MkIV fuel gauge problems

[MkIV Lux]

The fuel gauge on my MkIV (AKL1417 /T-dash / under boot tank) no longer gives sign of life. No defect indications before. Has anyone had same experience? Where do I find the electric wire connections with the sensor?

[jbottini]

pull down fabric flap at top center of boot/trunk area when viewing car from rear. I believe you'll find tank leads there. Have you been doing any under dash work lately that may have losened wires? Check fuses also, although as I recall there are several gages on one fuse.let us all know how it works out.  While on topic...my gauge reads 1/4 tank below actual contant...any ideas?

[MkIV Lux]

quote:Originally posted by jbottini
   
pull down fabric flap at top center of boot/trunk area when viewing car from rear. I believe you'll find tank leads there. Have you been doing any under dash work lately that may have losened wires? Check fuses also, although as I recall there are several gages on one fuse.let us all know how it works out.  While on topic...my gauge reads 1/4 tank below actual contant...any ideas?
   

   No recent work done on the wiring. All had worked fine as usual when I been to Brooklands in July.
   My boot is fully alum lined. Don't think I can have access through there. Tank is the flat under boot version. Pick-up fuel line to pumps and return line are at front left of the tank. There is a rubber sleeved wire line that goes to front middle of the tank on top, but I cannot see the connexions to tank. Will have to have a second check using mirrors. Fuel sensor must be at front end of the tank, since when it is half full, the red "tank reserve" indicator lights up every time I accelerate quickly (fuel flowing away from sensor).

[ANF289]

quote:Originally posted by jbottini
   
pull down fabric flap at top center of boot/trunk area when viewing car from rear. I believe you'll find tank leads there. Have you been doing any under dash work lately that may have losened wires? Check fuses also, although as I recall there are several gages on one fuse.let us all know how it works out.  While on topic...my gauge reads 1/4 tank below actual contant...any ideas?
   

   Jim,
   My fuel gauge reads about the same.  Anyone know if the sensor is a simple float design?  If so, bending the float arm down a bit should compensate.   On the brighter side, based on recent GPS readings, the speedometer is surprisingly accurate!
   Art

[MkIV Lux]

quote:... based on recent GPS readings, the speedometer is surprisingly accurate!
   Art
   
   

   
   I can confirm. My speedometer is fully accurate!

[Mark IV]

Fuel sender is old style rheostat wiper design. You can test your gauge by grounding it to the chassis, guage should go to "full" when grounded. If the gauge is OK, test the continuity of the wires to the sender, if OK, the sender is suspect. You can repalce the sender with a newer solid state type unit.

[dkp_cobra]

quote:Originally posted by jbottini
   
pull down fabric flap at top center of boot/trunk area when viewing car from rear. I believe you'll find tank leads there. Have you been doing any under dash work lately that may have losened wires? Check fuses also, although as I recall there are several gages on one fuse.let us all know how it works out.  While on topic...my gauge reads 1/4 tank below actual contant...any ideas?
   

   
   Jim,
   
   I know that from my old VW Golf. It was a problem with a bad (corroded) connection to ground. With a new cable to ground everthing worked fine. The problem is that with the corroded clips the restistance becomes greater. Since the gauge messures the restistance the corroded clip is a "quarter of your tank".
   
   Hope this helps a little bit.
   
   Regards, Peter

[MkIV Lux]

I tried to read the wiring diagram provided in the Owner's Handbook and compare to the situation behind the dash (have not yet found what maintains the T-dash in the middle at the gearbox cover).
   
   
   
   There are five coloured wires to the fuel gauge, which if I interpret the wiring diagram correctly (using magnifying glass), have the following purpose (I will attached a pic I have drawn, on left the excerpt from wiring diagram, on rigth the fuel gauge as seen from behind):
   
   
   - top black: grounding of instrument light
   - top red: feed of instrument light
   - bottom black: grounding of fuel gauge
   - bottom light green: feed from voltage stabilizer
   - bottom green: feed from fuel sensor
   
   So far all connexions seem OK
   
   So I will have to dig further tomorrow.

