I have an 88, stock L98, originally a 4+3. About a yr ago I decided to convert it to an automatic. I purchased another 88 with a 700R4 for a parts donor. I pulled the trans and took it to a shop that was recommended to me. I told the guy what I had planned and he asked what I intended to do about the lock up. I told him I hadnt decided and he suggested that he could convert it to full hydraulic lock up and I wouldnt have to do anything else, so I told him to do that.

The conversion got delayed and I just started it about 3 wks ago. I finished it up today and took it for a test drive. Now to get to the reason for this post. The tranny goes into gear just fine. Firm but not to hard. 1st to 2nd and 2nd to 3rd shifts are great. Would not go into overdrive. I adjusted the TV cable every way I could and it would change the shift points and how hard it shifted but still would not go into overdrive. I finally decided to take it on the hyway and see what it would do. It stays in 3rd until it hits 60 mph then shifts to overdrive. As long as it stays over 55 mph it will stay in overdrive. As soon as it gets below 55 it shifts back to 3rd. RPMs change from 2400 to 1800.

I now have the TV cable adjusted back the way I understand it should be and the 1-2 and 2-3 shifts are great but it still will not go into overdrive until 60 mph. I have a temp gauge installed in the lower line about 6 ins from the tranny and it stays between 140 and 150.

I tried to contact the guy that rebuild the trans but he is no longer in the area. Does anyone have any idea what the problem could be. Could it have something to do with the hydraulic lock up he did. I have no idea what he actually did to it. I did have the pan off and there are no electrical wires there.

Any help would be greatly appreciated.

 

There are som differances in the internal valves between a Corvette th700r4 and a non Corvette as the Corvette is reaching top speed in overdrive and all other should shift to 3rd at WOT, because that is their "top speed" gear.

TV cable should be adjusted as tight as possible. Actually it should be adjusted too tight. When flooring after adlustment it is designed to ratchet back to correct position automatically.

I have never heard about a "full hydraulic lock-up" conversion. I also don't see why it would be needed.
No computer is needed to use lockup as it is completley stock. There is an internal grounding switch when in 4th gear that could be used to engage lockup solenoid. Power to solenoid should go thru a brake switch to disengage lockup during braking. You could also add a manual swich to disable lockup. Internal wiring in the transmission could be as stock.

Without knowing what was changed in the transmission it is impossible to know if it is related to the problem or not.

 

did you switch the doner car's prom with automatic to your car?

 

The transmission is out of an 88 corvette. I verified it is an actual corvette trans by the case and the date code shows it should be the original.

I did not switch the prom since it has no wires going to it I didnt figure it was necessary.

 

In theory, the hydraulic lockup has nothing to do with the 3-4 shift. However, your guy has converted an '88 transmission that was computer controlled lockup to an old style valve body controlled lockup. I can't attest to the compatibility of the old style valve body on the auxiliary valve body style wormtrack. I also can't attest to his skill level or what he may or may not have done to the associated valves and springs. One thing I CAN tell you is that unless you are anxious to burn up the clutch packs in your transmission, adjust the TV cable to specs and LEAVE IT ALONE. Contrary to many false beliefs here, the TV cable is not an adjustment for shift points - it is there to properly adjust line pressure.

 

When I originally set the TV cable I did it with the pan off and I believe I had it set right. I now have it back to where it was then. I do plan to leave it alone now.

I have been doing some research and it appears anything that he did was done to the valve body. If I were to come up with another corvette valve body and wire harness and put it back to stock do you think that should correct the problem?

 

proms are not the same for auto and standard

 

There are two things going on here. You have the overdrive gear and you have the converter lockup. Make sure you try to understand what is going on with the tranny. They will both affect rpm. You state that below 55 mph the tranny goes from od to 3rd gear and the rpms change from 2400 to 1800. Should be the other way around.

 

You are correct about the rpms. I meant the rpms drop when it goes into overdrive. I just got back from another test drive and the rpms are actually closer to 2600 before it goes into OD. They drop to 1800 rpms at 60 mph after going into OD.

