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Hi gang, been awhile since I last logged in. Anyhow, I have a problem with my 85 700r4 overdrive. I just had the tranny completely overhauled and he bench tested it thoroughly. So the tranny and torq converter are not the issue.
The problem: When starting out from a stop all works just fine. I get solid OD lock up. So, I know the OD does work. The problem occurs when at highway speed and the brake is applied and then speed is resumed without making a complete stop, the OD does not want to re-engage. I know there are several things that are inputs to the computer for this to happen. My thinking now is the TPS (throttle position sensor). I am wondering if any of you have any ideas where to start. Or if anyone has had a similar problem that has solved it. Any and all help would be greatly appreciated.
Is it overdrive or torque converter lock up you are talking about? I'd try adjusting the tv cable, your 85 doesn't have a fully computer controlled trans, only the lock up feature is computer controlled.
Hi gang, been awhile since I last logged in. Anyhow, I have a problem with my 85 700r4 overdrive. I just had the tranny completely overhauled and he bench tested it thoroughly. So the tranny and torq converter are not the issue.
The problem: When starting out from a stop all works just fine. I get solid OD lock up. So, I know the OD does work. The problem occurs when at highway speed and the brake is applied and then speed is resumed without making a complete stop, the OD does not want to re-engage. I know there are several things that are inputs to the computer for this to happen. My thinking now is the TPS (throttle position sensor). I am wondering if any of you have any ideas where to start. Or if anyone has had a similar problem that has solved it. Any and all help would be greatly appreciated.
Sounds like you are talking about the torque converter lockup, not the OD. The brake switch may need adjusting. Put your toe under the brake pedal and lift and see what happens.
Torque converter lock up and OD are two different things.
An automatic transmission is basically like having two fans in line with each other, and one fan is blowing air onto the other fan. It causes the other fan to spin. The clutches/fins inside the torque converter catch the ATF, and spin at the designed stall speed (when you press the throttle and the RPMs blip a bit, and then settle back down once throttle is released). So the engine speed and transmission speed can be different. Once the lockup occurs, the engine and transmission speed are matched, feeling like an extra "gear".
I may have some of the technical terms incorrect, but the basic principle of how the auto transmission works is solid.
Torque converter lock up and OD are two different things.
An automatic transmission is basically like having two fans in line with each other, and one fan is blowing air onto the other fan. It causes the other fan to spin. The clutches/fins inside the torque converter catch the ATF, and spin at the designed stall speed (when you press the throttle and the RPMs blip a bit, and then settle back down once throttle is released). So the engine speed and transmission speed can be different. Once the lockup occurs, the engine and transmission speed are matched, feeling like an extra "gear".
I may have some of the technical terms incorrect, but the basic principle of how the auto transmission works is solid.
Yeah, technical terms...
There's some older language that may lead to confusion.
In torque converters, once an equilibrium of speed and load are achieved the drive and the driven elements of the converter operate at pretty much the same speed. This was often referred to as "lockup." However, in the case of more modern converters there is a separate clutch inside the converter that when activated, mechanically locks the two components together. This is also a "lockup," but it's an actual physical lock between the components just like a clutch on a manual transmission, so there are no losses in the transmission of power.
Wasn't really looking for a lesson on all the detail. Just was looking for some help on a possible failure that won't activate the TCC after touching the brake while cruising. I did check the brake switch and it is operating just like it should. I have diligently checked everything else. And as I have already stated, everything works perfectly from a stop. So I know that everything else is working like it should. I am beginning to think it is either the PROM, the ECM, or both. I was kinda hoping some else may have had this problem and could confirm my thinking. Sorry for leading you all down the wrong path.
Originally Posted by ex-x-fire
Is it overdrive or torque converter lock up you are talking about? I'd try adjusting the tv cable, your 85 doesn't have a fully computer controlled trans, only the lock up feature is computer controlled.
Wasn't really looking for a lesson on all the detail.
If you don't understand how these things work, how are you supposed to diagnose a fix? While there are far too many people here that watch too many Youtube videos and then come on here and spew nonsense, finding a solution to your problem starts with understanding how certain systems operate.
OK, so I misspoke. Got it, TCC lockup is not OD. Sorry.
Seems I have control problem, everything seems to work just fine starting out from a stop. This tells me that all the switches, inputs, and wiring are working, or am I missing something? Ex-X-Fire said the tcc is controlled by the ECM. In trying to understand this, the does the prom do any of the control or is it just the ECM? Maybe I should just replace both?
