aupaul

What is a 4+3 Transmission?

ErminewhiteC2

Corvette 4 + 3 Overdrives


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We have been rebuilding Corvette 4 +3 Overdrives since 1984. This transmission incorporates a Super T10 4 speed with an automatic overdrive. The overdrive can come on in second, third, and fourth gears.... thus 4 + 3! It is only used in Corvettes made in 1984 to 1988 and is not recommended to be swapped into street rod applications.

The 4+3 Overdrive was plagued with many design problems in 1984 to late 1985 cars. The main problems were an underdesigned bearing in the carrier assembly, a direct clutch thrust washer breakage problem, and a pump sealing problem. The carrier bearing failed because of poor bearing design, bad preload, and poor lube to the bearing. This was resolved with GM part number 14105940. It incorporated a heavy duty bearing, and feed oil to the bearing. It was up to the technician to preload it correctly. When a carrier goes bad you will hear bearing noise in direct drive and it will go away in overdrive. Sometimes it can sound like a tire whirring noise. The Direct Clutch thrust washer would break because it could spin in the case. Once broken it would jam in between the overdrive piston and case. This would cause the overdrive to get stuck in two modes and burn up. The new piston GM# 14091496 resolves this problem by putting anti-spin tangs on it. It was redesigned by us for GM again in 1997 to utilize better heat treating capabilites. The pump assembly on 1984 and early 1985 cars used a rear pump cover with no seal between the output shaft and cover. The new cover uses a teflon sealing ring to prevent pressure loss and pump whine. The GM number for the new spool is 14091349.



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Below is a list of some a the parts and services we offer the Corvettes with 4+3 Overdrives. Late 1986 to 1988 Corvettes fail because of lack of service to the overdrive and 4 speed. It is recommended that oil and filters be changed in both 4 speed and overdrive every 10,000 miles. If you overheat your car always change all lube in the engine and gear box.



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Complete remanufactured Overdrive assembly with 1 year warranty - CALL: 631-957-9427
Overdrive soft parts rebuild kit. ( Clutches, gaskets, seals, filter ) - $95.00
Overdrive pan gasket and filter kit. - $18.75
New design carrier, complete - NA
Updated carrier bearing, not available from GM - $NA
Updated direct clutch thrust washer - $NA
Updated pump assemblies - $NA
All other hard parts always available.
Computer chips for better drivability 1985-1988 years.
We have sold our hard parts and rebuilding service to SK speed at SKSPEED.COM



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We have been rebuilding these units as we said since 1984. We also have built them for the Corvette Challenge cars in 1988 as well as upgraded many of the Callaway twin turbo Corvettes built in 1987 and 1988. Call 631-957-9427 for tech questions. If you have any technical questions please E-Mail US to get questions answered via Email.


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amtronic1

Lots of good information there.
Here is my experience with the 4+3 in my '88 coupe.
Bought the car new in Dec of 87. Currently has 150k miles on the odometer. From day 1, the transmission allowed OD in all 4 gears, making it a 4+4. The lube has been changed once in 22 years. In the last year, a small leak has developed at the output shaft. It never has been a speed shifter, it requires you to shift deliberately so no WFO shifting since I learned that early on.

aupaul

i am restomoding a 62. I think I want a Dana 44. I found one and the seller says it is still in the car with a 4+3. Do I want to try and put that in my 62? Thanks ...

OCS1667

I had a 86 that I purchased new. I would select overdrive while driving on the highway at 65 +. As I remember it would go into the overdrive, and then without any warning it would kit out of the overdrive, and as I remember, it would not hold when I tried to engage it again. I took it in many times for service, but they were never able to find the problem or fix it.
Dennis

Black_Magic

I had an '87 Convert and the 4+3 was close to the top of the list of BAD feature/function on that car. It defaulted to overdrive so you would always have to remember to get out of O/D every time you started the car. One time (if I remember correcly) a module went south and it spent most of it's time in O/D which did not improve the driving experience.

George

nassau66427

I had one in an '84 I bought new. Kept it for about 6yrs/60,000 miles. I did have the transmission replaced under warranty but it wasn't related to the overdrive.

It was a lot of fun to play with and kept me from getting bored while driving. If you wanted to you could treat it like a 7 speed; 1, 2, 2od, 3, 3od, 4, 4od.

One of the neatest features was that it would automatically shift into overdrive at 115mph.

capevettes

Ermine White obviously knows his stuff on the 4 plus 3. They are notoriously problematic unless you find someone like him to build it properly. Did it twice on the last 85 I owned. Most of the early cars (84,85) had problems before 30,000 miles. I think I would look to other trannies for your Vette.

ErminewhiteC2

I can't take credit. I just googled it, copied and pasted the article

amtronic1

Another problem - On my '88, the OD is controlled by a button on the shift ****. It actually is a rod that pushes on a microswitch down low. My switch cracked, no one makes a replacement. I went to the manufacturer and they stopped making this model switch years ago. I had to resort to putting a button on the console to control the OD function.
On mine, from when the car was new, if you pressed the button twice before starting off the OD would not kick on by itself.

BrettG

I believe the OD would only apply when the engine was at operating temp. (can't recall the exact temp.), at least on an 86 Vert I owned years ago......

JohnZ

It would be charitable to say that the 4+3 transmission was a screaming nightmare in terms of field failures and warranty costs.

capevettes

The computer would not enable the OD until the engine reached a temp of 140 degrees. Except for early cars, through mid year 1985, the OD was controlled by the driver with the switch on top of the shifter. The earlier 84 and 85 had a switch on the console. I only used the OD on the highway.

Qblue92

Who thought it would be a good idea to put an OD on 2nd gear??

amtronic1

A former Mack truck driver that liked to split shift - it was everything they could do to keep him from putting two shift levers in there.

1COOL60