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I have an '81 w123 240d and the glow plug relay is bad. I have one from a '84 300sd,they are very similar and I'm wondering if I can use it in the 240d. If you are holding either relay in your hand with the strip fuse on the bottom the 4 prong connection just above the strip fuse is the same on both. Just above that are 4 more metal prongs that are the same on both relays. On the very top of the 300sd one there is a metal prong on the right and a plastic one on the left. On the very top of the 240d one there is only a plastic prong on the right on no metal prong. If I break the top plastic prong from the 300sd relay, I could plug the male connector into the 300sd relay.Would that work? The connection for the 5th glow plug would be connected to nothing. Also, has anyone bypassed the relay with a button switch and heavy duty relay such as a starter solenoid? If so , how would you make the connections between the power supply and the plug connectors? Finally can I take the bad relay apart and fix it it? Thanks for any advice.
 

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I have an '81 w123 240d and the glow plug relay is bad. I have one from a '84 300sd,they are very similar and I'm wondering if I can use it in the 240d. If you are holding either relay in your hand with the strip fuse on the bottom the 4 prong connection just above the strip fuse is the same on both. Just above that are 4 more metal prongs that are the same on both relays. On the very top of the 300sd one there is a metal prong on the right and a plastic one on the left. On the very top of the 240d one there is only a plastic prong on the right on no metal prong. If I break the top plastic prong from the 300sd relay, I could plug the male connector into the 300sd relay.Would that work? The connection for the 5th glow plug would be connected to nothing. Also, has anyone bypassed the relay with a button switch and heavy duty relay such as a starter solenoid? If so , how would you make the connections between the power supply and the plug connectors? Finally can I take the bad relay apart and fix it it? Thanks for any advice.
I believe you are correct that you can use the later model relay.

The electrical connector that plugs in near the strip fuse on the 240 will have a wire that goes to a temp sensor on the engine. The later model does not need that wire as the temp sensor is inside of the relay.
 

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You may need to make sure the numbers on the small electrical connectors match the 4 pins on the relay.
 
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