Pre-select gearbox question. DB18

[Dave Glyde]

Sometimes I have a problem selecting reverse gear and have to go into a forward gear move slightly then go straight to reverse to get the gear. Other times it will go straight into reverse.

Also when selecting top gear there is often a short delay then there is gentle surge like an automatic gearbox as it finally drops into gear.

Before I start fiddling around do these problems sound like selector linkage issues or gearbox adjustment issues ?

[captain bobo]

Dave, in my experience it could be both!

It is quite common to have difficulties selecting reverse if the linkages are worn or out of adjustment.

The "surge" that you reference can be a symptom of incorrect type of oil in the fluid flywheel or the level too low. it could also be the bands need adjusting.

Without wishing to insult you, have you tried the re-calibration process? If you don't know what it is, it means having the engine turned off (with car stationary) and selecting 1st gear then fully depressing and releasing the "clutch" pedal about 10 times. Repeat this process for all gears in sequence (i.e. 2nd, 3rd, 4th, Reverse). Apologies if you have already tried this.

[Simon Hyslop]

Generally this is caused by the adjustment being out of sync between the quadrant on the wheel and the quadrant on the gearbox. Each of the rods between the control tube bottom lever and the gearbox quadrant has ball joints and it is a good idea to look at whether there is slack in the joints and if there is undue slack, adjust them if the joints are of the adjustable variety or replace them if they are non-adjustable.

Once you are happy with the joints, time to look at the adjustment. The official method is to disconnect the joint at the steering column, move the selector quadrant on the gearbox to reverse then select reverse on the wheel. Adjust the joint in or out to be a good fit in its socket. Disconnect it again. Repeat the process except in top gear.

That's the official method. What I have found is that the linkage between the frame to the gearbox can require separate adjustment if it has had a joint or joints replaced. It must be able to pull and push all the way from first to reverse and if replacement joints are longer or shorter than the originals, the overall length must be restored to its original value otherwise the mechanism feel it comes to a stop but it's just run out of travel. What may not be immediately apparent is that the total movement of the quadrant on the gearbox is actually very small and so equally small discrepancies in the dimensions lead to selection troubles.

The delay is something I also experience in the LD10, but not in the LA10, from time to time. The top gear unit is a small multiplate clutch which is worked by balls which move in a spiral groove. It is quite different to the mechanism on the other gears. I suspect (but without proof) that the balls stick due to lack of use. The alternative is that the plates are slow to move together. It's a while since I had one of them apart so it would be good to hear from anyone who has had one apart recently and knows for sure.

[qantasqf1]

I agree your reverse problem is probably due to worn ball joints or bad linkage adjustment. Simon, as far as top is concerned I would doubt that the operating rings' ball bearings are the problem since they operate every time top is selected or deselected and the relative travel of the 2 rings is pretty large. However, it certainly sounds like the top gear clutch is lazy so it's possible the tongues of the driving and driven plates are the problem - they can wear and there's a hell of a lot of them! Additionally, there are a series of brass thimbles (6, if memory serves) that house the clutch disengagement springs. They're a pretty good fit and if they stick then the clutch operating ring has to overcome any load here as well as overcoming the load applied by the springs themselves. If you can live with a lazy top that's fine because If I'm correct (no guarantees!) then it's gearbox removal and strip time.
Controversy Corner:
You could try this: drain the gearbox oil and discard it - after all that should be done every 10,000 miles or so anyway. Before refilling it, though, fill the box with flushing oil (even paraffin would do) and run the engine in neutral for about 15 minutes. After that, idle the engine and select and deselect top a few times to work the oil into the clutch plates and help free up the clutch plates' tongues/disengagement springs. Drain the box and refill it with SAE 30 in the usual way...and you never know!
Cheers
Steve

[Simon Hyslop]

Thanks Steve, that's very useful information on the top gear mechanism. The detail on it had gone from my memory!

[Dave Glyde]

Many thanks guys for your advice. I will check the linkage regards reverse, just strange that 60% of the time it will select reverse straight away. With regards to the top gear being sluggish will it cause damage to drive the car for a while ? The reason I ask is that I have just finished the restoration and the car has been off the road for at least 30 years and so far I have only covered about 150 still finishing it. Maybe needs bit of running in if its been stationary all this time. Oil is new and correct grade.

Captain Bobo, I will try your calibration idea as that is straight forward and no harm in trying it ( and I certainly wasn't insulted by your helpful advice). I saw in searching the old forum you posted the same problem back a few years ago. Can you recall how you resolved it ?

Thanks again guys. Will keep you posted how I get on .

[captain bobo]

I solved the reverse gear selection problem by disconnecting the linkage at the gearbox and checking the alignment of the linkage to each position of the lever on the gearbox. There was a small misalignment in every gear (caused, I think, by the engine & gearbox mounts gradually collapsing with age). For some reason that I don't fully understand the reverse selection was more sensitive to "not being selected", I never had a problem selecting the other gears. Adjusting the linkage to exactly match the positions on the gearbox lever eliminated the problem.

I don't know what caused the "surge" that I mentioned in the old posting you refer to, but I replaced the fluid flywheel oil and the gearbox oil which seemed to eliminate the surge.

Good luck!

[qantasqf1]

Hi Dave. I doubt the lazy action of top gear will cause you problems. The fact that the car has been in storage for such a long time may be significant as mechanical things can get sticky when they're not used! Keep driving the car in the normal manner and top may well come good.
Cheers
Steve