Smith fuel gauge goes by rev

[umbyfer]

hello,

my Smiths fuel instruments on elan sprint, goeas following the rev engine. it means that at at minimum rev the istruments works well indicating the correct level of fuel, as when the engine is off. But open gas at 1500 rev or more, the fuel gauge moves slow up to full. stopping the car at minimum rev the indicator goes down to the correct level of fuel.

I changed the fuel sender unit by a new one; i replaced the black wiring from sender unit to ground.
Why it happens? where is the problem? how to repair it?

thank you to all!!

[ericbushby]

Hi,
Behind the speedo and tachometer look for a small metal can about 4cm by 1.5 with three terminals on. This is the instrument voltage regulator. It takes the car voltage of 12 to 14 volts and produces a steady 10 volts out. Some old versions switch on and off to give an average of 10 volts. Maybe this is failing and giving too much output when stressed.
Mine is a Series 3 so there may be differences, but it gives you somewhere to start.
Eric in Burnley
1967 S3SE DHC

[umbyfer]

Hi, thank you for the advice. How can i check if it is faild?

[ericbushby]

Hi
If you can get at it, you can check the voltage at the gauge itself or at the fuel level sender. One terminal on the sender should be something under 10 volts but steady or switching as described before. the other terminal goes to the chassis.
Which ever system you have, watch it`s behaviour as you rev the engine. It should stay steady. If some thing changes as you increase the revs, I would suspect the regulator.
They are readily available or can be repaired if you prefer to do it yourself.
Best of luck
Eric

[bob_rich]

Hi

The instrument voltage regulators are sometimes difficult to check because the bimetallic switching action of the instrument voltage regulators switch between the battery voltage (which can go from 12V to as much as 15V) to zero voltage with the design set to average 10V on the long term. Full gauges are very slow responding perhaps taking many seconds to average 10V and so this slow switching action is fine for such gauges as fuel or engine temperature which generally do not change rapidly in a time period of 10,s of seconds. If you use a meter to check the 10V you may find the needle (or the digits on a digital display) keep changing with the switching action making it difficult to determine if it is holding an average of 10V.

Most users replace the bimetallic regulators with semiconductor units which hold a very accurate 10V for a wide range of input voltages. The semiconductors are small low cost and can be installed in the case of the old regulator
or many people now sell them ready assembled. They can be obtained for positive or negative earth applications.

If you type " 10V instrument regulator " into the search bar on this forum you should see a number of posts with hopefully useful ideas

Hope this helps best of luck

Bob

[umbyfer]

Hi,

thanks to all for the advices. Please note that ONLY the fuel instruments "feels - is sensitive" to the engine REV. all the others works well.
If the problem is the voltage stabilizer, why the other instruments works well?

[ericbushby]

Hi,
I do not have a drawing for the Sprint version, but on the S4 circuit diagram only the fuel gauge is powered from the 10 volt stabiliser.
The water temperature gauge is driven by a capillary bulb. Other instruments are not voltage sensitive.
Also I now notice that the 10v goes to the gauge first and then to the sender. The other sender terminal is connected to the chassis. As the sender resistor is probably the highest resistance in the circuit, the test I suggested would still work OK. I have edited my first post to correct this.
Cheers
Eric

[Craven]

Look at the relation between the increase in fuel gauge reading and the ignition warning light, if the gauge is steady until the ignition warning light starts to dim but after that point the gauge starts to rise then fairly certain the aforementioned voltage stabiliser has failed.
Water temperature gauge is a similar gauge just may be less sensitive, anyone know the resistance of the temperature sender.

[ericbushby]

Craven,
Thank you, I did not know that Sprints had an electric temperature gauge.
Eric

[billwill]

Hi,

thanks to all for the advices. Please note that ONLY the fuel instruments "feels - is sensitive" to the engine REV. all the others works well.
If the problem is the voltage stabilizer, why the other instruments works well?

Because it's the only instrument on your car that is SLOW and needs Stabilized supply.

[ericbushby]

That is a lot more that I didn`t know about Sprints. Thank you Bill
Eric

[umbyfer]

Hi,

thanks to all for the advices. Please note that ONLY the fuel instruments "feels - is sensitive" to the engine REV. all the others works well.
If the problem is the voltage stabilizer, why the other instruments works well?

Because it's the only instrument on your car that is SLOW and needs Stabilized supply.

is the diagram for plus 2? not for S4/Sprint...

[umbyfer]

hello,

i checked the voltage out of the stabilizer voltage and i fond 12,30 Volt with ignition light ON but engine not running; then i checked again with running engine and the voltage goes up to 14-15-16 volt in relation to the rev.
I assume it is fault and must be replaced.
Here the pictures of my Smiths voltage stabilizer.
i don't know what is the "grey cylinder" on the ground wiring, it seems a condenser as a filter...

[ericbushby]

There you go. Fit A new one.
The `cylinder` will be a suppression capacitor and may not be needed with a modern electronic regulator.
It appears to be marked `Dubilier` who are capacitor manufacturers. It will do no harm to leave it in there.
If you are into these things it is possible to fit modern components inside the box, but they are not expensive to buy.
Eric in Burnley
1967 S3SE DHC

[umbyfer]

There you go. Fit A new one.
The `cylinder` will be a suppression capacitor and may not be needed with a modern electronic regulator.
It appears to be marked `Dubilier` who are capacitor manufacturers. It will do no harm to leave it in there.
If you are into these things it is possible to fit modern components inside the box, but they are not expensive to buy.
Eric in Burnley
1967 S3SE DHC

Great! tank you. Any suggestions where to buy it?