I replaced the Vehicle SpeedSensor that’s on the tail shaft of gear box nothing changed. I would like to know if there is another sensor that affects the speedometer possibly inside the transmission ( 4L60E ) ? Also thought about the wheel speed sensor does it affect the speedometer ? The truck hasn’t thrown any check engine codes. Now this I’m not sure about but transmission may be getting hot because of this as my starter has started to drag when cranking truck when engine is hot , may just be time for a starter , I haven’t driven it enough to be sure about the transmission temp. One more thing if this leads me to a wheel speed sensor how do I tell which one it is? I have the original size tires and wheel on truck.
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Welcome to Motor Vehicle Maintenance & Repair!– Pᴀᴜʟsᴛᴇʀ2 ♦Jul 16 at 12:16
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Was the reason for changing out the VSS because it failed or because you were having this 10mph difference? You also stated your Tahoe has stock tire sizes. I'm reading that 97 Tahoes should have 245/75R16 sized tires. Is that what you have?– Pᴀᴜʟsᴛᴇʀ2 ♦Jul 16 at 13:59
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The reason I changed the VSS was because of the increased speedometer reading and I purchased a genuine GM part.– CouchRiderJul 17 at 13:42
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245/75R16 is the tire size I have on the truck.– CouchRiderJul 17 at 13:44
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TY. I was just checking to make sure of the reasoning and size.– Pᴀᴜʟsᴛᴇʀ2 ♦Jul 17 at 13:45
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1 Answer
Your vehicle has a module called a VSSB (vehicle speed sensor buffer) or a DRAC (digital ratio adapter controller). This is a small circuit board inside a non-waterproof snap-shut case that's usually in the vicinity of the PCM. Possibly behind the glove box.
The module has permanently soldered jumpers that establish a ratio between the number of pulses detected by the vehicle speed sensor and the pulses sent to the PCM. The pulse ratio that's used depends on the gear ratio in the differential(s) and the circumference of the tires that were installed at the factory. The module's pulse ratio can't be easily changed; it would require cutting circuit board traces and/or soldering jumpers.
However, aftermarket modules (replacement VSSBs or DRACs) with DIP switches are available to easily calibrate the speedometer for different tire sizes or differential gear ratios. Useful when changing from highway tires to mud tires, for example.
As far as I know, wheel speed sensors have no effect on speedometer readings, as their pulses feed the ABS module.
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Correct, the wheel speed sensors have nothing to do with speedometer reading on the GM trucks of this era.– Pᴀᴜʟsᴛᴇʀ2 ♦Jul 16 at 13:57
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Where would I purchase one of the aftermarket modules used to replace the VSSB / DRAC ? Are the modules fairly straightforward as far as installation? Jul 17 at 13:47
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I’m wondering what causes the reading to be off in the first place , could the torque converter clutch possibly be malfunctioning slightly and that cause the increased speedometer reading.. Jul 17 at 14:03
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The "Vehicle SpeedSensor that’s on the tail shaft of gear box" comes after the torque converter so slippage will have no effect on indicated road speed, but slippage would affect rpm vs road speed. The most common reason for discrepancy between speedometer speed and GPS speed is tire circumference. Two different models of tire of the same size can have enough difference in circumference to affect speedometer readings.– MTAJul 17 at 14:57