A-type overdrive problem when hot

[Sloop]

My overdrive (TR 3A) works reasonably quickly when the car, and gearbox, are cold.  As they warm up overdrive reaction to the dashboard switch becomes slower, and finally stops.  Engaging takes longer - it normally disengages immediately.  When the car is at normal operating temperature it ceases to engage at all.

With the engine off I can hear the relay working in 2nd, 3rd and 4th, though there isn't any 'thump' from the relay.  

I presume the problem is either not enough oil, or the wrong kind.  So two questions:  do I need to drain the gearbox or can I top up, and what oil do you recommend?  I'm in SW France, so ambient temperatures in summer can be high.

Thanks!

John

[RogerH]

Hi John,

if you can hear the OD relay click but not the solenoid clunk then the problem is not oil but probably electrical

I doubt if it is wiring as that should not be affected by heat (usually)

The inhibit switches may need  adjustment (they can be affected by heat)

Does the problem happen in all possible gears or just 4th.

 

Roger

[peter clarke]

What oil do you have in the box. straight 30 engine oil will thin when it heats up and you may have pressure loss at the pump/piston seals due to a bit of wear, if not using a 75/90 gl4 gear oil try it and see if it makes a difference before anything else.

[John McCormack]

9 hours ago, peter clarke said:

What oil do you have in the box. straight 30 engine oil will thin when it heats up and you may have pressure loss at the pump/piston seals due to a bit of wear, if not using a 75/90 gl4 gear oil try it and see if it makes a difference before anything else.

My gearbox packed up in the mid 1990s. The people who overhauled it said a contributing factor was that it had been filled with hypoid EP90 diff oil. I'd had the car serviced for a few years by a local garage when I was based on a Navy base with nowhere to work on the car myself.

I'm not too knowledgeable on oils and their qualities but, on the recommendation of those who do know, I now use a GL4 oil (Castrol VMX80) and previously a Castrol 25-50 engine oil. No problems with the box or overdrive since that overhaul way back in 1995, and well over 100,000 miles.

[Sloop]

Thanks gents.  The problem is in all gears and is definitely heat-related (and reversible when the car cools down) so I'm still inclined to think it's oil.

I'll find a medium-viscosity gearbox oil, drain the box and refill to see what happens.  No hoist available so it will be a warm afternoon with ramps and axle stands!

Will report back...

[RogerH]

Hi Sloop,

think about this step by step.

You say that when it is not working you can here the relay 'click' in but not the solenoid come in with a clunk.

How can it be the oil !!!!

One area that repeatedly occurs is that the inhibit switches being set to a marginal position.

But you experience this is ALL gears.

You need to remove the GB tunnel and closely inspect the wiring on the two inhibit switches.  

Using a mulitmeter check that the switches are working when HOT 

 

Roger

 

[Ralph Whitaker]

I would also suggest that as your draining the oil anyway, remove the large brass nut from the bottom of the o/d and have a look at the filter and magnetic rings if fitted. A blocked filter would probably have the opposite effect from your problem, in that the cold, thicker oil would struggle to get through the filter if blocked, but warmer thinner oil might pass easier, but an examination of the filter might give an insight into the condition of the o/d. For example lots of cork like bits would indicate that the brake rings are wearing or breaking up.

Ralph.

Edited June 1, 2022 by Ralph Whitaker

[ianc]

You say the problem exists on 2nd, 3rd and 4th gears, and that would mean that two different isolator switches on the gearbox cover would have to be misbehaving - somewhat unlikely.

If the relay is clicking but the solenoid fails to operate with a clunk, then you may have a defective solenoid.

Ian Cornish

[Sloop]

At last got round to changing the gearbox oil yesterday - to Dynolite Gear Oil 40 as recommended by Moss.  The old oil (whatever it was) looked surprisingly clean.  A lovely job with the car on ramps and axle stands...

Also checked the operation of the solenoid, which was initially a bit reluctant but then appeared to operate correctly, with all the right inhibitors working.  The plunger retracted quite quickly and returned more slowly - I guess this is normal, and disengaging the o/d has never been a problem anyway.  I checked and cleaned all electrical joints.

Test run afterwards seemed initially good, with o/d engaging and disengaging quickly in 2nd, 3rd and 4th but once everything had warmed up properly there was a delay after switching, then eventually no engagement at all in any gear.  

So it's not the oil, and the electrics appear to be working correctly.  Checked the relay, which is clicking OK and giving the right readings on a meter.

So I suppose it must be the solenoid.  Does anyone know if it is possible (or sensible) to change it from underneath, without removing the tunnel?  I'd prefer not to disturb that if possible.

Thanks!

[stuart]

19 minutes ago, Sloop said:

At last got round to changing the gearbox oil yesterday - to Dynolite Gear Oil 40 as recommended by Moss.  The old oil (whatever it was) looked surprisingly clean.  A lovely job with the car on ramps and axle stands...

