Burned fuel gauge

[sevrien]

Hi everybody

Last weekend my fuel gauge burned out for some reason.
Before that, the gauge wasnt reading acurate, and was stuck on full now and then.
Does anyone have an idea where to look at to fix it?

greetings

[MarkyB]

If it's from a later car I wouldn't bother, plenty of people have spare speedos and it's rare to have a problem with the petrol gauge.

[sevrien]

Unfortunatly it's a '51 MM series

[MarkyB]

Oh, is it the type that fits into the dash rather than in the speedo?

You could put an ad in the wanted section, I'm sure it's a rarity for them to burn out.

Have you taken it apart to see what happened?

[sevrien]

it is indeed the type that fits into the dash,
havent taken it apart yet.
I wondered if someone had the same problem and what could be the cause.

[phurn]

My guess would be some exposed wires that short together every now and then that carry toomuch current for the fuel gauge.

[sevrien]

I've measured the curren at the two connections (battery and tank) on the fuel gauge, it gives a constant 12 V
moving the floater of the sender doens't give much of a difference. I still have to check if the sender cable doesn't make a false contact somewhere so it doesn't cause a short circuit.

To be continued

[phurn]

voltage is a potential accross two points, not a current, which would be measured in amps anf *through* something, not accross it as you do with voltage, so you cannot check it in that way.

[sevrien]

Hello everybody,

I've done some more measurements.
The Ohm value between the sender body (ground) and the contact point for the cable going to the gauge variated between 3,5 and 83,8 from full up to full down.

When connecting the negative pole of a 12V power supply to the sender body and then measuring the voltage between the connection point of the sender and the positve pole of the power supply it gave a constant value of 12 V either with the floater up or down. (quite weird i guess)

Connecting a power supply to the gauge and turning the voltage gradualy up from 1V to 12, the gauge reacted quite fine, from empty easy to full.

There's no failure of the sender cable (no short circuit)

So for some reason, the sender gives a constant 12V to the gauge, and it just can't take it.

What should I do, replacing the sender? or is there another cause?

greetings

[//no.comment]

the reason your getting a 12v reading across the gauge is because this is what it should be (i think)
I don't know if you've ever herd of ohms law but it states V=IR (voltage = current x voltage )
The sender varies the resistance, which in turn changes the current flowing to the petrol gauge, the petrol gauge lever is then deflected more due to the increase in current (showing how much petrol is in the tank). from ohms law if the current changes inversely to the resistance the voltage can stay the same, this would explain the readings your getting.

the reason your getting a change in reading by varying the voltage input via a power supply you are in fact varying the current as well (voltage is related to current as shown above)

I don't know what the correct resistance values are for the sender is perhaps its not giving the full scale of resistance values, thus not enough current?

are you sure there's no short in the sender cable not even a little intermittent one this would cause the signal wire to be completely isolated from the sender, therefore it would not have any affect as it would b earthed along the wires length somewhere bypassing re sender

anyway tell me what you think, just a little thought that might help you solve the problem! I hope ive made myself clear, I might not be right, but its something to think about!

[sevrien]

Thanks for the replies,

in the end just replacing the gauge did the thrick.