Which starter switch (solenoid)?

I have a question about the correct Motorcraft starter solenoid/switch for my 1987 Turbo Coupe.

I hesitate to use the word “solenoid” as technically that is the small device that typically sits on top of the starter and engages the starter pinion gear with the flywheel—what I need information on is the small round relay that is mounted on the left front fender within the engine compartment. This connects the battery to the starter and causes it to crank the engine—it’s technically a “switch” or starter relay.

I recently had one “go rogue” on me (this would be the second time in a few years) and refuse to disengage the starter from the flywheel. This was—as before—a NAPA one. Yeah, you might think, “Well Forrest, if it happened before perhaps you should stay away from that supplier”, right? I didn’t think it could happen again—what are the odds, right? Guess again.

So, looking at the Motorcraft options, I see one that has the four terminals (including the never-used “I” terminal)—WO133-1698717-MTR, for a 1987 Turbo Coupe, with VIN letter W…and one that has just three, which makes more sense to me. This one is F5TU-11450-BA (or also E9TZ-11450-B)—also known as SW-1951-C. According to the Amazon listing, this last one will NOT fit a 1987 TC, again with VIN “W”. It says it’s for a 1988 TC with VIN letter “T”.

It’s not just the Amazon listing that says that.

Why? Or, more precisely, why is it incompatible with a 1987?

Is it a voltage/current issue? Aren’t the two model years essentially the same?

I am most curious as to what this is about.

I had the same issue with my 88 several years ago. I believe I replaced it with the 4 terminal version and it would occasionally act up as well. I added a quick disconnect to the negative battery terminal which saved my bacon a few times. 
Not one for putting up with things not working as intended, I changed/cleaned all the battery cables/terminals as well as the heavy cable going to the starter and the problem never returned. 
Just for fun, I reinstalled the original solenoid and that still worked properly. I guess the added resistance of the oxidation on the connections contributed to the problem?
One of the heavy battery cables looked to be in good condition but upon closer examination, there was a lot of corrosion (green crud) under the insulation which was only apparent after stripping the insulation back. I replaced this cable as well.
Good luck in your attempt at solving this. Random electrical gremlins are a pain to hunt down.

Thanks for your answer. After this happened for the first time years ago I replaced all the battery cables and the ignition switch, making sure everything was clean, greased (to prevent corrosion) and tight.

The fact that it happened despite that just a few years later tells me it is the starter relay/switch itself, especially since it was the exact same item both times.

I have had the solenoid stick a few times over the last 40+ years in various Fords causing the starter to run continuously. I good hit with my hand, a wrench, etc and the solenoid would return to normal operation. I do carry a spare solenoid with me just in case.

Many years ago had a starter motor stick on in my long gone 71 Thunderbird 429. Grabbed a hammer from the trunk and gave the starter motor a good hit or two which disengaged the starter. Never had that problem again.