Post War Diesel Horn repair

If the horn works OK when it is hot wired, then you have bad electrical contact somewhere.
Since you've replaced the horn and relay, the problem should not be the relay contacts, or the connection provided by the horn mounting screws. I guess I would check the relay bracket, which also serves as part of the battery bracket. Also the battery door (cover). They might need cleaning or polishing.

Even a brand new whistle/horn relay may have dirty contacts! Since you have tried and have had the horn work when hot wired it has to be as C.W. pointed out a bad connection somewhere! Clean the battery holder and clean the contacts and give it another try!

One common problem area on the relays is the contact between the armature (part that moves up and down), and the body of the relay.

Sometimes I remove the armature, and polish the copper contacts on the bottom, as well as the part of the relay body that accepts it. (Careful, it is easy to damage the copper).

Other times I solder a few strands of flexible wire to the bottom of the relay armature and the back of the relay. It is important that the wire doesn't impede the movement of the armature, or touch somewhere it shouldn't.

Thanks guys, "it's always the little stuff"! I will give these suggestions a try tomorrow in the shop. Fingers crossed, too.

When you say "hot wired", does that mean running it straight off track power or connected directly to a D cell. Anything in the electrical pathway that is corroded will stop the 1.5V of the battery from operating the horn. I've been attaching wires like CW suggests and also add a wire to ground from one of the horn attachment screws. It can only help. Those silly bike horns can be a real PITA. 

Roger

Watch this repair video..

https://www.youtube.com/watch?v=3LsDKsNBBKQ

The frame of the locomotive is used as the negative return for the battery. Since the horn is powered by only 1.5 volts, the electrical path for the battery to the horn must be perfect for the horn to work properly. In addition to the suggestions about cleaning the relay contacts, dis-assemble the entire horn and mounting bracket, relay, and the battery spring. Clean and polish up the mating surfaces of all the above parts and reassemble. If the horn sounds properly when powered directly from the battery, there is no need to adjust it, or dis-assemble it. Slight corrosion of the chassis and horn circuit is the major culprit of poor horn performance.

Larry

Ok let's break this down a little, you say it starts to beep then it squawks? I not sure how to interpret 'Squawk" but I am taking that there is AC track voltage leaking into the battery horn circuit. You said you replaced the horn on a 602 Seaboard switcher, which means you also replaced the battery bracket since it is a assembly. I assume you purchased a reproduction horn and bracket and not a original. Check to be sure the insulation on the back side of the bracket is good. That could be a problem area if it is a repo part. Is the long folded cardboard piece in place between the contact spring and relay? Be sure the horn relay does not touch the metal frame anywhere. Make sure the horn relay is not touching the e-unit ground lug on the side. Also check to see if the rivet on the battery contact spring has its insulation, might need to check that there is no continuity between the rivet and battery bracket.