Hello gang!
This year I have done several repairs and preventive maintenance on my 91 Olds 88 Royale 3.8. I had to replace the starter, entire exhaust system including a new Cat, TPS, O2 sensor. I had the tranny fluid changed this summer along with a full tune up including plugs and wires, fuel filter. Right now I am at 198874 miles and the car still runs great, however for about a month now whenever I am at a long stop light, my check engine light comes on but once I start accelerating it goes away. So yesterday I had to get an oil change and I had my mechanic hook up his Snap On scan tool to see what codes I have and it looks like I have 5 of them! The codes are 24-Speed sensor, 27,28,29, for 2, 3, 4 gears and finally 31 neutral safety switch. So we started inspecting the connections to speed sensor and neutral safety switch, everything looked good. Then my mechanic took a road test with his scan tool and the speedometer matched everything that the scan tool displayed as well as the PNRD12 gears when checking the shifting. He doesn't do transmissions except for changing the fluid and filter. He thinks I should keep an eye on it, and if we have to, we will start with the speed sensor before anything else. I did an additional test by trying to start the car in each gear and it only starts in park or neutral like its supposed to, and I have not had any problems when starting the car either. I did have to replace the neutral switch about 5 or 6 years ago and it actually caused a no start problem unless you found the sweet spot. I have been getting decent gas mileage and everything else has been acting fine, so I am just not sure what this could be. Is it possible that the ECM is faulty? I would appreciate your guy'* thoughts on this issue.
Thanks!!!

Sorry nobody has responded to your post yet, I am not sure what the issue could be, but I am not good with transmissions.
Hopefully somebody will post something helpful soon, hang tight.

Here is an update: I went to work yesterday and the dreaded SES came on again at several long stop lights, then went away after a few seconds. I checked for codes with the "paperclip" trick and found the same batch as before, 24,27,28,29,31. My mechanic told me to go ahead and order a speed sensor and see what happens after that. I also think there is a possibility that the ECM could be acting up due to the 5 different codes that showed up. But hopefully not. Its weird because the car has no trouble starting, it runs great, never dies or stalls, and I maintain it very well. I understand that transmissions are VERY sophisticated and I barely understand the basics of them. Any feedback is appreciated!

It could be the ECM, or maybe all of those switches share something in common, I don't have a wiring diagram or I'd try to see if any share a common ground or 12v, it could be something like a ground wire running to all of those may have a little corrosion on it, only guessing though.
If the codes were were for several different things I'd lean more towards the ECM, but again all of those codes relate to switches.

Hopefully somebody can dig up a wiring diagram, or if you have a book for your car, see if you can find wiring diagrams for each switch, and try following each to see if any share anything in common..
I wish I could help more.

Thanks for the advice!! I do have a factory service manual for this car but its somewhat limited on info but I am going to go thru it some more and start checking for bad grounds especially under both front door thresholds. I picked up the new speed sensor today and I will have my mechanic install it on Tuesday. I hoping that helps this issue. I didn't get the SES light today so I am keeping a close eye on it. I will keep you posted!

You are welcome, hopefully it is something as simple as a little corrosion on a ground buss, let me know how it all goes, good luck!

Well this morning, I had a difficult time getting the car started but I had to give gas to keep it running, then it was fine after that. I checked for codes and I got 26, 27, 28, and 31. However there was no code 24 for VSS sensor. Last night I checked for ground issues but found none, I also examined the connectors for the VSS sensor, neutral safety switch, and the ECM. I think the ECM is possibly starting to fail, since its pushing 25 yrs. I will keep you all posted.

Good Evening folks.
After the hard starting issue I had on Saturday morning, I checked under the hood and noticed that the grey plastic cover that goes over the intake and fuel injector had a brown discoloration on the corner right next to the EGR valve, meaning that it probably had exhaust gasses shooting from the side, possibly causing the hard starting issue. So I decided to go the junk yard and get a new EGR valve because this one has been a nothing but trouble since I replaced it 3 years ago. I found one off of an 89 Buick with only 90000 miles on it, also I found an ECM off of an 87 Olds 88 for $20 Bucks and I figured that it would be good to have just in case. Later that night I took the new EGR valve apart and cleaned it up super well and its working just fine so far. Then yesterday I got the new speed sensor installed , and it got rid of the code 24 and 31, however I still had codes 27,28 and 29 present after disconnecting the battery. So today I disconnected both battery terminals for a little while and I checked all of the connections to the ECM along with the fuses and relays behind the glove box and everything checked out fine. After I reconnected the battery cables I took a couple of drives and drove in each gear for a few minutes each and they all seem to be doing what they are supposed to. When I got back, I checked for codes again and all I have now is code 29. I have not seen the check engine light since replacing the speed sensor. :?

Happy Thanksgiving all!! So here is the newest update on my 91 Olds. A week and half ago I took my car to a shop that can do more diagnostic testing than my mechanic friend can. Anyways I told them about all of the trouble codes I have been getting and the trouble I had with my Idle. They had to replace the Idle Air Control Motor, which fixed that issue, also they hooked up their scan tool and ran the same tests as my mechanic did. They are thinking that the ECM could be failing. However after all of that, I checked for codes and I am still getting the 24,27,28,29 and 31 ones like before. The SES light still likes to come on whenever I am at a long stop light, but as soon as I start going again, it goes away. Since I have that spare ECM I found at the junk yard, I was wondering if I should swap them out and see if that changes anything? From what the FSM tells me, it says to use the M-CAL chip from the original ECM. What do you guys think?

As long as the ECM is the same number, just transfer the Memcal.....

The late 80'* and early 90'* ECM'* were problems.....