Hello everyone, I’m hoping to learn more about my car’s transmission/gearing. I’m pretty clueless when it comes to anything transmission or drivetrain. I have a 1989 Trans Am GTA, 350 w/ 700r4 auto 4 speed. Unsure of the gears.

What is the difference between the overdrive setting on the shift panel and the “overdrive gear” aka 4th gear?

I’ve always driven the car in overdrive, because it keeps the rpm’s low for commuting. On the highway however, in overdrive setting, it seems to rev pretty high (around 3k at 80mph and 3800 at 100mph). It never seems to reach that 4th gear, I’m assuming it’s because that’s reserved for 110-140mph.

I’m trying to get my rpm’s lower for highway cruising, so would it be better to drive it in “drive” so that I can hit that 4th “overdrive gear” quicker? Overdrive feels like I’m stuck in that 3rd gear all the way till 100+mph

thanks and apologies for the confusing question lol

 

3rd gear is 1:1 ratio. 4th (overdrive) gear is 0.7:1.

Your RPMs do seem quite high to be in 4th gear. Unless someone put some very steep gears in the differential.

It may not be getting into 4th gear. It should be shifting into 4th probably as low as 40 mph if you have the shifter in the OD position. To be sure, what you can do is put it into 3rd gear, then get onto the freeway and shift it into OD. If the RPMs don't drop, then you know you have some sort of issue with the OD gear.

First thing I'd though check is the fluid level before doing anything else. Get that car warmed up in the driveway, then shift into and out of gear a few times. Then check the fluid level.

Also check the TV cable adjustment. The general way to do that is to (with the engine off) depress the button at the throttle bracket and push the inner sleeve all the way in. Then move the throttle linkage (either via the foot pedal or by hand at the throttle body) to WOT and and let the sleeve pop out. There's probably a video on youtube to show this (TH700R4 TV cable adjustment).

 

Doea it have 3 distinct upshifts?
Is lockup working. Shifter in OD, hold throttle steady, tap the brakes. Then release. Does RPM increase, then come back down?

If you have all 4 gears and lockup, you may just have a rear gear change.

Generally, 26" tires give about 1000 x gear ratio at 70mph.
0.70 4th gear * 3.73 is 2600. With 4.30 gears, 3000.

 

You should be able to feel 4 discrete transmission "events" in "overdrive": 1-2, 2-3, 3-4, and converter clutch apply. 4th is "overdrive".

4th should drop the RPMs by about 35% or so from 3rd, subject to variations caused by the converter. When the clutch locks up it should drop a couple hundred more. Your car should have come with either 2.77 or 3.27 gears, but of course, there might well be something else in it now. With 2.77 your 80 mph RPMs should probably be in the 1900 - 2000 kind of range, or maybe 2300ish with 3.27. This of course measured with something ACCURATE, not the stock wavy-pointer "sporty" trim piece in the dash, that is well and widely known to fail by reading higher than reality. If that's all you're basing you're question on, one of the first thing you should do is, verify that it's right, since there's a very good chance that it's not. You might just have a tach problem and not a "RPM" problem at all.

 

I'm interested to see what you guys are able to figure out here. I seem to be having the same issue,,, doesn't seem to be going into 4th. Feels like it should shift at about 40MPH with normal driving but nothing, RPM just keeps climbing. about 2,400 RPM at 55MPH, about 2,600 RPM at 60MPH, about 2,800 at 65MPH, about 3,000 at 70MPH.

I also switched from D to OD at highway speeds and no drop in RPM.

I have no idea what gears I'm running.

From reading/research, culprits I've seen suggested have been TV Cable adjustments, TV Valve, Torque Converter Clutch Solenoid and wiring or the Torque Converter.

 

Sounds like you have the infamous 2.73 10-bolt in the rear.

