Hey guys, I came across an article on the web attributed to:

CARS & PARTS CORVETTE
by Richard F. Newton

In this article he discusses the Doug Nash 4+3 Overdrive unit and describes a modification that allows full manual control in *ALL* four gears.

The problem is, he sort of glosses over the details about how the OD is supposed to work after the modification. For the life of me I can't figure out how his mod would be anything but a *BAD* idea.

Anyway, here's the excerpt:

A Major Modification
I discovered this one by accident. Well, not quite by accident. Actually Chris Petris, who used to build transmissions for the Corvette Challenge, helped me with this. There's a switch on the side of the transmission that tells the computer which gear is engaged. This second gear switch is one of the first things to go bad on the 4+3. The switch is engaged every single time you shift the transmission, whether the overdrive is engaged or not. It's no wonder this is usually the first item to wear out.

When my second gear switch went bad the car simply wouldn't engage overdrive properly. I could hit the switch on the console to turn on the OD and it might not actually engage until I drove some 20 miles down the road. You can imagine how aggravating this was. When Chris and I couldn't locate a replacement switch right away, we simply grounded the switch by running a very short shunt from one terminal to the other. This effectively made the OD a completely mechanical unit, operated only by the interior switch, which in my case is on the console.

Eliminating this switch made all the difference in the world. This was like giving me a new Corvette. This same modification was made to the World Challenge Corvettes. While Chevrolet designed all the electronics to pass the EPA fuel mileage cycle, we simply want a useful transmission. The 4+3 actually works better if you take some of the sophistication out of the system.

What we did was fool the computer into thinking that I'm always in second gear or higher. This switch is on the left side of the transmission, and can be seen easily if you have the car on a lift. This switch moved around a little during the years, but it was always the switch toward the rear. Remove the wiring harness from the switch and hook it out of the way with a tie-wrap. You won't be using this harness. Now make a little jumper wire that fits into the two terminals. You can actually remove the switch from the transmission and make the jumper wire on your workbench. It shouldn't take more than 15 minutes.

Ok, here's my problem with this mod:

If you fool the computer into thinking it is always in second gear or higher then the computer will activate the OD control solenoid which lets hydraulic pressure reach the clutch pack which in turn engages the overdrive. The problem is, there's no hydraulic pressure available to engage the clutch pack until the output shaft starts to spin. It's not spinning when the car is sitting still! So, the way I figure it, the car starts out in 1st gear non-overdrive and as soon as the hydraulic pressure rises to the point where the OD can engage, it shifts into 1st gear w/overdrive.

That transitional period where there is just enough pressure to engage the overdrive is what bothers me. Wouldn't you run the risk of burning up the clutch pack if you accellerate heavily while in this state?

Maybe I'm misunderstanding some fundamental issue here but it seems to me that the 1st gear lockout designed into the 4+3 is there for just this reason.

Anythoughts?

 

The short answer is that this guy doesn't have the advantage of years of experience reading & posting on Corvette Forum. A big "Thanks!" to Troy for offering this great place to share ideas... :seeya

All manner of permutations have been offered, but the consensus of opinion to get a "command" operated OD is to jumper A7 to C8 at the ECM harness, and install a 1985 (On/Off) OD switch.

The OD will stay in whatever mode you left it in; it will operate in any/all gears (even Reverse); and it will be available whenever you want it.

The ECM is completely left out of the circuit.

It's cheap, and you *will* be amazed and delighted at the "new" way your trans performs. :flag

 

Rich, can I assume from your post that you have made the modifications you describe or know someone who has?

I'm still skeptical because there is no way the overdrive will work from a dead-stop. The output shaft must be spinning before there is any hydraulic pressure to operate the overdrive. Maybe the pressure rises so quickly that it seems like the overdrive is functional in first gear or reverse but I don't buy that yet. I'll agree with you on one point though... I *will* be amazed if this works.

 

Well, yes in a way. Late 85 got the momentary switch, so the cars defautled to ON at startup. But yes 84/85E would do it also if the switch was in the on position. It was the predecessor to CAGS.

