glock19

I realize it is a little unit that attaches to the trans but
Is it 1st gear is the same and 2 3 4 have hi/lo
or is it a 4 speed with a 3 speed spliter like a dump truck
or is it something all together diffrent
Also what kinda GM 4 speed is it anyway
going to look at a couple of cars this week and both are 4+3s
thanks

Mike_88Z51

scorp508 is THE MAN when it comes to 4+3 knowledge. I can tell you a few things. It is essentially a Super T-10 4 speed manual transmission with an overdrive unit attached. While it technically has only 3 overdrive speeds for 2nd, 3rd, and 4th, you can get the overdrive to operate while in 1st under some circumstances. The operation and specs for the 4+ 3 changed quite a bit during its 5 year span of use. See the chart on the differences at http://www.5speeds.com/43table.htm

While the 4+3 isn't all that great a transmission setup, I LIKE my 4+3. In 84 and 85 the overdrive button was on the console I believe. It was later changed to a momentary contact switch under the button at the top of the shifter ****. This give you a fighter pilot sort of shift **** that allows you to drop from 2hi to 2low with the flick of your thumb as you slow to make a turn and back to 2hi after the turn. Kinda cool. You can also use the button to turn off (or back on) the OD function on a more permanent basis as you drive.

If you are on the highway doing say 50 in 4hi and you stomp on the gas the OD kicks out automatically sort of like a passing gear in an auto tranny, and then kicks back in when you let off. Again it's kinda cool.

rws.1

Mike88Z51 gives a good overview.
In addition, one does not have to depress the clutch in order to engage the OD, but it appears that doing so increases its longevity.
Most transmission problems with the 4+3 seem to be with the overdrive unit itself.

Rich Silvestris

Mike forgot to mention that if you depress the gas pedal a little extra in first, that the OD doesn't kick in until 4th. It's kinda like it knows what you're up to.

tjwong

Thats because after you hit the button the ECM applies power to the OD solenoid. So it does know what you are up to via TPS position and vehicle speed sensors

CFI-EFI

It is like a truck with a two speed diff, or splitter. The unit on the back of the conventional 4 speed is a planetary geared, hydraulically shifted, two speed trans. It is direct drive and overdrive. The ECM controls the shifting based on the input from several sources. Basically it is a 2 speed automatic trans. The first few years even have a TV cable like the 700R4. Being an automatic and working off of a front pump like an auto, shifting into OD is prevented, because the input shaft and pump don't generally turn fast enough with the trans in first, to generate sufficient volume and pressure to shift in to OD safely, without slippage.

RACE ON!!!

scorp508

Good great info has been posted so far. What year are the cars you will be test driving? This is important to know as 4+3 operation changed nearly every year of its existence I think that 1987 and 1988 are pretty much the same.

Read up on this chart so that you'll know if the tranny is operating as it should be or not when you drive the cars.

http://www.5speeds.com/43table.htm

There are also 2 different overdrive units. If you look on the RPO sheet for the car (Under the center console lid or on the bottom of one of the rear storage lids) on a 4+3 equipped car you will see MK2 or MH5. The gear ratios for these are...

MK2 = 0.68 : 1
MH5 = 0.60 : 1

The MK2 from what I know came in Z51 and possibly Z52 cars as well. It gives a hair better acceleration.

4bblC4

The 4 speed tranny, however, is NOT a super T10. It is a regular T10 aluminum case tranny.

Therefore, it is not as strong as a super T10 by any stretch of the imagination.

glock19

thanks for the info going to look at a 88 Z51 and a regular late alum head 86
anything i should look out for in particular on 4+3s
I have messed with , R200s, R700s, turbo 350/400s and T5s but this is a new one for me

scorp508

The OD should engage pretty smoothly at speed. Most of us use the clutch when engaging it, but for a test drive I wouldn't so that you can see how it is operating.

If you DON'T turn off O/D when slowing to a stop it'll eventually turn off when you get (I think) to 3 MPH. In this circumstance you CAN have O/D on while in 1st gear, but any acceleration (check the chart for RPM) will knock it out.

Kanmer

The filter on the 4+3 needs to be changed every 10,000 miles. Find out how often it was changed and when the last time it was changed. That is important.

CentralCoaster

What is your source? The supers had higher nickel content in the gears, I don't know other differences.

Doug Nash Engineering manufactured the case and built them. I presume he sourced parts from Borg Warner, but I coud be wrong there.

It's also the weakest of the super T10s because of the smaller teeth count ( =smaller gears)

Droptop Bombers

I have an 86 with the 4+3 and the switch in the shifter is not working. I press the button and nothing happens. I had someone test it and they said the switch is dead. I can't find one on the net. any suggestions. The OD works fine, it goes into OD in second, third and forth unless I get on the gas. I'm rather new to the vette forum and car for that matter so I hope I don't sound too stupid. Also how do you get the cable out of the shifter? I you can help that would be great. these forum are a real help!

scorp508

Hows a <$20 fix sound to ya?

http://www.newark.com/

For 1986-1988 4+3 Switch use P/N: 61F753

It is $12.30 now. You will have to solder the harness to it (no big deal at all). Otherwise it is $125 from Ecklers if you want the silly little harness included.

http://www.newark.com/NewarkWebComme...SKU=61F753&N=4

Droptop Bombers

i can't believe i spent over 3 hours trying to find the part and here is an easy fix. it should work great! thanks alot, you guys rule.

c4cruiser

The Doug Nash 4+3 also went into the 87 Callaway Twin Turbo cars that generated 345HP and 465 ft-lbs of torque. I don't believe that Callaway did any mods to the tranny so it had to be strong enough from Doug Nash/GM to handle that output. The 4+3 trannies that went into these cars had the MH5 version with the 0.60 OD ratio.

Mike_88Z51

I am not an expert by any means, but I have read several places that it was a Super T-10. The guys who were deeply involved with repairing the OD units and worked in conjunction with GM on improving some of the OD parts in later years at http://www.5speeds.com/ state:
"We have been rebuilding Corvette 4 +3 Overdrives since 1984. This transmission incorporates a Super T10 4 speed with an automatic overdrive."

scorp508

When I sheered the teeth off of 1st gear and the countershaft gear all the replacement parts were Super T10 parts. This was at 166k miles in stock form.