Corvette Forums - Corvette Enthusiast Site - 4+3 Transmission Problems? Maybe not Fatal

My 86 coupe has the 4+3 transmission. In the past 6 months its ability to automatically shift into OD deteriorated to the point of not working. The car drove fine in standard transmission mode.
I had a Chevy dealer install a new ON/OFF switch on the gearshift lever. It didn't change the problem. They told me that according to the Chevy diagnostic info in their manuals, the most likely suspects for my symptoms were: the solenoid and the pressure switch, both inside the case. And neither part was available from Chevy anymore.
I decided to check for parts and to check for a second opinion. I called SK Speed. This gets more interesting.
SK Speed told me that the 86-88 transmissions seldom had trouble with those parts (the 84 -85 models had some wear bearing problems, later fixed). They recommended some things that I will prioritize below:
1. Remove, clean and reinstall all ground wire connections to the car, chassis, motor, frame, wherever you can see them. A 20 year old car might not be making good ground connections anymore.
2. Replace the $15 Overdrive Relay under the hood before you spend hundreds or thousands on a transmission overhaul. (This was the best advice of the decade!)
3. Remove the trans plate and check the ohm reading on the solenoid. If it is between 15-25 ohms, the solenoid is OK.
4. Put a short piece of jumper wire across the terminals on the pressure switch, reassemble, and give it a test drive.

The OD Relay switch idea was an amazing fix for my problems, and at $15 at NAPA, I think all 4+3 owners should consider replacing their Relay switch, too.

Many writers on this forum comment about using the clutch to shift into overdrive because it shifts so hard (clunks down abruptly), both into and out of OD. My new OD Relay smoothed out the automatic shift into and out of OD almost to the point that you don't notice it happening.

On my 86, there are 3 relays under the hood on the drivers side of the engine compartment that look the same. One is mounted on the wheel well. I'm not sure what that one controls. There are 2 relays mounted piggy-back style on the firewall, between the brake booster and the distributor. The one mounted against the firewall is (I believe) a fuel pump relay. The one mounted away from the firewall, and much easier to remove, is the OD Relay. It took about 10 minutes to remove the two bolts holding it to the bracket, move it out and up a little so I could remove the electrical connector from the bottom of it, reconnect the electrical connection to the new relay, and put the two bolts back on.

The transmission worked exactly as described in the owners manual when I started the car and took a test drive. At about 124 degrees (I was in 3rd gear watching for results) it dropped quietly into OD. I came to a stop sign, shifted into low and took off in normal drive. As I let the clutch out in my shift to 2nd, the OD light came on with no clunk or fuss. Third and 4th behaved just as well. Two miles down the road I pulled into a driveway and stopped. I clicked the OD OFF, backed out and took off in 1st. I shifted through 2nd, 3rd and 4th with no OD engaged. Then, at 45 mph, I clicked the OD button without using the clutch. It dropped quietly and easily into OD. Two miles later I nailed it hard; it dropped out of OD, rpms jumped quickly, and at 60 mph I backed off the accelerator. It settled right back into OD. Later, after taking off from another stop sign, I nailed it in 2nd gear. It dropped out of OD, accelerated hard, I shifted into 3rd, and it stayed out of OD until I went to 4th gear. Great manners and great driveability.

Now that it is behaving this well, I can appreciated the comments from those 4+3 owners who say they turn OD off while they are driving in city traffic, and only use OD for open road driving. Later that same day, I turned OD off and way able to cruise in 30mph traffic in 3rd and 35mph traffic in 4th gear. I liked the rpm ranges I was using, and I like the idea of saving the wear and tear of shifting in and out of OD in stop and go traffic. I also noticed that 2nd gear OD is almost 'luggish' on rpms. I hope this helps some of you