Also, on Corvette Central's website they state that the life of the PROM was intended to be 5 years. Any more information on this would be greatly appreciated as well.
Gentlemen, I have been struggling with this for a year now. And I am getting very frustrated. Everywhere I turn, I cannot find a component failure and am slowly and painstakingly being pointed in the direction of the ECM and/or the PROM. I cannot find any information on the computer program for this control. I have tried through my poor electrical knowledge to diagnose this problem. I have painstakingly gone over and over the manual's diagnosis flow chart. I am coming to the conclusion that I have to bite the bullet and spend the money on an ECM and PROM. I mean, the car is 38 years old. I could easily spend more money on a shop to find the problem and still have to buy an ECM and PROM.
OK, so I misspoke. Got it, TCC lockup is not OD. Sorry.
Seems I have control problem, everything seems to work just fine starting out from a stop. This tells me that all the switches, inputs, and wiring are working, or am I missing something? Ex-X-Fire said the tcc is controlled by the ECM. In trying to understand this, the does the prom do any of the control or is it just the ECM? Maybe I should just replace both?
Also, on Corvette Central's website they state that the life of the PROM was intended to be 5 years. Any more information on this would be greatly appreciated as well.
Gentlemen, I have been struggling with this for a year now. And I am getting very frustrated. Everywhere I turn, I cannot find a component failure and am slowly and painstakingly being pointed in the direction of the ECM and/or the PROM. I cannot find any information on the computer program for this control. I have tried through my poor electrical knowledge to diagnose this problem. I have painstakingly gone over and over the manual's diagnosis flow chart. I am coming to the conclusion that I have to bite the bullet and spend the money on an ECM and PROM. I mean, the car is 38 years old. I could easily spend more money on a shop to find the problem and still have to buy an ECM and PROM.
While anything is possible, I think the ECM or PROM failure is the least likely. There are far more things that can cause your problem. I don't know what you have or have not checked but: Step 1 is to verify if the signal is actually getting to the transmission. Check the connection for loose fitting or dirty, frayed wires at the transmission wire termination. Set up a test by running 3 jumper wires into the drivers spot and do some checks with a test light.(you will have to splice into the wires at the termination plug,) Wire "A" is the one from your brake switch. With the car running, this should have + voltage until you step on the brake. When you release the brake, it should go + again. The other two need to be checked while driving.to see if the trans is getting the proper signal when commanded. If all check out good, then the problem lies within the transmission(faulty switch, sticking valve). I have included a couple pages from my ATSG manual. DO NOT ASSUME THE VALVE BODY DIAGRAM IS FOR YOUR MODEL. THERE ARE TEN OR MORE VARIANTS AND THIS IS FOR EXAMPLE ONLY.
EDIT: Post the model number of your transmission and I will post the appropriate diagram.
Step 1 is to verify if the signal is actually getting to the transmission. Check the connection for loose fitting or dirty, frayed wires at the transmission wire termination.
Yes, I have verified that the wire from the ECM to the transmission is good. I did this with a continuity check from end to end. Also, the TCC activates from a stop just fine.
Originally Posted by arbee
Set up a test by running 3 jumper wires into the drivers spot and do some checks with a test light.(you will have to splice into the wires at the termination plug,) Wire "A" is the one from your brake switch. With the car running, this should have + voltage until you step on the brake. When you release the brake, it should go + again. The other two need to be checked while driving.to see if the trans is getting the proper signal when commanded. If all check out good, then the problem lies within the transmission(faulty switch, sticking valve). I have included a couple pages from my ATSG manual. DO NOT ASSUME THE VALVE BODY DIAGRAM IS FOR YOUR MODEL. THERE ARE TEN OR MORE VARIANTS AND THIS IS FOR EXAMPLE ONLY.
When I start from a stop all works perfectly. All gear shifts and the TCC activates normally. This is consistently repeatable. The TCC works every time. It locks up after 4th gear engages and 40mph is achieved as it should. If starting out from a stop and getting to speed with TCC lock up and hit the brake and come to a stop and start out again everything will work perfectly. If starting out from a stop and get up to speed with TCC lock up and hit the brake and keep cruising, TCC will not reengage. This is what I am struggling with. What now tells the TCC to lock backup? The PROM, a relay, the ECM? Something isn't closing the logic loop in this condition. Why do I have to come to a complete stop to reset the logic? Even if it is a bad brake switch, why is it allowing TCC engagement from a stop. Isn't the switch just and open, close condition? It doesn't provide logic, just a signal. If the switch is momentary condition during cruising, it should just delay the reset for a longer time similar to a full stop.
