Help with inaccurate speedometer
Help with inaccurate speedometer I have a 1999 T/A with an inaccurate speedometer and I can not seem to figure out why. The speedometer is around 7-9 mph lower than actual speed. Actual speed is being measured by GARMIN GPS.
I do have GU5 on the door and verified by measuring the rear wheel rotation to the driveshaft rotation that I do have a 3.24 gear ratio. I have a basic HP tune (thanks to Mmjr) and he did verify that the speedometer is showing the right speed as determined by the PCM. In other words, the HP tuning computer is showing the same reported speed that the speedometer is showing. This speedometer error existed on both the stock wheels and the new C6 wheels that I installed. At the same time the C6 wheels went on the PCM was also tuned to reflect the larger wheel diameter. I have replaced the only seemingly culprit - vehicle speed sensor and that has not solved the problem.
Any ideas?
I do have GU5 on the door and verified by measuring the rear wheel rotation to the driveshaft rotation that I do have a 3.24 gear ratio. I have a basic HP tune (thanks to Mmjr) and he did verify that the speedometer is showing the right speed as determined by the PCM. In other words, the HP tuning computer is showing the same reported speed that the speedometer is showing. This speedometer error existed on both the stock wheels and the new C6 wheels that I installed. At the same time the C6 wheels went on the PCM was also tuned to reflect the larger wheel diameter. I have replaced the only seemingly culprit - vehicle speed sensor and that has not solved the problem.
Any ideas?
EFI Live has a speedo tuning tool, specifically for calibrating in the dash when the PCM and dash are showing different speeds.
he said that they are the same...
What I usually do for this is, set the proper gear ratio, and play with tire height until the speedo matches the GPS. HPT will automatically rescale all of the auto tranny parameters if you want it to.
Teching In
Joined: Oct 2005
Posts: 29
Likes: 0
From: Naples, FL
If you have an auto trans leave the tire size and gear at what they really are and change the PPM output from the PCM to a number other than 4000PPM which is stock. I am using a cable-x and it was not driving the speedo at the right speed even though it was set to 4000PPM input. Too hard to get to it to play with the dip switches so I just hooked up EFI live and compared pcm readout to speedo read out and made corrections to the PPM output to compensate. I think I ended up using a number around 3600 PPM for my speedo. Way too easy!
Keith
Keith
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Verdad Gallardo Teching In
Joined: Oct 2005
Posts: 29
Likes: 0
From: Naples, FL
Right, if he thinks the Garmin reading is more accurate to true speed than what the PCM is giving him then change the PCM output to give the speed reading he wants!
"A man with a watch always knows what time it is. A man with two watches is never quite sure."
Keith
"A man with a watch always knows what time it is. A man with two watches is never quite sure."
Keith
Frost - that is correct. Speedometer reading and PCM reading is the same - per HP Tuner. GPS is showing much higher than indicated. Almost 10mph at highway speed. I have replaced the VSS and verified that I do have the stock gearing - GU5 (3.23).
My shifting is a bit firm...I would suspect a shift kit. I originally suspected that the PCM had a tune in it, which would have explained the hard shifts and inaccurate speedo. I assumed someone programmed the PCM for lower gears without actually making the gear change. That actually wouldn't have suprised me.
Mrr23 came to my neck of the woods and turned off the EGR/AIR. He didn't see an aftermarket tune onboard...so that dashed my original theory.
My shifting is a bit firm...I would suspect a shift kit. I originally suspected that the PCM had a tune in it, which would have explained the hard shifts and inaccurate speedo. I assumed someone programmed the PCM for lower gears without actually making the gear change. That actually wouldn't have suprised me.
Mrr23 came to my neck of the woods and turned off the EGR/AIR. He didn't see an aftermarket tune onboard...so that dashed my original theory.
corect. tune was original untouched. PCM and dash read the same. so, that leaves a signal input to the PCM as the cause. this is why he replaced the speedo snesor.
seems that did not solve it.
seems that did not solve it.
I guess there really leaves only two possibilities. Either I have a fault at the VSS - like the ring gear that the VSS reads off of is damaged or missing some teeth. Or I am retarded and didn't check my gear ratio right. I guess there is the slim possibility that I have a set of gears that is less than 3.23. I will life the rear end and check the driveshaft / rear wheel rotation tomorrow.