 

[QUOTEMy 85 was a late 85 with the switch in the shifter. It did *NOT* default to on when the car was started.[/QUOTE]

I leanred something new. :)

 

I am one that bought the magazine and also bought the jumper switch for the 1st gear switch(?) that Chris sells at the Corvette Clinic. Although my OD (early '85, switch on the console) already performed like the one described (only engages when I have the OD 'on' on the console), I have had the OD kick on/off while driving down the highway (very annoying - more on this later). I figured that installing the switch might help correct it (it only cost about $20, so why not?). Anyways, what it did for my situation was eliminate the OD altogether. I could not get it to engage at all. I believe I installed it correctly, but I also believe that the part is not at fault either. The jumper is now back in its envelope, and the OD has been working fine this spring, and I can't wait for the weather in Michigan to get above 50 degrees for more than 2 days in a row so I can enjoy my new Bilsteins.
Now for the OD engaging/disengaging. Scorp posted a while back (sorry, don't know when but I printed the post and replies) about the harness connecting to the relay being 'pulled' and causing what I described. I haven't confirmed this on my car yet, but I am 95% positive that this is what I have been experiencing. Since my OD is working in the manner in which I want it to (I don't know why, and if it works, why ask questions?), I have that 1st gear switch sitting doing nothing and it can be had for $20, including shipping. Just drop me an email and I'll mail it to you. I think the benefits of having the OD under the driver's command are self evident.
I wave!
grsteve

 

Yes I have, on two cars now (mine and another). Works fine.

No argument here. I don't want my OD to go on when I switch the car on - so I leave the ('85) switch "Off".

In fact, I rarely use OD at all. Occasionally I switch it on while driving, just to keep the parts lubricated. I don't do any freeway driving, maybe 50 miles a year. No use for an overdrive.

This is a recurring thread/question. The archives should have more on it. (Some with my questions!) :D

 

You guys mention not engaging O.D. with drive train under load. Have you had bad experience with this or what. Is'nt the o.d. just like an auto trans?
I race mine in SCCA and constantly go from 4th to o.d. with foot to floor.
Bad idea?

 

I'm not sure on that one either. I've done the same without issue (so far).

 

Guys! I think we've managed to discuss everything except for the point I was trying to make in my original post:

There is no way that the overdrive can work from a dead stop. There is no hydraulic pressure available to engage the overdrive until the output shaft is spinning. Therefore the modification described in the article is most likely bogus and possibly damaging.

What really blows my mind is this paragraph from the article:

I wish he actually described how the overdrive operated after this modification rather than blowing smoke like "Eliminating this switch made all the difference in the world." and "This was like giving me a new Corvette." There's not much meat there....

As for the tangential issues raise here, I agree that having complete control of the overdrive is a great idea. I also agree that you probably can make the overdrive last much longer if you don't allow it to shift when under heavy acceleration.

But I have a lot of problems with this modification even though it comes from what appears to be a reputable source.


[Modified by LWesthaver, 9:24 PM 4/2/2002]


[Modified by LWesthaver, 10:18 AM 4/3/2002]

 

I posted this question some time ago about doing the same thing to an '84. Anyone found the answer yet?

 

Let me jump in here to add a little to this talk, first if the switch iis left on when you come to a stop it will still come out of O/D and yes it's because of the pressure switch which has to have 75psi or better to work and trust me it doesn't take that much road speed for this to happen. And SK is wrong it's not coming into second O/D that wears out the O/D unit, if you notice most of the time you let the ECM control the O/D it goes into O/D while shifting into second not after so the shift happens not under power but during the shift. It has more to do with early design flaws and lack of service, and leaks that go undetected, you only have a small oil supply so if it starts to brake down you will have problems. Also the reason they are expensive to repair has more to do with the shops that do it taking advantage of the few out there, lets face it an overhaul kits costs $150.00(it can be had for less at some places) and in most cases that is all that would be needed, now if you have hard part failures then it could get expensive. Also if you where to use stock GM clutches and seals then it could get expensive clutches alone could run $150.00! But with the parts out there now it sould be around $600.00 to fix it tops not counting removal. I used to get 4.6 hours warrenty time to rebuild one that includes R&R trust me it takes more like 8.0 to do it, but that's GM. But anyway getting back 84 to 86 can be converted by bypassing the ECM 86 you will also have to change the shifter switch to a late 85 switch, 87 and 88 because of the move of the O/D light to the LCD need a reprogramed PROM to do it correctly.

