What size fuses?

[RobTR3]

Took the TR3 out today and the overdrive didn't work.

On my return, I did a quick electrical check with the engine off , car in 2nd, ignition on and the O/D switch pulled. Couldn't here the solenoid kick in. 

On checking the fuse box , fuse between A1 and A2 had blown. Took it out and it was a 25a fuse. The 2nd fuse, which was Ok, is 15a - A3 terminal. The moss catalogue says the two fuses should be 50a and 35a.

Looking at the wiring into the fuse box I am assuming where I had a 25a fuse, that should be 50a (A1 - A2), and where I have a 15a (A3) that fuse that should be 35a. Is this correct?

The other question of course is why did it blow - because it is too low a rating? It's been there before I bought the car over 2 years ago.

Any comments, much appreciated.

Rob

[RobH]

If you search the forum there have been a  couple of threads on this topic recently.

 

There is confusion because older automotive fuses are rated for 'blow' current while modern industrial fuses are rated for maximum working current.

It all depends on whether your existing fuses are rated for working current or 'blowing' current.  A fuse rated as 25A working will take that current indefinitely and will require a 3 or 4 times overload to blow in a reasonably short time (though they do fail with long use and old age as yours probably has). 

The Moss book must be referring to fuses rated for 'blowing' current, as to use a 50A running-current fuse would imply it will not blow until the current reaches >100 Amps  which doesn't provide any worthwhile protection.

 Some fuses are marked with both ratings but unless the fuse is specifically marked 'X amps blow' you should take it that the marking refers to running current. Make sure you fit the right type.

 

[bullstreetboy]

As I recall, the Overdrive solenoid is 2 stage - first stage requires a larger current to pull the actuator valve and therefore engage overdrive - and the second  is just to hold the solenoid in place. May be worth checking the adjustment of the solenoid if the fuse keeps blowing.

[RobH]

Adjustment will make no difference to the maximum current drawn.  The pull-in coil takes around 17A and the holding coil around 1A.  If the internal switch fails to operate, the continuous 17A will eventually cook the solenoid but it won't blow the fuse unless the coil shorts out.  A wiring fault to earth will blow it but this is getting ahead of ourselves as Rob has't complained about continuous failures so far.

Edited September 26, 2018 by RobH

[RobTR3]

I should have looked a little closer at the item description in the Moss catalogue. The fuses are 35amp - 17amp continuous and 50amp - 25amp continuous. So 35amp and 50amp are the 'blow' rating.

I have a spare fuse with a rating of 35amp blow and 17amp continuous and so I have replaced my 15amp fuse with this one. I have a few 35amp fuses just marked as such, but I assume these are the 'blow' rating and I have temporarily used one to replace the blown fuse until I get the correct one at 50amp as above.

The solenoid now kicks in under test so I'll see how things go.

No previous history of blown fuses so I'll initially put it down to wear and an the low amp fuse.

Many thanks for the replies.

Rob 

[Guest]

35Amp is the max. Higher rates will melt the plastic around wiring! Most cars have old and brittle wiring, prone to 'fatal injuries' when too hot...  Don't overdo it with a higher rated fuse!

When I restored my car, I replaced the wiring from a 3 fuse-system to a 12 fuse + 4 relais-system from Advance Auto Wiring in the USA

 

[Rodbr]

Sounds exactly the same problem I had a few weeks back. I suggest you put a fuse in and check the operation of the overdrive. If it works switch it off quick.

It would be worthwhile then removing the solenoid and checking the resistance differential to operate and the hold in coil. Check Utube videos on checking the solenoid.

I am convinced that the plunger inside has stuck inside the body and your are getting full pull power which is not being switched to the hold in coil. If the fuse is to high a setting and the high current for pull in current applied you may have an issue with the Lucas smoking rope syndrome.

All things being equal and not having changed the overdrive adjustment a new solenoid fitted may solve the issue.

[RobTR3]

Well, after a couple of further blown fuses and a new overdrive relay switch, it was the solenoid after all. Installed a new one and no further blown fuses.

Having said that, the wiring from the relay terminal 30 or c1 was connected to A2 on the fuse box instead of 'A' on the control box!. However, it had been wired this way before I bought the car and had happily worked for the couple of years I have owned the car. I have now wired it correctly to 'A' on the control box. Could this have caused a problem over time?

I can now here the relay 'click off' momentarily after I switch off the overdrive, which I did not here whilst I had the problem.

Another point when adjusting the solenoid, as I have discovered before, the factory setting i.e. 3/16" dowel etc in the adjusting lever to the overdrive casing doesn't work. The lever needs to be set about a centimetre past the casing hole. Anyone managed to succeed with factory adjustment?

Rob

[RogerH]

Hi Rob,

       'A' on the control box is the same potential as 'A2' on the fuse box.

'A2'  then puts the fuse in the way. The rapid high pulse when the solenoid pulls in is probably what was killing your fuse.

This shows the wiring quite clearly -

http://www.advanceautowire.com/tr24a.pdf

 

Roger

[RobTR3]

Thanks for that Roger. I can see from the diagram it's the same result in a roundabout circuit. At least the way it was wired via the fuse box gave an immediate warning of a faulty solenoid,  by blowing the fuse.

Rob