richmolander

I picked up a 85 in disastrous condition (thought it was cheap until I was able to get the hood open). It will be a rebuild.
It has a Doug Nash 4+3 and no one seems to know much about it.
How does the Overdrive work? Do you consecutively press the button to go through the overdrives after 4th gear?

c4cruiser

Here's a link to the OD operation for the various years it was offered. The OD should be ON until you turn it off. There are a number of factors that determine how it works.

I would recommend doing a fluid and filter change as part of your rebuild. The OD unit uses Dexron II/III ATF (not synthetic) and a filter that is the same one used in Ford FMX auto trans found in some late 60's thru early '70's Cougars, Fairlanes and even a few F-150's.

Here's some part numbers for the filter:
Carquest 85926
AC Delco TF 184
FRAM FT1024
NAPA ATP14717
Ford D0AZ-7A098-A

Figure $10 for a filter and it will take just over 2 quarts of ATF.

Cant' hurt to change the gear oil in the 4-speed. A good quality 80W-90 GL5 gear oil is used there. Just make sure you can remove the fill plug before draining the trans.

HlhnEast

On the '85 the 4+3 is controlled by a switch on the console so it will be on or off depending on the position of this switch. There is also a solenoid in the circuit so if it just isnt going into overdrive, check this solenoid and the wiring as well.

You are correct, there arent many folks out there that can work on the OD portion of the tranny set up. The 4 speed portion is a Super T10 that has been around for a while. Depending on the shop you find that can do the work, it may be more cost effective to convert the car to the later 6 speed or do a Tremec 5 speed. Explore your options and budget before jumping into the 4+3 rebuild.

You have jumped into a rebuild of the heart not one that will make you a bunch of money. With the tranny and depending on what else, paint, interior, engine, your project will be expensive unless you can do all the work yourself. Even then its a labor of love.

Good luck and post up pics as your project progresses. We are pulling for you!

Karvette

The switch on the console was only on the early 85's. Late 85's had the button on the shifter. It is really not a bad setup if you understand it, and know how to use it. I have been driving mine since 1985 and have never had any problems with it.

QCVette

On my '85 the OD switch was on the shifter ****. Pressing the button toggles between the OD system enabled or disabled. If the system is enabled, the computer controls whether the OD engages or not based on the chart that c4cruiser linked to.

All the 4+3's had the same 1st through 4th gear ratios, but the OD ratio can vary. The Z51 cars had a .68 ratio while the others had a .60 ratio.

I had a Z51 '85 with the .68 ratio and an '87 base suspension with the 0.60 ratio.

Check the SPID label, the MH5 = 0.60 and the MK2 = 0.68 ratio.

Looking at the transmissions there is frequently nothing to distinguish between them visually between the two OD ratios, however I have heard from a transmission shop that used to specialize in these 4+3's that some had a marking on the top of the 4 speed section of the case that identified the SPID marking. It was not on all of them.

In practice, I used it like a 5 speed. I use 1st through 4th and then engage the OD for 5th.

I also found the ratio combinations a little strange. If you look at the chart below it will show the overall ratio from the combinations of gears with and without OD.

HlhnEast

Wish my switch was on the shifter.

whalepirot

My technique for using the console switch was to drop my wrist onto it; on or off. Easy.