If it's not shifting into 4th, then it's not a converter clutch problem. It's a 4th gear problem. Cause is almost certainly something to do with the band in the transmission (4th applies the forward clutch, the 3-4 clutch, and the band). Since 3rd works then the 3-4 clutch is working, which is actually one of the main failure points in that transmission. You might want to try replacing the servo with a "Corvette" servo, no guarantees but it might solve it, since the servo applies the band. Probably not but maybe. Otherwise it's probably about transmission time. Also, if your filter is real dirty, then the whole system may not be able to flow enough fluid to work all 3 of those frictions at the same time; you could try a new filter, but if it's so funkified that it can't flow fluid, then the usual explanation is that the fluid is full of clutch material, and even if it gets 4th to work again, it's not long for the world, and you're back to needing a new trans. If your fluid looks bad then the band is probably smoked. My truck (04 w/ 4L60E, same clutches and whatnot just different controls) just experienced that failure, and I'm in the middle of building a new trans for it. It still goes into 4th, and then the converter clutch applies afterwards, once in a while, but mostly it doesn't. I just drive it around in 3rd.

 

I appreciate your quick and detailed response. My heart sank reading it though. Just checked my Service Parts Identification tag. GU5 -- Rear Axle Ratio (3.23). That's the gears? Sorry, I'm new to this. Does 3.23 make any difference in what you are thinking the problem to be, you were under the impression it was 2.73?

Transmission had fluid changed, filter changed, fluid flushed in 3/22.

 

Yup, that's the gears. You have the better ones. No, has no bearing on your problem; only, on the relative RPMs you are reporting.

What's been done in the past doesn't matter now. What matters is what you do going forward. You can try changing it and see if it helps. If it does, great, at least for a short while. If not, oh well.

 

Again, thank you for your knowledge. I will do a trans service again and keep my finger crossed. Next step would be to attempt the Corvette servo you suggested. Are you talking something like a 700R4/4L60E High Performance Billet Corvette Servo Assembly? Need something for 1-4 shifts? Sorry again for my ignorance.

You still don't think it could be the torque converter and not the trans (wishful thinking on my part, I know)? I did just have to do my motor mounts, so not sure if the engine shifting in the bay might have done some damage there.

 

I had a no OD situation on my DIY 700R4 rebuild using Dana's elite kit. I have a couple threads here accumulating great suggestions on what to check before I tore into it again.

In my case like Sofa referred to I forgot to install a nickel-thin washer in the new servo assembly, so there was too much play in the pin movement that resulting in the band not applying.

Of course you aren't missing a washer but between band wear, dirt, and maybe an aged, leaking o-ring or seal that band is prolly not grabbing the drum.

My 2c.

 

No of course not.

If it doesn't shift into 4th, it's a 4th gear problem. If it goes into 4th but the TCC doesn't lock up, it's a TCC problem. Not sure how this can possibly be unclear, or how it could be possible to blame the TCC for a 4th gear failure. Makes no sense.

No, motor mounts won't prevent a transmission from shifting into 4th gear. Note that I am NOT saying "your motor mounts are fine", NOT saying "don't bother with motor mounts", NOT saying "motor mounts don't go bad", NOT NOT NOT NOT NOT. All I'm saying is, bad motor mounts can't make a transmission lose 4th gear.

 

Great, thank you for confirming what Sofa had suggested. I will do the trans service and keep my fingers crossed. From there would do the servo and again keep my fingers crossed. Hopefully those items would cure the issue, prior to having to do the trans.

Thanks again for your insight.

 

OK, thank you for your clarification and insight, very much appreciated. As I had said, I am new to all this, so, some things that seem simple can be unclear to me.

As to the motor mounts, they were bad, the engine was shifting in the bay at acceleration. I have just had them replaced. Above, I was wondering if the engine shifting in the bay could have caused damage to the torque converter; but, as you have informed, this is really an irrelevant question/concern to the issue at hand.

Thanks again for your expertise and patience.

 

The motor and trans are bolted solidly together, and the TC is wedged between them. You could probably pick up the whole thing right out of there and shake it like a paint can, and the TC wouldn't even notice, let alone care. Bad motor mounts won't affect it at all. Just like they won't affect 4th gear in any way that can make it fail.

 

I'm not trying to get into a pissing match with you, and, genuinely appreciate your thoughts on the issues I am having. Again, I am new and learning.

Some information I am finding seems to contradict your assessment that this could not possibly be a TCC issue and can only be a 4th gear problem.

