Bucs34

Hi everyone,

I recently bought a 1985 Z51 Corvette with a 4+3 manual tranny. It has a little over 90k on it.

Ever since I bought it, the transmission has been acting funny. By that I mean that the OD seems to kick on and off sometimes by itself. Also, 2nd gear randomly grinds. It will just be a short grind then it will go in. This even happens if I try to baby the shift from 1 to 2.

I asked some mechanics around where I live, in MA, but nobody seems to have ever seen this transmission before. Any Ideas on what could be causing these problems and how to fix them?

Thanks in advance for the help!

rodj

The O/d is operated by the computer when certain conditions are met; speed / engine temp / throttle opening. By default the O/d is always on (or available) unless you manually disengage it by pressing the button ( when working).
Once disengaged, pressing the button will engage O/d so you have full manual control.
The O/d should come on automatically when engine warms up and drop out if you floor it hard enough. If the O/d is operating correctly and randomly dropping in /out could be bad connection somewhere.
Check would be to see if the wire is still connected to O/d unit firmly Then check the O/d relay on firewall .The early style relay connectors have problems with insulation stripping back allowing bad connections and shorts.

If you do not know the history of car well worth dropping the pan on O/d , replacing filter and new trans fluid.
Read
http://corvetteobsession.homestead.c...formation.html

4+3 trans in composed of two parts ; a 4 spd gearbox and an O/d unit on the rear.
Gearbox part is a regular BW Super T10 4 spd; any shop should be familar with it. Sounds like syncros in gearbox are worn or shift linkages worn/ badly out of adjustment

jhammons01

The O/D dropping out could be as simple as the Temps drop below 160°....a sensor tells the comp to drop out do to it being too cold.

Then Check the switch in the shifter. That could be an issue

The 2nd gear grind could be a few things. The Shift fork may be worn, the synchronize rings are going or your clutch is dragging or getting old and ready to go......OR...the Clutch Master cylinder or slave is going out.

Some of those things are bad, some are easy. What ever it is, it will show itself pretty soon...just brace yourself.

Bucs34

The previous owner replaced the clutch so I dont think thats the problem. However, are there any easy ways to check and see which of the other options, if any, is causing the 2nd gear grind?

Also, any links about DIY transmission service for the manual and o/d components would be greatly appreciated.

rodj

Min operating temp for '85 140

First up you need to determine if "the OD seems to kick on and off by itself" is part of it's normal expected operation or not.
Having not owned one before you don't know what to expect.

If you read the what it "should do" here
http://www.5speeds.com/43table.htm
you can work out if it is operating properly

scorp508

Where in MA are you?

Go through the link rodj gave you to figure out if it is operating normalky or not first. My 4+3 confused the hell out of me until I saw that sheet.

Bucs34

I'm in ludlow, near the springfield area.

And these o/d units are pretty confusing haha.

coupeguy2001

Before you do anything else, make sure it's got enough fluid. These trannys are known to be leakers, and if the fluid slevel falls below a certain amount, the pressure switch inside can kick out the OD so it doesn't burn up the clutches.
Once you know there's enough fluid, start from square one.
Oh yeah, the trans takes gear oil, and the od takes Dexron III auto trans fluid

jhammons01

^^You left out the fun part.....

OP, To check the O/D fluid...the technique is similar to the way you do the 4-spd or the differential. There is a fill plug on the side, you have to lift up the car "level" and get under there and pull the plug and see if any drips out. If not, can you feel it with your pinky finger.....if not then you need to fill it up again. It only holds 1 3/4 of quart.

Don't be caught checking under the hood checking for a dipstick....like me

c4cruiser

Grinding going into 2nd is typically a syncho issue. There may also be excessive wear on the shift forks. Is the tranny also hard to shift into 2nd or just the grinding going into 2nd? In any event the 4-speed portion is not much different than any other B-W Super T-10.

If you intend to do any work on the car yourself, you should get a Factory Service Manual (FSM). You will find detailed instructions on repairs to each component in the car including the manual tranny. You can usually find them in the C4 Parts For Sale section or even e-Bay. They are a two-book set so make sure you get both.

Unless you have some experience with automatic transmissions, I would not recommend any DIY activity on the OD portion. There are special tools needed to do rebuilds and it can be an expensive proposition to rebuild just the OD unit.

Bucs34

I will definately look into getting the FSM.

As for the 2nd grear, I would say that it is hard to get into 2nd. I've always been able to get the car to go into the gear but I have to be firm.

Chatman

Your description is like my car's behavior. When going between 1st and 2nd when overdrive is engaged, I have to shift slowly or I can get a clunky point -- sorry, it's hard to describe but it isn't gear grinding. With overdrive turned off, it isn't a problem.

So far, it sounds to me as if you car is probably behaving normally if the overdrive engagement and disengagement is associated with temperature and acceleration.

MBDiagMan

Lots of good responses, especially coupeguy getting to the basics of checking the fluid. He did NOT mention, however, to check the clutch fluid in the master cylinder on the firewall. If the reservoir is full then your second gear shift problem is likely internal.

scorp508

I should have mentioned my 2nd gear has the opposite issue. It shifts fine even under full load, but on overrun it pops out.

Bucs34

After a long spread of rain, today I was finally able to take a look at my car.

I looked over the O/D operation sheet that rodj gave me a link to and Im fairly certain that my unit is in fact operating correctly.

Also, I tried to check the clutch fluid in the resevoir on the firewal but I could not get the cap off. I could see through the side that the fluid was at the "full" level but im still not sure why the cap wouldnt budge.

jhammons01

Yeah, that cap gets stuck with the heat. You may have to use a large pair of channel locks and a towel to gently budge that thing loose. Mine did that as well.

If is full, then there is not really a chance that it is the Fluid.

There is most likely a corroded or loose contact somewhere.....good luck finding that one.

flynn508

Lots of good responses; I thought the same thing about my Nash, until I realized it was working the way it was designed. One thing I would do, is only use the OD in 4th. The gear ratios are fine 1-3 when cruising around town. I only use the OD on the highway in 4th, and always use the clutch when engaging/disengaging the OD.

scorp508

Be careful with that plastic cap and plastic tank!

Bucs34

I got the cap off and the fluid is full.

On the topic of not using the o/d until 4th gear, how can I do that? Since the o/d operates on its own is there a way to turn the unit off?

skybolt31

there is a button on the console or on the top of the shifter depending on what year your car is. Pushing the button engages or disengages the OD, depending on whether it is engaged or disengaged at the time. If you do nothing, once the car is warmed up and you are out of first gear, the car will engage the OD. So when you then pull up to your first stop light, push the button and take it out of OD mode. Then just shift up through the gears normally as if you had a 4 spd. Then when in 4th, clutch it and hit the button for OD. That is the way that I used to use mine, make it into a 5 speed. The only problem was when I would forget to hit the button when I was stopped and then I would shift to second and it would engage the OD, the jump from 1st to 2nd OD was a big one..................