4+3 overdrive slips
Thread Starter
7th Gear
Joined: Jun 2007
Posts: 7
Likes: 0
4+3 overdrive slips
I'm new to this forum, and need to ask a question. I have an early production '85 coupe with the 4+3 tranny (has 34k miles). Last Fall I replaced the tranny fluid (I know, big mistake). Shortly after that, the car would occasionally slip out of overdrive. This could last from 1 second to several minutes. It has started to become more often and longer in duration. When the engine is cold, the O/D works fine. Seems to be more of an issue when engine is well up to temp. I don't believe it's the switch (on console). One mechanic I spoke with said it might be the solenoid. Another reference said it may be low on fluid. I have to do some research before I give the car to an unknown tech to repair. Anyone got any ideas? Thanks.
Team Owner
Joined: Jan 2003
Posts: 20,315
Likes: 245
From: Summerland B.C. Canada
Start by looking at the OD relay. The wires may be shorting due to the wire insulation shrinking back. Here is some info from my 86, may help.
http://members.shaw.ca/agent86/Overd...iagnostics.pdf
http://members.shaw.ca/agent86/OverD...Electrical.pdf
http://members.shaw.ca/agent86/Overd...iagnostics.pdf
http://members.shaw.ca/agent86/OverD...Electrical.pdf
Thread Starter
7th Gear
Joined: Jun 2007
Posts: 7
Likes: 0
Just wanted to give a quick update - I had the car at the tech's last week - they did a complete electrical check following the schematics you shared. Electrically, everything is in order. There is power to the solenoid, but it is not going anywhere. My first thought is a faulty solenoid. However, I also read somewhere that it could be the ECM. I'm inclined to try the solenoid first. Any thoughts? Thanks.
Team Owner
Joined: Dec 1999
Posts: 34,873
Likes: 487
From: Lacey WA RVN 68-69
NCM Sinkhole Donor
The solenoid controls the action of the OD unit so that is probably the next place to start. There is also an internal pressure switch which may be failing. Both components are inside the OD pan so it must be dropped to get to them. You might replace both components.
This is a good time to replace the filter and have new fluid installed. Don't bother with buying the $20 to $25 filter from the various Corvette accessories catalogs. The filter is the same one used in late 60's to early 70's Ford FMX trannies and was found in some F150's, Cougars and Fairlanes.
Go to a local Carquest store and get the filter, part # 85926. You could also cross-reference that number to a Fram, Wix or other brand. Cost will be $6 to $8.
The OD unit takes just over 2 quarts of Dexron II/III fluid. Don't use synthetic ATF!!
This is a good time to replace the filter and have new fluid installed. Don't bother with buying the $20 to $25 filter from the various Corvette accessories catalogs. The filter is the same one used in late 60's to early 70's Ford FMX trannies and was found in some F150's, Cougars and Fairlanes.
Go to a local Carquest store and get the filter, part # 85926. You could also cross-reference that number to a Fram, Wix or other brand. Cost will be $6 to $8.
The OD unit takes just over 2 quarts of Dexron II/III fluid. Don't use synthetic ATF!!
Burning Brakes
Joined: Jun 2005
Posts: 1,191
Likes: 15
From: Wallingford Vermont
I'll play devils advocate ... and say the OD is about to crap
Mine did this years ago. I figure a search should pull up all kinds of dark history of the dreaded Doug Nash. Suffix to say DNE had some quality control issues; some where built with main shaft preload (on a ball bearing
), some with lash. The carrier bearing had a (lack of) oiling issue, later carriers had an oil passage added.
Not sure who's rebuilding these lately... Beuller? Bueller?
Mine did this years ago. I figure a search should pull up all kinds of dark history of the dreaded Doug Nash. Suffix to say DNE had some quality control issues; some where built with main shaft preload (on a ball bearing
), some with lash. The carrier bearing had a (lack of) oiling issue, later carriers had an oil passage added. Not sure who's rebuilding these lately... Beuller? Bueller?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Intermediate
Joined: Jun 2007
Posts: 34
Likes: 0
From: Burton MI
Do remember, your 4+3 switching into overdrive is based on your speed. If you have a bad speed sensor or your digital speedo is not working properly, it will shift in and out of gear even though there is nothing wrong with your tranny. My speedo went bad and was droppin g to zero when the car got warm, this made my tranny think I was at "zero" and threw it out of Overdrive. As soon as the speed read my correct speed again, it threw me back into overdrive. Just a thought for you.
Thread Starter
7th Gear
Joined: Jun 2007
Posts: 7
Likes: 0
Thanks for all your responses. For the speed comment, my speedo is registering OK. It continues to show speed when I lose the OD. The solenoid is my next check, but I'm not sure where to get one. I have an early model (81xx s/n) so the switch is on the console. I did change the fluid last summer, but not the filter. Do you know where I can get the solenoid? Thanks again for the help.
Le Mans Master
Joined: May 2006
Posts: 9,226
Likes: 9
From: Irvine Ca
Beat me to it
http://www.skspeed.com/
Watch the solenoid testing. It can be tricky. you need to pull the pan off and test the solenoid from the inside
http://www.skspeed.com/
Watch the solenoid testing. It can be tricky. you need to pull the pan off and test the solenoid from the inside
Last edited by jhammons01; Jul 4, 2007 at 12:04 AM.
Thread Starter
7th Gear
Joined: Jun 2007
Posts: 7
Likes: 0
Thanks again. I will try to get this done next week. It's really frustrating. One other thing that confuses me - it seems to happen when the engine is at temp of about 208 degrees. When cold (145 deg.) it functions great. Does this make sense to you?
Thread Starter
7th Gear
Joined: Jun 2007
Posts: 7
Likes: 0
I have an '85 with 4+3 Nash transmission. I developed a problem where the OD will engage properly at about 145 degrees engine temp, but will kick out at about 192 deg. I have read different threads about filters, solenoids, switches, etc, but don't want to spend lots of money guessing. The car has 35k miles on it, and is still original. I did replace the OD filter and ATF about a month ago, but that didn't seem to make a difference. I have had the wiring checked, and the mechanic said it all looks fine. I have tried to find a solenoid, but don't know where to get one (S-K requires a phone call, which I haven't made yet). I'm not even sure where to look for it. I purchased a shop manual on dvd, but it is not real friendly. I have searched for the information but couldn't find it, probably because I don't know what GM calls the parts. I guess what I need ot know is: where is the solenoid (I imagine in the pan); What is the switch, or is it the same thing; where can I get these parts; anything else I should be looking for? Can anyone shed some light on this for me? Thanks in advance.
