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Overdrive - working but not really working

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fishyboy

Jedi Hopeful
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Hi all,
About 6 months ago I noticed a change in the effectiveness of the overdrive on my BT7. When I flick the switch I can hear the overdrive engage and the revs drop, but only very slightly (perhaps 100-200 rpm). Previously the drop in revs was very noticeable both in the engine pitch and the on the rev counter (500-800 rpm). Now there is very little change. A few months before I noticed this problem I had had the OD/Gearbox fluid changed using 20W50 engine oil.

Any ideas what might be going on or how how I could investigate the fault?
Thanks
Phil
 
You might try a heavier oil such as 30 or 40 weight racing oil.
 
I agree with Keoke, multi grade oils are not suitable for overdrives. But the small drop in revs seems curious. The overdrive has set ratio. If the o/d clutch is slipping the revs would be all over the place.
 
That's what mine was doing. Since it would partially work I assumed it was slippage of some type due to inadequate pressure. I tried adding fluid but that did not help. I was on a trip and by the time I drove the 200 miles home it had failed completely. It is possible I had multiple problems but I found that the solenoid had failed and the O/D would not engage at all.

Ultimately, I ended up doing the following:
Replacing solenoid, armature, and gasket with new unit from Moss. This made it click again but it would not consistently engage.

Removed and cleaned the plunger, spring, and ball bearing valve.
Replaced 90W gear oil with 30ND engine oil, cleaned magnets, and replaced bottom screw cap gasket.

I still was having engagement problems. My lever had some play so the classic drill bit in the alignment hole adjustment was not working.

It really helped me to jack up the rear of the car and run it in the garage in 3rd gear. Watch out for that rear tire! With all the noise it is hard to tell if the unit is engaging and your dashboard tach won't help. I used a bright light on the U-joint and looked for changes in the strobe effect.

After quite a bit of fiddling and a few drives with the tunnel off I found the sweet spot and it has been perfect for another 500 miles so far.
 
By far the most useful "Healey specific" tool I have, that I've actually only ever needed once, is a modified operating valve cap that has a hole drilled in the top and I welded a fitting to it so I can put a liquid filled 600 PSI pressure gage on it. It simply allows you to drive with the tunnel off and actually see if the OD oil pump pressure is being made. If the transmission is serviced and you don't see a good operating pressure you can trouble shoot from there. Adjustments may often be the problem and it's great when you finally get it spot on but if you aren't getting good pressure from the pump no adjustments you do will make it work and you need to look elsewhere. I spent a lot of time fiddling whch I should have been spending by biting the bullet and tearing into it. I found multiple problems, fixed them and have had no issues for years.
 
:iagree: With Jon:

I also made me one like that and be sure you get an oil filled gauge that reads from 0 to 1000 PSI .

It will read at or near midscale in use where it is most accurate.
 
roscoe said:
By far the most useful "Healey specific" tool I have, that I've actually only ever needed once, is a modified operating valve cap that has a hole drilled in the top and I welded a fitting to it so I can put a liquid filled 600 PSI pressure gage on it. It simply allows you to drive with the tunnel off and actually see if the OD oil pump pressure is being made. If the transmission is serviced and you don't see a good operating pressure you can trouble shoot from there. Adjustments may often be the problem and it's great when you finally get it spot on but if you aren't getting good pressure from the pump no adjustments you do will make it work and you need to look elsewhere. I spent a lot of time fiddling whch I should have been spending by biting the bullet and tearing into it. I found multiple problems, fixed them and have had no issues for years.
Click to expand...

Ditto . Being an HVAC tech makes this easy . Also the drill bit in the hole thing will never work there to much play wear etc in linkages . The secret is to have the operating lever lift the ball bearing 5/64" (I think that's the number ) and if you have pressure it will engage .
Use the search feature on here there's a ton of reading on overdrive fixes . There are also links to some really good articles on the myths and fixes for over drives . They are a very simple unit .
Just a hydraulic pump with a wet clutch and a bypass/engage valve . Nothing complicated .
 
In Norman Nock's Tech Talk notes on overdrive, he details a simple procedure for determining if your pump is working without having a pressure gauge. This was some comfort to me since I was concerned about my pump operating.

Remove the plug covering the plunger, spring, and ball bearing valve.

Jack up the rear of the car and chock one wheel so it cannot rotate.

Rotate the other wheel in the direction of travel as fast as possible by hand.

If the pump is working, you will see pulsations of oil at the valve opening.
 
Gearhead_Garage said:
In Norman Nock's Tech Talk notes on overdrive, he details a simple procedure for determining if your pump is working without having a pressure gauge. This was some comfort to me since I was concerned about my pump operating.

Remove the plug covering the plunger, spring, and ball bearing valve.

Jack up the rear of the car and chock one wheel so it cannot rotate.

Rotate the other wheel in the direction of travel as fast as possible by hand.

If the pump is working, you will see pulsations of oil at the valve opening.
Click to expand...

all this tells you is the pump is turning and moving oil . It tells you nothing about how much pressure it is capable of producing . You need 450psi pressure minimum out of the pump to overcome the spring pressure and engage the OD . If you can turn the wheels by hand and confirm that without a gauge then your hired !!!!
 
I believe I have described in a previous post the problems that I experienced with my overdrive. Briefly, it engaged when the trans oil was cold but when temperature was reached it would slip, then disengage. The problem was identified as loss of pressure with warm oil. I obtained a replacement accumulator from a British supplier. This part came with one o-ring groove, as used on later units (my car is a 1954 BN1). It wouldn't hold enough pressure until a second groove was machined and another o-ring fitted. It has worked perfectly for the last ten years.
 
Hi all,
I have now had chance to change the gearbox/overdrive oil from 20 weight to a heavier 30 weight, but with no change in the operation of the OD. I can hear the OD engage, but with only a slight decrease in revs. Certainly not the drop in revs I used to experience (up to about 12 months ago when the gearbox oil was changed to 20 weight).

I used Norman Nock's checklist to confirm the dash switch and solenoid are working. I haven't checked the relay as the wires to this were cut by a PO at least 25 years ago. Is this a big issue (the OD having worked for the previous 24 years).

Does anybody have any further suggestions? Could something have been dislodged when the initial 20 weight oil change was done some 12 months ago?

Phil
 
Did you clean the filter when you changed the oil .
Without a pressure gauge to confirm that the OD pump is producing enough pressure to operate the OD your just guessing at the problem . If the OD has never been overhauled then chances are the pump is worn the accumulator piston rings are worn the operating valve is worn etc etc etc . Time for a full OD overhaul .
 
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