Yes, my glow plug relay was working correctly as far as the glow plugs were concerned but the dash light itself was never coming on until I replaced that transistor. All a transistor is used for is an amplfier or a "switch" and in this instance it is a "switch" that operates off of the timer IC on the board and allows the lamp to come on and extinguish at the end of the timer for the relay. If your relay works intermittently but your not getting the light on all of the time it may just be the contacts in the relay itself need cleaning or you may have a GP about to go bad on you. If your relay works all of the time but you don't get a light on the dash, check your wiring to the lamp and the lamp itself and if that all checks fine it may be this problem here.

Search the site here for troubleshooting the relay and the lamp circuit and if everything checks out for you but you lamp will NOT come on I would bet on this transistor to be your problem.

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1981 300SD