TR2/3/3A Fuse to protect OD solenoid

I'm trying to get my A OD adjusted and after two bouts with some vintage Lucas Smoke escaping from my wiring, I decided to add a fuse to the hot side of the OD relay. I had a modern mini blade fuse holder with wires just long enough to reach where I needed to go. What size fuse should I put in this circuit?
I may be back with more OD adjustment questions shortly.
Thanks,

I used a cartridge type fuse that is an 8 amp Slow-Blow - I do not know if there is such a thing in a mini blade type.



I put the connectors on each end for a 'no-cut' installation:



The piece then connects in line with the relay and the existing brown lead:



I'm gonna miss the Radio Shack on the corner.

Like Geo, I used an 8 amp Slo-Blo (mostly because I had a bunch of them handy). However, I believe that even a 5 amp standard (fast blow) fuse should do. That's what Triumph used on early Stags with A-type overdrives. The solenoid holding coil only draws about 1 amp; while the pull-in coil should get disabled so quickly that even a "fast blow" fuse doesn't have time to respond.

That way, the fuse will blow if, for some reason, the pull-in coil contacts don't open; so the solenoid won't overheat and you might possibly be able to fix the problem that blocked the contacts from opening.

TR3driver said:

Like Geo, I used an 8 amp Slo-Blo (mostly because I had a bunch of them handy). However, I believe that even a 5 amp standard (fast blow) fuse should do. That's what Triumph used on early Stags with A-type overdrives. The solenoid holding coil only draws about 1 amp; while the pull-in coil should get disabled so quickly that even a "fast blow" fuse doesn't have time to respond.

That way, the fuse will blow if, for some reason, the pull-in coil contacts don't open; so the solenoid won't overheat and you might possibly be able to fix the problem that blocked the contacts from opening.

Click to expand...

I followed your suggestion Randall and have been running a 5 amp with no issue.
Tom

TR3driver said:

Like Geo, I used an 8 amp Slo-Blo (mostly because I had a bunch of them handy)...

Click to expand...

And, in case it isn't obvious, the reason I used an 8a Slow-Blow is because that's what Randall did. Good to know that a normal fuse works, should may the mini blade type the OP mentions a suitable set-up.

Great information. I think I'll get a 5a mini blade.
I love this forum!!

I was given an old set of overdrive wires that had been on the tranny side. I noticed that the two of the wires going to the lockout switches had been very hot at some point. Will the one fuse being on the input side of the relay prevent this from happening on the output side. I have no idea what caused the initial problem as this was pulled off a car many years ago.
Charley

charleyf said:

I was given an old set of overdrive wires that had been on the tranny side. I noticed that the two of the wires going to the lockout switches had been very hot at some point. Will the one fuse being on the input side of the relay prevent this from happening on the output side. I have no idea what caused the initial problem as this was pulled off a car many years ago.
Charley

Click to expand...

I'm puzzled about "two of the wires". Usually there is only one wire, then a short jumper from one switch to the other. Is there any chance you're looking at the wire that went to the solenoid instead of a lockout switch? It might have melted and merged with the other wire.

The fuse we've been talking about (at least the one I've been talking about) is for the circuit that supplies the solenoid. It will protect the output of the relay even though it's located in the power input to the relay.

But the lockout switches are on the coil (control) side of the relay, which is a different circuit. It seems unlikely to me that you would have a fault that would cause too much current through a lockout switch (maybe a short across the terminals of the relay?) but it would take a second fuse to protect against that. Or another solution would be to connect the operating switch on the dash into the green circuit (eg A4 on the fuse block) instead of the white circuit as original. That would mean your heater, turn signals, etc would quit working if there was a short in the OD relay coil, but that might be an acceptable risk.

I looked again at the wires. It looks like the color coding is not as found in later cars. But the wires that " lost their smoke" we're connected to the C1 and C2 terminals. So you are correct Randall, it was on the brown wire side.
Charley

Great information. I think I'll get a 5a mini blade.

JIM:

To insure your bases are properly covered I would stick with the Recommend 8 AMP Slo Blow fuse..

The inrush current in this Inductive type circuit is approximately 19 amperes but lasts only for a short time -Milli Seconds-as has been noted earlier.