when its off!

An analog meter connected to the stabilizer will show the regulator's pulsing as indicated by swings of the meter hand. How violently it swings is a function of the meter's 'damping'. (Its like a shock absorber built into the meter.) In short, neither meter will correctly indicate the average voltage but an analog meter will verify that the regulator is working.

I'm done now. Seriously, its never a good idea to encourage me.

-rick

-------- Original message --------

From: francis newman

Date:12/21/2014 1:55 PM (GMT-06:00)

To: Rick Lindsay

Cc: francis newman , The FerrariList

Subject: Re: [Ferrari] fuel gauge on 308

Great info Rick,

I will store that one away for future use - if needed.

Francis

On 21 Dec 2014, at 12:28, Rick Lindsay wrote:

> Regarding Ferrari gauges...
> 
> I have experienced only two kinds of gauge errors that were actually AT the gauge; total failure and indicator sticking at the minimum position then popping up when vibrated. The kind of error reported is almost certainly at the sender.
> 
> Comments and tests, using the details provided:
> 
> 1. The fuel gauge in a 308 is a galvanometer. Basically, its a voltmeter. If it goes to 'E' at rest, e.g. the ignition off, the internal mechanism has not failed, the gauge is not the problem. (BTW, most British car gauges work by a completely different principle.)
> 
> 2. It is very common for the sender to bind at some point. I have found this to happen with an old car where condensation has caused corrosion on the sender at a position where the arm long set. Witness my R-R's fuel level sender.
> 
> 3. The senders is just a variable resistor. It is a coil of wire with a contact that slides along the coil. Its position is set by a float on a stiff wire arm. As the fuel level changes, the float position changes as does its attached wire. Up gives low resistance, down gives high resistance. Ever notice how the apparent fuel level changes when cornering? The float is going up and down causing the resistance to change. 
> 
> 4. With the ignition on, unplug the harness wire from the sender. The gauge should go to 'E'. If it does, the sender is your problem.
> 
> 5. Senders are typically easy to repair. At least they are in the cars that I have owned. Just remove the retaining screws and manoeuvre the unit out. It will be stuck pretty good at the gasket. Also, it has that long arm and float attached so a bit of really careful twisting and turning is required.
> 
> 6. With the sender on the workbench,  slowly move the float through its full range if motion. I suspect you will find that it binds in that lower position. Clean the pivot and carefully clean the resistor. For the latter I recommend only a blast of brake cleaner. Once smooth full range of motion is achieved, oil the pivot and reinstall the sender.
> 
> 7. I don't know if new gaskets are available. I just cut my own from parts-store bulk gasket material. Those shops also sell fuel-safe sealant. Its a gasket dressing that is not attacked by fuel. That's what I use on the gasket, to reseal the sender to the tank. Certainly don't want any fuel leaks there to prove Clyde right! :-P
> 
> 8. You may find that the sender is held in place with a retainer ring rather than screws. Procedure is the same except for attachment method. There's probably a ground wire on the sender too, and a low fuel warning light circuit. Just keep the wires and connectors on their correct positions and the rest is pretty easy. A photo or three in advance aids with reassembly and reconnection. 
> 
> 9. The 308 is more forgiving than say a Testarossa, with regard to parts removal and replacement. I expect the sender can be removed with the tank in place, but I can't guarantee that. When I rebuilt my Testarossa - as Clyde can attest - everything has to be assembled in the correct order or that last screw, bolt or nut can't be fitted! I learned this the hard way! I swear, at times it felt like the radio knobs needed to be fitted before some of the FI parts would go on! I bet I assembled that engine five times!
> 
> 10. So to summarize, remove the sender wire from the sender to see if the gauge goes to 'E'. If so, the sender needs to be cleaned, repaired or replaced. See above.
> 
> -rick
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