XK 150S overdrive not working properly

Before I restored my XK150, the electric overdrive worked properly, i.e., flip the switch and it engaged. To disengage, you merely switched to off. Now, it engages properly, but when I back off the gas - even a little - it goes back into 4th gear on its own. I’m pretty sure this is not how it is supposed to operate. Any thoughts on what might be wrong. Note: the solenoid is new and all the wiring and fluid levels have been checked.


Hi Jerry,

I have no direct experience, as my o/d is functional. I understand that the solenoid has two windings, one to perform the initial activation and one to hold it in position. The former cuts out when the o/d actually engages. Perhaps the holding winding is not working.

There is also a gearbox inhibitor switch to prevent o/d working in all but 3rd and 4th gears, and a throttle inhibitor switch. Perhaps these or their wiring are faulty.


How low is your speed when it’s popping out of overdrive?
We tend to drive a 65 year old car like a modern one.
Overdrive should be switched on around 60 plus
If your doing 40 your dumping the gears
It’s should drop off if your that slow
Make any sense?
Also it should not engage in anything but 4th
Good luck
Sorry Welcome aboard!!!

Let’s bear in mind he probably has a 4.09 rear gear so the output shaft of the tranny will be turning quite a bit faster than a shorter rear gear car would require. I see no reason why the pump and accumulator system should not be able to keep up the pressure even at moderate speeds, though the OP did not say at what engine RPM this problem is cropping up.

I would also suspect the “holding coil” as one of the possible causes here.


Thanks. I will check on these.


It pops out at all speeds…low or high. I have driven as fast as 65 mph and it pops out…really creates a jerk in the gearbox…which is another reason to know that it should not be this way. Before all this, I could drive at quite low speeds (35mph?) and it would stay engaged. I know that is hard on the gears but I just wanted to see if it worked. I cannot recall it ever popping out of OD at low speeds, but maybe I have just forgotten. Also, the owners manual says nothing about OD popping out when you reduce the pressure on the accelerator. It only speak of the “in” and “out” function and it should be used at speeds of 50mph and higher.


It happens at all engine speeds when the OD is engaged.

It could be the supply(from the oil pump) check ball is not sealing properly, leakage past the accumulator piston rings(or O-rings), leakage past the operating pistons rings(or O-rings), holding coil, (although one would think that problem would trigger the “pull-in” coil repeatedly), or the complete electrical system, including the shift rod switch and relays.

It is fairly easy to check the solenoid operation from under the car…and if not the problem, to remove the accumulator cover to check the piston rings, heavy spring and check ball, spring and ball seat.

Hi Jerry,

I was wrong, the solenoid has only a single winding.

So solenoid is obviously working for o/d to engage. Question is, is the solenoid losing power or is the issue in the hydraulic circuit?

I’ll look at the electric circuit to see what could cause loss of power. There are the switches I mentioned plus a relay

Sorry for the bad info.

Hi Jerry,

I have tried to figure out the electrical circuit, and it is possible that the relay is missing a jumper wire between the C4 terminal and the W2 terminal, or the internal contacts for C4 are corroded. The C4 to W1 connection “latches” the relay in the overdrive engaged state. Without this, the relay will disengage the overdrive whenever the throttle is closed. However, the overdrive would re-engage when the throttle is reapplied.

This failure would only occur if your throttle switch is installed and working. My car, as many others, has had the switch removed.

I found the internal wiring of the relay in a Sunbeam Alpine workshop manual:

When the overdrive switch is set to “in” and the mechanical throttle is not closed (causing the throttle switch to be electrically closed) then the relay winding is energized. This closing of the relay contacts means power is routed from C2 to both C1 (the solenoid) and C4, which is connected to W1. This means that if the throttle is now closed (causing the throttle switch electrically to open), power is still fed to W1 from C4, keeping the relay energized and hence the solenoid also energized.

When the overdrive switch is thrown to “out” and the throttle is not closed (so the throttle switch is closed), then W1 is electrically connected to W2. This connects the two ends of the relay windings - i.e. shorts the coil out - so the relay contacts open disconnecting power to C1, the solenoid.

