1985 4+3 OD Sluggish - CorvetteForum - Chevrolet Corvette Forum Discussion

Notices
C4 Tech/Performance L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

1985 4+3 OD Sluggish

Old 02-12-2018, 10:40 AM
  #1  
Bfenty
CF Senior Member
Thread Starter
 
Bfenty's Avatar
 
Member Since: Mar 2017
Location: Nashville TN
Posts: 1,296
Thanked 69 Times in 66 Posts
2017 C4 of Year Finalist
Default 1985 4+3 OD Sluggish

For a while now, my OD has taken a moment to engage. I chalked this up to the OD not wanting to engage while I have my foot down or the RPMs are too low or something. It has started to get more sluggish engaging recently, however.

This morning I really couldn't get it to come on at all. Yes, the car was at temp (180 F) and I was doing about 55 at ~2200 RPM (forget exact RPM but the OD should definitely engage at that point).

It is kinda cold here today, ~32 F, which seems to always have a negative effect on the OD performance.

Only time I could get it to engage fully was coasting downhill at the above speed/RPM. On level ground, even coasting, it wouldn't engage. The OD light on the dash comes on, so I know the computer THINKS it should be on.

Do I just need to add fluid to the unit? I suppose if I'm going to add fluid I should drop the pan first and change the fluid entirely. Anything else I should check?
Bfenty is offline  
Old 02-12-2018, 01:06 PM
  #2  
84 4+3
CF Senior Member
 
Member Since: Aug 2017
Location: New Jersey
Posts: 852
Thanked 97 Times in 95 Posts
Default

Do 85s have kickdown cables for the overdrive? May need to be adjusted possibly as well. Just saying.
84 4+3 is offline  
The Following User Says Thank You to 84 4+3 For This Useful Post:
Bfenty (02-12-2018)
Old 02-12-2018, 01:54 PM
  #3  
Bfenty
CF Senior Member
Thread Starter
 
Bfenty's Avatar
 
Member Since: Mar 2017
Location: Nashville TN
Posts: 1,296
Thanked 69 Times in 66 Posts
2017 C4 of Year Finalist
Default

Originally Posted by 84 4+3 View Post
Do 85s have kickdown cables for the overdrive? May need to be adjusted possibly as well. Just saying.
Sorry to be dumb...but what is a kickdown cable?
Bfenty is offline  
Old 02-12-2018, 02:06 PM
  #4  
Bfenty
CF Senior Member
Thread Starter
 
Bfenty's Avatar
 
Member Since: Mar 2017
Location: Nashville TN
Posts: 1,296
Thanked 69 Times in 66 Posts
2017 C4 of Year Finalist
Default

Ok did some Googling, looks like the kickdown cable only tells the O/D unit to shut off under full throttle. I don't THINK that's the issue based on the behavior.
Bfenty is offline  
Old 02-12-2018, 05:00 PM
  #5  
84 4+3
CF Senior Member
 
Member Since: Aug 2017
Location: New Jersey
Posts: 852
Thanked 97 Times in 95 Posts
Default

Originally Posted by Bfenty View Post
Ok did some Googling, looks like the kickdown cable only tells the O/D unit to shut off under full throttle. I don't THINK that's the issue based on the behavior.
Ah, see I was thinking it was what set kickdown at any position. It's been a while since I really messed with it.

If you suspect that you are low on fluid I recommend that you check it. It's doing more harm than good if that's the case...
84 4+3 is offline  
The Following User Says Thank You to 84 4+3 For This Useful Post:
Bfenty (02-13-2018)
Old 02-12-2018, 06:43 PM
  #6  
VikingTrad3r
CF Senior Member
Support Corvetteforum!
 
VikingTrad3r's Avatar
 
Member Since: Apr 2014
Posts: 4,341
Thanked 351 Times in 331 Posts
Default

Originally Posted by Bfenty View Post
For a while now, my OD has taken a moment to engage. I chalked this up to the OD not wanting to engage while I have my foot down or the RPMs are too low or something. It has started to get more sluggish engaging recently, however.

