Can you use a 300D GP relay in a 240D?
 

I just upgraded my 1978's glow plugs to the pencil style and decided to try out a relay I had from an '83 300D. Pretty everybody who talks about doing the relay upgrade seems to be saying they used one that comes with a Bosch kit that has 4 pins for the GPs. I just hooked up my 5 pin relay and hooked up four of the five terminal ends to the GPs. The fifth is just hanging there for now. I had bench tested the relay using this method, and it worked perfectly, so I figured the relay must not care if there is one unconnected.
Now that I've installed it in the car, the relay works (I hear it click on and off), and the car starts surprisingly well, but the dash light doesn't come on.
Any idea if the reason the light isn't coming on is because the relay knows there's an unconnected terminal? Any idea how to make it come on?
Thanks.
Mike
P.S.: Sorry for the millionth GP upgrade post, but I don't see the answer to this anywhere else.

The relay has a current sensor. The way it works is there is a set of copper bars passing in opposite directions near a reed switch.

The current going to GP #1 is the reference. It passes current in one way. I think they make 2 passes with the #1 current going past the sensor.

The current to GPs 2-5 is the comparison. It is collectively passed by the sensor the other direction.

If all is well the magnetic fields cancel out between the two paths and the reed switch does not detect the magnetic field. However if the current is out of balance then the reed switch sends a signal to the logic which prevents the glow lamp from turning on.

Double check your bulb, jump pins 1 and 3 on the control connector harness plug, and make sure the dash light comes on. Key must be in position II.

I am fairly certain they used the same GP relay on the 240D and 300D, they just don't hook up #5 in the 240D. There is enough slop in the detection circuit that it really can't tell if you are missing #5 or not.

However what is important is that you have one plug on pin #1 and then the other 3 plugs on pins 2 thru 5, it does not matter which.

If you had hooked all 4 plugs up to 2 thru 5 then you would be getting a detection fault. It needs to see current out of #1 and compare that to #2 thru #5 to work correctly.

i am using an 85 GP relay in my 77 240D. everything works - it had to be to wired up to match. use the pin numbers and schematics to make sure all the lines are connected properly. i'm sure it wouldn't just plug in, but if you are comfortable taking apart the benz plug (disassemblable) and moving terminals around to match it will be fine. a few simplifications are possible, e.g. leaving the indicator disconnected, or combining the two power wires, but if you're going to the trouble of connecting it, do it right....

Thank you both for your informative responses. The 300D relay works great with the 1-4 terminals hooked up on the five terminal harness that came out of the 300D. I taped up the end of the #5 terminal, and the GP light comes (and goes out) on just as it should.
I had also tested this by hooking a 5th GP up to that #1 terminal and just grounded the threads. The light worked that way, too, of course.
It's nice to know that we can upgrade to pencil glow plugs with some parts from the junk yard.
Thanks, again.
Mike