TheGreek!

I'm still on a learning curve with my 85 I recently picked up, I have some questions about the 4+3 trans in it. First, where do I add fluid to it and how full does it need to be? Second, it's supposed to be a two speed overdrive but when I engage it I don't feel it shift, it only seems to have one overdrive speed. When I engage the overdrive is it supposed to go into it's first gear and then shift into it's second gear shortly afterwards? How does this thing work? Any help in teaching me how this thing is supposed to work and how to test it's two gears is greatly appreciated.

84 4+3

So when you flip the switch it should go from direct drive, just the 4 speed with no overdrive straight to the rear, to overdrive. That's it. Speed 1 is straight through and 2 is overdrive. Then there is magic on the down shifts etc. You aren't feeling it engage at all is what I'm guessing?

As for filling I believe on the passengers side rear of the trans is a full plug for the overdrive unit. I believe full is to the top of the hole. So just a dribble running out.

TheGreek!

What is this "magic" on the down shifts that you speak of? As for it not engaging, the overdrive does work, I just mainly use it as a fifth gear. When I'm ready to go into overdrive when I'm in fourth gear I hit the clutch, flip the switch, and then let the clutch back out and then it's in overdrive. If I leave the switch on it works in second, third, and fourth like it's supposed to. When the switch is left on it's like having a "taller" second, third and fourth gear. I noticed that it has a downshift cable, is it supposed to work like a passing gear when the throttle is floored, meaning, is it supposed to disengage the overdrive while it's floored and then re-engage it when the throttle is backed off? I haven't floored it when it's in overdrive to test that yet so I'm not sure how it's supposed to work. As for the fill plug, I didn't see one on the passenger side but I do see one on the drivers side (I have the car up on jack stands right now because I'm changing the u-joints). It's not very high up on the unit, in fact it looks like the bottom of the plug is more or less parallel with the top of the pan. Anyway, thanks for the reply, anything I can learn about how this thing is supposed to work helps.

FOURSPEEDVETTE

The way you're describing how yours is operating is exactly how it operates. Nothing at all wrong with yours. When you floor it while the overdrive is engaged in 4th gear, it should disengage the overdrive automatically and engage it again automatically if you ease up on the accelerator, or accelerate above (I think)100 mph. When you fill the overdrive unit with fluid, make sure your wheels are off the ground and then spin one wheel by hand so the fluid is pushed throughout the entire system until it starts running out.

84 4+3

My bad, the 4 speed side plug is on the passengers side. It's been a while. I used magic because GM changed how it shifts down from overdrive several times meaning what is true of mine may not be for yours if that makes sense.

I agree it seems to be working properly.

jv9999

Change the OD fluid NOW. They are usually neglected resulting in early deaths. Make sure you can get the fill plug open before you pull the pan.
All power goes though clutch packs in the OD whether you're in OD or not.

Bfenty

THIS. I killed an OD unit because I didn't bother to change the fluid when I bought it. It's an easy thing, takes under an hour for the whole job.

Like previously mentioned, make sure you can get the fill plug out before dropping the pan. I can't remember but I believe the fill plug takes a special socket to remove (I KNOW the rear differential and transmission bolts do, can't remember if the OD does as well.)

You drop the pan, clean out the fluid, put the pan back on, and put fluid in until it won't take any more in the fill hole. Simple.

While you've got the car in the air, btw, I would highly recommend just doing the transmission oil and rear differential. None of it is hard. For the transmission you need to make sure you can remove the fill plug before removing the drain plug. Then just drain it and fill until it flows out the fill plug.

For the rear differential, you have to pump out the old fluid and put in new. It just uses 80w-90 gear oil, same as the transmission, but you need an additive that you can buy at O'Reilly's (not sure of other parts stores but probably). Again, just fill until it flows out and you're done.

All those jobs together probably will take you under two hours, the hardest part is getting it up in the air honestly. And that way you know that all the fluids are good and can enjoy just driving the car

c4cruiser

It's a good idea to change the OD fluid and filter every year or every 12K miles. Definitely remove the small fill plug before you remove the pan!! The fluid change process is fairly simple; get the car up in the air so that you can get under it and remove the OD unit pan. It holds about two quarts. Remove the small bolts and the pan should come off easily. If it sticks to the case, use a putty knife to gently break the seal.