I do understand the difference between lock up and overdrive. After the trans shifts into 3rd there is no drop in rpm at all until 60 mph when it goes into OD. Is it possible it is shifting and lock up at the same time.

Another thing that seems strange is that it dont matter how I drive it. I can have the throttle floored or I can be barely accelerating and it still shifts right at 60. I can then let completely off the throttle or let it slow down gradually and it still shifts down at 55 mph.

Thanks for the replys.

 

Is the converter clutch solenoid still installed, even if the wires are cut off? It is located in front of the valve body, secured with (2) 10mm head bolts, it is actually in the pump body.

If = yes, remove it and see if there is a check-ball on top of the solenoid. It will just be sitting there, or fall out when you remove the solenoid. This is an old-school method to achieve lock-up without computer control. If you find this mod, reinstall the solenoid without the check-ball and you won't have lock-up at any speed, but you can ascertain if the 3-4 shift is affected by the mod or not. (It shouldn't be.)

 

Thank you for that suggestion. I will check it out as soon as I get the car back up on blocks.

 

If you have a full pressure valve body, the TV cable adjustment will have little effect on the gear changes etc,
the gear changes and shift points are now effectively controlled by how the valve body is set up.
A full pressure valve body is a high performance improvement, so hardly surprising it doesn't want to go into 4th until a fairly high speed.

 

The OP nowhere states he has a full pressure valve body.

 

"I told him I hadnt decided and he suggested that he could convert it to full hydraulic lock up and I wouldnt have to do anything else, so I told him to do that."

This is why I made that comment.

 

Hydraulic lockup for the TCC vs computer controlled lockup is a separate issue from a full pressure valve body.

 

Adjusting tv cable from one extreme to the other with no change can also be a symptom of high pressure valve body.
I'm just suggesting a possible reason for what is happening, nobody including the op knows exactly what was done to that trans so its only guess work

 

Thank you for the replies. You guys are right about I have no idea what all he did to it. That is the problem. Everything seems to be pointing to the valve body. As "arbee" stated earlier he had to change the valve body to an older style in order to do the full hydraulic lock up.

What I am thinking now is to try to find an 88-89 valve body and wire harness and try to put it back to stock. Then I could use any of the available methods for the lockup.

Would there be anything else I would need to change other than the valve body?

 

You would need to address the computer prom and also have a brake switch installed to cut out the + feed to the TCC clutch. I don't know if your original trans had one for the OD unit.

 

I pulled the converter clutch solenoid and yes there was a check ball so I did as you suggested and removed it. There was no change in 3-4 shift.

Im going to try to come up with the correct valve body to put back in and see what happens.

thanks for all the comments.

 

I'll be damned. The old dog got another one right.

Can you tell if there is now no lock-up at all? With the check-ball over the solenoid, the converter clutch locks as soon as the transmission shifts into 2nd gear. So now, with no lock-up, the acceleration should now be more "lively" as the converter is now able to do it's job of multiplying torque in the higher gears. << This is of course if there were no other changes to the valve body. With the shifter in D2, road speed fast enough to be in 2nd gear, adding throttle should increase the RPM by a couple of hundred when the converter 'slips'. With the check-ball there should have been no increase in RPM with throttle, as the converter was locked to the engine RPM.

My thought regarding the 3-4 shift is that it has something to do with the governor. But I really don't know. I'm not a transmission guy. (Ask arbee, he'll agree).

If you obtain a different valve body also get the separator plate and wiring harness that goes with it. If you can, also get the governor that was working with the valve body and separator plate. When you remove the valve body and separator plate, look carefully at the "worm tracks" in the case for any epoxy filler or chiseled out partitions, or holes drilled between passages. << These "mods" will have to be repaired to restore operation with the replacement valve body. There are acceptable methods to restore the passages without replacing the case. Usually JB weld works if it's really clean and oil free.

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