[MkIV Lux]

Over night I received this friendly e-mail message from Horst in Germany (dart427):
   
   (translation) "..... under the rubber floor mat in the middle of the boot, you find a round shaped aluminium cover that is the trap to the tank sensor and its connection wires. You probably find corrosion there."
   
   So I'll dig into the boot floor tonight.
   
   Many thanks Horst

[1984MkIV]

quote:
   (translation) "..... under the rubber floor mat in the middle of the boot, you find a round shaped aluminium cover that is the trap to the tank sensor and its connection wires. You probably find corrosion there."
   
   So I'll dig into the boot floor tonight.
   
   Many thanks Horst
   

   
   Horst always uses the trap to measure the fuel using his finger ... [] []
   
   Another "simple and stupid" method was used on the Ducati Imola:
   
   
   
   You can see the fuel level shining through the unpainted stripe in the middle of the tank.

[MkIV Lux]

quote:...
   Another "simple and stupid" method was used on the Ducati Imola:
   
   You can see the fuel level shining through the unpainted stripe in the middle of the tank.
   

   
   dead accurate, and corrosion proof []

[MkIV Lux]

So following Horst's advice, I dug into the boot tonight, and this is what I discovered! Surprise that it had worked fine for so long!
   
   
   
   After cleaning the surface, the following three connections are found:
   - black: grounding
   - green: connected to the white spot sensor
   - yellow: connected to the red spot sensor
   
   
   
   
   Trying to clean the connections, of course the grounding connection broke off, while the red spot sensor connection is quite loose[][]
   
   So there is number of questions coming up:
   
   - the metal top plate looks like quite corroded and I guess I better have it replaced complete with the sensors, or is it one sensor with an input line (red spot) and output line (white spot)---> I assume the plate and sensor connections and the actual lever type sensor underneath are one part
   - where to source such sensor
   - meanwhile, if I fully isolate the three lines, can I drive safely without the grounding to this plate[?], in other words: is this grounding connection only grounding the sensor or does it have another safety function to ground the aluminium tank[?] I assume it is only grounding the sensor. Can anyone confirm this?
   At least meanwhile I could continue using the Mk IV, calculating miles to the next fuel stop in the absence of the gauge.
   [?]

[MkIV Lux]

quote:....
   - the metal top plate looks like quite corroded and I guess I better have it replaced complete with the sensors, or is it one sensor with an input line (red spot) and output line (white spot)---> I assume the plate and sensor connections and the actual lever type sensor underneath are one part
   - where to source such sensor
   - meanwhile, if I fully isolate the three lines, can I drive safely without the grounding to this plate[?], in other words: is this grounding connection only grounding the sensor or does it have another safety function to ground the aluminium tank[?] I assume it is only grounding the sensor. Can anyone confirm this?
   At least meanwhile I could continue using the Mk IV, calculating miles to the next fuel stop in the absence of the gauge.
   [?]
   

   
   Further investigation with my local garage reveals to me the functioning of the fuel sensor and its three connections:
   - black: ground-in connected to sensor plate
   - green: ground-out connects sensor (white spot) to the fuel gauge, wich measures resistance that changes with fuel level
   - yellow: ground-out connects sensor (red spot) to reserve indicator ligt on dash when reserve level is hit
   
   No "hot current" goes through these loops.
   
   All is very logical [^]
   
   So I can use the car while keeping in mind to fill up every 350 kms to be on the safe side, while waiting to find a replacement sensor.

[AKL 1333]

Hallo Constant,
   kannst du nicht einfach die Platte mit dem Sensor rausschrauben und  die Kontakte anfrischen und etwas auflöten?
   Liebe Grüße
   Jürgen

[MkIV Lux]

quote:Originally posted by AKL 1333
   
Hallo Constant,
   kannst du nicht einfach die Platte mit dem Sensor rausschrauben und  die Kontakte anfrischen und etwas auflöten?
   Liebe Grüße
   Jürgen
   

   
   Hello Jürgen,
   
   Yes I could take the plate and sensor out and repair it. But before doing so I rather have an alternative ready to close the whole, in case repair is tricky. Can't leave the petrol tank open and when it is open, after having drained all fuel it would be wise to have the remaining petrol fumes be neutralized by inert gas. So it's not that easy. Better have this done by a specialist shop, I guess.