I have gone through all of the service manual diagnostics several times. I have checked wiring, switches, sensors, and keep coming up empty, because everything works except this particular condition. I will recheck anything again if you really think I missed something. Again, the TCC works and everything checks out in the start condition.
Originally Posted by arbee
EDIT: Post the model number of your transmission and I will post the appropriate diagram.
How do I determine a Type I or Type II tranny? I see there is an overdrive relay switch for the manual trans, but the service manual doesn't list one for the automatic. Am I missing something? A bad relay? Thanks for your time and posts. Greatly appreciated.
Has it done this since the trans rebuild ?? If so why didn't you take it back to the shop that rebuilt the tranny ??
No, it was what prompted the rebuild. The rebuild was bench tested and works perfectly in all respects. The rebuilder is working with me. I pulled the tranny and installed it, he just did the rebuild and bench test.
Originally Posted by WVZR-1
A revised calibration PROM has been released which is intended to improve driveability on 1985 Corvettes with Automatic transmission.
This PROM contains changes to improve tip in response when accelerating and also raises torque converter clutch engagement speeds to 65 KPH (41 MPH).
This PROM P/N is 1227805.
Pretty sure I have the updated PROM. The TCC activates after reaching 40mph. And was working fine until it now.
Yes, I have verified that the wire from the ECM to the transmission is good. I did this with a continuity check from end to end. Also, the TCC activates from a stop just fine.
When I start from a stop all works perfectly. All gear shifts and the TCC activates normally. This is consistently repeatable. The TCC works every time. It locks up after 4th gear engages and 40mph is achieved as it should. If starting out from a stop and getting to speed with TCC lock up and hit the brake and come to a stop and start out again everything will work perfectly. If starting out from a stop and get up to speed with TCC lock up and hit the brake and keep cruising, TCC will not reengage. This is what I am struggling with.
What now tells the TCC to lock backup?
The brake switch supplies the + feed to the internal solenoid. Go for a drive and maintain, say 60 MPH. Don't let off the gas but momentarily touch and release the gas pedal. Your tach should climb and fall a couple hundred RPM. This will confirm if your brake switch is operating correctly. If the tach doesn't go down when releasing the brake, that doesn't necessarily mean the brake switch is bad. That's why I said to hook up the test light. If you release the brake and the tach doesn't go down but the test light goes on, it means the + signal has been restored but there is a problem inside the trans.
The PROM, a relay, the ECM? Something isn't closing the logic loop in this condition.
In conjunction with the above noted + signal, the trans indicates that conditions have been met and the ECM sends the - signal(Method varies, that's why I said post your trans model number.) I wouldn't get a woody over thinking it is electrical but this should be ruled out first. Test, test, test. There is also TCC valves in the valve body and/or pump that may have cross leaks, sticking valves, broken return spring etc.
Why do I have to come to a complete stop to reset the logic?
Could be many things. Maybe while at a certain speed, a cross leak is occurring and is sticking a valve. May "unstick" when pressure goes down. ????????
Even if it is a bad brake switch, why is it allowing TCC engagement from a stop. Isn't the switch just and open, close condition? It doesn't provide logic, just a signal. If the switch is momentary condition during cruising, it should just delay the reset for a longer time similar to a full stop.
I have gone through all of the service manual diagnostics several times. I have checked wiring, switches, sensors, and keep coming up empty, because everything works except this particular condition. I will recheck anything again if you really think I missed something. Again, the TCC works and everything checks out in the start condition.
How do I determine a Type I or Type II tranny?
I don't know what you mean by "Type 1 or Type 11". You say you have a 700R4. Look on the back right hand side of the transmission right where the oilpan meets the case. There are some letters and numbers there. Post them.
I see there is an overdrive relay switch for the manual trans, but the service manual doesn't list one for the automatic. Am I missing something? A bad relay? Thanks for your time and posts. Greatly appreciated.
There once was a C4 corvette traveling down a road,
all the parts were working and singing happily, they
suddenly came upon a corvette god and all the parts
stopped working and bowed except pin F on the ALDL,
he threw his finger in the air at the corvette god and
yelled at him, "I won't bow down today corvette god,
not today".
The corvette god became angry and pointed his finger
at pin F and said (in a hollow 55 gal drum voice) "you shall
never speak to me that way again pin F" and he thrust
his staff at pin F and a bolt of lightning flew from the
the staff and struck pin F.
To this day, Pin F can not speak what voltage it has on it,
Instead, all owners must use a DVM to tell what it is.
08-10-2023, 10:18 AM
1985 700r4 TCC issue
