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I was wondering why my 92 TA always feels so slow and why other driver will overtake me even when I am over the speed limit. The reason is, my speedo is way off!, it reads too high. I recognized this issue last year that something is wrong with my car but yesterday I realized that the speedo is wrong. I've tested with GPS how fast I am. The speedo reads correct until about 40 mph, over 40 mph it reads higher and higher. At 60 mph I am driving 50 mph, at 80 mph I am driving 60... going 100 mph I am driving 75... someone else had this issue? Car is a 5.0 TPI with the 140 mph speedo. Rear end is 3.23 and the driven gear on the speed sensor seems to be correct for the rear end. Any ideas?
Usually a drive/driven gear that isn't right will read a constant error off from the real speed. Might be time to look at the components on the driver board in the speedometer.
If your driving in third gear and reach 70 MPH before you manually shift into 4th,...... what does your Tachometer read ?? ( should say 3,230 RPM if Tach is accurate and gear is 3.23. )
What color / tooth count in the DRIVEN gear ?? ( should be Dark Grey and 44 teeth with 245/50/16's )
Not like you've got a speedometer cable that might be binding or gears in the speedo head that could be worn. If all things are 'as they should be',...........I'd suspect the electric transmission VSS before all other parts. It's a "Permanent Magnet Signal Generator" that should produce 4,000 Pulses Per Mile; Maybe not producing the proper signal at a rotation rate above "X" revolutions per mile ?? They arn't know to fail and cause the problem you describe,....... but that doesn't mean anything.
If the VSS checks out and is OK (& If all things are 'as they should be' ) then I'd think VSS Buffer might be suspect, and then last-but-not least,..... the speedo head itself.
There is no speedo buffer on speed density TPI cars. The ECM does the math.
Usually when the VSS itself fails it'll cause the speedometer to read wonky. LIke it'll drop like a rock to 0 and then shoot back up to the speed you're going. Or it'll puke altogether. Fortunately when it craps the bed it'll usually set a code. Wiring problems would do something similar.
If your driving in third gear and reach 70 MPH before you manually shift into 4th,...... what does your Tachometer read ?? ( should say 3,230 RPM if Tach is accurate and gear is 3.23. )
What color / tooth count in the DRIVEN gear ?? ( should be Dark Grey and 44 teeth with 245/50/16's )
Not like you've got a speedometer cable that might be binding or gears in the speedo head that could be worn. If all things are 'as they should be',...........I'd suspect the electric transmission VSS before all other parts. It's a "Permanent Magnet Signal Generator" that should produce 4,000 Pulses Per Mile; Maybe not producing the proper signal at a rotation rate above "X" revolutions per mile ?? They arn't know to fail and cause the problem you describe,....... but that doesn't mean anything.
If the VSS checks out and is OK (& If all things are 'as they should be' ) then I'd think VSS Buffer might be suspect, and then last-but-not least,..... the speedo head itself.
Sorry, forgot to mention. The car has original crosslace rims with 245/50R16. I have to check the tach in 3rd, but I am 100% sure it has a 3.23 read end. I think I have to check the driven gear, I definitely know it has the red 17 teeth drive gear. But I am absolutely unsure what driven gear is installed. Guess you pointed me in the right direction. I will take out the VSS an count the teeth...
Originally Posted by Drew
There is no speedo buffer on speed density TPI cars. The ECM does the math.
Usually when the VSS itself fails it'll cause the speedometer to read wonky. LIke it'll drop like a rock to 0 and then shoot back up to the speed you're going. Or it'll puke altogether. Fortunately when it craps the bed it'll usually set a code. Wiring problems would do something similar.
Absolutely steady, not wonky, no trouble code... I guess I might have the wrong driven gear. Will check that...
From the mph numbers you posted, you are reading 25% off.
100 mph reading is actually 75 - 25% off.
80 is 60 - 25% off.
60 you said is 50, but 25% off comes out to 45.
Sorry, forgot to mention. The car has original crosslace rims with 245/50R16. I have to check the tach in 3rd, but I am 100% sure it has a 3.23 read end. I think I have to check the driven gear, I definitely know it has the red 17 teeth drive gear. But I am absolutely unsure what driven gear is installed. Guess you pointed me in the right direction. I will take out the VSS an count the teeth...
Absolutely steady, not wonky, no trouble code... I guess I might have the wrong driven gear. Will check that...
FWIW, I went from 2.73 to 3.23 (stock 245/50-16 tires) with a red 17-tooth drive gear and required a 45-tooth driven gear to get my speedometer accurate again. Compared to GPS showing 70 mph, my speedometer now reads ~71.5 mph, so about 2% off. At low speeds the error is negligible. Also required a new VSS that would accept the higher-tooth driven gear.
FWIW, I went from 2.73 to 3.23 (stock 245/50-16 tires) with a red 17-tooth drive gear and required a 45-tooth driven gear to get my speedometer accurate again. Compared to GPS showing 70 mph, my speedometer now reads ~71.5 mph, so about 2% off. At low speeds the error is negligible. Also required a new VSS that would accept the higher-tooth driven gear.
What do you mean by "VSS that accept the higher-tooth driven gear"? Are they different?!
Yes. 35 to 39 tooth driven gear require a different VSS than 40-45 tooth driven gear. The shaft is positioned differently to account for the difference in the diameter of the gear. I went from a 17/38 to a 17/45, so I had to change to a different VSS. If you can maintain the current "range" of driven gear by replacing the drive gear, then you wouldn't have to change the VSS. You could, potentially, swap out the drive gear to a 15 tooth and then you would need a 38 tooth driven, so your current VSS would work. Just a lot more work to swap out a drive gear. https://www.thirdgen.org/forums/tran...eedometer.html
Guys, you rock. I am embarassed to admit, I had the wrong driven gear in my transmission. The transmission is from a 91 model, and obviously I didn't check the gears... Oh my, this is so embarassing... I own thirdgens since 2005 but that never happended to me. I disassembled the VSS today and found a 17 teeth drive gear... and a brown 38 teeth driven gear. Installed on a black VSS. So I digged into my spare parts and found a white VSS and a grey 44 teeth driven gear and also a blue 39 teeth gear, don't remember where I got it from... And now I also understand the differences in the VSS between black and white. The 44 teeth gear is a lot bigger than the 38 teeth gear.
Thank you all for your help guys! The easiest solutions are often the best solutions. I couldn't do a test drive after installing the white VSS, was running out of time, I don't know if it will work, must have been in my storage for years... But I will find out by tomorrow.
Good deal! For some reason the 44-tooth driven gear didn't quite make it for me. Not sure why, but my speedometer was still reading about 4% high, so I swapped in a 45-tooth (which is the largest they make) and it pretty much nailed it. Maybe my 245/50-16 General tires are a little bigger (or smaller?). I even tried a different speedometer-drive PC board, but it didn't help.
04-03-2020, 05:30 PM
