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Hi All

Well this started a number of months ago when my overdrive started dropping out randomly when in 4th OD. I was sure it was an electrical issue as it could work fine for hours of driving. It appeared to be worse when hot and tended to happen when the revs dropped while still in fourth. I checked all the electric and couldn’t fine a problem and as we were about to head off to Spain and France with the car I decided to leave it alone. We covered 2380 miles, 3 Tr6s with no problems, however my overdrive continued misbehaving, a little worse in the hot weather. However we arrived back in Portsmouth and drove straight home, 505 miles and it performed perfectly. 
I have a number of jobs to do on the car this winter; including recovering the central dash support, so I decided to remove the gearbox cover to try and sort out this problem. I checked the electrics and everything works correctly, so I fitted my pressure gauge and disconnected the propshaft. 
At tick over in neutral the pressure is 100psi

In 4th gear at 2500 to 3000rpm the pressure is 450psi

engage Overdrive it drops momentarily to 200psi then back to 450psi

disengage OD and the pressure doesn’t move. 
 

So far so good. However once I reduce the revs the pressure immediately drops to 100psi and if I then turn off the engine it drops to zero within a minute. 
 

So my question is, am I correct in thinking the pressure should be maintained, and if so what could be causing it to drop. 
 

Thanks

Cameron

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The OD oil pump only operates when the you are in gear, the pump cam is on the gearbox output shaft so it will even pump up pressure when in reverse.

If all your seals and operating valves are in good condition it should hold pressure for a few hours, your pressure looks good so your problem could be the limit switches not making good contact. I would take them apart and check to make sure they are in good shape

 

Good luck Graham

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Cameron,

at what lower revs does the pressure drop? 
As Graham says, it should hold pressure, looks like yours is leaking somewhere.

Regards,

Waldi

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Hi All

Thanks for the reply’s. I spoke to one of the overdrive service companies today about the pressure issue. Basically they confirmed that the overdrive should maintain the pressure, however the fact it doesn’t wouldn’t stop it performing perfectly. The pressure is probably leaking past the accumulator or relief valve, however in their opinion not worth worrying about. 
I removed the switches today and the 3/4 gear one was not right and had a high resistance, so probably the problem. I will also renew the wiring while I’m there. 

 

Waldi , The pressure remains high while in gear even at idle (propshaft disconnects)

Its when going into neutral it drops to 100psi ,then to zero within a minute of turning off the ignition. 
 

Thanks

Cameron

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Hi Cameron,

if the pressure stays high even at low revs, I would not do anything to that. Since you have detected high resistance on the switches that is a more likely cause. I found several of my switches to have this, and cleaning them with a contact spray fixed them all: “zero ohm” resistance.

The switches can additionally be adjusted with the amount (thickness) of fibre rings between switch and cover. 
 

And check all connectors on the switched are still ok (a tight fit when pushing them on the switches), several of mine were loose, so I replaced those.
 

Regards,

Waldi

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