Fuel Sender Unit

cdxdillan · December 18, 2009

Hi, has anyone got any info with regards to testing the fuel sender unit on a 93 LS400.

steve2006 · December 18, 2009

Hi,

Before you go diving in there is a known problem on the 93-94 Instrument clusters that give incorrect readings on the fuel gauge and there is a relatively simple fix for it if you can solder or people on this forum who can do the repair for you.

cdxdillan · December 19, 2009

hi, can't seem to find anything, temp guage and temp sender yes, fuel no, maybe I'm doing something wrong?

steve2006 · December 19, 2009

Sorry you have lost me here, you cannot find your fuel gauge? or you cannot find the fuel sender unit?

The fuel sender unit is inside the fuel tank, access is by removing the rear seat base and upright to gain access to the fuel tank.

cdxdillan · December 19, 2009

Hi steve,

what I meant was I couldn't find any info on the forum about testing the fuel sender, however, since my last post I have found the test data in the workshop manual I have on CD, it was under the heading Combination meter (the instrument cluster to most people), all the info for testing the instrument cluster are there i.e fuel guage etc., it also includes all the tests for items that send info to the cluster i.e. fuel and temp senders. thing is the manual is for a 2000 car not a 93 though I suspect the resistance readings will be the same or very similar, when weather permits I will check things out and maybe post my findings.

Just one thing Steve, looking at the component layout for the fuel tank it would appear that access to the fuel pump is via the rear seat, and the access to the fuel sender is via the boot as this is located on the opposite side of the tank? is there a difference between the 93 and later car fuel tanks?

Nottingham Lad · December 19, 2009

Just one thing Steve, looking at the component layout for the fuel tank it would appear that access to the fuel pump is via the rear seat, and the access to the fuel sender is via the boot as this is located on the opposite side of the tank? is there a difference between the 93 and later car fuel tanks?

Mine is an 95 LS400 Series 3 and I can confirm that access to the tank and sender is via the boot and you only need to pull the upholstered panel out the back.

Where the pump is I don't know, could it be in the tank with access through the sender aperture?

Roy

cdxdillan · December 20, 2009

Thanks Roy,

you've confirmed my thoughts on the sender location, therefore, the fuel pump is on opposite side of tank and would be accessed by removing the rear seat.

chris vince · December 20, 2009

Thanks Roy,
you've confirmed my thoughts on the sender location, therefore, the fuel pump is on opposite side of tank and would be accessed by removing the rear seat.

Does your dash light up within 1-2 seconds from an overnight chill?

If not there is a very good chance that your guage problem is related to the dash lighting curse.

I have repaired dozens of cars with this problem, lots have had related fuel guage misreading, centre consul lighting failure and even radio lighting failure problems.

Chris Vince

cdxdillan · December 20, 2009

Hi Chris,

there are no issues with any of the guages lighting up there are also no problems with any LCD's or backlights, it's just the fuel guage it takes a long time to register. I will check guage operation from the fuel sender by shorting at the appropriate connections as per the manual when the weather improves.

By the way Chris I was going to contact you with regards to a quote for a repair on the usual temp/clock LCD problem on my better halfs 93 LS, what is your e/mail address?

steve2006 · December 20, 2009

There are 2 separate issues with the instrument panel, one as you know is the lighting up problem, the other is an inaccurate fuel guage both of these problems are caused by different faulty capacitors within the dash.

The point Chris and myself are trying to explain is that it could be your dash causing the fuel guage problem even though it is lighting up correctly.

If you need to contact Chris send him a PM.

cdxdillan · December 30, 2009

Hi,

thanks for the replies the fuel guage problem is now sorted, replaced 2 capacitors in the instrument cluster total cost of parts 27P from Maplins.

for the record, testing the sender unit is relatively easy using a multimeter.

working inside the boot,

remove 4 screws holding the top of felt rear panel then pull panel forwards, there is enough access without removing bottom screws and the panel altogether,

the sender is in the centre of the tank,

unplug the wiring connector,

there are 3 terminals on the sender, (the outer 2 are connected across a fixed resistance), the middle terminal is the wiper which sweeps across the fixed resistance, its position on the fixed resistance depends on how much fuel is in the tank, (google linear potentiometers for more detailed info).

testing across the outer 2 terminals the reading should be approx 300 Ohms (constant)

if the tank is say half full, the wiper will be swept approx mid way across the fixed resistor, therefore, testing between the centre terminal and one of the outer terminals the reading should be approx 150 Ohms, likewise, testing between the centre terminal and the other outer terminal it should also read approx 150 Ohms,

if the tank is full, using the above tests you should get 21 Ohm, 279 Ohm, tank empty 279 Ohm, 21 Ohm so as long as the readings between the centre and the 2 outer terminals add up to approx 300 your sender should be ok.

the tests are with the sender still fitted in the tank, to test a sender throughout its entire range (between empty and full) you would have to remove it from the tank and then move the float while testing.