Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

Glow Plugs

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See Figure 1

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Fig. Fig. 1: Cross-sectional view of an installed glow plug and its related components

The glow plugs provide a means of ignition during starting and perform the same function as normal spark plugs, although they do so in a different manner.

TESTING



The light on the dashboard which indicates when the glow plugs are hot enough to fire can also serve as a troubleshooting aid. If the light does not glow, it usually indicates a faulty plug.

  1. Test the plugs by having an assistant hold the starting knob in the preheat position while shorting the plugs to ground, in turn, with a screwdriver. Each plug should produce a spark if working properly. While bridging the connections, the light on the dashboard should light.
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  3. If, after disconnecting the ground lead of the preheating system, the light still stays lit, a short circuit in the system is indicated. This is usually caused by a carbon-fouled plug electrode or by a lead touching the cylinder head. Check the leads first.
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  5. If they seem O.K., pull the knob to the preheat position and disconnect one plug power lead at a time, starting from the ground end, until the light goes out, indicating the faulty plug.
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  7. Glow plugs can be cleaned, but it is better to replace them if they are badly fouled.
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REMOVAL & INSTALLATION



See Figures 2 through 6

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Fig. Fig. 2: To remove a glow plug, disengage the electrical connection and remove the nut on the end of the plug



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Fig. Fig. 3: 1. Glow plug 2. Ceramic insulator 3. Electrical connection 4. Bus bar Remove the ceramic insulator ...



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Fig. Fig. 4: ... and disconnect the bus bars from the glow plug



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Fig. Fig. 5: Use a wrench to loosen the glow plug



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Fig. Fig. 6: Remove the glow plug from the cylinder head

  1. Loosen the cable, if this has not been done already, by removing the knurled nut.
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  3. Unscrew the other nuts and remove the insulators and the bus bars.
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  5. Using a 21 mm. socket, unscrew and remove the glow plugs.
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  7. Before installing new plugs, clean the ducts and prechamber bores with a stiff bristle brush or a small scraper. The ball pin in the prechamber is easy to break, so don't go much deeper than 2 in. into the plug hole.
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  9. Crank the engine a few times to blow out any carbon particles loosened by the scraping, then insert the plugs. Do not exceed 35 ft lbs torque.
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  11. It might be a good idea to recheck these new plugs to ensure that all connections are tight and not grounded and that the plugs are not faulty.
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