My 81 300DT just burnt thru 2 glow plugs of the 5 I replaced last winter. Because of time constraints, I used 2 of the old plugs (still good) and things were ok for a few days. Now my relay will not kick in and I get no light on the dash. The bulb is good and all the tests for the preglow system are positive. I read where the relay senses the the voltage(or is it resistance) between the #1 plug and the rest and if there is a mismatch, it may not work. Does this sound like what is happening or should I pony up and replace the relay and the plugs. The part shop sells a bosch relay that seems to be cheap by comparison.

Was one of the plugs you replaced with the old one number one? If so, screw a new one into the number one hole.

Good luck,

If you narrow it down to the relay, before you buy a new one open it up and remove the circuit board. On the board you will see a black barrel shaped resistor mounted on end. Look at the bottom and you will probably find solder breaks. Resolder and it will be as good as new. The diesel vibration tends to crack the solder. I have repaired at least five boards this way. All had the same break.
John Olszewski

relay kit
 

I've used a couple of the kits and I think they are a great improvement. As I understand it, the glow plugs are redesigned and the whole thing works great on my daughter's '77 240D. My was only about $100 for plugs, relay, and wiring harness.

I made up a jumper using #12 solid wire and two hefty alligator clips that I carry at all times in my diesel. When you don't get a glow light on the dash, take the jumper and sequentially jump or bypass each plug until you find an open one by having the light come on. (Of course this won't work if two glow plugs are open.) Using the jumper, you can get the remaining plugs to glow and get you started - roughly for a few seconds until the open cylinder finally achieves combusion. I've never had to replace the actual relay, my problem has always been a case of open glow plugs. Like a light bulb, they can go at any time without warning. To definitively test the relay, jump all of the plugs at once with a longer lead and see if the light comes on. That should tell you quickly if it is o.k.

You have gotten some good suggestions, but most of them apply to the earlier loop type(serial) glow plugs. The '81 has parallel glow plugs.

Check the bar fuse inside the relay. I have never personally seen one of these relays go bad.

Good luck,

thanks for the help. this is a great source of info and support. Ray