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GKN Overdrive Service


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James. Yes again :lol:

At least if you take a direct feed to the solenoid you know if it's the overdrive or the electronics.

David.

Funney thing but the overdrive engages quicker without the electronics fitted than with.

mike

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It's a couple of decades since I was running the 1798cc MGB, but I seem to recall that the most common cause of OD malfunction, as mentioned in Club magazines, was an electrical fault, so that mirrors the experiences recounted by others in this thread.

I also met someone whose OD problem was cured when the gearbox was topped up with oil.

At that time, there was no electronic control, so I have no experience of that.

Because the MGB didn't have a low ratio gearbox it was enough to limit the operation to 3rd and 4th, when the road speed, and thus gearbox output shaft speed, was sufficient to drive the pump fast enough to create the pressure required.

To accomadate the increased torque the MGB V8 had OD only on 4th gear.

The (MGB) OD manual I have does show test pressures, but to quote them here would be misleading. It's better for the information to be missing or accurate, rather than 'perhaps'.

Cheers.

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Well i thought it was sorted after the checks above and OD would engage at any speed above 30 as expected.

But after a trip out yesterday and today its back to its old habits of surging and having to be above 55mph.

It makes sense with the oil pump being an issue as you need the top end speed to get it to work.

Also if you are going too slow ie >30 but < 55 and switch the OD on - nothing happens until you turn if off then it will surge IN then off again, is this becasue as the OD is turned off it closes the bypass valve and momentarily there is enough hydraulic pressure there to engage it.

thoughts

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sounds like either the solenoid is playing up or a fault in the ECU box.

???? really

I follow Daivds logic in that the pump etc needs to be working at a faster speed in order to get the system to engage.

if it was the ECU then surely it wouldn't let the OD engage at all? Everything with it seems to work ie red light comes on if your less than 30mph and it disengages it if you drop below 20mph and light comes on (on the rare times it works properly), light doesn't come on when above 30 and switch pressed as you would expect.

If its the solenoid how come it always then engages at 55mph? to me it sounds like it needs to be doing that speed to get the pump to push through sufficient fluid to get it to work (ie engage)

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Sound likes oil pump, solenoid valve or relief valve leaking past requiring high revs to overcome the internal spring pressure of the clutch. I haven't extracted them in my unit so can't say what the problem may be (needs a special spanner making up to do it).

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The UK oil Gear Vendors recommend for the Laycock unit is castrol TAF-X, fully synthetic 75W-90

That sounds a lot thicker than ATF but may be worth a try for diagnostics.

Have a look here http://www.volvoclub.org.uk/faq/Transmissi...asicDiagnostics

Also even better http://www.buckeyetriumphs.org/technical/jod/JOD1/JOD1.htm

The set up on the Type J shown on the sites is the same as the Defender but without the modern electronics. Also recommends 80W90!

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i reckon its a partial blockage of the oil passage or similar, its currently dislodged when it goes back in the way i dont have enough oil Pressure unless over 55mph.

contacts and solenoid were checked and cleaned as per your original suggestion.

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