Overdrive question

[Bumpa]

The overdrive on my 1977 1850HL behaves perfectly in all normal round town and local driving. But yesterday I went to Edinburgh in it to see my son, a drive of about 80 miles mostly on motorways. The car was in OD top gear for many miles travelling comfortably at 70 mph. When I reached the city and flicked the switch to disengage the OD, nothing happened. Repeated flicking of the switch produced no effect. Leaving the switch in the OFF position, I reckon it was a full five minutes before I felt the slight jolt as it disengaged. From then on the OD behaved as it should, flicking in and out easily. Exactly the same thing happened on the way home in the evening.

Although I have worked on 60s and 70s cars for 50 years, overdrive has always been a mystery to me. Can anyone tell me what the likely cause will be and how to fix it please? Just for info, the switch and all the OD wiring were renewed by me about 18 months ago.

[cliftyhanger]

Could be sticky o rings on the solenoid? Bit odd that it was only after prolonged use, then ok again.
Maybe rig a light connected across the solenoid terminals to check if it is electrical supply or solenoid.
Not sure if the solenoids can be cleaned/serviced though. New are eye wateringly expensive too.

[Richard the old one]

When I had a similar problem on one of my 1500HLs I had the solenoid changed as I could not get the solenoid out and this resolved the problem but the official service manual states
"Solenoid operating valve overhaul -Should it be necessary to clean the operating valve, immerse this part of the solenoid valve in paraffin until the valve is clean. Examine the "O" rings (comment but there is only one shown) on the solenoid valve for damage, and renew together with the sealing washer if necessary. Fit the solenoid and operation valve.

I seem to remember you need a very thin spanner 1 in (25mm) A.F and possibly a Crow's foot spanner to get the solenoid out because again in the manual it states "Do not attempt to remove the solenoid by gripping the cylindrical body as this is very easily damaged.

I believe the parts required are ; washer copper NKC 108, "0" Ring NKC107

Remember also Not to drive in reverse if you think O.D is still engaged as it will wreck the O.D.

[Mad Mart]

Could just be a sticky inhibitor switch.

[GTS290N]

I had the same when the engine was hot, the solenoid would not disengage. It was a faulty solenoid.

[Bumpa]

I assume the gearbox oil has to be drained before removing the solenoid. It's not external is it? When you replaced the solenoid, did you buy a new one or a refurbished one?

[cliftyhanger]

It could be sticky o rings?
May be worth asking it out, cleaning it? (It it possible to take it apart? And fit some new o rings.
New solenoids seem expensive...

[GTS290N]

I assume the gearbox oil has to be drained before removing the solenoid. It's not external is it? When you replaced the solenoid, did you buy a new one or a refurbished one?

Yes you will lose oil, easier to drain the box. I used another second hand solenoid from a very noisy box. At the time new solenoids were £70 from Overdrive Repair Services Sheffield, and I didn't have £70.

[80Sprint]

If you are underneath and already draining the oil I would also take the cover off the overdrive and clean out the filter.

[Richard the old one]

I agree with 80 sprint if you feel confident it would be a good time to clean the filters but remember there are two and the high pressure filter is a bit more difficult to do but I have found that it is the filter that seems to collect the most bits.

[Bumpa]

Thanks Richard. I'll see if I can undo that high pressure filter. I haven't got any spanner that would fit the solenoid with the gearbox in the car. There's just no room between the OD and the chassis rail and exhaust pipe. I have ordered one of the special short spanners made for this job.

[Richard the old one]

I will post a photo of the tool that I made to get to get the hp filter out and a photo of a filter that I took out to clean.

[Bumpa]

I have made a tool not dissimilar to yours this afternoon and got the filter out. Mine was a bit cloggy, but not as bad as yours was.

I can't get the solenoid off because it is impossible to fit a spanner of any sort into the space between the overdrive and the chassis rail. I have had to resort to ordering the specialist tool from James Paddock. Hope it doesn't take long to arrive. Silverstone Classic is getting quite close now.