Hey Gang.....Calling All DN 4+3 Gurus
I just bought my 1st Corvette, a '86 Coupe L98 w/ the DN 4+3. The price was right and as a ancient auto tech/instructor it's going to be a DD-ish car. I bought it to DRIVE and enjoy.


Now back to my question(s).


The system does not shift into overdrive. The light in the message center comes on but clutching makes no RPM difference. However, when the O/D light is on, backing off the throttle for decal causes the car to freewheel. Clicking the button cancels O/D and the car decels normally. The car also backs up normally. I'm running right at 3k RPM @ 70mph.


I've had the car 2wks and I'm getting ready to service it. I've ordered/received the O/D filter kit and my shop manual CD has come in.


What do I need to look for as I change the fluid/filter? What failures seem to be the most common?




Thanks in advance for the information.
Jonathan.....

 

The condition of the ATF may help determine what's going on--hopefully it still has some red to it. Fingers crossed that the fluid was just a little low, but it sounds more like symptoms of a burnt up internal clutch Could be caused by poor/irregular maintenance by the PO, or an unaddressed external ATF leak. May also be internal seepage from the T10, though that problem is more common with earlier cars. Supposedly the offending design flaw was corrected before your '86 rolled off the line, but seals do fail...you'll know for sure if there's gear oil in the fluid


If you're feeling lucky and the old fluid doesn't look too bad, replace the filter, fill her up, and pray that O/D engages. If not, you've got more digging to do.

 

I agree with the above, it has all the symptoms of burned clutches and seals. You may get lucky and it be low on fluid, but that may not be the case. When the light comes on it indicates all the electrics are working, it's now a mechanical issue, you may get lucky.

 

A fluid change is relatively easy to do and maybe a lack of fluid is part of the problem. Is there any signs of leaking ATF around the OD case?

You have to remove the pan to change the fluid. There are two metal lines from the OD unit to the radiator. Disconnect the two lines and use low pressure air to blow out the lines. Then re-connect. Pull the old filter straight down. If the "kit" came with a new rubber grommet, insert it into the hole in the filter and push the filter back on the tube.

When you remove the pan, make sure all traces of old gasket material is gone from both the pan rail and the OD case. Make sure the pan rail is straight with no bends or the holes aren't pushed up from excessive bolt tightening. Remove the round magnet and clean the pan and replace the magnet.

I like to use black RTV sealant instead of the cork gasket. Apply a 1/8" bead around the rim of the pan and around the holes and re-attach to the OD. You don't have to torque the bolts just get them equally tight.

Use DexIII/Mercon ATF fluid that is not synthetic. You will need a pump to get fluid into the OD pan. Get one of the $5.00 Harbor Freight pumps. Once you have the fluid in, with the rear wheels off the ground, turn a rear wheel by hand for about 8-10 revolutions. This will push new fluid thru the two lines that go to the radiator. Then top off the OD unit. Should take just under 2.5 quarts to do everything.

Here's a link to OD operation and common problems.

 

Odds are you're going to need to rebuild it: http://www.corvetteactioncenter.com/...-unit-610.html

 

...but no harm in trying a fluid & filter change, as long as the OP is willing to pay for fluid twice if it fails to resolve the problem. It's a fairly quick job too.

One other thing--some (all?) of the filter kits do not include a new grommet, which is critical as it seals the screen and fixes it in place. Be gentle during removal (I wasn't particularly cautious, but I was lucky ), and you should be able to remove the grommet and reuse it in your new screen. If you are not so fortunate, sourcing discussion here: https://www.corvetteforum.com/forums...et-update.html

 

The filter kit from Eckler's came with the new grommet. I'm going to do the service this week and keep my fingers crossed.


Does anyone do a quality reman on these units or would I be better served to have mine rebuilt? The o/h kit is $275 plus shipping from Eckler's.

 

If you follow the information provided in the above Action Center document or service manual it may be some thing you can do yourself. They are not too difficult to repair. The problem is getting hard parts. I can almost guarantee you will need the rear thrust bearing, and possibly the front bearing. When you drop the pan look at any metal, that will give you some information to make a decision. If you are handy you can repair it.

 

I'm an ancient MoPar tech (1974-87) then taught automotive in the stage tech college system (1987-95) then finished out at the Chrysler Tech Center in Cincinnati (1995-2000).


I read up on the FSM and it don't look like it takes too many special tools.

 

I think you could do it, an option would be to talk to a trans shop. This job is maybe 25% of normal auto trans rebuild. Good luck!

 

I bought an 87 and the od did not work but it was an electrical problem.I put the rear in jack stands and put 12v on the od unit and it worked.

 

UPDATE
I did the fluid/filter change and the pan was clean w/ no fragments of clutch disc mat'ls showing. System operation didn't change.

OPERATION QUESTION If the O/D light is on, the car will free-wheel on decel when I back off the throttle. Turning off the O/D causes the car to decel normally. Sprag clutch?

got some electrical work to do in that the back-up lamp switch came loose from the switch and the connector melted against the header. There may be a partial break in the wiring somewhere.

 

Sounds like it's time for a teardown & inspection. I believe the O/D light serves as a "fail safe" to indicate that the circuit is intact...but it doesn't necessarily make the O/D electronically functional. A shorted or stuck solenoid which fails to actuate as intended may still allow current to pass.

Mayhem and misdirection always accompany melted wires and shorts, so you should of course correct that first...then dig into the O/D.

 

Regarding the "operation question". You are correct. When the unit is in OD it is driving through the sprag in the OD unit. The sprag is a one way clutch, thus the lack of decal. When you knock off OD you are running via a traditional direct drive transmission. Will your car back up an incline?

 

Frank
The car backs up perfectly.

I was browsing thru the wiring diagram (the 1st thing I ordered when I got the car was the FSM).

I found the O/D relay and the green wire from the relay to the solenoid branches off to the O/D light. Electrically, I'm down to a stuck solenoid inside the unit or a break in the green wire from the relay to the solenoid.

 

I have an 84 serv. manual which is slightly different than the 85 and forward units (more gear switches). There is a flow chart in the emission's section that may help you diagnose the electrics. The solenoid has a ball check under it, possibly hanging open? Keep us posted.