MG Car Repair: fuel pump voltage, volt power supply, voltage drop test

Question
Have A 1980 MGB it was running for awhile and it got real hot, due to a stuck thermostat. I shut it down, had it towed to the house. Changed the thermostat. Everything looked fine, tried to start it but no luck. Found out I only have around 4.5 to 5 volts going to the pump. I know the pump works because I hooked up a 12 volt power supply to it and it starts great. Now any ideas where I'm loosing the voltage to the pump. I have 12 volts from the battery to fuse block/panel also have 12 volts going to the ignition relay. Any ideas would be great. Thanks Tom

Answer
Hi Tom,

When you know that you have a low voltage at any load you need to lay out your wiring diagram and start at the load (your pump) and do a "voltage drop test" on the wiring. Since you have already tested the pump connect the negative test lead of your volt meter to the pump power wire and the positive lead at the next junction toward the power supply and turn on the pump. This will measure the amount of voltage drop in that section of the wire. You will need to set your voltmeter on a low volt scale. You should see a very low voltage in every section.

Your test of checking voltage at the fuse block is not a valid test unless the fuel pump was turned on because with no load you are only testing "potential" If you only had one small strand on fine wire making the connection it will show a full 12v with no load.

By applying the load (pump on) and testing each section of the wire from the load (pump) to the source (battery) You have to find the problem (high resistance)weather it is a connection, a broken wire or a bad switch connection. But you have to test each section one at a time along the rout. On a "voltage drop test" you are not testing between the power wire to ground, you are testing between the power wire and the same power wire further up the line. When you find the bad section you will see a voltage of more then .5v usually.

You shouldn't have to test the ground circuit because you said that you ran a jumper hot wire and the pump ran ok.

In the dealerships I usually just did the voltage drop test about half way in the circuit so as to isolate the problem into a smaller section and then divided it in half again to work faster.

Look at your diagram and note the fuel pump circuit (load to power) and do the voltage drop test about half way to see in which half the problem is in. Remember to turn the pump on so you have a load while testing.

Howard