I have a no start problem with my 87 300td. Turning the ignition on, the glow plug light comes on and I hear the glow time relay “clunk” on. Light goes off after about 10 seconds, and after about 20 seconds later I hear the relay clunk back off. Car just cranks and cranks...no fire at all. It’s about 50 degrees today so I attempted to continue cranking to build heat in the pre chambers...lo and behold after about 60 seconds of cranking, it very slowly starts to fire, and with enough continued cranking it eventually starts.

The glow plug system is clearly not working...yet there are no blown glow fuses and the time relay controls the glow light properly and “clunks” on and off where I expect it should.

Is there an alternate failure where the glow time relay still seems to function properly yet doesn’t warm the glow plugs? I had this problem a week ago with the same symptoms but it was too cold outside to warm the prechambers by just extended cranking. Miraculously, the next day it started immediately at an ambient temp of just 14 degrees and start and the preglow worked perfectly.

Is there a glow time relay failure mode that matches these symptoms?

Thanks,

J.G.