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Topic: Doug Nash 4 + 3 Transmission Overdrive Questions!!

in Forum: C4 Driveline Components


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Doug Nash 4 + 3 Transmission Overdrive Questions!! (1/11)
 8/12/12 10:19am
87 SharkTail
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Minster, OH - USA

Vette(s):
Beautiful Original Condition 1987 C4 Convertible, Bright Red exterior, Red interior, 4+3 Doug Nash Manual transmission, with 62K miles. Garage housed and dry roads driven. Fun to cruise along in, but always ready to go!


Joined: 9/6/2011
Posts: 17

To All C4 Members
 
I would appreciate any additional comments or data from members regarding this topic.
 
All early C4 manual transmission Corvettes have the Doug Nash 4+3 transmission, which automatically drops the 2nd, 3rd, and 4th shifted gear into Overdrive after accelleration has been completed, and Cruising has begun.
 
The purpose of this design, as I know of it, was to reduce engine RPM, and thus also increase fuel economy, in response to the new EPA mileage regulatioins put into effect during this time period. Later C4 manual transmission designs went to a 6 speed manual transmission to reduce engine rpm.
 
Regarding the 4 +3 manual transmission, there is a general trend for the 4 +3 Overdrive (Overdrive switch) to not work on continuing basis over time. If this happens, the 4+3 transmissioin becomes a basic 4 speed manual transmission with no overdrive.
 
My questions for your comments and data input are this:
 
If this occurs, what would be the general rpm consequences of having only the 4 speed, with no overdrive.
 
How much additional rpm would be experienced (2nd, 3rd, 4th gears) over the (2nd+Overdr, 3rd+Overdr, 4th+Overdr) rpms.
 
Will the additional rpm levels, be such, that it produces and increased wear or life of the transmission or engine?
 
What would be the expected reduction in mileage from the additional rpms.
 
What would be the main factors to start looking at, if the Overdrive starts to intermittantly not automatically actuate?
 
Is this a Computer programming glitch issue?
 
Is this a gear shift embedded Overdrive dissable switch issue?
 
Is this a mechanical binding issue in the overdrive itself?
 
Are their other issues to consider?
 
Again, and comments, experiences, repair cost history, information references, and data regarding this topic would be most helpful.
 
My understanding is that the 1984s and 1985s had most of the mechanical issues producing this effect, but the overdrive design was improved to eliminate/reduce these issues in the 1986, and 1987 models.
 
Bottom line, if Overdrive is not engaged, will the additional rpm created, produce any significant areas of concern.
 
Lastly, if needed, can the 4+3 be swapped out for the newer 6 speed transmission, and what would be the anticipated cost if done?
 
Thanks to all for your comments !
 
Best Regards,
Dan
 
 
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Re: Doug Nash 4 + 3 Transmission Overdrive Questions!! (2/11)
 8/14/12 11:03pm
davep85c4
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Joined: 4/4/2009
Posts: 254

Wow. It would take a few hours to answer all those questions. I have the knowledge on this subject to do so, but for the traffic on this site, not worth it.  I'll hit a couple of highlights:

EDIT: 8/14: I just removed a response that took over an hour to write 2 days ago that pretty much addressed all your questions, including several hints to diagnose your intermittent engagement. A peeve of mine is to take an hour to formulate a response, and the Original Poster never comes back and acknowledges the effort.

|UPDATED|8/14/2012 8:03:44 PM (AZT)|/UPDATED|
Re: Doug Nash 4 + 3 Transmission Overdrive Questions!! (3/11)
 8/14/12 11:39pm
eddie20890
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lanoka harbor, NJ - USA

Vette(s):
1990 L-98 white/ red/black leather interior targa top auto 1975 L48 blue/black leather interior t tops auto


Joined: 3/22/2009
Posts: 1363

DaveP85C4 said: Wow. It would take a few hours to answer all those questions. I have the knowledge on this subject to do so, but for the traffic on this site, not worth it.  I'll hit a couple of highlights:

EDIT: 8/14: I just removed a response that took over an hour to write 2 days ago that pretty much addressed all your questions, including several hints to diagnose your intermittent engagement. A peeve of mine is to take an hour to formulate a response, and the Original Poster never comes back and acknowledges the effort.



too bad you removed your response. understand about original poster not coming back but there may be others that could have used the info you posted. it might have even helped the traffic on this site as someone passing thru would see the intelligent answers and might think this is the place to hang out. just an observation here.



