1981 300D glow plug relay
 

I need to kwnow the exact output voltage at terminal G of a GP relay for this car. According Haynes manual the voltage that reachs the serial glow plugs (filament type) is between 9.5 - 10 volts. However, when I mesured the voltage at G terminal, it shows me 12 volts. These glow plugs works with a nominal voltage of 0.9v. I'm afraid to burn the GP if i turn on the system. Thanks in advance for information about this topic. Best regards,
Luis.

81 300D should have the pencil style glow plugs and the relay should have an output of 12 volts. Are you saying you have the serial glow plugs and they're stamped as 0.9V?

Hi!.. That's right. This old engine seems to be a 1981 300D model (european version) and has a serial glow plugs stamped as 0.9v. I guess it's much older than 1981. According Haynes wiring diagram it's an early model 123 (UK). I have another MB, 1979 240D (US) that uses a serial glow plugs, but GP relay it's complete different (also controls a buzzer). Any way, the question is if this GP can handle a 12 v from the relay.

What voltage is written on your relay? You probably don't have the correct one.

You should be reading battery voltage at the relay output, whatever that is. It will read 9-10 volts when the glowplug chain is working, as you are loading down the battery. If the engine is off, and the glowplugs are not working you will read 12V at the relay.

Thanks... I think this is the right explanation, since the same happens on 240D older model with manual starting knob and a buit-in fuse located on firewall. Any way I'll put an 80amp. fuse and try to power on the engine. Best regards.