‘A’ type compact overdrive issues

[Flostles]

Hi

I’m currently experiencing issues with an ‘A’ type compact overdrive fitted to a 2.4 ltr ‘67 MK II. A ferret around the internet does not seem to have come up with results similar to these issues.

When cold it will swap in and out of overdrive with no issues. If selected whilst cold and road conditions allow then overdrive will remain engaged. Once hot the overdrive will not engage with no click being heard from the solenoid.

First thoughts were low oil pressure. After topping up the gearbox oil the problem remains.

It does not appear to be down to the electrics as 12v is being passed through to the solenoid when both cold and hot. Hopefully that means the 4th gear switch can be eliminated.

I suspect the next thing to try is a replacement solenoid, but before going down that route (and associated configuration) is there anything else I can try?

Best regards...

Ross

[piman]

Hello Ross,

you need to get the gearbox really hot and then get under the car and have someone operate the overdrive, it will be evident if the solenoid is functioning, as you will see the arm move (Opposite side of the overdrive to the solenoid). The setting can also be checked as there is an alignment hole in the arm and gearbox casting. You need a 3/16" rod or drill and it should pass through the arm to the gearbox casing. If not then the arm needs adjusting. Much cheaper than buying a new solenoid as a fault finding method. If you have access to a multimeter , check the current draw of the solenoid, initially it should be about 10 amps dropping down to about 1 or so when fully engaged. (It is a two part winding with an internal switch to cut out the high power pull in winding)

If the solrnoid is working and correctly set there is a pressure relief valve and a port on top of the overdrive casting to do a pressure check.
I suspect that your overdrive is low on pressure. It really depends on your facilities and also I don't remember how easily it is to access these bits? A workshop manual is very usefull, well essential really.

Alec

[PPV]

When you say that 12v is being passed down to the solenoid when hot are you actually checking the supply at the solenoid? Just a thought as it may be a problem with the supply as mechanically it seems strange that it works when cold and not hot and there are not any strange noises coming from that area. It seems solenoid/ supply issue to me, at least it is easier to check them before taking the next step.

[Flostles]

When you say that 12v is being passed down to the solenoid when hot are you actually checking the supply at the solenoid?

At the moment I have disconnected the solenoid and connected a multi-meter in its place. A very rough and ready way of checking, but hopefully rule out electrical gremlins.

[piman]

Hello Ross,

a multimeter won't necessarily give an accurate result. If you left the solenoid in circuit you will get a proper reading. The reason being when the circuit is open and no current is flowing, a poor connection in circuit will not show up and you will read full system voltage.

There is little reason that the solenoid will work when cold but not when the gearbox is hot. What that does affect is the oil pressure which is why I made the earlier remark.

Alec

[PPV]

I agree with Alec, that is why I suggested just checking that the supply and solenoid was good as then the cause is likely to be oil system related.

[Flostles]

Hi

Thanks for all your responses to date.

Connecting the multi-meter in line gives a reading of ~1 amp when the solenoid is engaged (I did not notice the 10 amp initial spike, presumably the meter being too "laggy").

When cold I get 1 amp, a satisfying click and the overdrive works.

When hot I get 1 amp, no click and no overdrive.

I presume a reading of 1 amp indicates the solenoid has withdrawn the plunger and gone onto the maintenance winding in both cases. Am I right in thinking that even if the oil pressure was low you should still here a click as the plunger withdraws? I'm not hearing a click when hot, most strange...

Going back to Alec's original post. I'm not seeing an arm on the opposite side to the solenoid. Did they enclose the mechanism on later compact units?