Wheres the overdrive unit and how do i test it?

have you told any of the tranny places of problem when they were replacing or working on vehicle ---has the tcm been tested ---and scan for any trouble codes

sounds like a electrical problem but could be the little oil pump in the overdrive. first check the solenoid on the overdrive unit is being turned on with ign on. in 3rd or 4th gear use a test lamp or similar. the most likely cause is the switches on the gearbox that will not allow the overdrive to function in the incorrect gear. but could be the pump stuck or blocked filter

the over drive is controlled by a servo/solenoid that sometimes fails
It could be a TCM problem
It is not an indication of a failing transmission unless you have more indications you have not mentioned and if diagnosed by an accredited transmission shop should be fairly cheap to fix.
It is worth fixing

There are a couple of things to check on the M-46 with Type P overdrive (I think yours is a type P and not a type J):

1) Check the electrical connections from the switch on the top of the gearshift knob to the solenoid in the transmission with a VOM. Switch depressed - 12 VDC. Switch not depressed - 0 VDC. If OK - you are getting power to the solenoid that activates the overdrive.
2) Test the solenoid. Either use the switch on the top of the gearshift or use a local power source (12 VDC) wired to the solenoid. Listen for a light click. If you do not hear a click; the solenoid is probably defective. Test with a known good unit.
3) If the solenoid is working (and you are sure it is working) then check the oil level of the overdrive unit. These units use Dexron II/III (check your manual for the correct oil type) and ensure the oil level is correct.
4) These overdrive units are a lot like conventional automatic transmissions and are constructed as such. Consequently, the home mechanic should confine troubleshooting to the solenoid and correct oil level. The Type P units are slightly more durable than the earlier type J units but in either instance rely on fairly rigorous maintenance.

Despite the overdrive icon and manufacturers calling it overdrive, there is no overdrive unit on your Oldsmobile. What is mistakenly referred to as an overdrive unit is the lock up torque converter. Over a certain speed (usually about 35 mph), and absent a call for heavy acceleration, the torgue converter is hydraulically locked so that the drive and driven vanes in the converter spin as a single unit. The lock-up is controlled by a solenoid valve in the side pan of the transmission. The most common problem with the solenoids is that they fail to release and keep the converter locked up which stalls the engine at stops. The usual cure is to replace the solonoid

Yes there is but that does not control the overdrive engagement. no a resistance test can't be done on that unit.

You problem is more involved and will require a transmission expert to look at it and diagnose it properly.

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/sean_54c3a3dc48a7773c

the overdrive unit is at the tail of the transmision. if its a 4wd its inbetween the trani and the transfer case

Replace the overdrive unit case. It isn't hard to remove the parts and switch them. OR replace the overdrive unit with another one. I can tell you how to disassemble the unit. You will be suprised how easy it is.

Under truck on left side of trans.is a 4 prong plug unplug that and turn ignition to on and test light each one of them plugs one should be hot, if none of them light test light than, that is the reason it won't shift to overdrive, If one does light, than the problem is not there. Now under hood on top of intake close to front of engine there is what looks like a temp. sending unit with a 2 wire plug going to it, when that goes bad it will not send a signal to trans. to let it shift to overdrive when engine gets to temp.

yes I will mail the information.

I will do some copies and send to you as soon I can.