4+3 transmission O/D not working

black86roadster · 03-04-03, 08:10 PM

I recently purchased a 86 pace car convertible, 70k miles, black on graphite. When I initially test drove the car it seemed to show decent performance, however the SES light was on and it seemed a run a tad rough, I felt the car had some potential so I handed over $8,000 and drove it home. The car was 100% stock condition, no perf. mods whatsoever. I recently replaced mass air sensor & relays, Idle aircontrol motor, EGR switch harness, & bolted on a SLP cold air induction as well as a Flowmaster cat-back exhaust system. The car is finally starting to behave like a Corvette,
This is the part I feel like crap about: during the test drive (mostly on minor arterial roads) I never had the car over 60 mph so I was not paying any attention to the O/D unit and whether or not is was engaging. And it was not, I see Mid America Corvette has an limited supply of 4+3 overdrive units for around $1,500 then add at least a grand for install, ouch! Is it possible to swap A 89' six speed and never think about the 4+3 again? or is the rebuild O/D unit the only option. Help!

Ken · 03-04-03, 09:48 PM

I put a Richmond 6-Speed (ROD) in mine.

There is a difference in cost if you want to swap the later ZF 6-speed instead of a ROD but I forget what the difference is at the moment. I think it's cheaper to go with the ROD than the ZF but I could be wrong.

The overdrive unit may not be the bad actor here; there are several other things that should be checked first, such as switches, relays and wiring, among a few other things. What exactly is the problem with yours?

black86roadster · 03-04-03, 10:09 PM

thanx for the reply,

the 4-sp tranny works great, so does the clutch, no whines shreaks or shutters, smooth as our Subarus, but with the ability to release gobs of torque.

At 50 mph push the botton and the Rpms do not fall, nor is there any compression felt when dis-engaged. note: the D light will appear intermitenly.

My last car GTA trans-am w/ 700r auto, cruised at approx. 2200 rpm @ 70 mph. (both 5.7 L-98) I know these are totaly different cars, but ...

I will check function of the switchs & wiring, Thanks Ken

Ken · 03-04-03, 10:19 PM

Definately check the little things first. It sure sounds to me like an electrical problem rather than mechanical.

Just as an FYI though, when the overdrive fails, the transmission and clutch assembly will continue to function properly, although you will lose reverse.

Keep us informed.

_ken

It just dawned on me that you might not know where everything is, but then I realized that from the sound of your first post you know what you're doing and have the right books. Am I correct?

black86roadster · 03-04-03, 10:44 PM

Yea this one is over my head, I will most definetly be searching out a decent local transmission shop.

I am fairly familiar with whats under the hood, I have had several L-98 powered cars, this is my first Corvette,

Thanx for the Richmond link,

If their 6-sp is priced anywhere near the original O/D unit I will go with the Richmond.

Ken · 03-04-03, 10:48 PM

Be very, very careful of who you choose to work on the DN 4+3. Not the trans necessarily, it's a Super T-10, but the OD unit will drive most mechanics nuts because they don't understand it that well. Be selective and choose someone you can trust.

_ken

DkBG · 03-04-03, 10:49 PM

When my brother was haveing trouble with his 4+3 it turned out to be a relay on the firewall . $12 bucks and it works fine now . I always like to try the cheap fixes first . Cliff

Top · 03-04-03, 11:36 PM

Go with DkBG first. Go buy a relay. It is mounted just to the left of the brake master cylinder as you look into the engine compartment from the hood. There is another relay behind it. You want to replace the front one. Replace it yourself. It's a simple plug in and replace. That fixed mine. The relay was 18.22 here in Vegas. GM part number is 14078915. I bet it is the relay. When you put the relay in make sure you clean the wire harness that plugs into it to insure good contacts.

The OD is automatically enabled when you start the car. Once you reach approx 128 in temp the OD should kick in automatically unless you hit the switch to turn it off. After that you should be able to 'kick it in and out' as you please using the switch on top of the stick.

However, now if that doesn't fix it, next step is as simple as low fluid in the OD. We have Convertibles so here is where the fun comes in. You must remove the X brace under the car to get the OD pan off to change the filter. You can get to the plug to add fluid but it is very tight to get at. So you can try to check the level without changing the filter to see if that works. Then I would shell out the 180 (mostly labor) or so to change the filter and magnet in the pan. This also happened to me. Had a small leak at the cooler lines and my fluid slowly leaked out. In a straight line the car was fine, but if I 'hit it' hard into corners and the fluid would 'swish' around I would lose the OD for brief periods until the fluid settled down again.

Only then would I dig in my wallet for a Richmond 6spd.

In fact, because you have reverse, I am SO sure it is one of the two above I will buy you dinner and drinks the next time your in Vegas IF I am wrong.

Good Luck

larry bud · 06-08-03, 10:56 AM

Ken,

You say that you put a Richmond 6-Speed in your C4. Do you have any more details on this site (I've been searching the forums)? Any mods that had to be done?

thanks.

Ken · 06-08-03, 06:16 PM

Quote:

Originally posted by larry bud
Ken,

You say that you put a Richmond 6-Speed in your C4. Do you have any more details on this site (I've been searching the forums)? Any mods that had to be done?

thanks.

Corvette Fever had a couple of issues that covered swapping transmissions, both for the ROD 6-speed as well as for the ZF 6-speed. The July 1998 issue has an article on the C4 Richmond 6-speed manual transmission installation, and I know there is another issue covering the same swap but I can't remember and the magazines are in the garage.

CF had another article in April '95 for the Doug Nash 4+3 swap for ZF 6-speed.

Good luck.

_ken

WhalePirot · 06-09-03, 07:03 PM

The O/D relay is the same as the fuel pump relay. Swap the connections before throwing parts at it. There have been some problems with the relays, I hear. Mine are still okay.

Many things control the O/D being available. I'd check the fluid level real soon, as the light flashing indicates that it is trying to come on, hence the signal is getting to the unit, at least part time. Low fluid in that second unit will behave as you describe.

Mine used to switch on and off, cruise control ON at 70. I fixed it that time by changing spark plug wires. My guess is that the ECM was getting an open loop indication (too rich from cam, etc. mods) which the aged wires aggravated. The ECM looks for closed loop; not just temperature. I swear, I only changed the wires!

The Ritchie 6-spd. requires cutting the interior pan to fit, in addition to other mods. I was going that way until I heard from a local shop that he'd pulled about half out, due to gear noise.

Don't bash the Nash. It's a tough little unit that'll handle more power than the Super T-10 part can handle. I talked with Paul at: http://www.5speeds.com/

Good guy; great work!

Mike

Ken · 06-09-03, 08:05 PM

Quote:

Originally posted by WhalePirot
The O/D relay is the same as the fuel pump relay. Swap the connections before throwing parts at it. There have been some problems with the relays...

Mike, if only I had known that before I spent thousands of dollars on mine! I think that was the problem with mine the whole time.

Trouble is, none of the "experts" diagnosed it either, so I shouldn't feel too bad I guess about not finding it myself.

Oh well, live and learn, eh?

_ken