question about external starter solenoid (64.5 mustang))

So we're finishing up this mustang, total resto/rod that seems to have a taste for starter solenoids.

it has under 1000 miles on it with nearly all new stuff, and maybe 50-100 miles ago when we turned the kill switch to ON the engine started cranking with or without the ignition switch on. it was shorted through the high side of the solenoid, changed it, and since it's been great for a large number of starts (tuning, moving, etc.) but not a lot of miles driven yet. the other day we were out putting a few miles on it, flawlessly, came back to the shop and shut it off. when we tried to move it maybe 15 mins later the same problem was back, cranks with cutoff live but regardless of ignition switch. if the ignition is on it'll start but the starter will stay on. we disconnected all 4 wires from the solenoid and it to has a short between the two high side terminals...

do we just keep throwing solenoids at it? (not acceptable for the customer really) i dont think i was overheating from exhaust as it felt like room temp to the skin, but perhaps there's too much current draw to the starter and it's welding it shut? we're going to check starting current draw with an amp clamp this week but might the GRM braintrust have some ideas for us to check out further?

I think, from your description, that the solenoids are failing closed. I doubt that over drawing would really do it, but it is possible. Otherwise, maybe you're just buying junk solenoids? I know everything comes from China today. Maybe try a new solenoid from NAPA, who seem to source the best that's available.

I wonder if there's something screwy with the small terminals. Apologies if you already know this, but one small terminal is the start signal IN, the other one is the ballast resistor bypass so the ignition gets full voltage when cranking.

the first to die was a FoMoCo model, the second i dont know, but i'll try to get the new one from napa.

i havent personally diagnosed any part of this yet, just watching from a curiosity point of view thus far, but i imagine i'll be the one put on it when i get back from kansas on tuesday. and unfortunately the guy who put together the electrical system initially is no longer working with us so i'll be starting from scratch. but i'll definitely have to take a look real close look at those small terminal leads and make sure it's all ok, thanks guys!

Don't discount the possibility that it is the ignition switch.

If you can recreate the problem pull the wire from the "S" terminal while it is occuring---if it stops cranking it is not the solenoid and you need to determine why the wire is hot when the ignition switch is not in "Start". If it continues to crank without the "S" wire it points to the solenoid.

Good luck,

Jeff

JKleiner beat me to it. Low voltage/high amperage (long time cranking with a crappy battery) will weld the contacts but that won't happen while driving.