The "Doug Nash 4+3"...

[Ken]

Following up on the transmission saga...

As it turns out, after going through the latest in the ongoing electrical gremlin problems I've had, the overdrive may never have been bad in the first place.

As I was trying to diagnose the electrical problem with my fuel pump, I came across another in the long list of relays on the Corvette. There is a relay for the overdrive unit in a manual transmission (4+3) located right beside and sharing a common bracket with a relay for the fuel pump. As a matter-of-fact, it is the same part number. But do you think that knowledge was readily available in the manual(s)? No! :eyerole

At any rate, when I got the car back the other day and ran around for a while, the overdrive behaved exactly as it was designed. In effect, a twelve dollar part cost me around thirty-five hundred dollars! At least now I know the transmission is good and can be re-sold to benefit another C4.

Now I gotta decide to rebuild the L98 to spec, or simply re-sell the engine as-is. :eyerole

_ken

 

[David-F]

The 4+3 was built by Doug Nash they bought the tooling and what not from Borg Warner but when they got it the tooling was worn out so GM had to loan them the money to retool to go into production, this is one of the reasons they went out of busness when GM went with the ZF in 89.
The 84 and early 85 were the problem units, but through out 85 new revised parts went into production starting with the revised direct clutco bearing thrust washer with the apply piston locking tabs, next came the revised carrier bearing and with it most of the problems went away.
If I were to get a low mileage 84 or 85 I would remove and rebuild the unit before anything could fail, it is much better to repair before something failed then wait untill something does happen, many of the parts are out of production and there is no knowing if some will ever be made again.
Also 84 to 86 can be made to operate bypassing the ECM with very little needed only the 86 would require the change of shifter switch from momentary contact to and on off switch like used on later 85, yes it's only in 85 that some had console switches and shifter switches after that they were all on the shifter and all 84 had console switches. The problem with 87 and 88 is that the O/D light was moved from the teltail assembly to the LCD cluster above the tach to were now the ECM comands the light on through the data stream to the LCD but 84 to 86 used a ligh that got powered up from the O/D relay that powered the O/D unit so by by-passing the ECM 84 to 86 can be converted and the O/D light would still work correctly, but 87 and 88 would require a reprogramed PROM.
If there is someone out there that need more info. on a 4+3 just give me an e-mail I have repaire hundreds of them and am able to help people do their own repair or what ever they are not very hard to do if you are able to get the correct tool and unless ther are major part failure can be repaire for under $600.00.

David Fulcher

 

[Ken]

Thanks for the info David, we get questions all the time on the DN 4+3 so any help that you can provide is much appreciated.

_ken

 

[rrubel]

So, Dave and anyone else... someone mentioned that the 4+3 might not be good for autocrossing. I assume that would be because of the constant shifting? Anyone think that just turning the OD off for every run (ie, leaving it in direct) would put low or no stress on the OD system?
[RICHR]

 

[maxrevs85]

I have an 85 4+3. How do you bypass the ecm? And, what kind of torque are they rated for?
I drove mine very little after I bought it . It seemed to work fine . But I believe the only time I had OD was when I activated the switch, in any gear.
Does the kit rebuild the 4sp. and the OD unit?
Oh why oh why couldnt they have just put in a nice simple 5th gear od with no wires !! :eyerole

 

[David-F]

There is no reason why you can't use a 4+3 in an autoX just keep the O/D off and you will not have a problem, if you do fear it coming on and have not converted it over to manual control just unplug the relay while you are racing.

David Fulcher

 

[David-F]

maxrevs85;
When I get home tonight I'll dig out my 85 wiring diagram and tell you just wires you need to cut and splice to turn your O/D to manual control, it's real easy and then you can use it just like 5th gear.

David Fulcher

 

[maxrevs85]

David ,thanks a bunch . Thats the way i want to use it. I like to drive it like a hotrod but, the extra gear is a big relief.

 

[David-F]

It turns out I had the wiring diagram here at work, this is real easy drop the ECM find terminals C7(light blue) and A7(tan with black stripe) cut the wire a few inches from the ECM and take the body harness wires and splice them together and thats it, to check just turn the key on and watch the telltail assembly to see that the O/D light turns on and off if it does so will the O/D and the ECM is unable to do it for you.

David Fulcher

 

[maxrevs85]

Thanks again David . I have it writen down for future reference. I have the tranny out ,but havent looked it over yet. have no idea if its been worked on before or if it has any upgrades

 

[rockytoptilidie]

so, while driving, unless I'm on the highway, I should keep it in UD?
Also, sometimes she grinds when I'm going into reverse-does this mean the clutch is going? How much will that cost me?
Thanks in advance!

 

[Edmond]

Hi Rocky and welcome to the Corvette Action Center.

I just noticed that it's been more than a year since someone last made a reply!

Sorry I can't help with your question.

 

[rockytoptilidie]

I know I know!!! I just found this in another message board, and it had some great insight. Obviously, this is the place for knowledge. I've been lurking around for the last couple of weeks, but I wanted to ask a question that I should probably know, and know that I wouldn't get ripped for it! Actually, I was told that before I go into reverse, put it in first, and then reverse. Didn't grind, worked like a charmp.

 

[95ZR1#418]

I loved the 4+3 in my '88 Z51 coupe, the only other choice was the automatic and I would never do that. Simple maintenance is the key to longevity. The previous owner and myself religiously changed the overdrive fluid every 15-20 ,ooo miles with new filter(~ $20.00 or less). You don't use the gasket that comes with the filter.

