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I think lionelhutz has hit the nail on the head. The OP said that the selector will only drop down one spot from what he thinks is "Drive". You can only drop down one position when you are in selected second gear, therefore, although the selector quadrant MIGHT be indicating DRIVE, I think that the transmission is actually in SECOND. To verify this, the OP should try pushing the selector into what it says is NEUTRAL while driving and see if the revs drop. If so, all he needs to do is to adjust the selector cable so the indicator quadrant is showing the correct gear.
I apologize for the delay in responding, but the weather here's been beyond crappy until today, and this is my tenth night at work.
I finally jacked up the car and took a good, hard look underneath. The transmission's oil pan definitively indicates a THM 350. It matches that diagram's example of it exactly -- thirteen bolts, identical shape, and even the slight curvature at its front end.
I examined the modulator at the transmission (in place, as it's supposed to be, on the passenger's side at the transmission's rear), and as far as I could inspect it, it was in good shape, though hardly clean. The hose connecting it to the engine was entirely rubber -- and as I understand it, they were originally metallic pipes.
However, it was also secure on both ends, and the material was in thoroughly good condition. The previous owner improvised with the fastenings somewhat, though -- the hose is attached to the modulator by one of those clamps you screw tighter.
As for the shifter, it's unmistakably P --- R - N - D (first click after N) - LO (last position; allows shift from first to second). The shifter's surround is worn and reads nothing. When I was beneath the car, I manually matched the clicks and positions to those on the shifter.
When I'm driving the car, and I move upwards once from Drive, it always reliably drops into Neutral. When I then pull the shifter back one click, it lands on Drive/a forward gear, and continues to exhibit the over-revving problem.
The only thing I haven't had time to attempt since reading this thread for new replies is engage the brakes and depress the gas pedal until the car wants to move to test the torque converter. I'll try that as soon as I leave for work in a few hours.
I also captured video of those scenarios AtL requested. I'll upload them as soon as I can and post them here.
You spun the driveshaft just over 3 turns to get BOTH wheels to turn 1 revolution, correct?
Originally Posted by ajrothm
Jack the car up, mark one side of the driveshaft with something (paint), put it in neutral and turn one of the tires 1 full revolution, then count how many times the driveshaft turns.... That will give you your ratio.
This test is wrong.
If you only spin one wheel then you need to spin it two revolutions when counting the driveshaft revolutions.
If you have a positraction differential where both wheels are connected together and spin at the same time then you spin both wheels one turn when counting the driveshaft revolutions.
When you only spin a single wheel, the side and spider gears in the differential make the axle and wheel have 2X the rpm's of the ring gear.
Last edited by lionelhutz; 09-08-2016 at 01:03 PM.
1. None of the images of a 1976 shifter lens show a LO gear they all show 1 2 3. Can you post a picture.
2. What is the size of your rear tire, 205/70x15 etc.?
3. Have you used a GPS to verify that your speedometer is reading correctly.
4. When you counted the rotation of the driveshaft, was the trani in neutral and did you observe both tires rotating together?
The way the shifter sits in the last position makes it all look badly misaligned. I don't know why it looks that way. Again, manually checking the clicks/positions underneath the car yielded no new information -- it all matches to what the shifter does, and there's nothing I can push to that's further than the last position I can use now. Additionally, between Drive (or what I think is Drive) and Neutral, there's nothing. I thought the shifter might be skipping it, but I couldn't find anything.
2. What is the size of your rear tire, 205/70x15 etc.?
I don't recall. I'll check, but they should be standard.
3. Have you used a GPS to verify that your speedometer is reading correctly.
Always.
4. When you counted the rotation of the driveshaft, was the trani in neutral and did you observe both tires rotating together?
It was definitely in Neutral, but I never paid attention to whether both were spinning simultaneously. Apologies. I'll raise the car and redo the test for sake of certainty.
I see what appears to be trans fluid on the mounting bolts there, whats your fluid level like when you check the dipstick while idling in park?