David Fulcher

 

David, glad you chimed-in. Based on empirical evidence, the hydraulic pressure must rise to an acceptable level (75psi) somewhere between a dead-stop and the second gear shift point. When the hydraulic pressure reaches 75psi is not known. You say it doesn't take much road speed to get the pressure up and this may be true. But do you think that it is safe to perform the modification described in my first post given what you know? I don't think so.

I agree with you here, I've never experienced my 4+3 shifting into overdrive *after* 2nd gear is engaged. I will allow that it can happen though. If you can shift from 1st gear to 2nd gear in a shorter period of time than that required for the overdrive to engage then there will be slippage and wear occuring in the overdrive. However, quick-shifting a 4+3 isn't easy to do, at least not on my car.

When you say converted, do you mean converting the overdrive to fully manual operation??? Full manual control seems like an ok idea in 2nd thru 4th gear but the question remains, is it a good idea to trick the overdrive into engaging while the transmission is in 1st gear?

 

Ok, this is just getting silly. Of course I've experienced that. I've also engaged the overdrive via the shift **** switch after shifting into 2nd gear... but that's not the point.


Good grief.... I *don't* want to do that. I started this thread because I discovered an article from a reputable source that proposed a modification that seemed to be a very bad idea. I wanted to find out if I was missing something in my understanding of how the 4+3 overdrive works.

I wouldn't stick a screwdriver in my eye either, but if someone reputable stepped forward and declared it to be a great idea I'd probably still want to discuss the matter even though I believed it to be ludicrous. Who knows, maybe there *is* a good reason to stick a screwdriver in my eye. :jester

I had hoped that this thread would shed some light on the inner workings of the 4+3 from a technical standpoint. Instead it sounds like that old joke (Patient) Doc, it really hurts when I do this... (Doc) Then don't do that!

So, my question remains, is it a good idea or a bad idea to trick the overdrive into engaging while the transmission is in 1st gear? From a technical standpoint that is.



[Modified by LWesthaver, 3:04 PM 4/3/2002]

 

Go ahead and and make the change I highly recomend it all you have to do is remember to turn it off when you come to a stop but what I think you will find is that you will start to use the O/D as a 5th gear, then you can even use the clutch while you switch it on there by reducing O/D clutch wear. Also the big advantage is it will not come out when you least expect it to like pushing it through a turn that is going slightly up hill and you then get to the TPS setting when the O/D cuts out now the back end brakes loose and who knows where you may end up. There has been coments that people are concerned about starting out in O/D first as stated earler this can not happen second if you have a manual control you will probably turn it off way before you may come to a stop,but if you do forget it still will not start out in 1st O/D. If you would like to talk farther about the O/D operation we can just ask away, but as for the change to manual control it is the way to go.

David Fulcher

 

I also am a believer in manual control, since mine acts in this manner. I can count the times on one hand that I have forgotten to turn the 'OD' to the off position. I notice it immediately when shifting to 2nd gear, since the car doesn't have the normal go to it. I will wait until the car is in first gear, coming to a stop, and when the light goes off, then I will switch it off. Its just another thing I have to remember, but now it is almost habit.

 

...And since the physical mod is already described above, the only thing left is the switch, which is available from Newark Electronics http://www.newark.com as part # 61F752 for the '85 (On/Off) switch; 61F753 for the '86 (Momentary Contact) switch (if you want to just replace the OEM switch).

Either one is ~$12...Better order a couple; Scorp burns them out fast... :D

 

Must be inferior parts! :D