I am a second owner of a 1985 Corvette with the 4+3 transmission. For some history, I purchased the car from the original owner back in 1991 with 22,000 miles. The car now has 87,000 miles and still looks great. About 3000 miles after ownership, the O/D completely self destructed. The front carrier bearing went out and literally locked the transmission out. I then purchased a 1988 rebuilt O/D for roughly $800 and paid a mechanic $250 to complete the swap. The O/D has worked flawlessly for over 60,000 miles or 18 years, but unfortunately has stopped working now. When I push the button on the shifter, the dash light illuminates but the O/D won’t engage in any of the top 3 gears. It did engage at one point when shifting from 1st to 2nd gear but disengaged, then engaged, and finally disengaged (all in about 3 seconds). Per the GM service manual, I checked for 12 volts at the solenoid lead with the rear wheels spinning at 20 MPH, engine temp at 185 F and when pressing the shifter switch. I also checked for 12 volts at the first gear switch lead. Both leads measured correctly. The fluid level is roughly 1/8 to 1/4-inch below the fill plug and was last changed about 1.5 years ago. ( I have religiously changed fluid and filter at 10,000 miles or every two years since the ‘88 unit was installed). The next sequential repair steps in the GM manual are to 1) replace solenoid, 2) install check ball at solenoid 3) replace pressure switch. 4) free up valve or replace valve body 5) replace drive pin or pump gears. My questions are:
Per the symptoms and checks completed, what is your opinion of the root cause for failure?
Are any of the parts listed above still available through GM or aftermarket shop?
To replace the solenoid, this looks a like a tricky job as it requires removal of the TV cable and valve body. The manual calls for special GM tools for setting TV cable, J34671-1,2 and J34529 to compress the shift valve and spring for removal of the solenoid. Are there other methods if these tools are not available? Sorry for the long note but wanted to provide complete details as I am hopeful for making this repair. Please offer all suggestions. Thanks fellow Vette owners.

 

As you have correctly determined that you have fluid and 12V at the solenoid connection when the ECM is commanding O/d ; only thing left can be the solenoid and associated operation .

Theses are the only places I know that carry parts for 4+3 O/d
http://www.skspeed.com/
http://www.pipersauto.com/4speeds.html

 

If you really have a 88 OD unit, you do not have a TV cable. Your 85 unit did. So you don't have to worry about that. Be sure to top off your fluid level, it needs to be all the way full. I would pull the pan and check the wiring, look for metal shavings, etc. If you maintenanced this unit regularly, I think it would be fine. My next guess would be the pressure switch. Maybe the solenoid is jammed up with filings. I also have the 88 unit in my 85.

 

When Mine did this......I finally dropped the pan and to my delight...the spade connector on the pressure switch had fallen off.

Yes...the operation was intermittent prior to it dropping off for good.

So, I was perplexed as was this forum......hope this helps.......cause you have an electrical issue as well.

 

Thanks for the advice...I will check those item first. If I test the solenoid for operation while still installed, would I be able to see any movement or at least hear it clicking? If I have to replace the solenoid, will I need the GM tools as discussed in the service manual? Thanks again for the help.

 

I would think that no special tools are required since you do not have a TV cable, but I have never replaced the solenoid, so I would not know for sure. I would also think that you would be able to hear it engaging, but maybe not if it is up on blocks and running. Maybe if you could apply 12 v to it while it is not running it might click? It might be easier to measure the resistance of the coil, if it comes in at a few ohms and not a open circuit, it should be good.

 

I took your advice and topped off the fluid last night. I estimate it took about 4 ounces. I started it up while on the jack stands and waited for the engine to reach about 170 F. I shifted in second gear and pressed the switch and to my surprise, it engaged. I repeated this about 10 times. Even though it was pretty late, I had to road test it. I drove it about 10 miles and it engaged on command. I was so tickled that I drove the car to work today. After 50 miles, still no problems. So the good news is it appears to have been related to low fluid level but the bad news is why it had leaked down to this level after 2000 miles. I'll have to just keep an eye on it for now and hopefully the leak rate does not increase. Thanks again for the helpful tip as I was ready to drop the pan and start swapping parts.

 

Your welcome! I'm no super genius, but I had noticed that I have to keep mine full to keep it happy.....you might check the 4 speed side, because it is possible that you might be leaking into the 4 speed. A more likely explanation is that when it was filled last, that the cooler and lines were somewhat empty when it was filled.