For the past year I've been trying to track down a no start gremlin on my 83 SD. Intermittently you can turn the key to start and there is not even a click at the ignition. All new glow plugs and all lighting works fine. I can jump the two wires at the junction box by the battery to bypass the ignition and it will crank. Now I've replaced the ignition lock and switch, adjusted the neutral safety switch and still have the issue. It happened to me last Friday after driving to the coast I stopped at the store got back in the car and nothing. I popped the hood attached my jumper wire and nothing, it would not crank. I tried again twice more and still nothing. On the fourth try I let the glow plugs cycle and jumped the junction box and it started only after the glow plug relay had clicked. I was tinkering with it a few minutes ago and sure enough the jumper wire would not start the car until the glow plug relay had clicked which is about 30 seconds after the glow plug light goes out. Is this typical for the glow plug relay to prevent the car from starting at the junction box? Could this be the source of the no start issue using the ignition as well? Any help would be appreciated.

I have not read of anyone posting this particular problem.

You might apply your Voltmeter to the Battery Terminals an have someone turn on the Glow Plugs and see how far the Voltage Drops. The Voltage should not drop much more than 11-11.5 Volts.
May not be your problem but worn Shifter Bushings can mimic a bad Neutral Safety Switch.

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84 300D, 82 Volvo 244Gl Diesel

This is not a problem with the glow plug relay. Even with a totally bad relay, or even a disconnected relay, the engine should still turn. And if you crank long enough (30-60 seconds) with healthy compression and mild ambient temps the engine should still start.

However, since you can repeat the process, it seems to be related in some way. I'd suspect a weak battery. When the glow plugs are lit it doesn't have enough juice to turn the starter. Only after the relay is no longer drawing on the battery does it have the power needed.

That's an easy place to start anyway. You can have it tested.

If it's not that, your cable connection to the starter may be dirty/corroded, same with your battery ground cable. Or you might have a faulty NSS or starter.

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

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Check your alternator output at idle with the glow relay off or disconnected. I had a lot of strange no crank, low charge issues that turned out to be a weak battery and multiple Bosch voltage regulators.

Regulator wasn't regulating, battery wasn't holding a charge but seemed to take a charge. I ended up replacing the battery and scalping a regulator from a parts car.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Just wondering if this happens only when the car is warm?

This is why I was suggesting to check for the Voltage Drop on the Battery.
Besides the normal Pre-glow/Glow Plug Cycle before you crank the Glow Plugs are also on when you turn the Key to start Cranking the Starter.

Once got 2 good starts out of a Battery and drove the Car in between the Starts. On the 3rd Start I got noting at all. A Voltmeter applied to the Battery showed 12 Volts making Me think My Ignition Switch was shot.
When I appled a Jumper Wire to the Starter Solenoid there was only a tiny Spark. So the Battery had Voltage but little or no amperage available.

I could not do the Voltage Drop test on the Batter as I was by myself and had no way to clip the Volt Meter to the Batter.

Across the Street was an Autozone and I bought another Battery and after that was replaced the Engine started right up.

It is not common for Batteries to fail like that but it happens.

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84 300D, 82 Volvo 244Gl Diesel