Volvo 740 GLT (1990) Overdrive Problem

[remonannetti]

I am having trouble engaging overdrive, it works when the car is cold and the transmission oil has not heated up. Once the car has heated up, I cannot engage it. Gearbox oil has never been changed since new (I have had the car from new), would changing it solve the problem? I know Volvo at this time used a Laycock overdrive unit which was bolted on to the gearbox, does this share the main gearbox oil or does it have its own supply? Any help would be much appreciated, the B234F engine is a delight but I would feel better cruising on motorways with the overdrive engaged!

[Laird Scooby]

Chances are the oil is low and thin by now as it's 30 years old!

Change the OD oil, the gearbox oil and while you're at it, the diff oil too as i doubt any of those have been touched either. Also might pay to change the engine oil/filter and then you know it's been done!

[griston64]

Originally Posted by remonannetti

I am having trouble engaging overdrive, it works when the car is cold and the transmission oil has not heated up. Once the car has heated up, I cannot engage it. Gearbox oil has never been changed since new (I have had the car from new), would changing it solve the problem? I know Volvo at this time used a Laycock overdrive unit which was bolted on to the gearbox, does this share the main gearbox oil or does it have its own supply? Any help would be much appreciated, the B234F engine is a delight but I would feel better cruising on motorways with the overdrive engaged!

If it's an M46 the overdrive shares the same oil as the gearbox. You remove the cover on bottom of overdrive unit to drain that part. Need a new gasket on that when you replace

[Laird Scooby]

Originally Posted by griston64

If it's an M46 the overdrive shares the same oil as the gearbox. You remove the cover on bottom of overdrive unit to drain that part. Need a new gasket on that when you replace

What the OP was referring to Mark is that some cars had shared fluid between the OD and the box, others had separate. Both types are M46 designation and both use Laycock overdrive units.

However one is one type, the other is a different type (think they're J and L types but can't remember) and without exhuming my Haynes manual again i'd only be guessing.

That said, because his car is a 1990, i'd guess he has separate fluid for the box and OD unit but i could be wrong.

Either way, fluid level and condition is the first thing to check, after all it works when cold so it tells us the electrics are ok and the hydraulics are capable of functioning so the fault is most likely in the fluid not holding pressure once hot.

[griston64]

Originally Posted by Laird Scooby

What the OP was referring to Mark is that some cars had shared fluid between the OD and the box, others had separate. Both types are M46 designation and both use Laycock overdrive units.

However one is one type, the other is a different type (think they're J and L types but can't remember) and without exhuming my Haynes manual again i'd only be guessing.

That said, because his car is a 1990, i'd guess he has separate fluid for the box and OD unit but i could be wrong.

Either way, fluid level and condition is the first thing to check, after all it works when cold so it tells us the electrics are ok and the hydraulics are capable of functioning so the fault is most likely in the fluid not holding pressure once hot.

Interesting. I thought they were all J type fitted to Volvo from the early 70's

[griston64]

Looks like they updated that to P type latterly. Not sure when that was but 1990 is is fairly late for Laycock in Volvo's

[loki_the_glt]

Probaly fitted because they came across them in the stores and decided to use them up...