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My 400 has gradually developed a problem where the starter won't actuate. I just get the sound of a relay click (on the firewall) when the key is turned on and again to start. Battery voltage 12.6v. Neutral select switch checks fine but the brown wires are jumpered for these tests.

Working my way up the chain, I'm getting no voltage to the starter solenoid terminal with the brown wire in any ignition switch position and no voltage at the neutral select switch. At the ignition switch I have 12.6V at the battery terminal, 12.6V at the accessory terminal when on, and nothing at the start terminal in any position. Is this pointing to a faulty ignition switch?

BTW, voltage at the resistor with the green wire to the other starter solenoid terminal is 5.6v whether in the on or start position, (12V on the other side of the resistor).

Very likely the ign switch. The heavy start current for the senior starter is provided by a fairly small contact in the ign switch that can arc, pit and burn away to almost nothing.

Some of the classic symptoms of a failing switch are when turning on there is a need to wiggle the key to make a connection or sometimes if the starter contact tries to weld or has excessive roughness due to pitting the starter wants to keep running because the spring cannot release the contact plate from the contact.

Here are photo of the ign switch. First shows the small starter contact that is roughly 1/4 the size of the others and how damage usually appears. Another shows how the switch makes all the connections.

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Thanks. It didn't take long going from intermittent to not working at all. I assume it's not practically repairable and I need a replacement switch, which I assume are all used, with luck of the draw as to the actual condition.

You might try the vendors and see if any NOS switches are still available but they will be expensive.

One of the vendors has reportedly repro'd the contact plate to start repairing switches but I have a couple of concerns on the plate I saw. Biggest one being it is seemingly made of ordinary brass which has a much lower melting point than whatever composition of phopho-bronze material the original plate is made of. Since even the original contact material has melting issues I would wonder about the longevity of ordinary brass contacts holding up if it is used in rebuilt switches.

It has also been reported that there is a factory bulletin suggesting a relay be added to the start circuit to get the heavy senior model solenoid current out of the ign switch. I have yet to find that bulletin but the idea is a good one. Hete is a diagram showing options on where and how to add such a relay. An ordinary 30/40 amp contact cube relay from the parts store would do the job.

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Looks like Tucson Packard rebuilds them for $200 but are out of stock. Maybe I'll just give them my old one. I found an NOS switch still in the box for $150 -- you're right, not cheap. An under dash relay would be a rather simple addition and are only about $5 so I may go ahead and add it.

I do think the relay is a good idea and like you said, not a lot of $ to add one. The relay in the drawing is the standard 5 pin with N.O. / N.C. contacts many stores carry. It is shown wired the correct way with power going to the common pin 30. In reality the 4 pin N.O. without the normally closed contact would work well in this application.

If you already have or can only buy the 5 pin relay I would add a piece of heat shrink over unused terminal 87a. Even though it is in the middle and sort of protected, I would still insulate it or you could reverse wires on 30 and 87 so the hot wire is confined to that one terminal when relay is off.

I ordered the four pin Gebildet JD1912 What are your thoughts on the best way to wire it with positive ground?

This is the standard diagram.
photos.app.goo.gl/BkcaTzAjAj72Brdq6

The specs on Amazon do not mention any built in surge protection diode and I don't see one on the diagram printed on top of the case. If there is no built in diode across the coil, relays do not care about polarity so wire it up as per the suggested location you are going for..

That was my thought. I'll let you know how it goes. It may be two weeks to receive the ignition switch and I'll have to transfer the key cylinder. Any wisdom on that short of taking it to a locksmith?

For the relay, connect 85 to ground/chassis, 86 to the starter terminal.

Though any relay or switch will work with reversed polarity, since DC current will always arc the same way when the switch is opened, sometimes switches designed for DC loads will be optimized for a particular current direction. Speaking in conventional current flow, the relay expects the current to flow from 30 to 87. Hence for a positive ground system, you want to connect the solenoid to 30 and connect 87 to battery.

I assume you ordered that NOS starter you mentioned? I have rekeyed mine in the past so I can help you through it, though I can't find the thread in which I wrote about it previously.