How to disable OD on the 4+3 manual tranny

[MacVette86]

I have an 86 with the Doug Nash 4+3. I will be using this as a boulevard cruiser almost 100% of the time and don't really need the overdrive. I've read so much about this tranny on this site that my eyes have litterally glazed over. My OD is not working and from what I've read it seems that there will always be problems. Given how I will use the car, is there a simple way to disable the OD completely so the auto part of the tranny just stays in the 1:1 ratio, essentially giving me a stright 4 speed?

BTW, I am having a custom chip burned for the car, if it's possible to permanently turn off the OD this way.

 

[Ken]

One thing though. You say the overdrive unit on yours is not working, but what exactly doesn't work? If the unit itself is wearing out, eventually you will lose reverse gear, and simply disabling the overdrive function won't help.

 

[nyernga]

One thing though. You say the overdrive unit on yours is not working, but what exactly doesn't work? If the unit itself is wearing out, eventually you will lose reverse gear, and simply disabling the overdrive function won't help.

And if there's anybody on this site that knows Doug Nash better then Doug Nash does.....it's Ken.

The OD is engaged by several parameters ie: temp and speed. I don't think the 86 has the switch thats mounted on the side of the unit that can be jumped out. I did that to mine a few years ago and what it does is turn the OD into a completely manual unit. IE: you no hit the switch,,,,she no go into OD.

Len

 

[MacVette86]

Well, the transmission itself works fine, and it shifts into reverse very easily. However, I was in fourth gear and hit the button on top of the shifter and nothing happened. The engine was up to temp and I was going about 50 MPH, so I assume it should have kicked in. Bottom line for me is that I don't mind servicing the OD unit by chnaging the fluid and filter from time to time, but I don't want to spend the rest of my life fussing over a feature I will rarely if ever use. So I'm wondering if theres an easy way to just ignore the overdrive

 

[WhalePirot]

I'm wondering if theres an easy way to just ignore the overdrive

Seems you already are!

The O/D issue may or may not be simple. It might just be a bad relay, but if it's not working and you don't need it, do you really care? Seems not, as you drive in 1:1, now.

The reverse shaft is part of the O/D assembly, inserted into the main unit; the reverse gear 'rides' on it. As a separate unit, otherwise, I don't see why it would cause the Super T-10 to fail. Keeping the O/D fluid clean and filled should ensure the unit continues to pass the power through just fine.

 

[nyernga]

However, I was in fourth gear and hit the button on top of the shifter and nothing happened. The engine was up to temp and I was going about 50 MPH, so I assume it should have kicked in.

It could very well be the switch itself. It's inside the shifter and not an easy to find part. The procedure for replacing it is simple enough. It's finding one that's difficult. If the engine was at 122 degrees it should have kicked in when you hit the button. I have to agree with whale though...if it's not working and you don't want to use it, then just keep it serviced with fluid and filter changes.

Len

 

[MacVette86]

So could I remove the switch on top of the shifter knob? Its not a very good place for it as I like to grab the shifter from the top. If I got rid of the switch I could out a nice shifter handle on.

 

[Agent 86]

If you need some schematics and diagnostics for the OD let me know. I have my entire service manual on my hard drive so emailing is a snap.

I will be using this as a boulevard cruiser almost 100% of the time

In Maple Ridge:W Or do you head west

 

[MacVette86]

So far I haven't wandered very far from Maple Ridge as I'm not that confident in the car mechanically. Surprisingly, it seems to run great but I'll feel better after I givea good tune up and change all the fluids, etc.

 

[WhalePirot]

Smart idea to get a 'base' for all the fluids, but once off the boulevard, you might want the overdrive. Actually, while many bemoan it, it is quite a good unit and will deliver very long life, if a bit more complicated than a straight manual box. My research has indicated that it is the tougher of the two tranny components, which is not the popular view here.

There are threads on this forum that refer to switch fixing, with a replacement spring. Others address other aspects of O/D repair. Te switch sends a request for O/D to the ECM, which then sends 12vdc to the relay; one of two identical relays near the brake booster, on the LF wheel well. You can simply swap pigtails on them and try to start the car. The other relay is for the fuel pump, so if the car starts, the relay is fine.

I cannot confirm the 122 degree as a hard number, as the ECM must be in closed loop, meaning the car is well warmed up and using the data sent from the various sensors to adjust mixture, etc. I once fixed my O/D by changing spark plug wires! The car was running so rich that the ECM was on the line between open and close loop (steady 70 mph, warmed up); therefore switching it on and off. It looked like an overdrive problem, but obviously was not. The plug wires were old and not delivering enough spark. For disbelievers, they were ALL that I changed. I was pleasantly surprised to have a normal O/D again.

Many C-4 owners alter the switching, by making the switch a simple, direct on/off, eliminating the ECM aspect of the logic. With such, you could use any switch, mounted anywhere. There would be no reason why you couldn't choose whatever shifter you choose, although there is no obstacle now, if you never want the O/D.

BTW, the stock shifter levers can be changed to reduce the throw slightly. I did this ages ago, and am one of the few 'fans' of the Nash. It is still in one of the baddest C-4s in this area.

