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1989 Corvette - stock L98 - 700R4 trans. Trans is not original, came from a 1992 truck. Trans has been working flawlessly for 10 years, then suddenly died.
When shifting into Drive or Reverse the trans hits the gear so hard the car will jump forward or backward unless you have your foot planted HARD on the brake pedal. But once in gear, when you let off the brake and give it some gas the engine revs and the car barely moves.
Fluid level is correct.
The best advice is to put a line pressure gauge on it and check the FSM trouble shoot for what they mean. It will point you in the direction of clutch slipping, rebuild. Or valve sticking put a new valve body in, etc. I replaced the valve body in my 4L80 before knowing about the line test and the valve body was not the issue.
A cracked or broken forward accumulator piston will NOT cause a loss of pressure to the forward clutch. What it will do is cause harsh forward apply because the accumulator cavity has become "oil-logged" and the spring becomes ineffective. No different than a water bumper on a water line in your house. The pressure will hammer but there is no loss to your taps. This does not explain the reverse harshness though. As stated, the first check would be to put a gauge on the line tap.
Yes, it will cause a loss of pressure in the forward clutch circuit. The accumulator is in this circuit. The accumulator cover has a hole it it. Pressure from behind the cracked piston now goes through it and out the hole in the cover.
You are describing correctly what would happen if the 1-2 or 3-4 accumulator pistons cracked. They don't have orifices to the outside. The forward accumulator, with its hole in the cover on the aux valve body does.
I just took these of an aux valve body I have in my stash:
I will investigate further. If so, I stand corrected.
Does it "barely move" in Reverse, or only "barely move" in Forward? Or "barely move" in either direction?
Define "barely move". Does the engine rev up, but the car not move (something is slipping), or the engine strains to move the car like the park brake is set?
If it is slipping, and only in Forward, I can think of a failure point that was in the 92 transmission: The forward clutch accumulator piston was originally plastic. It can crack, and will cause a pressure loss to the forward clutch. But after 30 years, it probably has been replaced with an aluminum piston at some point in the past. The forward accumulator can be accessed and inspected with the pan removed. Inspection does not require transmission removal. If you DO find a cracked plastic piston, and it IS slipping in Forward, the forward clutch is probably fried. Trans has to come out.
Harsh "Garage Shifts" (Ie, those you do in the garage, from P or N into R or D) are usually caused by high line pressure, which can be caused by Throttle Valve (TV) issues. The 700R4 / 4L60 is susceptible to TV problems. Usually a sticking TV valve in the valve body. The best way to identify what is going on is to put a pressure gauge on it, and run the pressure tests in the FSM. There are also pressure tables in the ATSG manuals which can be found online. TV valve remedies require removing the valve body from the transmission, but again the transmission can remain in the car.
There's some initial ideas
It barely moves in either direction. By barely moves, I mean the engine revs but something is slipping, delivering little torque to the driveshaft. My son drive the car 3 blocks up the street and turned around because he knew something wasn't right. It made it up and back under its own power, but the engine rpm was 2 or 3 times normal for the speed/gear.
Last edited by 1989Jim; Aug 11, 2022 at 03:22 PM.
Reason: additional info
Possibly, the stator sprag clutch in the Torque Converter let go. As a result, there is no torque multiplication in the converter. This would affect both forward and reverse. There is nothing in the transmission itself that can affect both directions with the same symptom. Reverse uses a different set of frictions than the forward gears do, (other than for D1 low range. I put this in for arbee, he likes to bust people for overlooking insignificant details.). The torque converter affects both directions. Torque converter problems could also cause harsh garage shift engagement.
Again, there could be TV issues, and TV affects both forward and reverse. Again, a pressure check is probably the place to begin.
It's great to see that as of late, you have done some reading and increased your knowledge base regarding these transmissions since you made these statements in the past that I had to correct(paraphrasing here)
"When you push your C4 backwards, the noise you hear is the rear roller clutch slipping". Wrong
"There is a small green O-ring in the input drum that if it leaks, causes the 3-4 clutch to slip". Wrong
"In first gear, the reverse input drum is turning twice that of engine speed". Wrong
There is something about "people who live in glass houses...". I do thank you though for correcting me above. All knowledge is good knowledge and the ability to admit being incorrect is far more virtuous than those that would go down fighting trying to perpetuate their claimed knowledge about EVERYTHING. Once again, I commend you.
So. Where are the corrections? This would be educational to others reading the thread. By not offering a courteous correction, you just come across as the dic that we all expect arbee to be.
The corrections were pointed out in the original threads. I would gladly quote them but someone has a fetish for deleting posts.
First, I will apologize for wasting your time. Most of what I said in my original post was wrong. My son (who owns and drives the car) told me what it was doing, which is what I posted originally.
Well... today I drove it myself just to verify the symptoms before diving down multiple rabbit holes. Here is what I found.
1) The trans works fine in reverse. It did not slam into gear any harder than it used to, and the car moves fine under power.
2) The trans does seem to hit hard going into 1st when sitting still. Could be an issue with the accumulator. I will be checking that.
3) The trans works FINE in 1st gear. The problem is it never shifts to 2nd. I mean never. That's why my son thought it was slipping, because it was reving so high at low speed.
So, I am now suspected a governor problem. Either the plastic gear lost its teeth, or the governor valve has become stuck. Since this problem just came on suddenly with no odd behavior before hand, I am suspecting the gear is stripped.
My first step is going to be to remove the governor and check the gear and the motion of the valve. That is relatively easy to do on the car and doesn't require removing the pan. I will let you know what I find.
As for slamming into 1st gear, my first objective is to get it shifting again, then I will tackle that as a secondary problem.
Thank you all for your insights and helpful comments.
I just had another idea. If the kickdown cable is somehow hung up with the cable pulled, it would keep the trans from shifting (at least at speeds I was driving at) and might also cause it to hit hard shifting into 1st, with the valve body thinking the engine is at full throttle.
Before I check the governor, I am going to make sure the kickdown cable is free and that nothing is hanging up were it goes into the trans. I have heard that can get stuck with dirt and dust over time, and pulling the cable to WOT and letting it snap back several times can free it.
I just had another idea. If the kickdown cable is somehow hung up with the cable pulled, it would keep the trans from shifting (at least at speeds I was driving at) and might also cause it to hit hard shifting into 1st, with the valve body thinking the engine is at full throttle.
Before I check the governor, I am going to make sure the kickdown cable is free and that nothing is hanging up were it goes into the trans. I have heard that can get stuck with dirt and dust over time, and pulling the cable to WOT and letting it snap back several times can free it.
I had this happen to me in my 85 a few years ago. The first few times I was able to get the valve unstuck by moving the tv cable with the engine running at idle and in trans in park. Maybe that makes no difference and you can do it with the car shut off, I'm sure someone can elaborate on that. Eventually the TV was completely stuck and no amount of cable movement would free it up. From what I understand this issue most often occurred on early 700R4s. I pulled things apart to install a "no-stick" TV valve and did a shift kit as well while I was in there. The valve was definitely stuck when I went to replace it.
700R4 problem
