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.

__________________
'76 240D-Sold
'78 240D-Sold
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet- under restoration
'86 560SL 124K Miles-Sold
'94 320E Wagon, 74K Miles-128K Miles JUNKED
'06 E350 Wagon, 84K Miles
07 SL550, 14K Miles