The Gauge Dilemma

Ok,  so one of the gauges isnít working on your dash.  You can probably square it away with the tips and guidance we supply here.

  First of all, most LBCs are similar in their instrumentation.  Weíre going to deal with TR6 and Spitfire here but some of this will apply to MGB and others.

  Two of the gauges donít run on 12 volts.  Yep.  The temp gauge and the fuel gauge run off whatís called a voltage reducer.  This voltage reducer is usually mounted on the back of the speedometer.  It takes in 12 volts from the system and gives off between 6 and 8 volts (depending on itís mood) to run the gas gauge and temperature gauge.

  If the voltage reducer is failing or has failed then neither the temp gauge nor fuel gauge will function.  We might have a wire off the voltage reducer causing our problem or we might have a faulty voltage reducer.  The good news is that the voltage reducers are available new, used, and in solid state after market varieties.

  If the temp gauge alone is faulty then you may have a bad sending unit.  The easy way to tell if the sender is bad is to hook the wire that goes to the sending unit on the engine direct to ground. Turn the ignition on.  Does the gauge go to ALL THE WAY HOT?  If it does then the gauge is fine and you probably have a bad sending unit.   If the gauge doesnít move then youíll probably find that youíve a bad gauge.

Fuel gauges are a different kettle of fish.  They get their power from the voltage reducer.  The second connection on the fuel gauge goes back to the sending unit in the fuel tank.  The sending unit changes the resistance to ground as the float goes lower in the tank.  When the tank is full the float is all the way up.  This provides almost a perfect ground to the system and our gas gauge indicates full. 

  So, our tests are as follows:

  Disconnect the lead from the fuel gauge that runs to the sending unit in the tank.  Ground the connector on the gauge (the one that would normally have the wire on it from the sending unit) to a good ground.  Turn on the key.  If the gauge goes to full then the gauge is ok.

  Double check to make sure that there is a good connection from the body of the fuel tank sending unit to ground.  This is imperative.  Without this connection your sending unit wonít work properly.  If you have a good ground from the body of the sending unit to the frame of the car AND the wire going up to the gauge is good then you will probably have a bad sending unit.