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Topic: 4+3 Trans

in Forum: C4 Driveline Components


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4+3 Trans (5/15)
 10/5/10 1:49pm
besbol
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Mineral, VA - USA

Vette(s):
1984 4+3 manual 2-tone Bronze Z51 2003 Electron Blue Convertible


Joined: 10/29/2009
Posts: 28

I agree that the OD filter and fluid should be totally changed. I've had my 84 for 11 months and put about 3K on it. Who knows when, if ever, that was done. Since I don't have a lift or garage to work on it, I think my local vette guy will get this task and change other fluids as well this late fall or winter.
Total miles on mine is just over 60K.


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4+3 Trans (6/15)
 10/5/10 9:48pm
Trooper
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Hermitage, TN - USA

Vette(s):
1990 C-4 Coupe Polo Green Saddle Interior L-98


Joined: 8/8/2010
Posts: 280

Ok, I am the non-mechanical guy in this group. What exactly is a 4 + 3 transmission? I have a 90 model couple with automatic, is this something I need to be learning about. None of the lights and information you guys are talking about seems familiar to me, but would like to be sure so I don't screw nothing up. SOMEBODY EDUCATE ME......Wits End

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4+3 Trans (7/15)
 10/5/10 10:50pm
Adams' Apple
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Duncanville, TX - USA

Vette(s):
1985 Coupe-L98/Auto,Bright Red/Carmine. 1974 Coupe-L48/4speed, Medium Red Metallic/Black Deluxe.


Joined: 3/18/2009
Posts: 2230

The 4+3 is a standard transmission(4 speed) with a two speed(direct drive & overdrive) trans bolted to the rear of it. The unit was designed/built by Doug Nash Transmission for the '84-'88 Corvette. It was a decent idea, and did work well to reduce the emissions, and allow the car to pass the Federal EPA fuel mileage, and emissions requirements.
The reader's digest version is, there is a normal 4 speed trans(basically a Borg Warner T-10) in the car, and it shifts just like any other 4 speed. The overdrive section is a separate trans, bolted to the rear of the 4 speed. This O/D unit can either be in direct drive, or in an overdrive mode, depending on engine temp, vehicle speed, and throttle input, and is controlled by the ECM(Engine Control Module). Under normal driving conditions, the O/D unit would shift into O/D mode shortly after the driver  would shift into second gear, and would stay engaged in O/D unless the accelerator pedal was mashed hard, or the driver "canceled" O/D with a switch, mounted either on the console/dash, or on the top of the shifter knob, depending on year model. If you take off with heavy throttle, the O/D will not kick in at all, until the throttle is let up on, or the engine rpms get too high.
 The "4+3" name came from the trans having the capability of having overdrive in 2nd-3rd-4th gears, so 4speed + O/D in 2-3-4 = 4+3.
It was more of a nuisance than anything for most folks. The later cars had better ECM control of the O/D, but it's still a pain, unless you keep your foot in it all the time.
The last year for the 4+3 was '88, then they went to the ZF built 6 speed. Your '90 automatic is just that...an automatic, overdrive transmission, known as the 700(or 700R4, if ya wanna get picky). You have nothing to worry about, other than normal, routine maintenance. 
Adams' Apple2010-10-05 19:53:10

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4+3 Trans (8/15)
 10/5/10 10:59pm
Trooper
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Hermitage, TN - USA

Vette(s):
1990 C-4 Coupe Polo Green Saddle Interior L-98


Joined: 8/8/2010
Posts: 280

Kewl, I appreciate that. And extremely good answer I might add, Your the man.

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4+3 Trans (9/15)
 4/15/11 11:13pm
evilram
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Chino Valley, AZ - USA

Vette(s):
Sold, sniff...1985 Cpe. White wlsaddle leather, 4+3, L98, Z51. Oh well..


Joined: 4/12/2011
Posts: 23

Hey, just to add a post (my first) the manual on my '85 (chilton) says that even if the OD is locked out but the shift button, it will still automatically shift when you exceed 100 mph., will check it out in another week or so, after I go over the car better..  I've had it over that on the way home when I bought it, but had the od on..
It's a weird setup, but I like it.. 
4+3 Trans (10/15)
 4/16/11 10:44am
Adams' Apple
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Duncanville, TX - USA

Vette(s):
1985 Coupe-L98/Auto,Bright Red/Carmine. 1974 Coupe-L48/4speed, Medium Red Metallic/Black Deluxe.


Joined: 3/18/2009
Posts: 2230

The ECM will indeed force a shift into O/D at high speeds like that, to keep the rpms out of the red zone. Full throttle will still keep the O/D canceled to a higher speed, tho.

