Hot overdrive wont engage

My XK150 overdrive works flawlessly, engaging and disengaging instantly at the flick of the switch that is until the engine (and presumably gearbox) gets very hot (which it does in our mid 30’s C summer temps). It will stay engaged alright even when hot, but once it’s hot it refuses to engage or re-engage.
I’ve confirmed that it’s not an electrical gremlin, so wondering if the gearbox oil is too thin (causing low O/D oil pressure) or if it’s an adjustment thing which possibly changes with temperature.
Appreciate any thoughts on what I should do next.

Just my two cents as the DB4 is basically the same set up but a different box.
Is the plunger FREED UP? Its a magnet set up which pulls the plunger up and down to kick it into overdrive. In expensive to replace.
Grant it , it can be the overdrive unit, but the solinoid is adjustable.
I think its the adjustment when hot.
I think those complete units are under 100 dollars.
Again,you can adjust it…
Good luck.
If the unit DID NOT SHIFT at all then the clutch packs are toast, your saying though it goes in…

Graham if have not already checked the oil level or had it replaced with 30 engine oil I would look at that. Does your 150 have a fill level plug on the side of the gearbox or are you using the dipstick inside the cabin? I fill mine to the level on the gearbox side. A clean gauze filter is something else to look at when the gearbox /overdrive is drained prior to replacing the oil.

Years ago, I put 10W-40 oil in my MGC overdrive. Worked great when cold but would not work hot. I guess the oil got to thin even though it was supposed to act as 40W when hot. Multigrade oils are also detergent oils so best to go with a non detergent straight weight oil. Try some 40 or 50 straight weight oil. If that does not take care of it, then you have some internal problems.

I have A type OD’s in a couple different marques. I had the same problem in the one in my Triumph. If your solenoid unit has ever been replaced, I have learned that in later units the pin in the body that switches the contact switch between the pulling coil and holding coil will swell with heat. Older units look like they used a bakelite material for the pin and newer units from the 70’s forward had a nylon plastic pin. Testing on the work bench I could duplicate the exact problem by using a heat gun. Cool, it would pull and release but after the temp would rise the pin would stick meaning that the contact switch would be held in the holding coil mode. Once the unit is stuck in this holding mode the solenoid plunger can’t be overcome the mechanical resistance of the operating lever of the OD unit. Once it cools down the plastic pin would release and the pulling coil circuit would be active and ready to engage when 12 volts is applied

Wow, some great advice here thank you all. I think I will change the gearbox and overdrive oil as a matter of course and clean that filter. then look at the adjustment and take a look at that solenoid. Will this oil work ok in the gearbox/overdrive?

Graham, All good advice…
Please make sure that oil is non synthetic and apl4 not 5.

Use this it’s like glue and makes the box perfectly quiet just get the right grade
Good luck
I promise you this stuff is the best I learned the hard was going red line synthetic it was terrible
I’m not sure if your overdrive is a different grade

SAE 30 is correct.

I never heard any reason why it would make any difference whether you use detergent or non-detergent, other than cost. Non-detergent SAE 30 is commonly used in air compressors.

The difference is the modern detergent oils eat away at the old brass synchros, also some of us have experienced terrible noise, whining and terrible shift points. Its too slippery!
Go on amazone and the above company surely makes it in 30 w…GL4 NOT GL5
You will think the transmission is rebuilt…Then go to the plunger/solinoid , Im betteing like others its time for a new one.
Good luck

We don’t have brass synchros, nor spiral nor hypoid gears in the gearbox; these GL4 & 5 are for rear axles.

Rob its the overall, seals, gears and all, overall I have found using the modern synthetic/redline and all wont work well…
But hey maybe I missed something.
That penngrade is like GLUE…
Let me clarify, We used old Castol back in the day 75/90, its now synthetic and gl5 .
We used them for the rear and the transmission, but it was NON SYNTHETIC and GL4.
Now also has posilock additive where it didnt years ago.
Just check if the 30w is just 30w…no posi lube,no gl5 on the bottle.
Thats all.

Fixed the problem!
Contrary to my first post it turned out to be electrical in nature but heat related. The overdrive relay mounted fairly close to the exhaust manifold was failing to close after it got very hot. I tagged and disconnected all the wires figured out where they all went and tested all the circuits, they were all fine, so I opened the relay itself (not easy) and tested it by blowing hot air over it with a heat gun, sure enough it worked ok at first then as it got hot the electromagnetic windings could not pull the contacts closed. The windings are centre tapped to ground from the W1, W2 connections so only half the coil actually pulls the contacts closed, by grounding out the W1 side (and disconnecting/abandoning/insulating the original wire to it) it was strong enough to work again.

Your relay coil now pulls a different current I wonder whether this is the difference between the 6ra and the 22ra relays:- one is for continuous use and the other for momentary use (the 22ra is a starter relay).

kind regards