[TR] Overdrive solenoid

[Randall]

> So, anybody know offhand how many amps the solenoid draws, including any
> spikes?  I imagine I'm saving the solenoid with all this, but then maybe
> it's the solenoid telling me it wants to self-destruct.

Yup, your fuse is actually doing it's job protecting the solenoid.  You've
got a serious problem if it is blowing 10 amp fuses during operation.

The A-type solenoid draws a big jolt of current when it is first engaged,
roughly 20 amps.  But that should last for only a fraction of a second, just
as long as it takes the plunger to move.  When the plunger hits home in the
solenoid body, it hits a pushrod that opens the contacts inside the end
cover, which in turn cut the current to the high current "pull-in" coil in
the solenoid.  That leaves only the low current "holding" coil in operation,
which only draws about 1 amp.

If anything happens to prevent the contacts from opening, that's when the
solenoid burns up as it cannot hope to dissipate the heat from continuous
operation of the pull-in coil.  In my case, it was a tiny bit of gravel or
sand that had worked its way into the housing and jammed the plunger.
Obviously, what you want is a fuse that will blow before the solenoid
overheats.

Fuses work on heat, so even a 5 amp fuse should take longer to blow at 20
amps than it takes the plunger to move.  Triumph used a Lucas 10 amp fuse
(which is roughly the equivalent of a 5 amp American fuse) for the OD
solenoid on a Stag.  I used an MDL-8 fuse just because I had a big box of
them on hand (a previous employer was throwing them out when the sonar line
closed), but I know of people that used AGC-5 with no problem.

Another thing that can happen: on some OD units there is supposed to be a
rubber bumper that limits how far the solenoid plunger can extend when the
solenoid is not powered.  If the rubber rots and falls out, the plunger may
extend so far that solenoid has a hard time picking it up against hydraulic
pressure.  Here's a photo where you can perhaps see my solution
http://s258.photobucket.com/user/TR3driver/media/A-type%20OD/DSCF0055.jpg.ht
ml
If that doesn't work, try this one:
http://tinyurl.com/znrrvsp

You can see a slightly different solution towards the end of the article at 
http://www.buckeyetriumphs.org/technical/AOD/AOD4/AOD4.htm

Randall