Also checked the operation of the solenoid, which was initially a bit reluctant but then appeared to operate correctly, with all the right inhibitors working.  The plunger retracted quite quickly and returned more slowly - I guess this is normal, and disengaging the o/d has never been a problem anyway.  I checked and cleaned all electrical joints.

Test run afterwards seemed initially good, with o/d engaging and disengaging quickly in 2nd, 3rd and 4th but once everything had warmed up properly there was a delay after switching, then eventually no engagement at all in any gear.  

So it's not the oil, and the electrics appear to be working correctly.  Checked the relay, which is clicking OK and giving the right readings on a meter.

So I suppose it must be the solenoid.  Does anyone know if it is possible (or sensible) to change it from underneath, without removing the tunnel?  I'd prefer not to disturb that if possible.

Thanks!

You will struggle to change it from underneath Im afraid. Have you checked the adjustment by checking the other side where you use a 3/16" pin through the cross lever as its possible that the adjustment has slipped, I know its possible to correct this from underneath.

Stuart.

[RogerH]

2 minutes ago, stuart said:

You will struggle to change it from underneath Im afraid. Have you checked the adjustment by checking the other side where you use a 3/16" pin through the cross lever as its possible that the adjustment has slipped, I know its possible to correct this from underneath.

Stuart.

+1

if the adjustment is marginal when cold then metal expansion, when hot could certainly stop it.

When using the 3/16" drill in the rigging arm consider moving the arm the meerest fraction further clockwise. 

Do take the tunnel off - that way you can have a really good look at what is going

 

Roger 

[Sloop]

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

[RogerH]

Hi  John,

the 12V supply to the solenoid is pretty simple and should not be the problem.

It could be that you have been very unlucky.

The pull in should be momentary (quite a high current -  22amps) and then flip to the hold in low current (1 amp)

Have you had a hunt around for a used solenoid.  Failing that get a new one and rig up a test circuit that could cycle it many times.

 

Roger

[RobH]

It could be that the solenoid internal contact was not operating, so that the pull-in coil was continuously powered when in OD. That will burn out the coil in time and may also explain why things failed to operate after a while when it was really hot.  The switch operates towards the end of the solenoid travel - could something be limiting the travel so the switch never operates?

edited to add:

If you still have an ammeter you can check this by operating the overdrive with the engine not running. You should see an initial 20 odd amp drain 'kick'; which quickly drops to a couple of amps.  If it remains at 20amps that is your problem. 

 

Edited August 29, 2023 by RobH

[Lebro]

+1 most likely 

Bob

[Sloop]

Thanks to all.  I've checked that the 'points' switch on the solenoids (original and replacement) were clean and working - but maybe something has been preventing the solenoid piston from going in far enough to open them  I'll get the ammeter out and investigate, but one way or another it looks like another new solenoid.

John

[stuart]

38 minutes ago, Sloop said:

Thanks to all.  I've checked that the 'points' switch on the solenoids (original and replacement) were clean and working - but maybe something has been preventing the solenoid piston from going in far enough to open them  I'll get the ammeter out and investigate, but one way or another it looks like another new solenoid.

John

Check adjustment on the actuating lever as that can stop it going in far enough to change coils.

Stuart.

[ianc]

Minor correction to Stuart's comment above.

The armature doesn't cause a change in coils - both coils are energised when overdrive is engaged (the coils are in parallel).

When the armature rises to the top of its travel, it lifts a contact which de-energises the pull-in coil, leaving just the hold-in coil energised.

Incidentally, to reduce wear on the relay's contact caused by the back e.m.f. as overdrive is disengaged (only oldies will understand this!), one can mount a small diode in parallel with the solenoid and in the reverse direction.   My car has eaten some 3 overdrive relays in 30 years and I am evaluating this modification, which I ought to have incorporated sooner as it is what I was doing some 60+ years ago when we built data loggers for tank farms using Post Office relays.

Ian Cornish

[RobH]

A brief explanation required here I think. 

When a coil (e.g solenoid or relay etc) has a voltage applied to it via a contact it sets up a magnetic field in the coil.  

When the voltage is removed as the contact opens, the magnetic field collapses very quickly and generates a voltage of opposite polarity to that which was applied and generally much larger. This is the 'back emf' (ElectroMotive Force) and is what causes contacts to burn because it strikes an arc as the contact opens. 

[emf ∝ BLV where B is the magnetic field strength, L is the length of the coil winding and V is the velocity of the change in field strength.   Since emf is proportional to velocity, the almost instantaneous collapse of the field as the contact opens can produce a spike of hundreds of volts. ]

By placing a diode across the coil in reverse polarity, the diode will conduct under the reverse voltage and reduce the back-emf to about 1 volt.  

Another means of protection is to have a low value capacitor (condenser) across the contact, as this provides a means for the back emf spike to bypass the contacts. That is the method used with the ignition points.

