Joined: 02 Jan 2008 Posts: 98 Location: Maidenhead
Posted: Sat Dec 26, 2009 10:19 am Post subject: Fuel Gauge Query
Despite have 4 such gauges 'on the shelf' none worked! Having tested the transmitter with a multimeter I established that all was fine at the tank end. Bought a 2nd hand gauge and it reads backwards! i.e. when the tank is full the gauge reads empty and vice versa. The replacement gauge is a Smiths from a Triumph Spitfire and the reading is at the top of gauge i.e the need points 'north'. The Smiths gauge that was in the car has the reading at the bottom and the needle points south. Would this make a difference? I have tried all the options of reversing wires, changing polarity etc but no difference. Any suggestions?
Joined: 27 Dec 2008 Posts: 1116 Location: Chesterfield
Posted: Sat Dec 26, 2009 1:26 pm Post subject:
I can't quite get my head round the effects of unknowingly replaceing a thermo-static gauge with a moving/balanced coil gauge or vice versa. Thermo-static gauges move slowly, Moving coil gauges move instantly.
You don't say what car you are fitting it to or mention its age. Until probably the mid-1950s, most gauges were moving coil. I think that your Spitfire gauge is almost certainly thermo-static.
I do know that a thermo-static gauge will show no reading if disconnected from the tank unit whereas a moving coil gauge will show FULL (Ignition "0n" of course). Thermo-static gauges require a sender unit which passes most current with the tank full. Balanced coil gauges need one which passes the least current (highest resistance) with the tank full. The orientation of the dial or its markings does not matter except perhaps visually. Unless of course you are mixing manufacturers and the parts are not compatible. I hope this helps.
Please excuse me, but I cannot resist an anecdote here.
It is not generally appreciated that a hearse is a GOODS VEHICLE, and as such is (was?) required to be frequently presented on a weigh bridge for checking by the D.O.T. A Sheerline Hearse we operated just broke the 2 ton limit which took it into the more expensive higher category. Yet we always got under 2 ton. How? Well we made sure the petrol tank was virtually empty and then disconnected the fuel gauge sender so the (moving coil) gauge showed full. Deducting the allowance for fuel load at 7.5 lbs. per gallon knocked over 100lbs off the weighbridge reading, bringing it just under 2 tons (LOB Rick!).
Also, I don't know whether regulations have changed, but a hearse fitted into no testing category and never needed an M.O.T. _________________ Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!
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