A518 won't shift into overdrive

Well, start by posting a wiring diagram.

You mean, this?

http://www.transmissioncenter.net/SwapInformation.htm

http://www.transmissioncenter.net/img3.jpg


Even though somewhat cobbled up, this is a fairly simple wiring problem of several switches, so either the "system" is not getting power, or the wiring has a problem, or one of the switches is bad, OR worst case, the transmission has a problem

You should be able to bypass the switches one at a time with clip leads, etc, or even run temporary wiring to get power to the trans itself to see if it will engage.

What you are doing is running POWER to the center terminal of the connector, and the vacuum switch switches this power ON to the OD when the vacuum is HIGH, BUT the OD does not turn on until the oil pressure switch on the transmission "makes." So BOTH switches must close to turn it on.

So if you bypass BOTH switches, the OD should be ON.

If it is not, you have either a power supply problem, a wiring problem, or a bad connection at the transmission, or an internal transmission problem

So get right down to the transmission and check that you are getting power to the connector with the switches bypassed, OR

get a spare connector and hook temporary wiring to it so you can hook it to 12V for testing.