I found this post giving tips for finding fault in the hydraulic circuit: [xk] 150 overdrive - #5 by Gary_Lindstrom.


Thanks to all for your suggestions. I did some fiddling with the OD and took it for a test drive. I believe it was working fine…it would not engage below about 45 mph (as I believe it should not) and did engage at speeds slightly above that. However, it still kicks back into 4th gear if I let up on the accelerator the least little bit, but it goes back into OD (on its own) if I press on the gas. Maybe this is the way it is supposed to work, I don’t know. If this is correct, when it is engaged it becomes kind of a 5th gear that comes and goes with speed…not unlike modern cars except for the lurch when it goes back into 4th. Nothing in the manual about that, though. I wish that was the end of the story. After about 20 miles, the OD ceased working at all and I can’t figure out why. I do have the wiring diagram for it (which I will try to include here if I can figure out how to do it). My mechanic has checked everything and can’t find anything wrong…except that it doesn’t work! I would really like to hear from a 150 owner who has the electric OD and hear how theirs actually operates when driving. I also have a 150S with manual OD and believe me, it is far less trouble than the electric one!

Thanks to all for your help and advice. I will keep you posted
Sorry…I tried to paste the wiring diagram like Clive did but couldn’t get it to work.


Shifting in and out of O/D on it’s own is NOT correct. Besides, if the thing was operating like an automatic O/D does, it would DOWNshift when you hit the gas and UPshift when you let off. It is sounding more and more like a pressure problem to me. You mentioned how the thing stopped working at all after 20 miles…hotter oil will be thinner, less pressure.

I am not necessarily advising you do this, but I would run the car up around to 75 or 80 to see if the O/D works properly at that higher engine RPM…more RPM = more pump strokes per given time, possibly compensating for leakage. This could tell you if it is an internal leakage problem.

The later 140’s were equipped with a throttle switch to help match revs when coming out of O/D…your car probably has this switch so something else to check.


The o/d should not drop in and out, mine does not.

I suggest rigging up a test light or volt meter to see if relay output to solenoid stays on when o/d drops out.

If it is a hydraulic problem, that may explain the failure once the oil got really warm. Alternatively the solenoid burnt out.



I’ve been hesitating to give my ideas regarding your problem, but I guess that you should not exclude a mechanical or better hydraulic problem in your box.
As you know there is an oil plunger pump and an accumulator inside the overdrive that builds-up (and keeps!) the pressure to switch into overdrive. Are you sure that nothing has been changed during the restoration? Has the oil in the overdrive become dirty and should it be changed? Has the adjustment of the operating valve been changed?
Your description of the problem may well have been caused by an oil pressure “on the edge”. When you slow down it’s too low and picks up again when you increase speed.

Bob K.


Thanks for your thoughts. They may be right on. I recall years ago I changed the gear oil in my 150S manual overdrive unit and afterwards it wouldn’t engage. After driving it for a while, one day it engaged and it has been fine ever since. Never new what the problem was and didn’t care since it was working. This may be the same issue. The oil was changed during the restoration and I’m pretty sure it is the correct oil but will check. A mystery to me was when the manual OD wouldn’t engage, the dipstick showed the proper level…as does this electric one. I’m going to take it out again today and see if the same thing happens…i.e., the OD unit engages but then does not after a while. So now I’m thinking more hydraulic issues than electrical.


Do you have the original xk gearbox fitted or has it been replaced with one from a saloon which also has a speed switch?

It’s the original gear box with OD.

Update on things today.

I took the car out this morning…nothing. The OD did not engage at any speed. So it’s back to square one using all the tips and advice I’ve received. To add insult to injury, my LF tire went flat just as I drove back into my garage. I don’t think I can blame that on the OD.

Thanksj for all your help. I will keep you posted.


SAE 30 or 10W30 engine oil, not 80 or 90 gear lube, which has a characteristic smell.

If I’m correct isn’t this the same basic set up as the Aston
Do you have the planetary clutch set up or pumpkin at the tail of the transmission?
Probably that clutch assembly is shot