'82 300 sd recently purchased as an engine donor for another project so I don't have any history with the car to trouble shoot. I've read countless GP threads and I'm still not sure where I'm at.

Car had been sitting 9 months. PO was out of state so no real history to go by. 40 degrees, put in a fresh battery and it fired up fairly quickly. Drove it home to pull the motor and decided to compression test it 1st to reassure myself I had a good motor. Couple days later after my diesel compression testor arrives I pulled the GP's which all visually inspected good and compression showed 1-343 2-340 3-340 4-360 5-363. I'm happy with those numbers so I put some anti-seize on the threads and re-installed the GP's. Didn't attempt to start it at this time. A few days later I attempt to start it at 35 degrees and nothing but cranking. No GP light comes on inside. Start reading GP threads.

Yesterday 25 degrees. Ohmed the GP's from the relay plug and everything checked out there also with good battery to the relay and past the fuse. I'm still confused. Before removing the GP's again for a bench test, I decide to bypass the relay system. Unplugged the GP wiring from the relay and ran jumper wires to all 5 plugs. Jumper cables to battery and 15 seconds later it cranks but stumbles for about 10 seconds before catching on then idles smooth (for an OM617). With the GP wiring unplugged from the relay the light now comes on inside?

Here's what I don't really understand.

1. Are GP's easily damaged during installation, specifically tightening the small nut for the wire?

2. If 1 GP is bad (open or shorted), does the GP light not come on in the dash?

3. If 1 GP is bad (open or shorted), does the relay not send power to the other 4?

4. The GP wiring felt brittle when I removed and reinstalled the GP's. Is this a common issue?

5. Am I missing something obvious here?

6. Since I'm repowering a non MB vehicle with the OM617, would I be better off scrapping the MB GP relay system and wire the GP's via a push button activated silenoid?

I am guessing that when you did the Compression Test you did not unplug the Glow Plug Relay Connector so that the Glow Plug Relay would not activate.

With the Glow Plug Wires hanging and touching the Engine Block when you did your Engine cranking the Glow Plug Relay turned on and you burned out the Strip Fuse on the Glow Plug Relay because the Glow Plug Wire end/s were grounded on the Engine Block.

Another thing that can happen is just due to age the Strip Fues on the Glow Plug Relay can crack.
I have also read of people finding the Strip Fuse Screws loose.

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84 300D, 82 Volvo 244Gl Diesel

I have the Push Button + Solenoid on my Volvo Diesel Glow Plugs and it has been flawless.
I got the Solenoid (often referred to as a Ford type Starter Solenoid) from the Junk Yard so the cost was minimal.

If you plan to transplant the Engine into a non-Mercedes vehicle there is going to be no Glow Plug Light or Mercedes Ignition Switch with the Pre-glow key pisition to connect the stock Glow Plug Relay to. That kind detracts from using the Stock Relay.

Besides if the Stock Relay is good you could sell it to pay for the Starter Solenoid.

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84 300D, 82 Volvo 244Gl Diesel

Thanks for the input 911. I'll recheck the fuse as reccomended and I since I have a Ford solenoid laying on the shelf, I think that system will be in my future.

What's the going rate for a used GP relay?

You can get some idea how much a used GP relay is worth by seeing how much they sell for on eBay.
But, it needs to be a working.

For my use the only advantage of a Sock Ralay (because I have been glowing until the Relay times out) is that you can be doing something else like wiping the Windshield while the Glow pre-glow is still on.
The other advantage of the stock Relay is if someone else drives your Car the Glow Plug light prompts them to pre-glow that amount of time before starting.

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84 300D, 82 Volvo 244Gl Diesel