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(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Re: 4+3 Trans (11/15)
 9/7/11 12:42am
87 SharkTail
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Minster, OH - USA

Vette(s):
Beautiful Original Condition 1987 C4 Convertible, Bright Red exterior, Red interior, 4+3 Doug Nash Manual transmission, with 62K miles. Garage housed and dry roads driven. Fun to cruise along in, but always ready to go!


Joined: 9/6/2011
Posts: 17

My 87 has the 4+3 manual transmission also. The OD was recently fixed, but the disengage switch on the shifter does not disengage the OD once there.
 
I need to downshift from 4od to 3, instead of disengaging the od (like downshifting from 4od to 4) from the column push button switch.
 
My info is that the standard disengage switch is no longer available, but a Radio Shack normally open (NO) mometary push button switch can be installed in its place.
 
Has anyone ever done this, and do you know of the RS model number needed?
 
Is the no availability info correct as well? 
 
Thanks
Re: 4+3 Trans (12/15)
 9/7/11 1:07pm
Adams' Apple
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Duncanville, TX - USA

Vette(s):
1985 Coupe-L98/Auto,Bright Red/Carmine. 1974 Coupe-L48/4speed, Medium Red Metallic/Black Deluxe.


Joined: 3/18/2009
Posts: 2230

Are you sure the switch itself is bad?
There is a relay involved in the circuit that could be flaky, or a fuse, wiring, or connector issue. Did the button work before the trans was worked on?


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Joel Adams  
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(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Re: 4+3 Trans (13/15)
 9/13/11 3:16am
evilram
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Chino Valley, AZ - USA

Vette(s):
Sold, sniff...1985 Cpe. White wlsaddle leather, 4+3, L98, Z51. Oh well..


Joined: 4/12/2011
Posts: 23

Hi Guys,  I have the 4+3 in my '85 with 162,000 miles on it and it still works good..Changing the fluid regularly helps, and taking it easy when you know it will shift is most important.
When you want to get on it, leave it off, and around town in traffic too.
The '85&^ shifter switch is still availiable from Corvette Central, with the right size end and wires. Still a simple small on/off switch, like would come with an alarm for valet, I use one to bypass my antenna motor because I always use my XM Roady thru the radio.
On long highway runs it helps, because at those speeds the downshift is enough for most passing situations, and yes, it engages about 105 mph even when switched off.
A two speed O/D unit added after the trans, or rearend was popular in a lot of foreign sport cars in the '60s. Had one in my '63 Volvo P1800..
Chevrolet's was basically a poor idea thrown together in a rush to meet fed. regulations.
I like it, but a manually controlled, simple O/D would be better.
Really, does anyone who enjoys their Corvette give a sh.. about mileage?.
Re: 4+3 Trans (14/15)
 1/3/16 11:16pm
JonJon
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Crittenden, KY - USA

Vette(s):
'86 Coupe Silver/Grey Manual Trans


Joined: 12/27/2015
Posts: 21

I bought my first 'vette last week.  It's a '86 coupe (silver/grey) w/ the Doug Nash 4+3 and I have an issue with the overdrive.
 
The OD light kicks on in 2nd/3rd/4th gear as it should but the overdrive unit doesn't shift.  I'm running 3k rpm at 70mph.  Backing off the throttle for decel causes the unit to freewheel and engine speed to drop to curb idle while I'm coasting.  Clicking the OD-off button on the shifter turns off the OD light and drops the unit out of overdrive where I then get engine braking.
 
I have no clue if the OD unit has ever been serviced.  I'm going to do that first and then presumably, I'm looking at rebuilding/replacing the OD unit.
 
Any help from the Doug Nash gurus is greatly appreciated.  I have the FSM ordered but it hasn't arrived yet.
Re: 4+3 Trans (15/15)
 1/10/16 12:58pm
C4Shane
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Charlotte, NC - USA

Vette(s):
-1993 LT-1, Coupe, 6speed, white/black -1986 L98, Coupe, 4+3, Black - SCCA TT Racer -1979 L48 Coupe, Auto, Red/doeskin


Joined: 5/15/2010
Posts: 26

As an FYI, if you want to defeat the OD and have it off all the time - remove the OD relay which is on the firewall.  It is bundled with the fuel pump relay and sits on a bracket above the valve cover on the drivers side.
 
Very handy if you plan to drive on track or in autocross events. 


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C4s: 1993 LT-1 coupe, 6-Speed, White w/black 1986 L-98 Coupe, 4+3 Trans, Satin Black w/red-black interior - SCCA Time Trial racecar.
C3: 1979 L-48 couple, auto, Red w/doeskin interior.
 
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