My 350SD had afterglow for approx 15 seconds with the original glow plug relay. I could always tell when the afterglow quit - the engine would start to shake and smoke. If I started it up and slowly drove away it would be just fine.

To solve this issue, I ordered a glow plug relay for a 1996 E300D which has the capability of a 60 second afterglow. When I replaced the relay, I removed the glow plug temp wire from the wiring harness and replaced it with a wire to a variable capacitor (from radio shack). The capacitor makes the car think that it is 30 degrees out and I get a long afterglow.

Starts like a champ, idles like a champ, does smoke like a champ and the engine definitely runs MUCH smoother when the afterglow stops.

I used an adjustable capacitor so I could adjust the glow period in the summer vs winter. Small price to pay but winters are so much nicer in it. PS, I don't adjust it. I'll just replace the glow plugs a little sooner.

The one catch with my approach is that the glow plug light stays on for a long period, where it used to be much quicker. On wamer winter days, I don't wait for the light to go off, because I know I have rigged it to be longer.....

I love afterglow. I have also retrofitted my 1978 300SD (w116) with a relay that has afterglow. I have great results with starting that too!!!