Does anyone know how to calibrate the speedo to give correct speed? Right now Speedo reads 85 when going 70 on freeway, speedometer only goes to 85.
Do I need to change out the VSS to one from a pontiac 6000 with a 4 speed, or go with the one for a th125C not sure what gear ratio is in transmission, but at 70 on freeway running about 1800-2000 RPM's
To figure out what gearing the 4T60 has you will need the information located on the transmission tag. The gears in the VSS can be interchanged for proper gearing.
I did take some time to study the circuit board for the speedometer, and did not get too far. The speedo takes the pulses, converts them to the sin/cos type needle movement (same type as the tach), and also sends the speed back out to the ECM, if I understand it right. But the two IC's on the board were custom markings, and I could not find any reference to them. So I could not make up a schematic of the board, and thus could not find any key points where adjustments could be made. (was looking for ways to add a pot like is done on the tach). I also looked on line to see if anyone else had any info on the circuit, to no avail. So unless someone can find some more data on the circuit, I think that it will remain an unadjustable item. I wonder how the speedo calibration shop did it?
tire size matters a lot. run online tire calc to find out how much. (Need tire size for donor car and current tire.)
Final gear and VSS does not matter. GEARS for VSS in old cars does matter and wrong set will often screw the speedo and ECM/PCM. Change tire size need to change gear(s) to fix this.
Optionally you can try aftermarket devices that make the problem electronic by charging VSS pulse per mile.
(Is why most new cars have tire size in the programming for BCM or PCM)
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The Fiero speedometer sender from the TH125H can interchange with the 4T60. If your speedometer is showing a faster speed you need a drive gear with more teeth . Try the assembly from your old transmission and hope that its the blue 30 tooth Fiero speed sensor gear. In the worst case both the gear on the governor assembly and the gear in the VSS will need to be changed to provide the proper VSS output. .
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Dennis hit the nail on the head it is the gears in the governor , changing the tire size or finding out the tire size of the donor car when got the transmission from a salvage dealer that the car was not even there is hard to do and have sold the old th125c. I made notes as to what worked and did not work for the 4T60 swap, some information from here and from other places on the interweb worked and some didn't. the gearing in the transmission governor is from a th440/4T60 1989 pontiac 6000, going to count gears and get some with 10% up and down and try different combinations to see what is the most accurate and go from there.
Google Dakota Digital and under automotive find the Universal Speedometer Signal Interface SGI-5E. I just swapped in the later model 6 speed manual transaxle which is entirely different in pulses per mile. The Fiero VSS feeds the speedometer and the speedometer feeds the needed info to the ECM and the cruise control. This means the two wires that need to be intercepted were found at the base of the steering column and the interface was installed there. They have good instructions and their tech support people gladly answered my 2 questions. I made the install and now my speedo and my GPS are with 1 mph at all speeds. Can easily be re-programmed if another swap is made in the future.
I think the Speedometer is going to read by how many RPMs the axle is turning.
Correct.
quote
Originally posted by Cajun:
The gears in the VSS can be interchanged for proper gearing.
Correct.
quote
Originally posted by Dennis LaGrua:
The Fiero speedometer sender from the TH125H can interchange with the 4T60. If your speedometer is showing a faster speed you need a drive gear with more teeth . Try the assembly from your old transmission and hope that its the blue 30 tooth Fiero speed sensor gear. In the worst case both the gear on the governor assembly and the gear in the VSS will need to be changed to provide the proper VSS output. .
I changed my Indy from 14" to 15" wheels and from TH125 to 4T60 automatic. I used the VSS from the TH125 but had to change the gear from white to blue.
MD9 TH125C Automatics will have the large GREEN 10 tooth drive gear. The smaller VSS speedo gear will be: BLUE 30 tooth - tire sizes P185/80R13, P185/75R14, P195/70R14, P215/60R15 WHITE 31 tooth - tire size P215/60R14
Later I changed to a Getrag 5-speed from a GT and used the VSS that came on that trans.
