I have a 1981 123 240d. The glow plug relay is not working and I have several questions. I have a relay from a 1984 300sd and I'm wondering if I can use the 300sd one on the 240d. They are almost the same except for that connectors on the top or the relay. If you're holding the removed relays in your hand with the strip fuse on the bottom, there is a 4 prong connection that is the same on both of them right above the strip fuse.The male connector on the 240d will plug into those 4 prongs on the 300sd relay. Above that are 4 prongs that are the same on both relays. On the 300sd above the 4 prongs there is a metal prong on the right and a plastic prong on the left. On the 240d above the 4 prongs there is only a plastic prong on the right. If I removed the plastic prong from the 300sd relay the male plug from the 240d would plug into that relay. Apparently if I did that, my 4 glow plugs in the 240d would be connected to the relay and the extra metal prong for the 5th cylinder would be connected to nothing. If I just broke off the plastic prong on the 300sd relay I could still successfully use it in the 300sd . Is there an unexpected (at least to me) problem with that? Also, has anyone bypassed the relay with a button switch and heavy duty relay such as a starter solenoid. If so, can you provide some details? Such as the connections between power supply and the plugs. Finally, if I take my bad relay apart , can I repair it? Thanks for any advise you can give.

 

No. Just get a good used one.