ok folks here it goes 1980 w123 240D w/pencil type glow plugs 12mm no kit. installed from factory...

problem: when i put the key in the ignition the pre glow light blinks on and off, instead of the steady glow

I replaced all 4 plugs and the light is still blinking on and off.

When the vehicle turns on the light goes off.

Yes the vehicle has a hard time starting

Any help

thanks