 

[MacVette86]

I actually like the 4 speed! It shifts fine and goes into reverse easily. I sometimes have some shudder getting off the line if first, but I assume thats me being a bit rusty as I haven't had a standard for a few years. I have many minor fixes to do to the car after I do all the basic stuff like tune up, fluids, etc so I'm just going to leave the OD aone for now. I did try to read all the threads on the OD unit, but there were so many conflicting ideas that I couldn't make any sense of it. Your idea of fooling the ECM into keeping it off sounds promising, but I can't find any actual instrictions on how to do that. Oh well, I'll keep looking. For now, I'm just having fun working on my vette

 

[nyernga]

That switch you "jump" out to make it a fully manual unit is located on the side of the unit itself on the 88's. I believe it's a different setup on earlier models. The 122 degrees is when mine ALWAYS goes into OD after first starting the car. I can almost pinpoint the exact store I'll be in front of when it happens since I only use my vette on weekends. Apparently the only input is temperature. Once I click out of OD the first time it doesn't happen again until I manually do so.
One other issue is when you jump the switch out, it gives you OD in first gear as well. From what I've read however, that's not good for the drivetrain.
The switch in the shifter actually is down along the shaft. The button ontop of the shifter pushed down on a small "T" rod and that activates the switch. Should you decide to just remove the "T" rod to avoid accidently going into OD, I highly recommend counting the number of turns it took to remove it. Then have that number tattoed on you so you NEVER forget it. It's a number you will have to have to reinstall the rod should you decide to.

Len

 

[rrubel]

If you really want to disable the O/D, pull the relay that Whalepirot was talking about (however, it's on the firewall between the wiper motor and the brake booster). Then you'll just have to deal with the annoying "UPSHIFT" light on the dash (which can be fixed with a piece of black electrical tape ).

Are you saying that you don't get OD at all, even when just driving the car "normally"? The computer should throw you into OD without your having to hit anything; if you DO hit the button while the OD is off, it'll STAY off until next time you restart the car or hit the button again.
[RICHR]

 

[WhalePirot]

the relay is on the firewall between the wiper motor and the brake booster).

Oops. I moved mine.

Mine used to come on at exactly 169 degrees, in front of the same store. My manual states that closed loop operation is the criteria. As my car ran more richly, with modifications, that temperature climbed. Eventually, I did the "spark plug wire" fix, which restored the 169.

 

[MacVette86]

As far as I can tell I never get overdrive and I've had th temp up to 190 F so it should have kicked in. Oh well, I knew the car was a "Jewel" when I bought it. I wanted to do the work myself. I figured I could buy a car in fairly good shape and then fix things as they broke, or buy an older car and fix everthing right the first time. Well I bought the older car and I got what I wished for. Its currently in pieces on my garage floor and I'm having a lot of fun

 

[nyernga]

As far as I can tell I never get overdrive and I've had th temp up to 190 F so it should have kicked in. Oh well, I knew the car was a "Jewel" when I bought it. I wanted to do the work myself. I figured I could buy a car in fairly good shape and then fix things as they broke, or buy an older car and fix everthing right the first time. Well I bought the older car and I got what I wished for. Its currently in pieces on my garage floor and I'm having a lot of fun

Take two of these and start fresh in the morning...

Len

 

[c4cruiser]

Here's a link with information on the operation of the 4+3 unit for the diffeernt years: http://www.5speeds.com/43table.htm
Your trouble sounds like a bad relay as OD operation is controlled by the ECM sending a signal to the relay and once the specific water temps for your year (as shown in the table) is reached, the unit is supposed to go into OD as a default. I can keep OD off in my 87 by simply hitting the switch before the water temps hits 122 degrees F. Some folks say that isn't correct but it does work for my car.

Check the relay as mentioned to see if that is the problem. These relays are not expensive (less than $20 US) and it's a good spare part to have in the tool chest. IIRC from reading a post on the CF, a good replacement for the shifter switch can be found at Radio Rhack. Also check the connections at the tranny to insure there are no broken wires or connectors.

As far as removing the button and replacing the handle, I have yet to see an aftermarket shift knob that will thread onto the factory shifter handle. The diameter is much larger than the typical shifter handle. The short throw levers work OK but they seem "notchy" after install.

The 4+3 is a good unit and as long as you change the OD fluid every 15-20K miles and clean the filter, it should work fine. The same OD unit was used on the Callaway cars and they had much more HP and torque than the factory L98 motors. In addition, the drop in RPM with the OD engaged will provide a substantial increase in fuel mileage. Simply depressing the gas pedal will cause a downshift in 2nd, 3rd or 4th gear and is a lot of fun!!

I use Mobil-1 synthetic ATF (Dexron II/III type) in the OD and a synthetic 75-90 gear oil in the 4-speed. Use black RTF sealant on the OD pan instead of the gasket. I bought a new filter from Mid America and swap between two of them. They can be cleaned and reused with the same o-ring.

 

[NevadaJim]

The 4+3 is a good unit and as long as you change the OD fluid every 15-20K miles and clean the filter, it should work fine. The same OD unit was used on the Callaway cars and they had much more HP and torque than the factory L98 motors. In addition, the drop in RPM with the OD engaged will provide a substantial increase in fuel mileage. Simply depressing the gas pedal will cause a downshift in 2nd, 3rd or 4th gear and is a lot of fun!!

I use Mobil-1 synthetic ATF (Dexron II/III type) in the OD and a synthetic 75-90 gear oil in the 4-speed. Use black RTF sealant on the OD pan instead of the gasket. I bought a new filter from Mid America and swap between two of them. They can be cleaned and reused with the same o-ring.

I agree; I have the 4+3 in my Callaway, that put out 454 ft lbls torque at the rear wheel and it works great -- and its a lot of fun to drive

 

[Agent 86]

Check the connector to OD relay. The wire insulation shrinks back leaving exposed wires that short.

Do take a look. And while your there check the fuel pump relay and both maf relays behind battery. All of them are prone to insulation shrinking leaving a dangerous situation.

 

[rrubel]

Do take a look. And while your there check the fuel pump relay and both maf relays behind battery. All of them are prone to insulation shrinking leaving a dangerous situation.

Big agreement here. Two years ago at Carlisle, my fuel pump relay shorted the two bare wires and almost left me stranded.
[RICHR]