Went to start my newly purchased 300CD with a brand new 5-day old battery.

Turn the key and nothing!

All accessory functions work perfectly fine.

No cranking or even a click from the starter when I turned the ignition.

Attempted to jump it just in case it was the battery.

After leaving it connected for 15 minutes I was left with no success.

What steps should I take to properly diagnose this?

No crank at all can often be a gear selector issue, and this would be my first guess with your situation. The NSS (neutral safety selector) is what assures the car isn’t in gear when trying to start. Sometimes the switch itself is bad, sometimes the gear selecting rods have worn out bushings. Assuming it’s actually in park or neutral, try jiggling the gear selector a little bit forward and back while turning the key.

Also try to start in neutral. Sometimes the safety neutral switch looses contact in park, but will work in neutral.
If that does not work. Find the square relay & swap it with another relay?
Another issue is the bushings of the gear shift linkage? They can wear out & prevent the car from going into park

square starting relay

If it is the NSS, sometimes you can find a sweet spot by shifting slightly along the shift pattern. If so it will start. Just keep shifting until you find a good spot.

Also it could just be worn out shifter bushings which prevents the transmission from knowing exactly what gear it is supposed to be in. (I know - ended a sentence with a preposition.) If so, easy fix.

Good lord. See if your 84 CD has a wiring junction block on the fender well near the battery.

See the pick in post 4 in the attached thread. You Can use a remote starter switch on any of the 2 screws to the rear which have positive and connect to the front one. Or you can less safely use a mumper wire. If that does not crank the engine you have some other issue like perhaps the starter or battery cable issues.

Also, the white wire on the front of the junction block comes up from the starter. With the key off there is no voltage there. With the key on if your neutral safety switch and those transmission bushings are doing their job you will get voltage there.

However, no voltage their can mean your ignition switch is having problems, the neutral safety switch is having problems or your exterior shifter bushings which can sometimes be entirely gone could be the problem or a combination of any or all of them.

Less frequent is I had the inside shifter bushings down inside of the base of the shifter crack and need to be replaced.

Don't be in a hurry to get it running.

http://www.peachparts.com/shopforum/diesel-discussion/261584-jump-83-300sd-starter.html

Also look under the car about where your feet are and see if the chassis to transmission bell housing ground strap is there. Remove it and clean the contact areas. A good idea to do the same with the battery cables.

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84 300D, 82 Volvo 244Gl Diesel

Agree with the jump box (junction block). Actually you can cross two screwdrivers and the starter will work. Just put one screwdriver on one of jump box screws and one on another and touch the screwdrivers together. I think I used to use (counting from the left) screw #1 and screw #3 but trial and error will work.

When I was refusing to believe that my new 6 month old NSS was bad (the original lasted 40 years). I jumped it many times.

Are you talking about this white wire????? I've been wondering what that thing is. With the car running, I had 12vts on both sides of that connector.

Well, I was only speaking of the terminal/junction block where the wires are screwed down to it. And mainly speaking of the most forward part of the junction block that has a lavender and white wire screwed to it.

What is normally supposed to happen is when you turn the key on to crank the voltage goes to the starter solenoid and engages that and the voltage comes up that white wire to the lavender wire and lavender wire turns your glow plugs on, so your glow plugs are on during cranking.

But when you are using a jumper wire on the terminal block you use that white wire and the lavender wire and it the voltage goes down to the starter solenoid cranking the starter and also by way of the lavender wire to the glow plug relay turning that on.

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84 300D, 82 Volvo 244Gl Diesel

So no clue wtf that thing is I'm showing unplugged???

Here is the jump box in my SD.

Attached Thumbnails

 

Under the battery tray.

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84 300D, 82 Volvo 244Gl Diesel

No but the same wiring harness maybe used on the gasoline versions.

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84 300D, 82 Volvo 244Gl Diesel