> An under the car inspection revealed that something was blocking the metal
> plunger from pulling fully into its bore.
>
> Forward several months later to last week when I finally replaced
> the thing
> and I can see that some white plastic (looks like nylon) melted
> and dripped
> down into the solenoid bore and formed a stalactite.
>
> Once hardened, the stalactite kept the plunger from coming all the way up
> and shutting off the high amperage "pull" circuit.
>
> This promptly fried the sucker pretty well.
>
> At least that's how I take it things happened. There could be some other
> possibilities.
Scott, my take is that your 'stalactite' is the actual pushrod; and that the
solenoid got hot enough to melt it. All it takes is a piece of grit from
the road finding it's way in there, which might have fallen out later.
> I've installed an inline fuse in the circuit . .but I don't know if it is
> sized correctly or is the right type.
>
> I guess what I'd like to have is something that will allow a momentary
> (fraction of a second) surge of high current (20amps?) for the
> pull circuit
> and also hold up under the low current (2 amps?) holding circuit.
What you want is called a 'slo-blo' fuse, they're pretty common in
non-automotive applications. You could also use an automotive circuit
breaker, they are thermally operated and take a long time to open; but I
don't know if you can find one small enough.
I am using a MDL 8 slo-blo fuse, mostly because that's what I had on hand.
It is rated to blow in 7 seconds at 20 amps, which I'm hoping is enough to
protect the solenoid (although I haven't tried it). Nelson Riedel has
written that the holding current is closer to 1 amp, so you could go with a
smaller fuse to get a quicker blow. An MDL 2 will typically carry 20 amps
for about 0.2 seconds, which should still be enough time for the solenoid to
move. You could also try an ordinary (not slo-blo) AGC 10; it will
typically last for 1 second at 20 amps (according to the Buss data sheet).
My fuse is in an in-line fuse holder, in the wire from the ammeter to the
relay under the dash (59 TR3A).
Randall
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