The fuse is good huh? Always suspect it first.
Loop style GPs are less than half an Ohm in resistance, just a little more than the resistance of the leads on an Ohmmeter. hard to get a valid reading on the typical Ohmeter, DVM's excluded.
Pencil style GPs are around 2.2 Ohms and are easily read on a decent Ohmeter like a Simpsoon 260. Forget it if its a Radio Shack $3.95 meter.
To test loop GP's you can connect a heavy wire (jumper cable of necessary but be careful!) from the + terminal on the battery to the GP nearest the firewall ( the one in the front is grounded) there should be a spark as current flows through the series connected plugs. I have used this to start a car when necessary (after the timer failed) yes the points take a beating in the timer box and can be burnished back to give some more life. Is the relay clicking? If not then its an indication the timer might be kaput. Or the timer relay may be receiving an "engine hot" signal, later models use a thermal switch to drop out the long ON hold time
addendum:
The wire should be at least as large as 8 Ga and can be connected to the battery + or to the three terminal connector block that the starter motor is connected to (on the right side of the engine).

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

Thanks for the responses.

I checked the fuse (and everything else) with a DVM.

The relay does click but stopped doing after my fooling with it for a bit. I assumed that there was some kind of protection built in so some idiot (me) would not be able to overheat the GPs by constantly turning the key on and off.

The fact that there are 4 used GPs in the drivers door is also a hint that there is something not quite right, but I don't know enough about these cars yet to take the hint. I suspect that the GPs in the engine are relatively new.

What gauge wire for the jumper? Battery cables

Dropout

Drop out -

Is the GP Relay on your model attached right below the steering column? It would be just the basic silver relay (no fuse attached). I have a '77 300D with the same set-up and I'm looking for the timer relay (which everyone keeps telling me is a screw down with an 80 amp fuse inside) however I see nothing like it under the dash.

Oh, and an 8-gauge wire will do fine to jump the plugs. I would attach the wire at the fuse block however - just to be safe.

Yes that's right. There are two relays inside so it's possible one could be the timer relay.

Even if the circuit feeding the plugs is okay and it has new plugs does not mean that everything is good. As I tried to explain earlier, crud packs in around the plugs and offers a path to ground. You have to remove the plugs and use the special reamer to clean the holes out or use a hardwood stick and lots of time. Don't get the bright idea to use a steel brush because a broken bristle could work its way in a cylinder and cause damage.

Good luck,

And that would cause 0 volts at the fuse? If so, does it make sense that I could check the relay etc by removing the wire from the fuse to the #4 GP and see if I get 12V?

What should the voltage be at the fuse when the GP's (series) are on?

What is the best way to check the bulb in the dash? Which wire from the relay goes to the bulb?

Thanks.

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Dropout

1978 240D work in progress