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Re: Doug Nash 4 + 3 Transmission Overdrive Questions!! (4/11)
 8/15/12 12:54am
87 SharkTail
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Minster, OH - USA

Vette(s):
Beautiful Original Condition 1987 C4 Convertible, Bright Red exterior, Red interior, 4+3 Doug Nash Manual transmission, with 62K miles. Garage housed and dry roads driven. Fun to cruise along in, but always ready to go!


Joined: 9/6/2011
Posts: 17

Dave and Eddie,
My thanks to Dave once again, as I posted my thanks for the information on the same day it was received.
 
I don't know why it would not show this on the system!
 
As further follow-up, I checked into the switch contact issue Dave suggested and found the broken contact as recommended.
 
All is now working good!
 
Again, thanks for all your helpful tips ,Dave. I made a copy of your comments to save as future reference, if a problem should again resurface.
 
Best Regards,
Dan
Re: Doug Nash 4 + 3 Transmission Overdrive Questions!! (5/11)
 8/15/12 5:49am
CaledoniaBrian
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, - Canada

Vette(s):
1988 Convertable


Joined: 11/3/2011
Posts: 150

I also am sadden for this post being removed. Although I do not own the Doug/Nash, I did find this post very informative.
 
There is a lot of times that when I am having difficulties with my car, I do a search; and some of the responses can be a few years old; just because they may be older post does not take away their value. The Internet is not a perfect place. I'm sure your response was well received by both the original poster and many more.
 


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Re: Doug Nash 4 + 3 Transmission Overdrive Questions!! (6/11)
 8/16/12 12:38am
davep85c4
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Joined: 4/4/2009
Posts: 254

Good. Glad you got it fixed. Enjoy your car.
Re: Doug Nash 4 + 3 Transmission Overdrive Questions!! (7/11)
 8/16/12 1:15am
87 SharkTail
Standard Member
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Minster, OH - USA

Vette(s):
Beautiful Original Condition 1987 C4 Convertible, Bright Red exterior, Red interior, 4+3 Doug Nash Manual transmission, with 62K miles. Garage housed and dry roads driven. Fun to cruise along in, but always ready to go!


Joined: 9/6/2011
Posts: 17

Thanks Dave,
 
If you would like, I can repost your comments for other members to use as well, since as I mentioned, I saved your comments for future use.
 
A lot of very good info in there!
 
I also put a paper copy in my garage toolbox for future reference.
 
Just let me know.
 
Best Regards,
Dan
Re: Doug Nash 4 + 3 Transmission Overdrive Questions!! (8/11)
 12/17/15 12:55pm
HarveyCheyneJr
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Joined: 12/17/2015
Posts: 1

Sorry to drag this thread back from the dead, but I am shopping for a C4 and was really hoping to read Dave's  explanation of how to troubleshoot the DN 4+3. If you still have the information, could we work something out for  me to read it?

thanks

Harvey
Re: Doug Nash 4 + 3 Transmission Overdrive Questions!! (9/11)
 4/4/16 2:47pm
fisherom
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Milwaukee, WI - USA

Vette(s):
1986, L98, 4+3 trans, Black in color


Joined: 4/4/2016
Posts: 1

I too would have liked to read the 4+3 explanation.

Especially since the time was taken to answer, and would have been greatly appreciated by me, and others, apparently.

Wally

P.S. I've heard many refer to the Doug Nash 4+3  as a 7 speed,
but in truth, only one overdrive gear ratio beyond 4th gear is available








|UPDATED|4/4/2016 11:47:19 AM (AZT)|/UPDATED|
Re: Doug Nash 4 + 3 Transmission Overdrive Questions!! (10/11)
 4/6/16 12:59pm
87C4TrackCar
Former Member

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Issaquah, WA - USA

Vette(s):
SOLD: 1987 Gold Metallic Coupe converted by the previous owner to a street legal track car for autocross use. Traded for an 09 Z51 Roadster - faster, plus easier 1.5 hr drive to-from track. Loved the C4 on the track; hated getting to-from track.


Joined: 4/6/2016
Posts: 4

Me 2...

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87C4TrackCar

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Re: Doug Nash 4 + 3 Transmission Overdrive Questions!! (11/11)
 4/20/16 3:52pm
cco
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Joined: 8/16/2013
Posts: 191

Me 2 -- so who's got the comments to post? Articles like that would certainly go a long way to upping traffic here.
 
Thanks to whomever takes the time to post that valuable info. He/she would get a lot of 'likes' from the people I know.
 
C.
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