Edited August 30, 2023 by RobH
typo

[ianc]

Correct, Rob - I was being lazy by not providing a technical explanation.

However, a condenser is much larger than a diode, and for the solenoid, mounting a diode in parallel with the solenoid is very easy.  I have put a double bullet connector to the solenoid's feed with the second bullet feeding the reversed diode, with the other end of the diode connected to the screw at the foot of the solenoid.  I have encased the diode and its connecting wires in a plastic sleeve so as to avoid accidental short-circuit.

Ian Cornish

[Nobbysr]

On 6/1/2022 at 1:15 PM, John McCormack said:

My gearbox packed up in the mid 1990s. The people who overhauled it said a contributing factor was that it had been filled with hypoid EP90 diff oil. I'd had the car serviced for a few years by a local garage when I was based on a Navy base with nowhere to work on the car myself.

I'm not too knowledgeable on oils and their qualities but, on the recommendation of those who do know, I now use a GL4 oil (Castrol VMX80) and previously a Castrol 25-50 engine oil. No problems with the box or overdrive since that overhaul way back in 1995, and well over 100,000 miles.

essentially all oils lose viscosity as they  get hot  and sae30 egine oil is about the same viscosity as an 85w gear oil at ambient temp(see Chart) howver diffferent types of oils will thicken at different viscosities as the oil gets cold. So a 10w/40 engine  oil will circulate  round an engine quicker that a 20w/50 especially on a cold winter morning 

An engine oil contains addatives to  neutralise acids formed during combustion (BN), Detergents which keep the engine clean and Dispersants to hold the particulates in suspension which allows the filter to remove debris and allow the particulates to be removed during an oil change. There are other addatives such as anti-wear, anti-scuff, viscosity improvers, ant-oxidants etc. The base oils used in engine oils also vary in quality  as do addative packs and approvals . Some Lubricants Blenders claim approval that their oils have never been tested to  including API

Gear oils have to cope with oil molecular shearing which causes some loss of  viscosity, exterme pressure loading , scuffing and  preventing metal to metal wear, so they will be subjected to gear specific test such a 4 ball antiwear and micro oxidation tests . There are claims that Sulpur Phospherous  EP addatives gear cause problems with overdive brake linnings and can cause pitting in bronze/brass material   but todays active Sulphur EP addatives are pretty benign an i would rather have ep protecton in the sprag clutch and various bearings 

But please be aware that oil addatives deplete during use  so extending the life of the oil is false eccomomy. if you would like additional oils added to the spread sheets just let me know and I'll load the technical data..  Have tried not throw too much data in this post   but if you have any specific questions  I'd be happy to answer them.  

hope this helps

Steve

Gearoil comparisons.xlsx engine oil comparison.xlsx

Edited August 31, 2023 by Nobbysr

[Charlie D]

On 8/30/2023 at 8:41 PM, RobH said:

...This is the 'back emf' (ElectroMotive Force) and is what causes contacts to burn because it strikes an arc as the contact opens. ..

By an amazing coincidence, just two days ago, I have had a back EMF problem causing errors on a self-playing glockenspiel that I’m building.

I have a microprocessor (Arduino) switching relays, which in turn operate solenoids, which strike the glockenspiel tubes.
I KNEW that I should have put diodes across the solenoids, but didn’t bother (lazy). Result being that the sparks from the relay contacts (I think) caused the software to crash after a few seconds of playing.

I even tried feeding the power supplies to the solenoids from a totally independent battery not electrically connected in any way to the Arduino power supply. The software still crashed.

Once diodes were installed, it all worked perfectly.

I guess the spark was sending out radiation strong enough to upset the electronics in the microprocessor.

Most people probably don't have a self playing glockenspiel inside their TRs, but I guess some people have home brewed electronic fuel injection. I wonder if EMF radiation from non-spark suppressed relays/solenoids could mess up their operation, leading to a lot of head scratching.

Considering that there are places on classic cars where relays operate inductive loads (e.g. the horn and overdrive solenoid) I’m surprised that diodes were not wired in as standard.

Maybe worth fitting a diode across any relay operated inductive load. I would guess an IN4007 would do, but maybe Bob or Rob could confirn.

Charlie

 

[RobH]

Yes a IN4007 is fine for this.  

When the sidescreen cars were made, semiconductor diodes were expensive, relatively new and voltage restricted.  Since there were no electronics anywhere on the cars (not even a radio on most) it probably never occurred to anyone to fit spark suppression other than where absolutely necessary  ( i.e. the ignition points ). 

As an aside, many years ago I fitted an aftermarket steering wheel to my Austin Healey, and this had an aluminium horn push. I usually drove with thin gloves in those days so all was fine until I submitted the car for an MOT.  My did the tester jump when he sounded the horn !   

[BlueTR3A-5EKT]

Does this image suggest modern plug in relays incorporate the diode being discussed?

 

[RobH]

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.