It leaks very slightly, and I top it up when it's needed. I check the level weekly, and it's never been anywhere near the "ADD" portion of the stick.
Instead of rotating the wheel and counting the driveshaft revolutions, do the opposite. Rotate the driveshaft and see how many turns are required to make the wheel rotate two times (or more) and then divide the rotations of the driveshaft by the number of turns of the wheel. You'll get a more accurate ratio that way. It's hard to tell the difference between 3.08 or 3.36 revs of the shaft, but it's easy to count 10 revs of the shaft and divide by three to get 3.33 (rounded to 3.36). Or 11 revs of the shaft and three turns of the wheel to get 3.70.
Instead of rotating the wheel and counting the driveshaft revolutions, do the opposite. Rotate the driveshaft and see how many turns are required to make the wheel rotate two times (or more) and then divide the rotations of the driveshaft by the number of turns of the wheel. You'll get a more accurate ratio that way. It's hard to tell the difference between 3.08 or 3.36 revs of the shaft, but it's easy to count 10 revs of the shaft and divide by three to get 3.33 (rounded to 3.36). Or 11 revs of the shaft and three turns of the wheel to get 3.70.
I'll perform the test thusly this time. Thank you. With my luck, though, I'm sure it still won't be the differential.
Extremely high rpm? Have you verified that your tachometer is indicating correctly?
No. It seems to underreport the lower range (which meant there was room for significant error elsewhere on the rev range), but the shop that looked at the transmission told me the problem is fairly severe and evident regardless, and that the vehicle wasn't behaving right, which was my suspicion. They cleared the transmission, but said something else must be wrong. Being a gearbox specialist, they declined to examine anything else.
But I tend to agree with their analysis. The transmission whines hysterically, and the engine sounds and feels like it's shaking apart itself and the car above 50-55mph. I've been in healthy Corvettes before. This isn't what it should be like.
For whatever reason, these clips I took of AtL's suggested tests don't even begin to capture the aforementioned whine and racket. I'll try to record those separately to give you an idea of the stresses the car's putting on itself when driven at any considerable speed.
-Take off from a stop at 1/4 throttle, again at 1/2, and again at full throttle.
1) Approx. 1/4 throttle: http://tinypic.com/r/fjqmhc/9
2) Approx. 1/2 throttle: http://tinypic.com/r/s124qr/9
3) 1/1 throttle: http://tinypic.com/r/2ur6otf/9 - (That drop in RPM near the end of the video occurs because I lifted my foot off of the gas pedal to prevent it going over redline. It wasn't another shift. I've taken it past before, and it makes no difference).
-Find a cruising speed in 3rd gear where the RPMs are around 2,000 rpm. Give 1/2 throttle. Do it again but this time give full throttle.
-Find a cruising speed in 1st gear where the RPM are around 3,500rpm, with the shifter manually placed in 1st (use 2nd if you have to). Left off the gas to decel, then give 1/2 throttle. Do the same thing but give full throttle.
1) Approx. 1/2 throttle and 1/1 throttle (played with both as best I could -- it's in second, since I can't prevent it from shifting away from first into second): http://tinypic.com/r/33e3dzr/9
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Thanks for all your help, guys. If we can't pinpoint the problem, I'll try to find somewhere with actual knowledge of American cars to replace things you've mentioned piece by piece. Worst-case scenario, I'll have to replace the entire transmission, I guess.
In the video you didn't push the shift **** button to go into low. Have you?
.......I examined the modulator at the transmission (in place, as it's supposed to be, on the passenger's side at the transmission's rear), and as far as I could inspect it, it was in good shape, though hardly clean. The hose connecting it to the engine was entirely rubber -- and as I understand it, they were originally metallic pipes.
However, it was also secure on both ends, and the material was in thoroughly good condition. The previous owner improvised with the fastenings somewhat, though -- the hose is attached to the modulator by one of those clamps you screw tighter.
