Recal VSSB

[GOT DSL]

Guys I need help.
Just replaced the tires on my Suburban, went from 245/75 to 265/75.
Read the articles and visited the posted web sites, thought I was ready to recal my VSSB. Pulled it out from under my ECM, opened it up, and there are 14 pins not the 7 I expected to find. Is there any info available that discribes how to recal with 14 pins?

[Kennedy]

This was covered in detail in the 1999 Features book. I believe the solution was to swap the VSSB with one that could be recalibrated. Those books are priceless... ;)

[piersbc]

From what I've read the right 7 are the ones you set to the new binary value. And the left 7 just mirror the right 7. An example would be if 1,3,5,7 are turned on for the right 7 then the left 7 would be 2,4,6 on and 1,3,5,7 off. Look at how the jumpers are set up on you VSSB board now. The left 7 should be a mirror of the right 7.

[piersbc]

WHAT I FOUND ON THE "NET"...


I couldn't find anything on this on the net, so I had to figure it out myself.

The following applies to a vssb unit in a '94 Chevy pickup. I have no idea if this applies to any other vssb unit.

The vssb serves as an electronic callibrator for the speed pickup putting out a standard value for the odometer and speedometer (as well as cruise control). If you change the wheels or gear ratios it will no longer be valid. It may be off anyway.

The unit on the '94 Chevy can be modified to change the callibration value. HOWEVER, you need to be familiar with electronic assembly, have good tools and understand safe handling practices for semiconductor devices. IF YOU DON'T, YOU RUN THE RISK OF RUINING YOUR VSSB.

First, determine the amount that your speedometer is currently off. In my case, I maintained a steady 60 mph reading on the speedometer and timed between mile markers on an Interstate. Do this over several miles and repeat a few times to get a good reading. If your speedometer callibration is correct, it should take 60 seconds to travel 1 mile. My speedometer was reading low and it took 56 seconds to travel a mile. This is about 64-65 mph and represents a reading that's about 7% low. Record this time.

Take out the vssb module from behind the glove box (warning, do this at your own risk, if you destroy your vssb, don't blame me) and open the plastic case. You'll see 2 chips and a row of jumpers along the back of the board. Turn the board so the jumpers are toward you and the connector is pointing away from you. Ignore for now the leftmost 7. Turn the rightmost 7 jumpers into a binary number by using a 1 where there is a jumper and 0 where there isn't one. The jumper in the middle has the highest significance. My unit had 1001001. Turn this into a decimal number, in this case 73. Record this with the time determined above.

If you want to increase the speedometer reading, you need to increase this number. If you want to decrease the reading, you will need to lower this number. In my case, every addition of 1 changed the speedo reading by +.6 mph; this will probably be different for your case depending on gear ratios and tire sizes.

I pulled out all of the jumpers and put in a couple of dip sockets to facilitate modifying the jumpers. If you solder to the board, you will need to have good equipment to avoid ruining the board. YOU HAVE BEEN WARNED. You will need to change jumpers probably twice. DIP switches might be nice, but I didn't have any that would allow 2 to be installed given the spacing on the board.

Pick a number that will get you close to what you want. It would be nice to get it dead on, but that's unlikely. Assume .5 - .6 mph change for every change of 1 in the callibration number. You really need a second data point anyway. I picked 82. Translate this to binary (1010010) and set the jumpers accordingly. The left set of jumpers now need to be set by inverting and mirroring the right set. This works out to 1011010 for the above set. There are 7 pins coming out of one of the chips that need to be connected to one of two commons and they mirror the two sets of 7 jumpers on the board. Check that each pin from the chip is only connected to one of the two commons. In my case the 14 jumpers then looked like 1011010 1010010.

Reinstall and go back on the road and again time a mile when the speedo is at 60 mph. In my case, it was taking 61.5 seconds / mile. You now have 2 data points for a linear function. Determine the equation of the line and figure out what the real value should be to be dead on at 60. In my case, it turned out to be 80. Modify the jumpers, recheck and you're done.

[GOT DSL]

Thanks for the recal info.
Based on the increase in tire size I figure my speedo is slow by about 4%.

Will follow your directions to recal my VSSB and let you know how it turns out.

Thanks again for the help!

[Govt issue]

I changed the tire size on my truck also. when i drive by those speed traps on the road that post your speed it is always right on. I wouldn't worry about it unless it bothers you.

[GOT DSL]

Changed the settings in my VSSB from 85 to 88 per the method you described. The adjustment put the speedo right on the money. Timed 1 mile at 60mph, and it was exactly 60 seconds. Checked RPM at 60mph and it was just what I had calculated it should be with the larger diameter tires. Think this project gets a check mark.

Thanks for all the help.
Could not have made these changes without the guys on this web site!