This morning I really couldn't get it to come on at all. Yes, the car was at temp (180 F) and I was doing about 55 at ~2200 RPM (forget exact RPM but the OD should definitely engage at that point).

It is kinda cold here today, ~32 F, which seems to always have a negative effect on the OD performance.

Only time I could get it to engage fully was coasting downhill at the above speed/RPM. On level ground, even coasting, it wouldn't engage. The OD light on the dash comes on, so I know the computer THINKS it should be on.

Do I just need to add fluid to the unit? I suppose if I'm going to add fluid I should drop the pan first and change the fluid entirely. Anything else I should check?

i forget if u already dropped the pan on this unit? have u checked fluid on it yet?
VikingTrad3r is offline  
The Following User Says Thank You to VikingTrad3r For This Useful Post:
Bfenty (02-13-2018)
Old 02-12-2018, 11:03 PM
  #7  
Bfenty
CF Senior Member
Thread Starter
 
Bfenty's Avatar
 
Member Since: Mar 2017
Location: Nashville TN
Posts: 1,296
Thanked 69 Times in 66 Posts
2017 C4 of Year Finalist
Default

Originally Posted by VikingTrad3r View Post



i forget if u already dropped the pan on this unit? have u checked fluid on it yet?
I have not. Between baby and bad weather it hasnít happened. Sounds like I need to park it until I can take a look. Iíve been planning on servicing the OD, manual trans and rear differential all at once sometime soon.

I assume that driving without the OD if it has low fluid will still damage the unit? It is still transferring power even if itís not engaged, right?
Bfenty is offline  
Old 02-12-2018, 11:11 PM
  #8  
Bfenty
CF Senior Member
Thread Starter
 
Bfenty's Avatar
 
Member Since: Mar 2017
Location: Nashville TN
Posts: 1,296
Thanked 69 Times in 66 Posts
2017 C4 of Year Finalist
Default

Reverse still works just fine too-which Iíve read is a good sign for the OD unit being ok.
Bfenty is offline  
Old 02-12-2018, 11:20 PM
  #9  
ihatebarkingdogs
CF Senior Member
 
Member Since: Nov 2016
Location: Torrance CA
Posts: 1,204
Thanked 204 Times in 191 Posts
Default

If it backs up, it just means the direct clutches are still Ok.
The 84-85 cable is a Throttle Valve cable. Just like a 700R4. It needs to be adjusted properly, as it affects the pressure at all speeds. They changed the internal pressure controls for 87-88 to not use the cable. I forget what they did in 86. I don't think it has the cable either. The cable can not shift the unit. Only the ECM can by turning the solenoid off and on. The cable is ONLY for pressure control based on throttle position. AGAIN: The cable can NOT shift the unit.


Now, to your problem: My GUESS (just a GUESS) is that the thrust washer is hanging up, causing the delayed engagement. Mine was doing the same thing near the end of a 5,400 mile in 11 days road trip 2 years ago. I changed the fluid and it was much better for the 2,400 miles in 3 days it did this past summer. There were some pretty big bronze specs in the pan, which I suspect are the tangs on the thrust washer. My unit was rebuilt with the upgraded parts in 1987 when the car was 2 years old. One of the upgrade parts is the thrust washer, but the washer may be wearing again.

Last edited by ihatebarkingdogs; 02-12-2018 at 11:30 PM.
ihatebarkingdogs is offline  
The Following User Says Thank You to ihatebarkingdogs For This Useful Post:
Bfenty (02-13-2018)
Old 02-12-2018, 11:28 PM
  #10  
VikingTrad3r
CF Senior Member
Support Corvetteforum!
 
VikingTrad3r's Avatar
 
Member Since: Apr 2014
Posts: 4,341
Thanked 351 Times in 331 Posts
Default

brandon i think there are gears still spinning. ut not sure about torque applied into the od unit in gears 1-4.

drip that pan and show us what you see!!
VikingTrad3r is offline  
The Following User Says Thank You to VikingTrad3r For This Useful Post:
Bfenty (02-13-2018)
Old 02-12-2018, 11:46 PM
  #11  
ihatebarkingdogs
CF Senior Member
 
Member Since: Nov 2016
Location: Torrance CA
Posts: 1,204
Thanked 204 Times in 191 Posts
Default

In Direct, the torque is passed through by a sprag clutch in the forward direction, when the engine is "pushing" the car. Because sprags over-run (don't pass torque, or dis-engage) in the opposite direction, the sprag over-runs, or doesn't allow engine braking OR the car to back up. So there is a set of clutches for Direct that are held by spring tension. This spring tension is adequate because the torque that can be applied to the clutch in reverse or deceleration is limited.