I'm by no means an expert on automatic transmissions, but what you said here, got me thinking. Yes, the vacuum line to the modulator was a steel line, when the car was new. There was also a SHORT piece of rubber hose, that connected the metal part to the vacuum source at the engine, and the vacuum modulator on the transmission.
My initial thought was that a length of rubber that long, may not be robust enough, and is collapsing under vacuum. As I was typing that, I also began thinking about where the pipe connects to the engine. Is it a "ported" source, or a "constant" source of vacuum, and could THIS be part of the problem, if the line is connected incorrectly?
Your tach is shot ,, it is a mess. You idle at 0 .. But the trans is fine it is doing what it should. And you have the correct selector 3 2 1 .. Thats drive = 3 , then secoond = 2 and first = 1
I'm by no means an expert on automatic transmissions, but what you said here, got me thinking. Yes, the vacuum line to the modulator was a steel line, when the car was new. There was also a SHORT piece of rubber hose, that connected the metal part to the vacuum source at the engine, and the vacuum modulator on the transmission.
My initial thought was that a length of rubber that long, may not be robust enough, and is collapsing under vacuum. As I was typing that, I also began thinking about where the pipe connects to the engine. Is it a "ported" source, or a "constant" source of vacuum, and could THIS be part of the problem, if the line is connected incorrectly?
Just a thought.....
Is there a picture of some sort showing where it's meant to connect so I can match it? I'll go take a photograph of where mine leads.
You could test the modulator by disconnecting it and plugging the line from the engine. If the shifts happen the same as it connected then it's not working.
The modulator should connect to a port in the intake manifold behind the carburetor. If not there, then a port on the carburetor should also work as long as it has vacuum at idle.
The rpm vs speed that you are describing does sound like Dad's car when we first fired it up on jack stands but before we connected the modulator. It did shift to 2nd gear but then it would rev really high to get to the 50-60mph range but not shift again into 3rd.
pull the vac line off the modulator. see if it has any fluid in the line it should be dry if not it is bad and will not shift to third. also as stated check level of fluid idling hot in park or neutral on level ground.
your tach. does appear to be reading incorrect rpms.
Again like stated change the vac line to steel the rubber might be collapsing not allowing to shift correctly.
also take the console plate off and try the shifter without it. I bet the plastic slider around the shifter is binding up not allowing it to shift all the way down into L1 position. I had that problem with mine.
Your tach is shot ,, it is a mess. You idle at 0 .. But the trans is fine it is doing what it should. And you have the correct selector 3 2 1 .. Thats drive = 3 , then secoond = 2 and first = 1
pull the vac line off the modulator. see if it has any fluid in the line it should be dry if not it is bad and will not shift to third. also as stated check level of fluid idling hot in park or neutral on level ground.
your tach. does appear to be reading incorrect rpms.
Again like stated change the vac line to steel the rubber might be collapsing not allowing to shift correctly.
also take the console plate off and try the shifter without it. I bet the plastic slider around the shifter is binding up not allowing it to shift all the way down into L1 position. I had that problem with mine.
Good luck
If I can source the necessary pipe, how difficult is it to install? What requires removal to get it into place? Does that clip in the drawing above mean it comes in two pieces, allowing for easy insertion of both parts into their respective positions?
From what I hear in the video it is shifting into third gear just fine. RPMs sound normal, at least until wind and road noise drown it out.
Check for a tach filter bolted to the block near the distributor.
If I can source the necessary pipe, how difficult is it to install? What requires removal to get it into place? Does that clip in the drawing above mean it comes in two pieces, allowing for easy insertion of both parts into their respective positions?
It's been a while since I've had a car with a non computer controlled, automatic transmission, but in my past experience, the steel line is one piece, bent to snake between the intake manifold and the transmission, with a short, maybe 1-1.5" long piece of rubber tubing at each end. I also seem to recall that the steel tubing was a "slip fit" into the fittings at each end, and the rubber tube acted as a "seal".
09-08-2016, 09:40 AM
Quadrant slipped???
