Rodger
True Classic
So I did some sleuthing to figure out the fuel gauge problem. First, I pulled the sender from the tank and tried it just manually moving the float through its range. At the full position, the gauge only registered a little over 3/4 full. The sending unit was a new one that I bought from MWB about 4 years ago, but this was its first use. I contacted Matt at MWB and he said he would stand behind it and send me a new one, but that he has sold a lot of them with no complaints about them reading low. I then thought it may be a gauge or wiring issue, so I dug out the original fuel gauge from the car that I was not using and hooked up the sending unit to that. It read full at full height of the float, so that meant I probably had a bad gauge or a bad connection that was adding resistance to the circuit.
I pulled the gauge cluster and connected the sending unit directly to the signal post of the fuel gauge. Again, it read full at full float height so I tried the spade connector on the cluster that leads to the gauge. Only 3/4 again. I thought I might have made a poor solder joint when I built my gauge cluster, so I redid those, but no change. That lead me to the connection of the signal post to the circuit board. Turns out that it was corroded under the nut and washer, so cleaned the board, put in a new washer and voila, all fixed.
While I had the cluster out, I used the sending unit to test the temperature gauge and it moved through its normal range with movement of the float. I then put the gauge cluster back in the car and connected the sending unit to the signal wire from the Fiat temperature gauge on the engine. Again, the gauge moved through its full range so that ruled out a wiring issue. My assumption at this point is that I did not run the car long enough for it to get warm enough to get the gauge to move. I had mounted the temperature sending unit in the outlet housing off of the head, so it really will not start to register until the coolant starts to move to the radiator. When I was trying to figure out where to mount the Fiat sending unit on the engine, I debated trying to drill and tap a hole in the head into the cooling passage, but chickened out on that idea. I had only run the engine long enough for the radiator to get just slightly warm, so next time I will run it longer and see what happens.
I pulled the gauge cluster and connected the sending unit directly to the signal post of the fuel gauge. Again, it read full at full float height so I tried the spade connector on the cluster that leads to the gauge. Only 3/4 again. I thought I might have made a poor solder joint when I built my gauge cluster, so I redid those, but no change. That lead me to the connection of the signal post to the circuit board. Turns out that it was corroded under the nut and washer, so cleaned the board, put in a new washer and voila, all fixed.
While I had the cluster out, I used the sending unit to test the temperature gauge and it moved through its normal range with movement of the float. I then put the gauge cluster back in the car and connected the sending unit to the signal wire from the Fiat temperature gauge on the engine. Again, the gauge moved through its full range so that ruled out a wiring issue. My assumption at this point is that I did not run the car long enough for it to get warm enough to get the gauge to move. I had mounted the temperature sending unit in the outlet housing off of the head, so it really will not start to register until the coolant starts to move to the radiator. When I was trying to figure out where to mount the Fiat sending unit on the engine, I debated trying to drill and tap a hole in the head into the cooling passage, but chickened out on that idea. I had only run the engine long enough for the radiator to get just slightly warm, so next time I will run it longer and see what happens.