There is an oil pump in the unit. It is driven by the rear wheels. It supplies the pressure to shift the unit, and apply the Overdrive clutch, but it also supplies lube oil. There is a bearing that is in play in Direct. I've read this bearing is very difficult to obtain (if at all). So you don't want to burn it up by running low on lube.


I think you have a mechanical problem involving the thrust washer. This is a well-known and documented problem with the early OD units. The part was upgraded, but I think it can still be problematic. Mine is.
ihatebarkingdogs is offline  
The Following User Says Thank You to ihatebarkingdogs For This Useful Post:
Bfenty (02-13-2018)
Old 02-13-2018, 07:48 AM
  #12  
Bfenty
CF Senior Member
Thread Starter
 
Bfenty's Avatar
 
Member Since: Mar 2017
Location: Nashville TN
Posts: 1,296
Thanked 69 Times in 66 Posts
2017 C4 of Year Finalist
Default

Wow-thanks all for the information. Iím learning a lot about how this OD works.

Lucky for me, I DO have a spare OD unit that I know is in great shape. Not sure I want to jump to just swapping it out but I have a backup if need be, or if thereís a part I need to pull. Iím ASSUMING itís difficult to just swap the unit out?

it might be a few weeks before I can drop the trans pan. Iíll park it until then and provide updates when I can. Thank you everyone for the advice!
Bfenty is offline  
Old 02-13-2018, 10:52 AM
  #13  
arbee
CF Senior Member
 
arbee's Avatar
 
Member Since: Feb 2003
Location: Saskatoon Sask.
Posts: 927
Thanked 56 Times in 52 Posts
Default

Originally Posted by Bfenty View Post
Wow-thanks all for the information. Iím learning a lot about how this OD works.

Lucky for me, I DO have a spare OD unit that I know is in great shape. Not sure I want to jump to just swapping it out but I have a backup if need be, or if thereís a part I need to pull. Iím ASSUMING itís difficult to just swap the unit out?

it might be a few weeks before I can drop the trans pan. Iíll park it until then and provide updates when I can. Thank you everyone for the advice!
Go to the following link. The video author has several videos including rebuild of this O/D unit.


arbee is online now  
The Following User Says Thank You to arbee For This Useful Post:
Bfenty (02-13-2018)
Old 02-13-2018, 12:04 PM
  #14  
Bfenty
CF Senior Member
Thread Starter
 
Bfenty's Avatar
 
Member Since: Mar 2017
Location: Nashville TN
Posts: 1,296
Thanked 69 Times in 66 Posts
2017 C4 of Year Finalist
Default

Originally Posted by arbee View Post
Go to the following link. The video author has several videos including rebuild of this O/D unit.


https://www.youtube.com/watch?v=cS6vA3UR4jM
Great information there, thank you.
Bfenty is offline  
Old 02-13-2018, 12:33 PM
  #15  
Bfenty
CF Senior Member
Thread Starter
 
Bfenty's Avatar
 
Member Since: Mar 2017
Location: Nashville TN
Posts: 1,296
Thanked 69 Times in 66 Posts
2017 C4 of Year Finalist
Default

one other thing I was thinking-is it possible/easy to add a drain plug to the pan once it's been dropped? I was thinking about doing that to make it easy to drain the fluid, and then just swap the fluid every time I change the oil or something. I know that's more often than necessary but I'd rather overdo it than ruin the OD.
Bfenty is offline  
Old 02-13-2018, 12:45 PM
  #16  
ihatebarkingdogs
CF Senior Member
 
Member Since: Nov 2016
Location: Torrance CA
Posts: 1,204
Thanked 204 Times in 191 Posts
Default

