[Green Machine]

Calibrating Speedometer
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

Items Needed:

• 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.

Determining Tire Size

• 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.

Calculating your Input Ratio

(63360) x (Gear Ratio) x (40)
(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.

Finding & Removing your VSSB (Vehicle Speed Sensor Buffer)

• 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.
• 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.
  • Unclip the White VSSB box and take it into the house.
• 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.

Removing Jumpers

• 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.

Installing DIP Switches

• 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:

Reinstalling your VSSB and Testing

• 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...

 

[94C1500]

Could just post this source

http://www.tbichips.com/drac/

 

[Green Machine]

Could just post this source

http://www.tbichips.com/drac/

That was the main source (pretty much only the chart and equation), but I also used a couple other sites to confirm details and such.

 

[94C1500]

The 94 and earlier have the DRAC, and its not under the PCM, just behind the glove box, so things a little different in adjustment on the DRAC.

 

[Green Machine]

The 94 and earlier have the DRAC, and its not under the PCM, just behind the glove box, so things a little different in adjustment on the DRAC.

the vssb/drac positions i was going to add but forgot. I also believe that the units you are talking about are the 7 jumper board? It sets up basically the same way, but you don't have the mirrored image for the other 7 jumpers.

 

[94C1500]

Yep, that link explains the difference.

 

[jetech1]

Do the gas powered trucks have the "same" VSSB unit? If so we could get one or two from the salvage yard to try the surgery on:idea:
I'd like to do this on mine but just a little "skerd" to hack it up

 

[Rafedial1]

nice work Mike! very clear and straightforward writeup

 

[dustyvinyl]

that's some good advice, thanks

 

[Green Machine]

Do the gas powered trucks have the "same" VSSB unit? If so we could get one or two from the salvage yard to try the surgery on:idea:
I'd like to do this on mine but just a little "skerd" to hack it up

Gassers should use the same VSSB. May want to find one with the same gear ratio, although I'm not 100% sure that really matters. I know one way to find out lol, and thats try it and see what happens .

 

[SmithvilleD]

I like the idea of getting a spare from the local Pick-n-Pull also, unless they charge too much.

The PCM's are typically gone, or one of the first things scavenged. Don't know how often the VSSB is still there, but I'm gonna start checking now.

If they're relatively cheap from the yard, makes sense to mod the spare, so you've got the original as backup, if the mod doesn't work out.

Also, one would think these things to be cheap enough to ship around, that once we've got a modded VSSB in & working, we might offer up the stock one to others wanting to do the same mod.

If the modded one works when you swap it in, I wouldn't think it failing in the future would be probable enough to necessitate keeping a backup?

 

[Green Machine]

At one point during this I though I should have got a spare first but I figured out what was wrong though lol. One of the side connections to the rest of the board wasn't connected anymore so I soldered a little bit and it works perfect.

I have read a few places and a couple people have found the VSSB for $5-10 at the junk yard. Not sure if every place would be like that... but you never know I guess. I would also venture to guess that the VSSB's are probably still in there. I wouldn't think they are as popular of an item as the PCM.

Not sure about you, but I dislike having parts I may not use so if I had an extra one I'd give it away. At one time I was thinking of doing these for some people but I'm not that good at soldering right now so it wouldn't look pretty lol. Although 11 out of the 12 connections looked like all the rest of the joints .

 

[SmithvilleD]

Soldering things on circuit boards is a bit like the trickier welding skills like welding Al. It's a skill rather related to doing it often enough to maintain the skill at a high level. Many can do it well, but I'm not that good until enough practice.

Regarding pulling old components off, I have seen component recyclers use a small, fairly precise little torch to carefully heat the solder/pins on the board backside of a component, then quickly tap the board upside down using the component's own inertia to pull it out.

It looked like some practice might be in order before trying it (& hopefully a simple, single layer board), but thought it worth mentioning. I was told doing this correctly puts less heat into surrounding components & therefore less risk damaging other board components.

 

[Green Machine]

Yeah, I agree that soldering can take practice to be good. I don't know if I explained in my post that this was the first time soldering on a board for me. The part about removing old components is very interesting. Not sure it would work on the jumpers though? They are much lighter then a capacitor.. which is probably what is most replaced on computer motherboards (I may have to do a few soon lol).

This was before I cleaned the back of the board up and before I fixed the one bad connection (bottom left). The jumpers are located at the bottom of the board in this picture and the only trouble I had was that one connection.. Could not get it to come out right but I finally got it and then had to add some more solder to connect it to the rest of the board on the left.


Note: This is the first time I have worked on a board and it has been a few years since i've soldered anything lol.

I'm just so happy that everything works perfectly so far. I feel pretty good about myself... that I accomplished something I've never done before lol... and its something I can do in the future. I had this guide written up on Saturday but I didn't want to post it until I got my VSSB working completely perfect.

 

[samonhed]

Re Cal with tec1 or tec2?

Is it possible to recal the vss with the tec1 or tec2 for a 94 OBD1?

 

[94C1500]

No, it has to be done mechanically as lined out in this thread. For a 94, you will likely have a DRAC module instead of the newer VSSB, same thing just a little different and you can see the details on the site I posted earlier. Its not really hard to just change it. Green Machine went to the trouble of making it adjustable with dip switches. But if you just want to change it, cut old jumpers that dont apply to new setting and just a dab of solder on a paperclip jumper or something where you need a new jumper.

 

[samonhed]

Pics of mine

Just some pic of mine before and after. Works great!

 

[detroitMachine]

I'm going to just bump this instead of making my own thread, that way all the information is together.

According to this link, which sums everything up nicely, it says 1990-1991 uses DRAC and 1992-1995 uses the VSSB. I have a 1993 so I am assuming in can use any 1992-1995 VSSB? Gas, diesel, auto, manual, does any of that matter or are the only differences in the pins which I will modify anyways?

I'd like to have a spare rather than modifying my original. I'm going from 245 to 265 and would like to make the change properly.

GM DRAC/VSSB Calibration

 

[Glagulator]

They are all the same.
You can use any VSSB 92-2002.
After 95 they were only used on diesels.
The only difference is the extra set of jumpers starting in 95.

 

[scosworth]

VSSB alteration

Great information! Changing tire size is tricky, but I just changed the rear axle ratio. That takes tire size out of the equation. It turned out that I just read the jumpers and found their listing on the chart and wrote down the decimal value of the calculation described. I was able to take the ratio of the axle ratios and multiply it times the original decimal value for my jumper settings to get the new decimal value of the calculation. The new decimal value then shows the new jumper settings.

At first I calculated the rolling tire circumference from the factory jumper settings for my axle and then crunched the numbers for the new axle ratio. The ratio of the calculations for the axles was the same as the ratio of the axle ratios. That makes it very simple and accurate when tire sizes aren't changed. The value I got was right between two values on the table, so I went with the one for a slightly lower ratio so the speedometer would read a hair higher than the vehicle is actually going. I checked it with GPS and it was within one mph. That's good enough for me!

Thanks!