C4 4+3 manual gearbox

[Henri 91EXP]

Hello all,

It has been a long time I've visited these forums. Mostly due to that my '91 is sold (traded for a Blazer) about a year ago.

Now, I do have a question about the 4+3 gearbox found in early C4's.
I've found a '86 conv with hardtop in excellent condition (38000 miles) and it has the 4+3 transmission. I know these are a pain to repair, but with this mileage, repair would be a long way away I guess...
Any ideas on this?

Furthermore, I'd like to hear first hand experience how this system works. It is a 'normal' four speed, with overdrive on the last three gears.
But how does this work? Is this overdrive engaged automatically, or by the driver itself? If automatically, is it something like the GAGS on the later 6-speeds, where the car itself decides what to do based on acceleration and a couple of other factors.
If it is engaged manually, is it by a button somewhere...?

Thanks for all info I can get on this subject! As there are not many 4+3's in the Netherlands I thought to ask around here too.

 

[JonM]

The 4+3 can be engaged manually or automaticly...their is a button on the shifter to turn it on and off. I have found with my 4+3 the best way to keep it up to par is, keep it clean. Change the fluid and filter every 10K miles.

Here are a couple sites:

http://www.gearzone.net/dne.htm

http://www.5speeds.com/nash.htm

http://www.5speeds.com/archive/27.html

 

[Vettelt193]

It's basically got an automatic tranny attached to a 4 speed manual. Like JonM said, it can work automatically, or push the button when you want to engage/disengage.

Also I fully agree with JonM in changing the fluid often. The killer of these is the fluid.

 

[Henri 91EXP]

Thanks, Jon!

I've already learned a lot more!
Please bear with me with my questions, because I really want to understand the functionality of the system.

You say it can be switched on and off. If it is on, then the OD shifting is done by the computer? There is no way OD can be engaged manually? Not that I want to, but I just want to understand it's operation.

If in off mode: it will never use OD?
When in off mode, what is the rpm of the engine at about 55 mph?
And when engaged OD, what is the rpm at 55 mph?

When in OD, if one shifts to another gear, will it stay in OD or will it disengage OD and, in the new gear, will determine to use OD or not for the then current circumstances?

Thanks!!!

 

[silver98]

Try this link to the owner's manuals: http://www.crossedflags.com/nuke/html/modules.php?name=Downloads&d_op=viewdownload&cid=7

 

[Henri 91EXP]

Thanks, but these are only from '93 upward. The 4+3 was installed up until '88.

Great site though! Didn't know it existed!

 

[JonM]

You say it can be switched on and off. If it is on, then the OD shifting is done by the computer? There is no way OD can be engaged manually? Not that I want to, but I just want to understand it's operation.

Yes, when the OD is switched on, it is controled by the ecm...you can turn the od unit on and off to engage the unit and disengage the unit...I would call that manually. You can also press the gas pedel to the floor while the od unit is engaged and it will kick down...sort of like a passing gear for an automatic.

If in off mode: it will never use OD?
When in off mode, what is the rpm of the engine at about 55 mph?
And when engaged OD, what is the rpm at 55 mph?

Coreect...off means its off...As for the RPM's I'll make note for you next time I have my car out...I don't know off the top of my head. Unless someone else remembers and chimes in.

When in OD, if one shifts to another gear, will it stay in OD or will it disengage OD and, in the new gear, will determine to use OD or not for the then current circumstances?

It all depends on your speed and throttle position. As I mentioned above rapid acceleration will cause the od to disengage...until the TP and Speed are brought back to normal then the OD will engage again.(in 4th gear) Also OD is automaticlly engaged at speeds above 110.

Remember I am going by what my car does (1984) yours may be a little different as GM did a few mods.

 

[Henri 91EXP]

Thanks! I'm getting the feel of it!

If you compare it to a normal stick shift car, is it equal in feel & operation?

 

[JonM]

Thanks! I'm getting the feel of it!

If you compare it to a normal stick shift car, is it equal in feel & operation?

 

[kalmwill]

The overdrive automatically engages when you shift into second gear after the engine temperature reaches about 56 degrees Celsius. It remains engaged throughout 2nd to 4th gear. It also kicks out when under heavy acceleration and re-engages when you back off on the throttle. First gear does not have overdrive so if you are stopped in traffic you start all over again. If you press the button on the gear shift lever the overdrive disengages and will stay disengaged until you press the button again. I personally like to disengage the overdrive while driving in the city. I find that the lower gear range is more suited to city traffic and I get a better throttle response. The overdrive feature is fantastic on the highway. For passing and for negotiating curves at highway speeds you can just downshift into a lower gear and smoothly power your way out. The higher gear range is definitely suited for a higher speed range. With a bit of experimenting you will be able to determine how the overdrive best suits your driving style. I hope this is of some help. Enjoy.

 

[rrubel]

It's actually not based on the ENGINE temp, but on the transmission temp. On a warm day, it'll go into O/D immediately even if the engine still says LO.
[RICHR]

 

[c4cruiser]

OD operation is based on coolant temps. The OD unit will not engage until coolant temps reach approximately 120 to 176 degrees depending on the year. If your car sits outside in the sun, the coolant may fery well reach 120 degrees even though the car hasn't run that day. Then the OD could engage almost immediately.

Here's a couple of links that show the differerences in the 4+3 over the years it was available:
http://www.5speeds.com/43table.htm
http://www.5speeds.com/dne.htm

The early models had the OD switch on the console and the later cars had a button on the shifter.

The tranny is a fairly stout unit as that tranny was also used in the 87 Callaway twin-turbo cars that had 340+ HP and gobs of torque. Those cars got a slightly different OD drive ratio but other than that, they were the same gear box as in regular 4+3 cars.

The key to long life of these trannys is maintenance. The OD unit used regular Dexron II auto transmission fluid and if you change fluid and filter every 20K miles (32km or so) it should last a long time.

The OD filter is the same filter as used in the Ford FMX automatic transmission which was widely used in late 60's thru early 70's V-8 Mectury Cougars, Ford Torino's and some 1/2 ton pickups. Get them from auto parts suppliers and it will cost about 6 to 8 dollars (US) as opposed to the $30 from the Vette accessory catalogs.

 

[rrubel]

Mmmff. My bad. Somehow I understood that same page as temp of the ATF rather than temp of the engine coolant (since the ATF also cools the overdrive).
[RICHR]