Originally Posted by Bfenty View Post
one other thing I was thinking-is it possible/easy to add a drain plug to the pan
You would then be overlooking the most important aspect of "changing the fluid". Being able to inspect WHAT is IN the pan. Hopefully nothing of course. But if there is any debris indicative of an impending failure, you want to know about it.
ihatebarkingdogs is offline  
The Following User Says Thank You to ihatebarkingdogs For This Useful Post:
Bfenty (02-13-2018)
Old 02-13-2018, 12:50 PM
  #17  
Bfenty
CF Senior Member
Thread Starter
 
Bfenty's Avatar
 
Member Since: Mar 2017
Location: Nashville TN
Posts: 1,296
Thanked 69 Times in 66 Posts
2017 C4 of Year Finalist
Default

Originally Posted by ihatebarkingdogs View Post
You would then be overlooking the most important aspect of "changing the fluid". Being able to inspect WHAT is IN the pan. Hopefully nothing of course. But if there is any debris indicative of an impending failure, you want to know about it.
True. So you don't recommend doing that, I take it.

I'm not saying I'd never drop the pan, just thought it might make it easier to swap fluid often. But I'm really not much of a mechanic so I'll take your advice.
Bfenty is offline  
Old 02-14-2018, 11:55 AM
  #18  
FOURSPEEDVETTE
CF Senior Member
 
FOURSPEEDVETTE's Avatar
 
Member Since: Oct 2007
Posts: 4,067
Thanked 110 Times in 88 Posts
Default

Way too much diagnostic information being thrown at you at this point of your investigation into your problem. Start with the very basic and inexpensive fluid and filter change. Your problem could very easily be only that your O/D fluid level is too low. It happened the same way to mine. I had a small leak and the level dropped too low. You can add some additional fluid, but you might as well just do the required fluid change. With your symptoms so far, I'd go that route.
FOURSPEEDVETTE is offline  
The Following User Says Thank You to FOURSPEEDVETTE For This Useful Post:
Bfenty (02-14-2018)
Old 02-14-2018, 12:07 PM
  #19  
arbee
CF Senior Member
 
arbee's Avatar
 
Member Since: Feb 2003
Location: Saskatoon Sask.
Posts: 927
Thanked 56 Times in 52 Posts
Default

Originally Posted by FOURSPEEDVETTE View Post
Way too much diagnostic information being thrown at you at this point of your investigation into your problem. Start with the very basic and inexpensive fluid and filter change. Your problem could very easily be only that your O/D fluid level is too low. It happened the same way to mine. I had a small leak and the level dropped too low. You can add some additional fluid, but you might as well just do the required fluid change. With your symptoms so far, I'd go that route.

BS. Too much information! What a ridiculous statement.
arbee is online now  
The Following User Says Thank You to arbee For This Useful Post:
Bfenty (02-14-2018)
Old 02-14-2018, 01:30 PM
  #20  
Bfenty
CF Senior Member
Thread Starter
 
Bfenty's Avatar
 
Member Since: Mar 2017
Location: Nashville TN
Posts: 1,296
Thanked 69 Times in 66 Posts
2017 C4 of Year Finalist
Default

Originally Posted by FOURSPEEDVETTE View Post
Way too much diagnostic information being thrown at you at this point of your investigation into your problem. Start with the very basic and inexpensive fluid and filter change. Your problem could very easily be only that your O/D fluid level is too low. It happened the same way to mine. I had a small leak and the level dropped too low. You can add some additional fluid, but you might as well just do the required fluid change. With your symptoms so far, I'd go that route.
Thanks! I'm a little slower to diagnose these days (ask VikingTrader-I used to diagnose and fix something in hours not weeks

Also there is NO SUCH THING as too much information! But you're right, I need to start with the basics. those videos were hugely helpful. I have a tendency to get paranoid and assume the worst-that my car is about to go up in a giant fireball or something.
Bfenty is offline  

Thread Tools
Search this Thread
Quick Reply: 1985 4+3 OD Sluggish


Sponsored Ads
Vendor Directory

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

© 2018 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: