Having trouble with parents 1991 350sdl. Won't start. Glow plug light on dash doesn't come on when you turn the key on but you can hear the relay pick up. 12 volts on glow plugs. Dash light does come on after about 20 seconds-when it should normally go off? Light comes on occasionally while going down the road. Car has ~100K miles on it and probably has never had any glow plug service. Don't know what the resistance of glow plugs should be or if this is a good test? Can anyone tell me how a gasoline person can test these plugs? Can I pull them out without removing the intake? Is it better to replace them or can they be cleaned and reused?
Thanks
KWB