QuestionQUESTION: I just bought a 1977 MGB. Test drive was great, bought and was taking it home. The car was running great then out of nowhere after 79 miles, it just stopped like it was out of gas. Added a gallon on side of road and car started and ran fine. Got to gas station and car took only 3 gallons. I can't tell if fuel gauge is broken (only reads 1/2) or if something else. Your site is great I've read some similar posts not sure where to begin.
ANSWER: Hi Hugh,
You are talking about two problems, car not starting and wrong gauge readings. You need to test only one circuit at a time.
The only way you can test the "No Start" condition is to have the car in the failed mode. Meaning you can't test a car for a no start condition if it is now starting.
The false gauge reading can be tested if it is still reading wrong. First remove the wire (green w/black tracer) from the tank sending unit and turn the key to ignition (not start) and note the gauge reading. The needle should deflect all the way. Then ground the wire and the needle should deflect all the way in the opposite direction. This is a rough test of the gauge and wiring.
Next drain the fuel down about half and jack that side of the car up high and remove the sending unit from the tank. Check to see that the arm moves freely up and down. (A common failure). Plug the tank hole with a rag or something to keep the fuel fumes in and connect the green w/black tracer wire to the sending unit and if it does not have a black ground wire on it connect a jumper as a ground and turn on the key and move the arm up and down and note the gauge reading. This is a test of the sending unit.
Sometimes you can get a false reading due to the 10v voltage stabilizer that powers the fuel and temp gauge. However, if the temp gauge is reading ok, it is no use testing the stabilizer because it powers both gauges with the same power.
Howard
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QUESTION: Thanks Howard. If the needle does not behave as you describe above then the gauge/wiring need to be replaced/checked? In the second part if the sending unit arm does not move freely then it needs to be replaced? If so, is this an easy to find part? Temp gauge seemed to be fine. Thanks! Hugh
ANSWER: Hugh, if the temp gauge is working then the power supply from the stabilizer is ok. If the wire test at the sending unit shows a fault then the gauge is bad and needs to be replaced before testing the sending unit. You can test the green w/black wire by removing it from the gauge and the sending unit and use it like a test jumper to power up a high amp bulb like a brake light bulb or a head light bulb. If you can power up a high amp bulb with the wire it is ok because the gauge is a very light load. I worked in several MG dealerships over the span of the years MG was alive in the US and I never found a bad wiring in the gauge circuit so it is the least likely. The most failures I found were sending units and next was the gauge and last was the stabilizer.
The order of testing should be,
1. note if the temp gauge is working ok
2. do the test of the gauge at the sending unit.
3. remove the sending unit and connect it up to test.
The reason for that order is because of the ease of the tests even though the sending unit is the most likely. In the dealerships we worked on commission so it didn't make sense to spend from 30 min to an hour draining gas, jacking up a car and removing and testing a sending unit when it could possibly be a 10 minute job to replace a gauge or a stabilizer. And a zero time to test the stabilizer just by looking at the temp gauge. And two minutes to test the gauge.
Howard
Just about every part on the car can be found from one of the many MG parts suppliers on the Internet. You can even buy a whole new body if you want. I have not used all of them but the ones that I know are reputable companies are, Engel Imports, Moss Motors, The Roadster Factory, Victoria British and British Parts North West. They have web sites and toll free numbers. There are many others that may be reputable too but I don't know them.
Howard
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QUESTION: Howard you are the man! Thanks again! Last question. Seems like it must be the sending unit because if the problem was the gauge, the engine wouldn't stop. Had it driving today and it was fine. Seems to want a topped off tank of gas, though. Would the weight of a full tank make a failing sending unit work until the weight of the gas was lifted as the fuel was burned? Or might "gunk" in the fuel tank be clogging things? I wonder if a can of gumout might fix it all.
AnswerThe fuel line and pickup have nothing to do with the gauge sending unit. They are two separate items. If the tank is full but it runs out of fuel when the level is only down half way something is wrong with the pick up tube or the pump and has nothing to do with what the gauge says. If it runs out of gas you need to put a stick in the filler neck to see how much fuel is really in the tank. Also try removing the gas cap when it runs out of gas to see if the vent system is clogged thus not allowing air to enter the tank to replace the fuel used. This will stop any fuel from leaving the tank and act like you ran out of gas.
Howard
