Fuel sender or fuel gauge at fault?

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MikeV8SE

MikeV8SE

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Hi all!

OK, my *last* problem (for now, I’m sure!) to sort on the RR is the fuel gauge, which doesn’t work.

Obviously I need to isolate whether it is the gauge or the sender that is at fault – gut feeling is the sender, as the guy I bought it off said the gauge worked when he put some fuel in for the MOT a week or so before I bought it.

Anyway, I read somewhere on here that the fuel pump and fuel sender are all one piece. However, in the trusty Haynes manual (£1.50 from Ebay, nice!) it looks like the fuel pump is on the top of the tank (access through the boot floor) and the sender is on the side of the tank, accessible from underneath the car. Which is correct? It’s a 1991 V8 if that helps.

Once I have located the sender, am I right in thinking it works like a resistor – so when the tank is full, the float moves up, there’s less resistance and the fuel gauge reads full? As it empties, the float drops, resistance increases and the gauge goes down. So, going back to school electronics, if I touch the two sender wires together, it should read full as there’s no resistance – or will it spark next to the fuel tank and end up in a horrible explosion?!

Any other advice on getting this sorted would be appreciated!!
 
MikeV8SE said:
Hi all!

OK, my *last* problem (for now, I’m sure!) to sort on the RR is the fuel gauge, which doesn’t work.

Obviously I need to isolate whether it is the gauge or the sender that is at fault – gut feeling is the sender, as the guy I bought it off said the gauge worked when he put some fuel in for the MOT a week or so before I bought it.

Anyway, I read somewhere on here that the fuel pump and fuel sender are all one piece. However, in the trusty Haynes manual (£1.50 from Ebay, nice!) it looks like the fuel pump is on the top of the tank (access through the boot floor) and the sender is on the side of the tank, accessible from underneath the car. Which is correct? It’s a 1991 V8 if that helps.

Once I have located the sender, am I right in thinking it works like a resistor – so when the tank is full, the float moves up, there’s less resistance and the fuel gauge reads full? As it empties, the float drops, resistance increases and the gauge goes down. So, going back to school electronics, if I touch the two sender wires together, it should read full as there’s no resistance – or will it spark next to the fuel tank and end up in a horrible explosion?!

Any other advice on getting this sorted would be appreciated!!
Click to expand...

First of all check for a live. If you put the live wire to ground you should get full deflection. If you then touch the wires together you should get full deflection (this proves the circuit). If you don't get anything it is a ground fault. If you get deflection sender is duff. PS don't hold wires to ground for a long time just long enough to prove the circuit.
 
Thanks for the reply! So to confirm, take off the +'ve and touch that to an earth somewhere (stupid question - where is a good earth around there? Chassis has been waxoiled so guess that won't be!!) - this should send the fuel gauge to maximum.

If this works, then touch the +'ve to the earth wire on the sender - again, this should send to +'ve if the gauge works. Do I need to disconnect the earth from the sender?

If both of these send the gauge to +'ve then the sender is at fault - correct?

Also, I usually disconnect the battery before messing with wires, but obviously that will stop it all working! Am I ok to leave it connected the whole time?

Cheers!
 
MikeV8SE said:
Thanks for the reply! So to confirm, take off the +'ve and touch that to an earth somewhere (stupid question - where is a good earth around there? Chassis has been waxoiled so guess that won't be!!) - this should send the fuel gauge to maximum.

If this works, then touch the +'ve to the earth wire on the sender - again, this should send to +'ve if the gauge works. Do I need to disconnect the earth from the sender?

If both of these send the gauge to +'ve then the sender is at fault - correct?

Also, I usually disconnect the battery before messing with wires, but obviously that will stop it all working! Am I ok to leave it connected the whole time?

Cheers!
Click to expand...

Yeah but further thoughts. It depends on how the thing is wired . Don't flash the live to earth just test for 12volts with a meter. Or you may just blow a fuse. The earth path maybe through the gauge to ground. So when you have found there is a live with volt meter touch the wires together and you should get deflection. If you do sender is duff.

PS You could also do a continuity test through the sender.
 
Last edited:
Whats a continuity test? Sorry, my knowledge of electronics goes back to school days, so is a little vague!!
 
MikeV8SE said:
Whats a continuity test? Sorry, my knowledge of electronics goes back to school days, so is a little vague!!
Click to expand...

You would need a continuity tester or a multi meter. It is a test to make sure there is a circuit through the sender. Obviously if there is no circuit it is duff. A basic sender would consist of a wire wrapped around a card connected to one side of the circuit and a wiper arm attached to the other side. As the wiper attached to the float moves up or down the coil it increases or decreases the voltage altering the the voltage through the gauge so showing the fuel level. If you have a multi tester you can do all the tests without putting anything live to earth. You could test the wires for voltage, earth continuity or circuit continuity easily without any danger of causing a spark.

PS. There is a very good vehicle tester sold by Machine Mart for around £23.00 it will do everything you will ever need on a vehicle. Clarke CA500.
 
Last edited:
Hi all,

Going back to this topic - my fuel gauge still doesn't work, so want to check the sender unit this weekend. Can anyone tell me where exactly it is located? The underside of my car has been waxoiled, so hard to tell what's what as it's all black now!!

Cheers!
 
MikeV8SE said:
Hi all,

Going back to this topic - my fuel gauge still doesn't work, so want to check the sender unit this weekend. Can anyone tell me where exactly it is located? The underside of my car has been waxoiled, so hard to tell what's what as it's all black now!!

Cheers!
Click to expand...

A 1991 should have a access point in boot floor, sender is part of pump. But i believe is replacable seperatly.
 
Thanks Wammers. I have accessed the fuel pump before through the boot floor access panel, had a fiddle with the wires but no loose connections. Cleaned up and re-wired the earth as that was quite corroded, but still no joy.

Haynes manual shows the sender seperate, on the side of the tank - was this for earlier models then? And does that mean on mine I will need to remove the pump from the tank to access the sender unit?
 
MikeV8SE said:
Thanks Wammers. I have accessed the fuel pump before through the boot floor access panel, had a fiddle with the wires but no loose connections. Cleaned up and re-wired the earth as that was quite corroded, but still no joy.

Haynes manual shows the sender seperate, on the side of the tank - was this for earlier models then? And does that mean on mine I will need to remove the pump from the tank to access the sender unit?
Click to expand...

As far as i am aware the sender is part of pump on the 1991 vehicle.Not sure about earlier versions. Think it was around 90-91 when the trap door was introduced, before that it did not have one. Think the fuel pump was on the chassis and sender was on side of tank. Maybe someone else has a better memory.
 
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