I am troubleshooting my 300D 2.5 which has a glow plug fault. I have ohm'd each of the glow plugs at the glow plug connector to the glow plug relay. Each of the 5 are within .7 ohm range. I have even purchased and installed a brand new Beru (not MB) glow plug relay. The OEM has an internal fuse while the Beru has an external fuse. I don't know if it really matters.
There are two 5 pin connectors on the glow plug relay. The larger one has 5 pins, one for each of the glow plugs in the engine. The other 5 pin connector is much smaller and one would guess it must be responsible for activating the relay and illuminating the dash glow light.
When I turn the key switch to glow, the dash glow light will sometimes light, but most of the time will not. If I turn the key back and forth multiple times, it will sometimes cause the dash glow light to illuminate as expected. But again, most of the time the dash glow light will not illuminate. When I measure the voltage at the relay, I am getting slightly more than 12v at pin 1 of the 5 connector on the glow plug relay. Yes, the 300D 2.5 turbo has a 5 pin connector at the relay instead of the 4 pin on earlier diesel models ( I have an '87 300D Turbo with a 4 pin connector).
Could the fault be at the key switch?
Does anyone know what each of the 5 pins on the connector to the glow plug relay is intended to do? What voltages they are to have at glow and cranking? And which of the 5 pins is responsible for lighting the dash glow light?
I am seeing threads with questions about the a faulty glow plug system. I have read them and tried the solution(s). So far no luck. I certainly would like to improve my diagnostic skills, but I am stumped.
If anyone could help, it would be greatly appreciated!

Since you have 5 good glow plugs and a new relay, I agree that it could be a wiring problem or an ignition switch problem. As I am on the road and away from my library, I can't advise you as the the wiring of the small 5-pin plug. The fifth wire might be for afterglow (engine temp sensor input?) but that's just a guess.

Once you figure out the wiring, you can try connecting the relay to 12 volts directly (not going through the ignition switch). The ignition switch is not terribly expensive.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Here are the relay pinouts:

1 = pink/red, +12V from fuse 7
2 = violet/white, +12V from key switch
3 = black, +12V to dash glow indicator
4 = brown, ground at W3 (beside glow relay)
5 = white, ground from preglow temp sensor B11/7 (beside thermostat)

Whaddaya know, Jeremy, B11/7 is the factory preglow temp sensor

Sixto
87 300D