Car has over 100K OD now and still tight tranny.

 

[WhalePirot]

Stop bashing the +3

I have a heavily hopped-up '84 with 105k and am running the 4+3. I love manual shift but also love the kick-down for highway passing. It's the best of both worlds, to me. The '84 logic was complicated and largely misunderstood. I read and understood the logic and used it to my driving advantage. GM simplified the that logic in '85.

I had some 'control' issues with the O/D; none of which were the unit's fault. ALL were external. One was a pinched brown wire (pax side, firewall) courtesy of the 'tech' at the Chevy dealer. Another was bad plug wires which fooled the ECM into thinking an open loop condition existed. Low ATF levels can prevent engagement in steady state, if low enough; or cycling on/off or disengagement during cornering. Low fluid levels prevent the internal pump from picking up enough fluid to pressurize the failsafe-direct-drive design. The dash light indicates that the unit is trying to switch to O/D; has the electrical signal.

Yes, the relays are the same for O/D and fuel pump. They also like to fail, I hear. Mine are still okay. Both are fed =12vdc fulltime and activate by grounding one lug, completing the circuit.

My O/D was rebuilt by perhaps the best; the shop who did the units for Callaway. Paul is busy but was more cooperative than most in patiently anwering my questions and helping me. Check: http://www.5speeds.com/ in Boca Raton, FL. I shipped mine across the country to him after receiving the rebuild kit and tearing the unit apart. I shipped the mess to him and had him beef it up as much as possible. He assured me that the drivetrain weakness was NOT the +3, but the T-10; specifically first gear. Quote: "You will not break this overdirve".

The worse problem is the 'super' T-10 portion, due to cost constraints, specifically, the thin case hardening on the gears. An old pro, (Anaheim, CA) redid my gearbox, which showed hairline cracks on nearly all teeth and wear-thru on 2nd. I am quite good with mechanical things, but could not tear down and rebuild either box, due to lack of a 'press' for 2nd gear and the tricky reassembly of the +3. It has about 13 springs which need to be held inside, each aligned with a corresponding hole on the other sub assembly, but impossible to reach and hold. Some things are worth having done by a pro.

Case hardening applies a hard material to a softer underlay. These gears last a while, but nothing like the old days. They don't have the torque capacity due to the relative thinness of the surface, but who needed much in a stock, early C4? The metal under it is lighter in weight and softer, unlike the 'old days'. Iwas told the stock box would handle 225 ft.-lbs. (1st gear limiting). Going to a new, close ratio gear set added only 100 to that low number. I stayed put, avoiding all the cutting involved with the Richie or the ZF.

The units were reattached by moi and work great!

Highly recommended is GL-6 spec gear oil in the T-10, which means Red Line or Torco brand oil. I believe the 'spin-down spec for reverse is 8 seconds.

Grinding may indicate a failing clutch or incomplete disengagement; possibly due to one hydraulic component of the release parts. I have replaced 3 or 4 clutch masters and slaves, likely due to the higher heating of the fluid, from my headers. I change that fluid when it darkens, indicating water adsorption; which causes corrosion pitting of the internals.

Understand UR car and enjoy it more!


Mike

one-time AirForce pilot

 

[Edmond]

I think one of the reasons why the 4+3 is shunned is because of it's short life span. If GM had continued to use it, and refining it, I'm sure it would have just as much respect as today's 6 speeds.

 

[rockytoptilidie]

Great info, guys. I believe a lot of my grinding was not fully engaging the clutch-It is by far the deepest clutch I've ever driven. Thanks a ton-this Air Force Sergeant appreciates it!

 

[djscomp]

I've had my 1988 4+3 for a few months and today my Overdrive came on and I cant get it to go off. In 1st gear it's fine and as soon as I shift to 2nd it goes into Overdrive. When I push the button nothing happens. While in 2nd thru 4th gear if I punch the gas the overdrive goes off but then comes back on again. Can anyone help? Thanks

 

[88Z51]

Newbie here. I just bought a 1988 Z51 (my first Corvette), black/black with the 4+3. Unfortunately, after only about 100 miles of relatively easy driving I still found the 1st gear weak link. I gotta say, I've had some manual trans. failures in the past, but nothing has sounded this bad (1st gear REALLY bangs and clangs)! Anyway, my question is when rebuilding the trans, is there anything that can be done to improve it? I do plan on autocrossing it / club track days and the like, but mostly street driving.

Also, the button on the shifter does not disengage the OD. Only hard throttle will kick it out of OD (any gear). Does this sound like the switch under the console or something else?
I've tried adjusting the T rod to no avail.

Any helpful hints or insight on beefing up / improving this transmission would be greatly appreciated.

Thanks much!

Greg.

 

[Ken]

Welcome To The Corvette Action Center 88Z51!

My original intent for my personalized plates for my Vette was going to be "87 Z51" but in California four digits and a letter are reserved for motorcycles. I had to settle for "1987 Z51".

... the button on the shifter does not disengage the OD. Only hard throttle will kick it out of OD (any gear). Does this sound like the switch under the console or something else?
I've tried adjusting the T rod to no avail.

You will get a lot of response on the 4+3 Greg, I'd rather let someone else handle the trans gear part. However, the overdrive operation, and also the rebuilding, is covered in our knowledgebase. You can view the information here at: 1984-1987: Technical Article: Rebuilding the Doug Nash 4+3 Overdrive Unit.

I'm glad to see that you are keeping the 4-4-2 tradition alive. :cool