{"id":41889,"date":"2011-05-22T00:00:26","date_gmt":"2011-05-22T00:00:26","guid":{"rendered":"https:\/\/namgar.com\/NAMGAR2020\/?p=41889"},"modified":"2011-05-22T00:00:26","modified_gmt":"2011-05-22T00:00:26","slug":"fuel-gauge-problems","status":"publish","type":"post","link":"https:\/\/namgar.com\/NAMGARMAIN\/fuel-gauge-problems\/","title":{"rendered":"Fuel Gauge Problems"},"content":{"rendered":"

VOL 26\/NUMBER 1<\/h4>\n

\"FuelMGAs and Magnettes do occasionally break down on the road. However, the most embarrassing situation is when you run out of gas. MGA fuel gauges are notorious for giving inaccurate readings, or no readings at all. In this tech article, Mike Ash covers all aspects of testing the gauge and the tank sender unit, and what can be done to ensure your gauge reads correctly.<\/p>\n

I do not recall with whom, but one of the parking lot discussions (at GT-25) centered around a participant’s fuel gauge and its inability to read less than half full, even when empty. Not being a technical expert in such matters, I could not recall the tests I had used on my fuel gauge problem some years ago, but knew that I had covered the topic quite extensively. The previous article was in MGA! Volume 15, Number 6, which would be about 10 years ago.<\/p>\n

\"FigureThe basic circuit for the fuel gauge is quite simple, and is shown in Figure 1 from the MGA 1600 wiring diagram. There is a green (G) wire from the fuse block (21) terminal A4 to the gauge (41), a green\/black (GB) wire from the gauge to the sending unit (42) in the tank, and the sending unit is grounded by being bolted to the tank. The case of the gauge is grounded as well, I believe by a black ground wire connected under its retaining nut. There is a connector in the green\/black wire, which is physically located with a bunch of other connectors somewhere near the starter switch.<\/p>\n

\"FigureBe sure that the gauge is correctly connected. (Figure 2) Looking at the face of the gauge, the left terminal is connected to the ignition (green) circuit, and the right terminal is connected to the sending unit (green\/ black). These connections are the same for both positive and negative ground systems. The terminals are identified on the back of the gauge, with a “B” for battery stamped on the case near the left terminal, and a “T” for tank stamped near the right terminal. However, these markings are very difficult to see when the gauge is installed in the car!<\/p>\n

Problems with the fuel gauge are either an incorrect or no reading, or an erratic and unstable reading on the gauge. Both of these problems can be caused by faults in the gauge, the sending unit in the tank, the wiring, or any combination of all three.<\/p>\n

As an initial system check, turn on the ignition and disconnect the wire from the tank unit, the gauge should read empty<\/strong>. If the gauge does not read empty, then the probable cause is a problem with the calibration of the gauge. Do not reconnect the wire, but ground the end to a good ground on or near the tank, the gauge should read full<\/strong>. If the gauge still reads empty, then the gauge may be bad, there may be no power to the gauge, or there is a problem with the wiring between the gauge and the tank unit. With the ignition still on, check that there are 12 volts on the left terminal of the gauge as you face the front of the gauge. If there are, ground the other terminal. If the gauge still does not read full, then the problem is with the gauge. If it does read full now, then there is a problem with the wiring between the gauge and the tank unit. If, when the wire to the tank unit was grounded, the gauge read something less than full, then there is a problem with the calibration of the gauge.<\/p>\n

\"FigureIf the preceding checks reveal that there is no indicated problem with the gauge but the gauge still does not read correctly, then the problem is most likely with the tank unit itself. (Figure 3) The tank unit is a cylindrical float attached to an arm that moves across a rheostat, or variable resistor. When the fuel level is low, the float is low, and the arm is at the high resistance end of the rheostat. When the fuel level is high, the float is high, and the arm is at the low resistance end of the rheostat. I checked four used tank units that I have squirreled away in the attic, and the general consensus was that the resistance of the tank unit is about 90 ohms<\/strong> when the tank is empty, and less that 10 ohms<\/strong> when full.<\/p>\n

