1988 740 Turbo OD Mystery *resolution added*
 

Hi Everyone,

I'm fairly new to Volvos. I recently bought a 1988 Volvo 740 Turbo, manual 4spd with the Laycock De Normanville OD unit. There's a problem:

The overdrive gets "stuck" sometimes. When in 4th gear and the car wants to see OD, I push the button (using clutch) and it engages without a problem, and the light on the dash comes on. If I immediately push it again (using clutch), it disengages. However, if I leave it in OD for a while at hwy speeds, then it will not respond when I attempt to disengage. The light comes off, but the car remains in OD.

If I turn off the button and shift down, and the OD will remain engaged for some time, usually proportional to how long I've been running it in OD. Unpredictably, the gears will "drop down" and the shift indicator will come on, signaling that the OD has disengaged. This occurs anywhere from 1st to 4th gear.

I changed all the fluids this last weekend, and the transmission was unfortunately slightly coloured up. The magnet plug had a very small amount of metal hairs on it, and the magnet behind the filter had some sludge that I cleaned out. It wasn't filthy, but it wasn't pristine. I changed it with ATF.

The change did not seem to make a difference during testing. After being engaged for a while, the OD got stuck. As a new twist, at one point the OD didn't engage at all. This has only happened once, but I haven't logged a ton of hwy driving time due to the OD issue.

The transmission fluid is now clean and the levels are good. The car's history shows that this has been a problem for both previous owners, and both owners had the OD relay checked and replaced (both times at the same shop).

If anyone has any ideas of what the cause could be I'd love to hear them. I'm going to keep the rebuild as a last resort. Thanks in advance for any and all replies.


p.s. I've searched the archives, and this problem is novel to this forum as far as I know.

The attached diagram is for the 240 so use it for a reference only. It sounds like power is being removed from the solenoid - since the light goes out, but the solenoid is sticking in the "on" position.

*fix*
 

This issue has been resolved. It turns out that the solenoid was faulty.

The old solenoid was replaced and put on a power supply for testing. It drew 2.2amps and got quite hot after a few minutes. When disengaged after 10 minutes or so, the solenoid stuck open.

Driving tests confirmed that it was the solenoid. With the used solenoid installed, the OD shifts perfectly every time. Hope this helps someone someday.