See: https://www.transmissionrepaircostguide.com/p1870/

I am going to get a scanner and check the trouble codes (should have done this already, but, don't have easy access to one).

As per the above link, if it's a P1870, symptoms can include:

• Lack of fourth gear/overdrive
• Increased engine RPMs
• Poor performance

Possible repairs include:

• Change the transmission fluid//flush the transmission
• Install a high quality in-line transmission filter
• Repair worn transmission valve bores
• Replace TCC clutch
• Reprogram ECU

 

I forgot to answer:

Yes you could get something like that. There are also aftermarket ones that are even more aggressive (tighten the band around the drum even tighter) than any Corvette one, such as the Sonnax and Superior/Fairbanks ones. Just a stock Vette servo is usually enough for most people though: most of us don't want our eye teeth yanked out by the 1-2 shift, which is what happens when you go overboard on that. The shift can be so violent that it breaks parts besides. Also, as mentioned, the band tightens around a drum; the drum actually warps inward, sort of "collapses", when the band burns up around it, from the heat; and it's worse if the band is pulled tighter. There are limits to how far you want to go with beefing up the servo.

 

You replied while I was replying.

P1870 means something like "too much RPM difference between input and output when they should be the same or very close". The ECM has enough wherewithal to be able to predict what the engine RPM should be, based on what gear it's in and whether the TCC is applied, and that's what the code is for. Which is why it doesn't tell you just exactly "the such and such is broken, go get a new one": several things can cause it. If the ECM tells the trans to shift into 4th and there's enough pressure in the 4th gear circuit to operate the manifold switch which tells the ECM that yes the trans is indeed in 4th, but in reality the 4th gear friction (the band) is slipping, then it will set that code. Same with the TCC: if the ECM commands it but the RPM remains too high because it's burned up, that will also set the code.

However all that may be, a failed TCC cannot cause 4th gear not to work. They are 2 separate subsystems within the transmission. They are interrelated and all that, but, 2 different things. If 4th gear is not working, it's because 4th gear is failed, not because of anything to do with the TCC. Which is ALSO not to say that they can't BOTH have failed; that remains a possibility. But regardless, if 4th gear doesn't work, it's because 4th gear is broke, not the TCC; and no matter what you do to the TCC in that case, 4th gear will remain broken.

And again, however all that may be, P1870 is a code in OBD2 systems, which your car doesn't have. Your car only has the older OBD1, and there is no code corresponding to P1870 in your ECM. Your ECM isn't that smart. Too old.

 

I appreciate your detailed response. This is all making my head hurt.

Even if I I am not able to pull the code, due to the capabilities/age of my ECM, my vehicle seems to be generally exhibiting some/most of the symptoms listed for P1870.

In laymen's terms, if a symptom of the P1870 code is "Lack of fourth gear/overdrive" and a possible repair would be "Replace TCC clutch", how could it be correct to say, "a failed TCC cannot cause 4th gear not to work" and that "no matter what you do to the TCC..., 4th gear will remain broken"?

Again, I genuinely appreciate you taking the time to share your knowledge. I am just trying to make sense of all this and, hopefully, get these repairs made economically.

 

Code P1870 covers both the TCC AND 4th gear, in the matter of RPM being outside the predicted range.

The ECM will display its understanding of EITHER potential problem with that code.

In one car, it can appear due to the one cause, and in another, by the other.

2 different things. 2 different symptoms. Yet they both produce one particular bit of numerical information that can point to either cause.

It would be a bit like a troubleshooting chart for "engine won't run". It might list various repair procedures: put gas in the tank, replace spark plugs, install new timing chain, and so forth. But if your problem is that the car ran out of gas, you can change spark plugs as many times as you want, and it's still never going to run. Even if the troubleshooting chart for "engine won't run" said "change spark plugs".

You can't fix a 4th gear problem by working on the TCC. Just like, if you have a TCC problem, you can't fix it by working on 4th gear, either. Even though they both pop up as possibilities on a troubleshooting chart for the same "P1870" type symptoms.

