Well everything we tested so far seems to be working and I pulled and tested the overdrive relay last night, per Pierce's instructions, and it didn't click so I'm actually thinking the relay might be bad at this point. I ordered a new one last night and hope to have it by Monday.

 

Heya Pierce any luck yet with that wiring diagram?

And since my overdrive technically works, and it’s the action of the gear shifter button that DISABLES the overdrive that’s not working, do you even think that a bad relay could be causing my issue?

 

if the relay isn't clicking when you push the button while the ignition is on, then yes, I suspect the relay.

you want an OEM Kaehler relay, but https://www.240turbo.com/volvorelays...verdriverelays says they are NLA, No Longer Available. :-/

(this is a good part of the reason why I no longer have any Volvos, parts availability is gone down the toilet).

 

Yeah I'm strongly suspecting it to be the relay right now as well. Unfortunately it won't arrive until Tuesday with the holiday on Monday.
I did find the Kaehler one you referred to on eBay. You can see it below.


To recap I'm not able to DISENGAGE my overdrive using the button on the gear shifter.

What we've checked so far:

• The fuse, near the driver door panel, is good.
• The plug, on the lower passenger side floor, when separated and tested with an ohm meter shows that the button on the gear shifter is working and sending signal.
• The plug that holds the relay, when the relay was removed, lit the O/D dash light up when jumpered.
• The relay itself did not have any clicks when plugged into its socket or when tested independently via a car battery.

I found the below wiring diagram on cleanframetrap.com. It confirmed everything that I had already sorted through about the wiring.

 

The Overdrive Relay corrected the issue.