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Calibrating Speedometer
Ability to adjust settings without using a programmer
Ability to adjust settings without using a programmer
This Guide should work on trucks from 1992-1999 (maybe 2000).
This mod is normally used for those people who run over sized tires or different rear end gears and want their speedometer to be accurate. I'm not sure if this will change what the PCM sees for the transmission but it will help correct your speedometer. You can also spend $150+ on a programmer or a preset VSSB which either way is a good way to go if you have the money. But, you can also calibrate your own VSSB (Vehicle Speed Sensor Buffer) for a few dollars.
*Caution* - I do not take any responsibility if you mess up your VSSB in any way.
Skills Needed:
- Ability to use a tape measure
- Ability to remove soldered devices
- Ability to solder devices
- Ability to put numbers in an equation and solve it
- Small Pliers
- Soldering Iron
- Solder
- A paperclip or a straight pin
- Tape Measure
- Calculator
- 6 position DIP Switch (2 switches for 14 position jumper board)
- Can be found at an online switch vendor
- I found mine on Ebay for a decent price
Vehicles Gear Ratio
- If you are sure that the truck's gear ratio is stock, look at the RPO's in your glove box.
- GU6 - 3.42:1
- GT4 - 3.73:1
- GT5 - 4.10:1
- If you are really unsure of the gear ratio you can pope off the differential cover and normally you can see markings on the ring gear that tell you what ratio it is.
- There are a few ways you can do this... but the best way is the first way that will be explained.
- Measuring one revolution of the tire
- Mark one tire on the bottom and Mark the ground at the same spot.
- Roll the tire one revolution.
- Mark the spot on the ground where the tire ended up at and measure the distance between the two markings on the ground.
- This is what you will use as your tire's rolling circumference.
- Calculating one revolution of the tire
- Measure the height of the tire and multiply that number by PI (3.14......).
- This will give you an estimated tire rolling circumference
- To show the difference in the two methods. My actual tire size was 99", but the calculated tire size was 103". So I prefer to actually measure the tire for an accurate jumper position.
(63360) x (Gear Ratio) x (40)
(tire rolling circumference) x (128000)
(tire rolling circumference) x (128000)
- Equation Explanation
- 63360 = inches per mile
- Gear ratio = trucks gear ratio
- 40 = Pulses per revolution or speed sensor, most GM speed sensors produce 40 pulses per revolution
- Tire rolling circumference = measurement you did in Inches!
- 128000 = pulses per mile
- Once you have done the calculations look below for the "Jumper Settings". Find the closest value to your answer.
- Read across the line of your value for the correct jumper settings.
- 1 means there should be a jumper across the terminals (closed)
- 0 means there should not be a jumper across the terminals (open)
- Jumpers are not numbered on the board itself, but look at the picture attached for an example on how to read the VSSB board.
- For 1994 and Earlier, The VSSB/DRAC is located behind the glovebox. You must unscrew/unbolt the VSSB/DRAC unit to remove.
- For 1995 and Later, See Below:
- Remove Your Glovebox.
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- This Device is normally found in a white box right under the PCM.
-
- You can try removing the VSSB by just unclipping it from the PCM tray.
- Or, you can try what I did...
- Remove the PCM, unclip all the connectors and undo the clamp(s) holding the PCM in place.
- Unbolt the 4 bolts holding down the PCM tray, see pictures for exact locations.
-
- Unclip the VSSB connector and remove the tray. The VSSB will be attached to the bottom of it.
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- Unclip the White VSSB box and take it into the house.
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- Open the white box and remove the VSSB unit.
- WARNING - This device is very sensitive to electric shock (as with most computer electronics). Do not let this board get in contact with ESD (Electro-static Discharge), grease, oil, water, or any other liquids.
- There are a few ways to remove the jumpers, but here is what I found worked best.
- *NOTE* Do not touch both #1 positions on the VSSB jumpers Jumper locations. These do not change and if you mess with them a 6 position DIP switch will not work for this project anymore.
- Cut each jumper that you are going to remove in the middle.
- Use your soldering iron to head up the connection and you should be able to pull the jumper out with the small pliers.
- Do that for all the jumpers you are removing.
- Next you must clean out each hole, you can do this by using a small straight pin (used for sewing) or using a small paper clip. I used both methods. Take care when you do this as you don't want to ruin the board or other devices.
- All the holes must be open enough to fit the new DIP switches and most of the left over solder should be removed so you don't short out the board and cause incorrect readings.
- You want to carefully place the DIP switches in the 2-7 positions on each side of the jumper set (Both #1's are not to be touched) see pictures for more detail.
- Flip the board over *with switches installed* and make sure that all the switch leads are through the holes.
- Carefully solder each connection making sure not to put to much solder on.
- After that is done, you can set your switches to the correct settings.
- Using the input ratio you got from the equation above, go to the Jumper Input Chart below and check for the input ratio that is closest to your answer.
- The chart should give you an answer like 1-0-1-1-0-1-0, that is the right bank (or the entire code for the 7 jumper board). The left bank will be a mirrored image that gives the opposite answer of the other side (see picture).
- The straight across answer will be 1-0-1-0-0-1-0-1-0-1-1-0-1-0
- Remember, the switches are installed so that both jumper #1 are untouched. So your switches will read:
- 1-0-1-0-0-1-*-*-0-1-1-0-1-0
- Example:
- Install your VSSB (in the white case) back into your truck. Installation is opposite of the removal procedure you used.
- Plug in the VSSB and take your truck for a test drive.
- To help check the accuracy of your speedometer, use a GPS or pace another vehicle.
- If your speedometer is still not accurate, you will have to remeasure your tire and redo your calculations. Then pull your VSSB back out and adjust the DIP switches to the next jumper setting
Notes:
- There are TWO different types of DRAC/VSSB Units in these trucks. You have a 7 position and a 14 position. The Steps above are for the 14 position switch.
- The 7 position mod is very similar. The "codes" for the 7 position board is the same as the 14 position, but you just don't have to have the mirrored opposite on the left side of the board.
- I believe only the earlier models (up to '92ish) have the 7 position boards. But not 100% sure.
- I take No responsibility on what you do to your VSSB of anything else done to your truck.
- This can be a difficult mod for those who have not soldered. I have not soldered on a board, and I ran into one problem but I found the problem and fixed it.
- If you feel uncomfortable doing this you can most likely have an electronic repair shop solder the connections for you.
- You can also probably take your VSSB to some automotive shops and have them reconfigure your VSSB for you. Not sure on the price of this though.
- Here is my experience (speedo wise). Before my speedometer was about 5mph slow. After this mod my speedometer is about .5 to 1 mph to fast. I believe that is pretty good considering the accuracy from factory. This will probably help me get a little more accurate fuel mileage calculations too
.
- I will try an help all those who have questions, but I do not know "a lot" about these modules.
- I got this information from a few different sites and added some more from my experience. Including this site.
For the next couple days I may rephrase some steps or correct any information that needs changed...
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