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99-02 Speedometer Calibration?

Discussion in 'Interior' started by Matthew Jeschke, Jul 5, 2020.

  1. Matthew Jeschke

    Matthew Jeschke Full Access Member

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    I am surfing the web and can find info to calibrate the speedometer for bigger (or smaller) tires on newer trucks but not the 2002 and older. Apparently the newer ones use a shop scan tool / programmer and can set it. However, I've no clue on the 2002 or older.

    How do I calibrate my speedometer for my 2001 Tahoe?

    This is a a bit of a follow up on my thread where I'm swapping in a HD cluster with transmission temperature gauge. Linked below:

    https://www.tahoeyukonforum.com/threads/2001-transmission-temp-cluster-from-hd.119263/#post-1453647

    I bought a rebuilt gauge with trans temp but they didn't bother to program the speedometer for my truck :(
     
  2. Matthew Jeschke

    Matthew Jeschke Full Access Member

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    I busted out my old tuning software. Haven't used this stuff in about 18 years. I had to update the software but amazingly it still works. I'm not for sure why the cluster is affecting the speed readout. I'm assuming the parameters are set in the ECU which I never changed?

    upload_2020-7-8_22-46-59.png
     
  3. Matthew Jeschke

    Matthew Jeschke Full Access Member

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    This is beyond weird. This is a copy of the current program loaded to the PCM. Why would it have two conflicting calibrations? I've never tuned this truck. I am not sure about former owner but as far as I know I have stock tire size? 265/70/16

    From figures above these two should be equal

    2,537.9881 (trans revs/mile) * 40 (VSS pulses/rev) = 101,519.524 (VSS pulses/mile)

    Column 2 says 97,277 (VSS pulses/mile)

    I don't understand how that could be. Conflicting data came down from the computer?

    Based off my tire size 265/70/16 - 659 tire revs/mile

    Gears are 3.73 trans revs : 1 tire rev

    I'm going to assume the computer uses column 2: 97,277 (VSS pulses/mile)

    97,277 (VSS pulses/mile) * (1 rev transmission / 40 VSS pulses) * (1 tire rev / 3.73 rev transmission) = 651 tire revs / mile

    651 computer / 659 actual = 0.987

    My GPS Measured speed was 69 MPH
    My speedometer registered speed was 60 MPH

    That's 0.869

    Is there anyway to check the Vehicle Speed Sensor? It obviously is reading, but do they get dirty or something? It's as though it's missing 5 of the 40 counts per revolution. That or there's issue in the air core motor control inside the cluster.
     
  4. drakon543

    drakon543 Full Access Member

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    alot of vehicles if you gps check the speedometer reading you will generally be off a mile or 2 atleast. then factor in different size tires can be a big change when you wouldn't think there should be such a big difference. 245 -> 265 equats to aprox 5mph off in my tahoe and a similar change in my civic size was about 6mph
     
  5. Matthew Jeschke

    Matthew Jeschke Full Access Member

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    Weird, when I replicate math the PCM is doing I get precisely the right speed. However, after changing the cluster I now get the wrong speed. Calibration is stored in the computer, not in the cluster.

    Sent from my SM-A505U using Tapatalk
     
  6. drakon543

    drakon543 Full Access Member

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    well your looking at perfect world exact math vs whatever the vss is sending vs whats actually making it to the cluster too. which is where the typical couple mph off as a typical comes into play also. you could technically have a slight issue with the vss, the wires(interference also), a slight bad connection at the computer throwing the reading off, or the cluster rebuild was done poorly. im unsure of any vss test that would work correctly. follow the wires for the vss make sure its not damaged or laying on anything that could add any interference. check the plug where it connects to the computer maybe some contact cleaner and a follow up application of dialectic grease to the plug. continue from there and check the wires to the cluster. open the cluster up and take a look and see if anything looks wrong. maybe something got installed a little off and the cluster won't know that so it could be throwing what its actually showing. if you have the original cluster put that back in and see if the problem is corrected. besides the extra temp gauge is everything else exactly the same or did you get a cluster with a different style speedo? i dont know anything about tuning when ive ever done speedo correction it was on a handheld with the easy mode correction. just trying to give you some options of where to start looking without just throwing parts at it. also your op asked about different tire sizes vs the speedo. regardless to the math your getting if you changed the tire size that needs to be corrected in the tune im not sure where.
     
  7. Matthew Jeschke

    Matthew Jeschke Full Access Member

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    Thanks :) Despite burning hundreds of dollars in what I thought would be an affordable temp gauge, is a fun opportunity to refresh with tuning haha I pulled out my old dusty HP Tuner module. Plugged it in, it now has a KILLER awesome scan / logging tool.

    - My next step is to read exactly what speed the computer thinks I'm going and compare that to GPS.

    I can read directly what the computer thinks the speed is (essentially it's VSS calculation's result for speed). I'll compare that to the cluster. If it doesn't match then it's unfortunately the cluster.

    - Speedometer rebuild guy says I can send it back for a replacement however :(

    I'm a bit leery about sending back to the shop that rebuilt it, although they said they'd fix or exchange for no charge. Reason is, I had to re-rebuild most of the cluster when I got it. The cluster they sent me was all cracked up. Granted cracks would have been hidden in trim, but doesn't really meet my quality standards. Especially when I paid $200 for a programmed cluster you can buy all day long for $100 un-programmed. Additionally, two air core motors were bad :(

    I luckily had the $80 junkyard cluster I destroyed trying to transfer memory chips / odometer and parts from it got the $200 one working.

    This morning I swamped the air core motor on the speedometer and got the speedometer's accuracy to go from 85% to 90% of GPS monitored. I have one more air core motor not sure if that will improve it anymore. I additionally, have anther gauge cluster motherboard, only I destroyed the daughter board trying to swap chips. I think the board still works only the odometer is toast on it as the chip I broke on daughter board controls the odometer. I may try that and see what speed it comes in at as well.
     
  8. Matthew Jeschke

    Matthew Jeschke Full Access Member

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    I tested output from the ECU computer. It was spot on with the GPS.

    After I tested the replacement air core motor a bit more it was also 85% of actual speed same as previous air core motor it had.

    Sheet. I think this cluster is defective :(
     
  9. Matthew Jeschke

    Matthew Jeschke Full Access Member

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    If this doesn't get weirder. I have a second cluster that only the gauges operate on. I put it in the truck and test drove it. Same result. It's also 85% of actual speed.

    I wonder if there's a difference between the factory Tahoe w/o temp gauge and the 2500 HD with Temp gauge? Something that would case the speedometers to register differently.

    Wiring diagram:

    [​IMG]
     
    Last edited: Jul 10, 2020
  10. Matthew Jeschke

    Matthew Jeschke Full Access Member

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    Did some more diagnosing:

    - PCM to the cluster connection 0.1 ohms
    - Certain VSS to PCM is good as that reports accurate speed.
    - Oscilloscope to measure waveform on VSS of cluster compared to GPS measured velocity. 56hrz 5V square wave at 50MPH and 28hrz at 25 MPH
    - Tested another cluster with same results

    Anybody know what the ideal signal from PCM to cluster / VSS pin looks like? I want to validate if my PCM is generating the right signal.
     
    Last edited: Jul 10, 2020

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