As a point of the usual repair procedure however, the converter should ALWAYS be replaced when replacing the transmission; so that at least is a given. However, if your transmission's 4th gear isn't working, and you pay somebody to swap the converter, 4th gear STILL isn't going to work; you'll have to have that work done TOO, and almost certainly, they will NOT honor the transmission warranty if the converter isn't replaced AGAIN.

 

Great, thank you. Sorry to say, I still don't understand your reasoning/conclusion; though, I very much appreciate your insight/experience.

In my mind, if A (no 4th gear/OD) is the problem, and B (replace TCC clutch) is a possible repair for the problems, and, repairing A (no 4th gear/OD) with B (replace TCC clutch) fixes the problems, then B (TCC clutch) was the cause of the problems (not 4th gear). Did you look at the link I provided to see if there was any viability there?

Again, I am new and unexperienced.

I'm going to run a scan with an OBD1 to see what that produces- hopefully this weekend. I will advise of those results and we can go from there.

I do very much appreciate you and your willingness to share your knowledge.

 

I may be totally ignorant on all this stuff but, from the information I've seen:

Symptoms

Symptoms of a P1870 include:

• Delayed shifts
• Harsh shifts
• Lack of fourth gear/overdrive
• Increased engine RPMs
• Poor performance

What Repairs Will Fix P1870?

• Change the transmission fluid//flush the transmission
• Install a high quality in-line transmission filter
• Repair worn transmission valve bores
• Replace TCC clutch
• Reprogram ECU

Seems to reason that if replacing TCC clutch remedies the lack of 4th gear/overdrive symptom, then, faulty TCC clutch is a possible cause of the issue. This is possibly not a loss of 4th gear/overdrive issue; but, the symptom of a faulty TCC clutch. Not saying that this is definitely the issue at hand, but, it is a possibility.

 

TW I have gone round and round with my multiple problems on my DIY rebuild taking advice from experts here on TGO. There are many goings on inside a trans, mechanical systems, hydraulic systems, planet gears linked together etc. Even as I am able to rebuild an engine no problem a trans rebuild was like alien technology to me. In fact there are video series like Trans Workbench that tell you straight out you can rebuild the trans without understanding how it works.

The advise you are getting is based on the unknown actual problem inside your trans.

Like you are getting folks here gave me great ideas. Based on experience and reason. None of them predicted how I messed it up myself or in your case what specifically has worn out (or broke) and is the reason for the fail.

You really have three choices, start parts swapping hoping for a fix (the servo is a pain but only a 2 on a trans scale of 1 to 10) or tear it down and rebuild it, or buy a rebuilt one. If you have a problem on an aged trans and fix it without rebuilding, what is the likelihood another deeper problem will surface? It's still basically the same old trans.

Trying to convince someone to agree you are right based on something you read without knowing anything about a trans is bad thinking. There are just too many things going in there to be certain.

I know I have been there.

PS if you do open it up and despite your fluid and filter changes, you will see tons of places where dirt has accumulated. There are lots of tiny holes that could get blocked by this dirt. Blocked ports cause things to go sideways.

 

Thanks TT, I know that Sofa is most likely correct. Maybe it's more me trying to convince myself that it isn't the actual trans. More in denial than anything. I know that you guys have probably forgotten more stuff about the mechanics of these cars than I actually know. I appreciate you guys and your assistance. I will update once I've been able to get to the bottom of this.

 

Those numbers seem nearly identical to mine- glad to see I'm not the only one. Anyway, I'm in no rush to "solve" this problem as I'm currently painting the car and handling various other issues. I appreciate that this forum took off, though.. It gives me plenty of stuff to check haha.

 

The Superior and the Sonnax Intermediate Servos were created for the DIY Trans Guys.

The Ratio Difference/ Increase over the Corvette Servo is almost completely unneeded.

By simply altering the Valve-Body Separator Plate, I can have a Corvette Servo Shift Firmer than either Aftermarket Servo.

The Aftermarket Servos alter the size of the Apply Piston Surface Area to increase the force generated by the Servo.
By simply increasing the Hydraulic Pressure in the Servo Apply Circuit, I create the same increase in force.
And it is easy to over do it!


Too much will result in 2 possibilities...
Break the Rear Tires loose...
Or if there is traction, Break the Transmission.