If the preglow light does not come on then first check the glow plugs. On these engines when a glowplug goes bad the light will not come on. On later engines around 1990 (= or -) the glow lamp will blink after the car starts, telling you that a glowplug is bad. But the older cars the light just will not come on.
To test the glow plugs, the easiest way is to use an ohm meter, less than 1 ohm is normal for a good plug. Test them thru the large connector on the preglow relay. If you have more than one abd, you might as well replace them all.