A-type overdrive problem when hot

[Charlie D]

Just as an experiment I connected a diode the wrong way around across a car battery.

(If you ever try this you really must wear  goggles...)

If you get it wrong on the relay mentioned above, and it is sealed, you either need to break it open to replace the diode or else throw it away.

Heed Rob's advice.

Edited September 5, 2023 by Charlie D

[BlueTR3A-5EKT]

4 hours ago, RobH said:

Yes Peter some do , -  but that means you have to be careful in getting the supply polarity of the relay correct.  Pin 86 MUST go to positive. 

Does your relay look like this?

Then I guess it’s cattle trucked

 

 

copied from here

http://au.rrforums.net/forum/messages/17001/12646.html

 

Edited September 5, 2023 by BlueTR3A-5EKT

[Lebro]

Don't throw it away though, you could fit a modern relay inside the case, & it will look retro !

Bob

[Ralph Whitaker]

On 8/29/2023 at 5:54 PM, Sloop said:

I don't seem to have given a final response on this topic - apologies for the discourtesy.  

In the end I changed the solenoid: it appeared that the 'pull-in' coil was not working properly.  For the first few months - with very little use in the depths of winter - all was well and the o/d worked fine.  The reason I've come back to find this topic is that the same problem has arisen again.

Applying 12v to the solenoid, removed from the car, nothing happens.  With the old solenoid the pull-in doesn't work but the holding coil seems to - I can't pull the piston out.  

I'm reluctant to buy another new solenoid without working out why the last ones seem to have burnt out the pull-in coil.  Does anyone have any ideas?

(Incidentally I've just spent two days doing 700km on French side-roads - it would have been much pleasanter, quieter and more economical with the o/d working!)

John

Hi John, I think your problem is due to incorrect setting up of the operating arm preventing the solenoid pulling fully up to operate the hold in switch, causing it to  overheat. 

 Fit the new solenoid and then carefully adjust the operating arm by the book, you should end up with the solenoid piston fully up in the solenoid at the same time as the small arm on the right of the O/D matches up with the setting holes by using a 3/16 drill. If after a road test the O/d is not engaging it is sometimes necessary to advance the small arm slightly past the setting holes to compensate for wear.

Ralph

[Sloop]

Back again...  now I'm really mystified.

I bought a new solenoid from Moss, and fitted it.  It didn't operate.  To cut what has become a very long story short, I have now established that the solenoid operates when in position on the car and connected directly to the battery with a long lead.  All readings on the relay are correct, and when the o/d switch is on there are 12V at the bullet connector to the solenoid - but nothing happens.  I know the relay is operating - I've opened it up, can see the switch working and have cleaned the points.  The gearbox-mounted inhibitors are all operating correctly: the o/d switch only gives power in 2nd, 3rd and 4th.  

Just as background and in case it helps, this saga started because the o/d failed as the car warmed up.  A new solenoid seemed to cure it - for a while.  In fact I've now found that that solenoid (which I've just replaced...) works perfectly well. 

So - why should 12V coming from the relay not work, when 12V from the battery operates the solenoid...?

John

 

[RobH]

As I understand your description you have had the solenoid mounted in its proper position and that it operates with a direct 12v feed but not when fed from the relay.  You have measured the relay output  at the bullet connector - but the crucial thing is did you do that with the solenoid connected or was it an open-circuit reading so there was no load on the relay?     

As has been explained before, voltmeters take next to no current so can indicate that voltage is present even if the connection is poor.    When fault finding It is best to check for voltage using something that draws a reasonable current - like a side-light bulb for instance with flying leads attached. 

Going on your description I would suspect the relay or perhaps the connections to it, despite your cleaning of the contacts.  

The pull-in current for the solenoid is around 20Amps so the connections all need to be low resistance from the pick-off at A1 on the control box onwards (assuming you still have one) . If you try operating the OD with the ignition on but the engine not running (pull the  white wire off the coil to save cooking it)  you should see the 20A shown as a 'flick' discharge on the ammeter as you operate the switch. 

 

 

Edited October 21, 2023 by RobH

[ianc]

Over the last 30 years, I have had at least three 6RA overdrive relays fail, much in the way yours seems to be failing.   The relay clicks and, with no load (i.e. solenoid) connected, 12V appears at the relay output.  But when loaded, relay failed to energise the solenoid.

Basically, relay knackered, and had to be replaced on each occasion.

To try and reduce the problem, I now have a small diode connected in reverse and in parallel with the solenoid in the hope that the back e.m.f., which occurs on de-energisation of the solenoid and which may well be the cause of relay contact failure, will be reduced.  Still too early to say whether this will reduce or even eliminate relay failure.

Ian Cornish

[ianc]

Looking back on my notes, I see that my TR4 had munched 5 of the 6RA relays in 30 years.

RobH, in topic "Does the overdrive have a relay" on January 13th 2016, wrote a better description of the quenching offered by a reversed diode.   Can be found by searching for "e.m.f." (takes a few seconds for the search).

Ian Cornish

[Sloop]

Thanks gents.  In fact I've tried connecting the solenoid to the input to the relay (to C1 as I recall) with no reaction: but just a quick touch of a wire from the solenoid direct to the battery gives a small spark and an immediate response.   But I guess, as you suggest, that the problem must be a poor connection somewhere down the line.  I'll rig up a load as you suggest, Rob, and see what happens. 

I wonder if the problem might be in the egg-shaped o/d switch....  Going back to the original problem (o/d failing to engage after about half an hour's driving, but working again when the car cools down), I suppose that something in the switch could be heating up and not making contact when the o/d is used for a while.

And when I finally track the problem down, I'll certainly think about a diode.

I'm away for a couple of weeks so won't be able to pursue things till early November.

John

[ianc]

The current through the dash-mounted overdrive switch is very small, but the switch can play up - after all, it's probably over 60 years old and (like the owners) they do wear out !

Moss sell a replacement switch (112474), but it is £70.  I think the actual switch inside the case is as the original, so could be fitted in the old case to preserve originality!

Ian Cornish