Hello everyone I posted a question last evening as to why the OD unit may be inoperable but I would like to give everyone some history on these units. The OD unit that we took out of a 1986 Z/51 car has 250K miles on it. It was rebuilt at 210K miles but not a total rebuild as some of the functioning parts remained. This info should clear up any questions to the reliability of these units and no the car was not Sunday driven.
Early OD units 1984, 1985, and most cast iron head 1986 C4's had call back issues and were returned to Doug Nash.This and IRS problems drove Nash out of business. I never knew the main reason these units failed but I believe with the failure of the unit in the 1986 we are working on will tell the story. The inoperable unit in the car now was bought on E-bay complete with the T-10 trans. and is not the failed OD unit.
OK, here goes, on the 4+3 trans the reverse gear is located in a spacing plate between the main trans. and the OD unit. What happened on this unit is a front bearing in the OD unit worked it's way out of the bearing race and became entangled with the reverse gear of the main 4 speed unit. There is no wear on the race, the bearing simply backed out and once this happens trust me you are going nowhere!
This particular Vette is a early production cast iron head 1986 which most likely was assembled with excess inventory of 4+3 units from the 1985 production run that had problems along with the 1984 cars as I stated earlier. You can check with any reputable 4+3 re-builder and they know the early units had problems. I do not know for sure if the bearing as the culprit but I do believe it is.
My brother-in-law checked with a few knowledgable re-builders and they said this bearing in the OD unit colliding with the reverse gear in the spacing section between the T-10 and The OD was the warranty issue GM had with Doug Nash.
Now, if anyone out there knows for certain if the pressure switch located under the filter of the OD unit can cause it not to engage into overdrive please post your reply.
We are getting real good at taking the transmission out of the car but it is time consuming and that spine is a real dog to get in and out so help would be appreciated.
As I stated when I joined the forum I am retired from General Motors and have much insight into Vette production and GM. So, take care all and be well.

 

Correct.
If the line pressure is to low O/d won't engage

Crap engineering
"The 4+3 Overdrive was plagued with many design problems in 1984 to late 1985 cars. The main problems were an under designed bearing in the carrier assembly, a direct clutch thrust washer breakage problem, and a pump sealing problem.
The carrier bearing failed because of poor bearing design, bad pre load, and poor lube to the bearing. This was resolved in later years with h GM part number 14105940. It incorporated a heavy duty bearing, and feed oil to the bearing. It was up to the technician to preload it correctly. When a carrier goes bad you will hear bearing noise in direct drive and it will go away in overdrive. Sometimes it can sound like a tire whirring noise.
The Direct Clutch thrust washer would break because it could spin in the case. Once broken it would jam in between the overdrive piston and case. This would cause the overdrive to get stuck in two modes and burn up.
The pump assembly on 1984 and early 1985 cars used a rear pump cover with no seal between the output shaft and cover."

 

Other than those problems, I have fixed 4 units, and in my experience, the seals get hard from heat and age, and they bypass enough fluid that the pump cannot maintain fluid pressure enough to push the piston against the clutches and springs. This allows the unit to slip, exaggerating the wear, and increasing the heat exponentially.
The units I have messed with, two were later units, and two were the early version.
Neither of the older units exhibited any bearing problem, just hardened seals and worn clutches.
The one I originally put in my truck was out of an 88, and the seals were so hard that in trying to remove them, they broke into little bits of plastic where I had tried to pry them out.
The other thing was the gasket was compromised on the plate under the valve body, and had a leak path.
In my estimation, the pump isn't big enough, and the clutches aren't heavy enough. The springs should be a wafer spring. not a bunch of little springs.

 

This is a little confusing. Reverse is located on the front (input) side of the spacer plate. The OD input bearing is on the back side of the spacer plate. How did the two become "entangled", did the bearing chew through the spacer plate?

 

Welcome Anthony, we appreciate your knowledge and insights. Keep an eye out and give us the benefit of your years with GM whenever you can.

 

Dan Z/51
Hello, the reverse gear is located in the spacer plate between the 2 units as I described. The OD unit "butts" right to the plate and as the front bearing of the OD units let loose and filled the main trans with automatic fluid and jammed the whole works up. I guess you just have to see it to believe what has happened as I described but it should not have happened. The case of the OD unit was not damaged as the car was only backing up slowly ( after getting new tires) if perhaps this happened at speed I am sure things would have been much much uglier.
I have a 4+3 myself in a 87 Z/51 and after seeing this and working on it I am a bit concerned but not worried. Basically as I first wrote the older units are better but this one that failed in the 86 had a total of 245K miles (not too shabby).
BTW, for those running a ZF 6 speed (89 & later) I was speaking to an owner at Carlise last weekend and he relayed to me that he did not feel his transmission was shifting properly. His solution was to place a call to engineers at ZF in Germany and what he was told that the unit runs better with 60 weight oil as opposed to the 40 weight GM originally used. The reason the owner was told was that GM could save money on each unit by using the lighter weight oil. I have not confirmed this info but thought I would pass it on as the owner of the 89 convertible I was speaking to seemed to have all his facts in order.
Remember, we all love these machines but they are after all a business produced to extract revenue and it is up to us as owners to make them what they should have been even if GM cut corners in the manufacturing process. Believe me, I have seen many corners cut in production through the years at GM. Hopefully, GM has learned a valuable lesson at the hands of the government and will put the best product possible in the hands of the public.

 

Here's what he is talking about, Hold on while I bring up the pic.

 

Hi Coupe Guy:

I should have posted pics of the problem but I did not take my digital with me on Sunday when we swapped the unit bought on e-bay into the 86. I will try to get some pictures of the OD unit with the bearing and related parts or have my brother-in-law e-mail them to me for posting. Nice set of Vettes, I am partial to the C4's without airbags and all that other stuff but do appreciate the performance of C5 AND C6 Vettes.
BTW, e-bay is a good place for parts but you have to be careful as the complete trans and OD unit came with the serial numbers filed off. Makes me wonder and so far the OD unit is inoperable as I posted but we are hoping it is related to the pressure solenoid, sure hope so. Take care.

Tony