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Can you hear & feel the tranny shift into 3rd?
4.11 or lower gears?
Torque convertor slipping?
Tach circuit board faulty?
Clutch packs slipping?
Bands not packs, sorry.
Can you hear & feel the tranny shift into 3rd?
4.11 or lower gears?
Torque convertor slipping?
Tach circuit board faulty?
Clutch packs slipping?
At first thank you very much for your help, really appreciated.
Here my reply:
Can you hear & feel the tranny shift into 3rd?
---- Yes
4.11 or lower gears?
---- I'm not an expert. How i can discover gear ratio?
Torque convertor slipping?
---- Seems not, but not sure
Tach circuit board faulty?
---- How can i prove it? in next days i will test speed with other cars running togheter.
Clutch packs slipping?
---- I don't know how to check or not expert to ear that.
The engine goes well, but my concerns are about low maximum speed; i did a full check of all liquids (trasmission included), and the garage (specialized in american cars) tried the cars and told me that all is fine; but they didn't go full gas to verify my doubt.
Do you have access to a timing light (or other meter) with a tachometer and compare with your internal tachometer?
if you can get car in air, how many times does the drive shaft turn compared to one revolution of wheel, that is your drive axle ratio to cut firm.
google c3 corvette
- how to measure axle ratio
- tachometer accuracy.
make sure you hear 3 shifts…
what does tachometer read ignition on but not started? Zero? Below or above zero?
how about some pics of ride, differential, transmission, engine?
4.11 gears will be 3000 rpm at 60 mph, which is a ballpark figure. Yes it depends on tire size, etc. but it's close enough to figure out your issue. Even 4.88 gears wont get you to 4500 at 60. That would need 6.xx gears. How likely is it you have those? Probably zero. You can do arithmetic in your head to tell you 4500 rpm at 60 means the trans isn't in high gear, your tach is whack, or you have a seriously bad torque converter (and you almost certainly don't).
In 1979 an L82 automatic received a 3.55 rear. If no one has been in there to change it then at 85mph (if that is correct) 27inch tires, your tach should read 3755 rpms, not 6000rpm.
Try bypassing the tach filter located under the dist cap . Unplug both ends and insert your tach wire directly into the distributor cap, drive the car and see if the rpms are more resonable. At 60mph it should read @2700 rpm. .
The engine does not sound like it is going that fast, maybe half.
So most likely the elec tach board or the filter is bad. They are both very common failures.
Just check engine rpms with another tach while stopped.
Check it at idle and revving some, to say 2500.
Here is an online calculator to tell you what your rpm should be: https://purperformance.com/p-29669-rpm-calculator.html
Stock Tires are around 27 inches tall usually
Gear are usually 3.08, 3.23, 3.55, or 3.70. Outside of that they are not very common.
Here are some calculated rpms at 70 mph.
3.08 rear = 2726 rpm
3.23 = 2860
3.55 = 3142
3.70 = 3275
As you can see, your indicated rpms are not very realistic, so the tach is probably not correct.
Your tach shows 5300 at 70, but it sounds closer to 2500-3000.
The engine does not sound like it is going that fast, maybe half.
So most likely the elec tach board or the filter is bad. They are both very common failures.
Just check engine rpms with another tach while stopped.
Check it at idle and revving some, to say 2500.
Here is an online calculator to tell you what your rpm should be: https://purperformance.com/p-29669-rpm-calculator.html
Stock Tires are around 27 inches tall usually
Gear are usually 3.08, 3.23, 3.55, or 3.70. Outside of that they are not very common.
Here are some calculated rpms at 70 mph.
3.08 rear = 2726 rpm
3.23 = 2860
3.55 = 3142
3.70 = 3275
As you can see, your indicated rpms are not very realistic, so the tach is probably not correct.
Your tach shows 5300 at 70, but it sounds closer to 2500-3000.
Sorry for delay, but what does it mean "the op is in italy?".
In the while i have measured tires:
tires are 66cm = 25.98 inches.
My tach measures 4500 rpm at 60mph, so from the calculator exit a read gear ration of 5.8. Is it possible?
The electric tach being off is a common failure point.
There are replacement circuit boards available. It's small.
You can check you true rear gearing by jacking up the entire back of the car, putting a chalk mark on the driveshaft, and counting how many times the driveshaft turns for 1 revolution of the rear tire. You have a posi so both rear tires turn together.
diff cover & spring mounts.
I also found this code list for a 79 Code: OM 3:36 positraction OH 3:55 positraction OJ 3:70 positraction
The rear codes are on a 1/2" wide section of the rear diff, on the main housing, directly on the bottom, directly in front of where the diff cover and spring mount.
(Usually too greasy to see without cleaning)
One of those 3 should be the standard gear for a 79 L82 auto. Most likely the 3:36
The electric tach being off is a common failure point.
There are replacement circuit boards available. It's small.
You can check you true rear gearing by jacking up the entire back of the car, putting a chalk mark on the driveshaft, and counting how many times the driveshaft turns for 1 revolution of the rear tire. You have a posi so both rear tires turn together.
diff cover & spring mounts.
I also found this code list for a 79 Code: OM 3:36 positraction OH 3:55 positraction OJ 3:70 positraction
The rear codes are on a 1/2" wide section of the rear diff, on the main housing, directly on the bottom, directly in front of where the diff cover and spring mount.
(Usually too greasy to see without cleaning)
One of those 3 should be the standard gear for a 79 L82 auto. Most likely the 3:36
thank you very much.
Today i went to workshop/mechanic and we try car together; seems that all is going fine in terms of shifting and engine sound; seems that tach show about 1500 rpm more that expected.
We will plug and external tach to double check next week. In case which part have i to buy to fix the tach?
Iacopo
Checking with an external tach is a good idea but first do what post #11 said to do-- bypass the tach filter since it could be just the tach filter..easy enough to check. If it is the tach filter it's easy enough to make one if you can do soldering and use heat-shrink.
Before the OP orders a $60- part that may cost much more landed in Italy, is there anything else it could be? Tach filter cap in the engine compartment (link below)?
01-13-2023, 02:55 PM
Understanding Speed and RPM in corvette L82 1979 3 speed automatic