Clean the pan and remove the round magnet. Clean that too and replace in the same position. Make sure all of the old gasket material is removed and the pan rail is clean. Straighten the pan rails if necessary. Clean the mating surface on the OD case and remove any gasket material that may be in the bolt holes.

Remove the filter by pulling straight down Remove the rubber bushing and set aside. The OD filter is the same one used in Ford FMX transmissions from the early 70's. It was used on some Fairlanes and the Mercury equivalent. It was also used on some F-150's. Here's some part numbers:
Carquest 85926, AC Delco TF-184, FRAM FT1024, and NAPA ATP14717. These filters do not come with the rubber bushing so if the one you pulled off the old filter is in good shape, it can be reused. Otherwise, buy a OD filter kit from one of the Corvette catalogs; it will have the bushing.

I would suggest using black RTV sealant in place of the cork gasket. Just run a small bead of RTV on the pan rails and install immediatley. Tighten the pan bolts equally but they don't have to be real tight. Maybe somebody can chime in with the torque specs??

Get 3 quarts of non-synthetic DEXRON III ATF. You will need a small hand-held pump like the type sold at Harbor Freight. Pump fluid into the until it starts to run out of the fill hole and replace the plug.

TheGreek!

The car has a new overdrive unit according to the previous owner. While the car was up in the air for the u-joint job I topped off the fluids in the trans, the overdrive unit, and the differential. I stuck my finger in the fill hole of the overdrive unit before I topped it off and the fluid was nice and bright red, it didn't look or smell burnt at all. I'll change the fluid and filter in the overdrive unit soon anyway but for now I just wanted to get the car out of the garage and back on the road, I need a break from working on it, lol.

Kevova

4 speed with electric od that engages in top 3 gears. In 2,3, or 4 if switch od is on, the od should be on. That's where the "4+3" came from. You won't feel it, you may notice tach reads lower. There is a feature which will disengage od on hard acceleration similar to a forced downshift in automatic. No declutching required.

Bfenty

FOURSPEEDVETTE

This is where lots of owners have had problems. Many owners who had their overdrive units go out or malfunction never used the clutch when switching in or out of overdrive. When these overdrive units were first designed, clutching wasn't thought to be necessary. GM didn't even understand the logic behind using the clutch. Now after all these years, clutching has been proven to soften the shift into or out of overdrive. Thus saving many failures from happening.

arbee

Not correct. You could actually remove the direct clutches totally and still move in the forward direction.

FOURSPEEDVETTE

Oh yea....I'm sure the OP understands all this.

arbee

How does this differ from a 4l60 shifting into 4th with a locked torque converter?

arbee

Might also cause a real bad day if you parked on a hill without the hand brake!

Kevova

Sprag transmitting torque????

Kevova

A sprag holds it doesn't transmit
so you don't care for my participation. After you shift to 2nd do you declutch to shift to 2nd od then declutch shift to 3rd and simultaneously turn off od. Then declutch to re-engage to go to 3rd od. Then shift to 4 as you again simultaneously disengage od. The declutch to go to 4th od. Seems like alot of unnecessary clutch operation, but if it makes you happy knock yourself out.

TheGreek!

If I'm not mistaken you only need to hit the clutch when you initailly engage the overdrive, once it's on it will stay on until it's turned off. That's why you don't have to hit the clutch to engage the overdrive in second, third, and fourth when starting off in first if the overdrive is already on.

FOURSPEEDVETTE

Plain and simple of it. Operate 1st through 4th gear just like any 4 speed transmission. You may not even want to use overdrive. But if you do want to, use the overdrive only as a fifth gear. You only need to use the clutch once when you switch the overdrive on after 4th gear, and once when you want to turn the overdrive function off. No one drives a 4+3 the way described here. Unless they're an idiot. It's not a 7 speed transmission. 2nd gear overdrive feels like 3rd gear. 3rd gear overdrive feels like 4th gear. And 4th gear overdrive feels like a fifth gear. Fairly simple.