Fuel gauge and speedo problem

[wraithghost]

Hi, My fuel Gauge seems to read OK before I turn the key but as soon as I turn the ignition it goes way past full out of view (I have a fullish tank). I only just got the car and I think they had the dash out a few times could it be that they have just cross connected something hopefully I am right in thinking without turning the key it should read way past empty and when I turn the key it should read what it actually has (full).

Also the speedo is really erratic most of the time It bounces all over the shop I could be doing between 30 - 100 and it makes a really loud annoying clicking sound.

Any help welcome, I am just trying to get an idea about how to fix it while the weather in the UK is rubbish.

Cheers.

[Pete in NH]

HI,

The speedometer issue could be as simple as a dry speedometer cable in need of some lubrication. The cable may be binding in the outer jacket and it winds up like a spring and then suddenly releases.

The fuel gauge may be a short circuit on the gauge feed wire from the tank sending unit, a defective gauge voltage regulator if the temperature gauge also goes full scale or a stuck sending unit. Try disconnecting the fuel gauge wire at the tank sending unit and see what happens.

[wraithghost]

thanks I will take a look

[wraithghost]

OK I took the speedo cable out and but some grease on it and took the car out again, the ticking noise is still pretty loud and the speedo reads about 20mph lower than it should. The bouncing about has been cured tho.

any thoughts on how to fix it?

[Pete in NH]

You may have the wrong speedometer drive gear in the transmission giving you the 20% low reading or there may be something wrong in the speedometer head itself. The drive gear number of teeth depends on tire size and rear axle ratio. there is a fairly large chart in the factory service manuals outlining the number of teeth on the speedometer drive gear by tire size and axle ratio.