1942 - 1947 Chevrolet Shop Manual



	12-29

	If the above conditions do not prevail and current does not pass through the reversing switch, check the operating link and over-travel spring for proper assembly. If assembly is correct and trouble still exists, replace switch. SPARK PLUGS The spark plugs are 10 millimeter AC-104. The gap setting is .040". Care must be used when installing spark plugs to prevent upsetting the gaps by overtightening the plugs. When installing spark plugs, use new gaskets and screw each plug in finger tight, then tighten with a wrench 1/2 to 3/4 turn beyond finger tight. If used gaskets are being reinstalled, screw each plug in finger tight, then tighten with a wrench 1/8 to 1/4 turn beyond finger tight. If a tension wrench is used, the plugs should not be tightened over 15 foot pounds maximum. The two important things to remember about spark plugs are that they be kept clean and that the gap between the electrodes is properly adjusted. The gap should be checked with a round feeler gauge and set at .040". Spark plugs have much more to do with the performance and economy of engines than is generally recognized. With high combustion chamber temperatures and the modern anti-knock fuels, spark plugs require much more attention because of the tendency for oxides to deposit on the porcelain. This oxide is a fine, white, powdery substance which becomes impinged upon the insulator, usually accumulating far up in the shell, but is also deposited upon the insulator tip. This coating, being dry, is easily removed and it is better to remove it when it first forms, so that it will not have an opportunity to accumulate and melt into a glaze. Spark plug cleaning, in an abrasive, air-blast type cleaner, every 3,000 to 4,000 miles, will take care of it. Under higher temperatures, the white oxide becomes fused or melted by combustion heat and forms a smooth, glassy coating over the insulator. An insulator in this degree of oxidation presents the appearance of having been coated with a heavy machine oil. It is light yellow in color, with a hard, shiny glaze. A plug in this condition will often perform satisfactorily when cool, but will miss badly as soon as it becomes warmed up. Fused oxide coating is very deceiving. Even after an apparently thorough cleaning, there is apt to remain on the insulator a practically invisible layer of oxide. The plug looks clean, but really isn't. The safest way to handle such plugs is to clean them until you are absolutely sure that they	are clean and then start all over and clean them again. This coating is difficult to remove and only a double dose of cleaning can insure satisfactory performance. If spark plugs are allowed to go for 8,000 to 10,000 miles without cleaning, the oxides will form into blisters and the only remedy for this condition is replacement of the plugs. THE GASOLINE GAUGE The gasoline gauge is composed of two units, the indicating or dash unit mounted on the instrument panel and the tank unit mounted in the gasoline tank. The circuit for this instrument passes through the ignition switch, therefore the gasoline gauge operates only when the ignition switch is "on." OPERATION When the gasoline tank is empty, current flows from the battery positive through the ammeter to the ignition switch and then to the gasoline gauge dash unit top terminal. The current now passes through the choke or limiting coil to the common connection between the two coils, which is the lower terminal on the dash unit. At this point the current is offered two paths, one through the operating coil of the dash unit and the other over the wire to the tank unit. When the gasoline tank is empty, the contact finger of the tank unit cuts out all the resistance in the tank unit. The largest pro-

		Fig. 61- Gasoline Gauge Circuits
		portion of the current will pass through the tank unit circuit and only a very small portion through the operating coil of the dash unit, with the result that there is not sufficient current being forced through this operating coil to move the hand in the dash unit. If the gasoline tank is half full, the cork float of the tank unit rises on the gasoline and moves the contact finger over the resistance, cutting resistance into the tank unit circuit as indicated by the lines in Fig. 61.