The best way to check the tank unit is to remove it from the tank; preferably when the fuel level is below a quarter full! With the unit out of the tank, leave the green\/black wire connected, and ground the case of the unit to the car body or chassis. Make sure this is a good ground, and be wary of generating any sparks that might ignite any gasoline fumes coming from the tank! With the tank unit connected in this way and the ignition on, move the tank unit arm over its full range and watch the gauge; you may need a helper for this. As you move the tank unit arm over its full range, the gauge should also move over its full range. For correct operation, the gauge should read empty or full at the exact point that the tank unit arm reaches the appropriate end of its travel. If the gauge does not appear to read correctly as the tank unit arm is moved over its full range, the problem could be with either the gauge or the tank unit. The tank unit is the most likely. Any erratic movement of the gauge as the tank unit arm is moved is probably caused by a bad or dirty connection in the circuit. If the external connections and car wiring all appear to be good, then there is probably a bad connection that is internal to the tank unit.<\/p>\n

When I wrote on this subject last time, I used the problems with the gas gauge reading in my own car as the basis. In that case, replacing the tank unit corrected the erratic movement of the gauge, but the gauge did not read either completely empty or completely full when the tank unit arm was moved over its full range. Replacement with a new gauge corrected that problem as well.<\/p>\n

There are two nuts on the back of the gauge that secure two coils inside the gauge. The electrical current passing through these coils attracts the needle in one direction or the other depending on the amount of current. The gauge is calibrated at the factory by the placement of these coils. It might be possible to recalibrate a gauge by loosening the nuts and moving the coils, but I have never had occasion to try it.<\/p>\n

After I had written on this subject last time, a member wrote to say that one possible reason for erratic behavior of the gauge needle is poor contact at the rheostat within the sending unit. This could be because of poor grounding of the moving contacts. The moving contacts of the rheostat are attached to the float arm and find a ground through the point where the arm pivots in the housing. Corrosion or looseness at the pivot point can possibly cause an intermittent ground, which in turn, would cause erratic behavior of the gauge needle. The member solved this problem by running a ground lead between the float arm and the unit housing. The ground lead should be a thin, flexible wire soldered to the float arm at one end and to a solder lug under a screw tapped into the housing at the other. The lead should be long enough to allow the float arm to travel over its full range. The added lead will then make a very positive ground, instead of relying on contact at the pivot point.<\/p>\n","protected":false},"excerpt":{"rendered":"

VOL 26\/NUMBER 1 MGAs and Magnettes do occasionally break down on the road. However, the<\/p>\n","protected":false},"author":1,"featured_media":44357,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"nf_dc_page":"","ngg_post_thumbnail":0,"footnotes":""},"categories":[14],"tags":[],"class_list":["post-41889","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-featured-tech-sessions"],"_links":{"self":[{"href":"https:\/\/namgar.com\/NAMGARMAIN\/wp-json\/wp\/v2\/posts\/41889","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/namgar.com\/NAMGARMAIN\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/namgar.com\/NAMGARMAIN\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/namgar.com\/NAMGARMAIN\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/namgar.com\/NAMGARMAIN\/wp-json\/wp\/v2\/comments?post=41889"}],"version-history":[{"count":0,"href":"https:\/\/namgar.com\/NAMGARMAIN\/wp-json\/wp\/v2\/posts\/41889\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/namgar.com\/NAMGARMAIN\/wp-json\/wp\/v2\/media\/44357"}],"wp:attachment":[{"href":"https:\/\/namgar.com\/NAMGARMAIN\/wp-json\/wp\/v2\/media?parent=41889"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/namgar.com\/NAMGARMAIN\/wp-json\/wp\/v2\/categories?post=41889"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/namgar.com\/NAMGARMAIN\/wp-json\/wp\/v2\/tags?post=41889"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}