Lagondaforum » Technical information

davidbracey	May 16, 2020 2:39 pm
Subject: Fuel senders I've finally decided to sort out my fuel gauge as it hasn't ever worked since I've owned the car. Before I tackled the LG45 I thought I'd have a go at my 1.5 litre Aston as it's got the same problem. The cable between sender and gauge is fine. The gauge moves from empty to full when I disconnect from the sender and connect to earth, which tells me that works. The sender measures a range of 0 - 60 Ohms when I move the float but the windings in the gauge are a bit 'scratchy' so I suspect it's past it. Questions 1.Is the 0 - 60 Ohm range I am reading likely to be correct for the car? Seems like an odd range to me. 2.Is there a way I can check the correct range for the gauge? 3.If 0 - 60 Ohm is correct, are replacements available? Thanks for any help that can be provided. David

davidbracey

May 16, 2020 2:39 pm

Subject: Fuel senders

I've finally decided to sort out my fuel gauge as it hasn't ever worked since I've owned the car. Before I tackled the LG45 I thought I'd have a go at my 1.5 litre Aston as it's got the same problem.

The cable between sender and gauge is fine. The gauge moves from empty to full when I disconnect from the sender and connect to earth, which tells me that works. The sender measures a range of 0 - 60 Ohms when I move the float but the windings in the gauge are a bit 'scratchy' so I suspect it's past it.

Questions

1.Is the 0 - 60 Ohm range I am reading likely to be correct for the car? Seems like an odd range to me.
2.Is there a way I can check the correct range for the gauge?
3.If 0 - 60 Ohm is correct, are replacements available?

Thanks for any help that can be provided.

David

h14	May 17, 2020 9:39 am
Subject: Re: Fuel senders Hi David, I believe the sender in my Riley RMB has an 85 ohm resistance, so 60 sounds fine. Reading your detailed findings, the gauge works, and I think you're saying that when the float is moved, you get a reading traversing from 0 - 60 ohms. If that's the case. I suspect your problem is a poor connection between the sender unit and earth. On my V12, I see a secondary wire has been added from the sender unit to the tank body, so it's not a rare problem! Indeed, as tanks are flexibly mounted via non-conducting materials, ie rubber and canvas, the actual earth path is primarily via the copper petrol feed pipes. If they've been replaced by plastic, there's your problem. In all cases the best resolution is to run an earth wire from the body of the sender (or of course earth terminal if it has one) to the chassis. If like my V12, there are plenty of screws into the chassis, for example holding clips for the loom, and petrol piping, that a terminal could be fixed to. Ideally, clean the chassis to bright steel and place the terminal first, before re-attaching the screw, washer & clip. Laurence

h14

May 17, 2020 9:39 am

Subject: Re: Fuel senders

Hi David,
I believe the sender in my Riley RMB has an 85 ohm resistance, so 60 sounds fine. Reading your detailed findings, the gauge works, and I think you're saying that when the float is moved, you get a reading traversing from 0 - 60 ohms. If that's the case. I suspect your problem is a poor connection between the sender unit and earth.

On my V12, I see a secondary wire has been added from the sender unit to the tank body, so it's not a rare problem! Indeed, as tanks are flexibly mounted via non-conducting materials, ie rubber and canvas, the actual earth path is primarily via the copper petrol feed pipes. If they've been replaced by plastic, there's your problem.

In all cases the best resolution is to run an earth wire from the body of the sender (or of course earth terminal if it has one) to the chassis. If like my V12, there are plenty of screws into the chassis, for example holding clips for the loom, and petrol piping, that a terminal could be fixed to. Ideally, clean the chassis to bright steel and place the terminal first, before re-attaching the screw, washer & clip.

Laurence

davidbracey	May 17, 2020 1:14 pm
Subject: Re: Fuel senders Hi Laurence, Absolutely correct about the earth. I discovered this myself and was just logging on to post about it. Thanks all the same. Best wishes, David

davidbracey

May 17, 2020 1:14 pm

Subject: Re: Fuel senders

Hi Laurence,

Absolutely correct about the earth. I discovered this myself and was just logging on to post about it. Thanks all the same.

Best wishes,

David

h14	May 18, 2020 9:25 am
Subject: Re: Fuel senders You're welcome, glad you got it sorted. Laurence

h14

May 18, 2020 9:25 am

Subject: Re: Fuel senders

You're welcome, glad you got it sorted.

Laurence

davidbracey	May 18, 2020 9:37 am
Subject: Re: Fuel senders It seems that basic auto-electrics is quite baffling for a lot of amateur mechanics but a few simple skills with a multi-meter would be invaluable. I don't think I have ever seen any guides or articles in the club mag that members might find useful.

davidbracey

May 18, 2020 9:37 am

Subject: Re: Fuel senders

It seems that basic auto-electrics is quite baffling for a lot of amateur mechanics but a few simple skills with a multi-meter would be invaluable. I don't think I have ever seen any guides or articles in the club mag that members might find useful.

h14	May 19, 2020 9:01 am
Subject: Re: Fuel senders Agreed. Part of the problem I suspect is that it's not "mechanical". Whilst that is correct, the reality is that many electrical problems turn out to be mechanical; a loose or dirty terminal, a broken wire, a sticking brush, etc. But there is still a perception that electrics are a black art, when really, most issues can be resolved just using common sense. As you say, using a multimeter opens up an easier route to diagnosis. One thing I learnt from the instructions was how to measure voltage drop, simply by setting to the low voltage scale and connecting the clips to the two sides of a connection ... I know I was surprised to see the needle move in a situation where I would have thought that impossible! Laurence

h14

May 19, 2020 9:01 am

Subject: Re: Fuel senders

Agreed. Part of the problem I suspect is that it's not "mechanical". Whilst that is correct, the reality is that many electrical problems turn out to be mechanical; a loose or dirty terminal, a broken wire, a sticking brush, etc. But there is still a perception that electrics are a black art, when really, most issues can be resolved just using common sense.
As you say, using a multimeter opens up an easier route to diagnosis. One thing I learnt from the instructions was how to measure voltage drop, simply by setting to the low voltage scale and connecting the clips to the two sides of a connection ... I know I was surprised to see the needle move in a situation where I would have thought that impossible!
Laurence