I jumped in the car this morning on my way to work and everything seemed to be running just fine on my 88 4+3 manual transmission Corvette. It was running fine that is until I put it into overdrive. Going about 50-55 in 4th (2200+/- rpm) I put it in OD and rpms drop to about 1200 rpm +/-. When it dropped into OD however the car would shake whenever I hit the gas. It felt kind of like the way a car shakes when you're learning how to drive a stick and can't quite figure it out. It didn't just shake for a second and then go. Once the rpms were up closer to 2000, overdrive seemed to work like normal.

I hope I did an okay just of desribing my problem and I hope someone may know a possible cause. I'm worried it will be something transmission relater which typically means $$ which I don't have much of right now.

Any help would be great, thanks!

 

You may be "lugging it" a little. By shifting into overdrive so early that your revs drop to 1200 RPM's you are lugging it too much. Try bringing your revs up a bit before shifting into OD so that when you hit OD you are at about 1500 plus RPM's.

I would also check the fluid level in your OD and do a filter and fluid change. Good luck

 

Thanks for the advice, I probably am "lugging it", but I haven't had this problem until today. But I'll try to stop doing that.

How do I check my OD fluid levels??

Thanks

 

There is a threaded sight/fill plug on the side of the OD unit. Open it and put your finger in and you should feel Automatic Transmission fluid. If not it may be dry or low. The OD unit has a small pan that houses the filter.

 

Does it do the same thing in 4th gear with no od at 1200RPM? The fact that the OD is engaging would indicate that the fluid level is alright (the OD unit only holds about 2 quarts). Is there any signs of leakage around the OD pan? Any leakage around the tailshaft seal?


If you haven't had the OD unit serviced, it won't hurt to do so. Use Dexron II/III for the fluid and get a filter from the local Carquest store Ask for part 85926. It's the same filter used in Ford FMX trannies found on late 60's to early 70's Torino's, Cougars, and some F150's.

Before a fluid change, make sure you can remove the fill plug! If you drop the pan and cn't get the fill plug out, you wil have a problem

The pan drops straight down by removing the bolts (use a large drain pane here!!) Remove the filter by cafefully pulling straight down. There is a rubber seal that may come off with the filter so just reinstall it on the new filter.

You can use very light air pressure (5 psi or so)to remove the old fluid from the cooler lines. Remove one line and blow out the fluid.

Use some black RTV sealant on the pan rails. Make sure the pan rails and tranny case flange are completely clean of old sealant or gasket and dry. Tighten the pan bolts to about 11-12 ft-lbs (make sure they are equally tight).

When you refill the pan, add fluid until some comes out the fill hole. Then spin one rear wheel by hand for a couple minutes; that will force fluid into the empty coolant lines if you hve drained them. Then top off the pan and re-install the fill plug.

You might want to change the fluid in the 4-speed portion too. There is a drain plug at the bottom and a fill plug on the passenger side. REmove the fill plug first then the drain plug. Re-fill with SAE 75W-90 gear oil. Do't use syntehtics in either unit.

 

okay.. I went out at lunch to see if I could better answer some questions. It will do the same thing whether it's in OD or not, sorry I didn't notice that before. You can feel it a little bit in 1st and 2nd gear, but in 3rd, 4th, and OD with the RPMs low (1200+/-) it shakes. I had done the timing a week or so ago and am planning on checking that again tonight when I get home.

Maybe this extra info will help clear some things up.

Thanks for the help, I'm obviously new at this.

 

Im having the exact same issue with mine and have come to the conclusion it's rebuild time. What did you end up doing?

 

don't jump to a rebuild yet...you'll know it when you need it...you'll lose all engine braking and reverse. i think he's throwing on the OD too soon as well. i keep mine in the 1700 range... at 1200 you'll start to get a lil stumbly.

any anyone with the 4+3 should know how to properly service the unit...simple fluid and filter change every 10K miles

 

Thanks for the advice Orlando but unfortunately i know what caused my problem, im just not sure what needs replacing/fixing. I recently swapped out my 165 ecm for a 7730 ecm and had everything working to my liking but thought i could improve things by tweeking the settings in the prom chip. The ecm started commanding the unit in and out of OD at 1 second intervals causing a pretty rough ride. Ever since the unit developed a shutter when you apply a load to it. I think its related to a pressure problem internally. Im hoping its just a soft part problem and not hard part damage, like the pump.

 

zf4+3....careful with that OD unit...there are several spring loaded components with A LOT of force behind them. I wouldn't mess with that unit in my garage...call S+K Speed in NY and talk to Brian. They are the only shop I would trust with any OD issues.

 

No worries, i had the unit safely apart and back together about 10 years ago when i swapped out the speedo gears after making a ring and pinnion change.