I vant to take your pulse; you can call me DRAC
Our Scouts, like a great many older US-manufactured vehicles, use a mechanical speedometer drive cable that attaches to the transfer case output shaft (or transmission output shaft for 2wd vehicles). The cable spins at 1000 revs per mile and that is what the speedometer is calibrated for.
The speedometer cable is driven by a 2-gear setup. The drive gear is mounted on the output shaft of the transmission and may be plastic or metal. Here's a picture I nabbed from the web.
The driven gear (usually plastic, but may be metal) is just adjacent to and interfaces with the drive gear. It is usually mounted in a small removable 'plug' housing. On the left are the drive gear and housing (plus a random bolt!); on the right is the installed housing with a speedo cable installed. These are random representative pics from the web - they are not all from the same vehicle.
Here they all are together in a cutaway view:
One can change the tooth count on the drive gear (not usually done) or the driven gear (much more common). Replacement gears are available on the net from places like
/www.transmissioncenter.net which usually also have an online calculator to figure out what replacement gear you need to get back to 1000 revs per mile. Another option is to add a 'speedometer corrector' device in-line with the speedo cable. This is just a little gearbox that adjusts the revs per mile by a fixed factor. You may already have one on your truck now. Good luck finding the proper little corrector box when you need it. P.S. You have to do this every time you change tire size or gearing.
The GM TBI computer wants to receive vehicle speed information in the form of 2000 electrical pules per mile. One may employ a pass-through adapter in-line with the speedo cable. The device produces a set number of pulses per revolution of the speedo cable (usually 2 or 4). Using a 2-pulse adaper would produce 2000 revs per mile - ASSUMING your speedo drive/driven gears are correct for your tire size and differential gearing. This allows you to continue to use your speedo cable for the speedometer while providing the 2000 pulse per mile signal the TBI computer wants to see. These are a great option if you're keeping the factory mechanical speedometer. The fuel injection FAQ has more on this piece. I think they are about $90 shipped.
Never being one to do anything the easy way, I did something different. "NO WAY, DUDE!" you shout at the top of your incredulous lungs. Yes, it is true. I'm doing something more complex and difficult for very little additional benefit.
Starting in 1990 GM switched to electronic speedometers instead of mechanical cable speedometers. The transmissions (and t-cases) now employed a 40-tooth drive gear on the output shaft and a magnetic pickup in the tailhousing. This 40 pulse signal was fed to a little 'pre-computer' computer called a DRAC (Digital Ratio Adapter Computer, or Controller, or Cantaloupe, or other C-word). The job of the DRAC is to convert the 40 pulse signal into three different outputs: (1) 2000 pulse per mile for the computer, (2) 4000 pulse per mile for the cruise control and the speedometer, and (3) 128,000 pulse per mile for the ABS.
The DRAC still needs to be set for your tire size and gear ratio - GM was dumb and hard wired each DRAC. Fortunately, one can re-wire the DRAC, or better yet, add dip-switches to the DRAC and have very simple user-adjustability. There are several write-ups on the web detailing this process. Here's a great one with pics and explanation:
A Great One
Of note, poor old DRAC only lasted 2 years (1990-1991). However, his son lived on and was called VSSB (Vehicle Speed Sensor Buffer). The only difference was VSSB had a little extra circuitry to also talk to the electronic transmissions offered beginning in 1992. You can still use a VSSB if you can't find a DRAC - even if you're not runnning an electronic tranmsision.
I want to use a DRAC/VSSB to get both 2000 pulses per mile (for the TBI computer) and 4000 pulses per mile for the stock Chevy cruise control. The Chevy cruise control is a stand-alone, servo-controlled unit (no vacuum lines or chambers!) which easily attaches to the TBI - it's stock GM equipment. It is also only $15 at the junkyard. You can even get one from a car with a longer cable so you can put the cruise box anywhere in the engine bay. This cruise unit does not interface with the TBI computer - just the DRAC/VSSB.
OK, so now you're wondering why not just add another cable speedo adapter to the mechanical speedo cable: One 2 pulse per rev unit for the TBI computer and one 4 pulse per rev unit for the cruise control. Beside being a hokey-long mess, especially if one were to also employ an in-line ratio corrector, each X pulse per rev unit is around $90 shipped. $180 for adapters? Too much! $15 for the Chevy cruise is better.
OK smart guy, how about $90 for the 2 pulse unit and add an aftermarket cruise control? You mean the $200 aftermarket cruise controls now available? The days of the $80 aftermarket Audiovox cruise are long gone. Too much! $15 for the Chevy cruise is better.
OK, so DRAC/VSSB it is!
You may recall, dear reader, that I hemmed and hawed about which t-case to use with the 700r4. The choices were basically an easily found NP208 or a rare NP241 (passenger drop). I went with the NP208 - even converting it to a fixed-yoke output. Well, it turns out that the NP208 never had a VSS. I should have held out for a passenger drop NP241 with the factory VSS. I wouldn't have had the cheap fixed- yoke conversion, but I wouldn't have had to do this:
Fortunately the NP208 and the NP241 (and many other NP transfer cases) use the same size and spline count output shafts - where the speedo drive goes. The NP241's metal 40 tooth VSS drive gear ('reluctor') fits on the NP208 output shaft just fine. It is, however, narrower (front to rear) than the plastic NP208 speedo drive gear. To make up the difference, I'll cut a little section off the plastic NP208 gear to act as a spacer.
NP208 Speedo Gear and output shaft:
The VSS pickup (sensor) normally screws into the output housing and sits a few gazillionths of an inch away from the 40 tooth reluctor. I looked at adding the VSS pickup through the mechanical speedo drive window. It was doable but the angle made it prohibitively difficult. Instead I chopped a new window into the housing with a cutoff wheel (4.5" grinder) and a dremel tool. I will have a piece of aluminum flat bar or angle welded in place 'water-tight' for no leaky-leaky of the gear oil. Then it's drill and tap time. Mounting depth will be close to correct and can be adjusted via copper or aluminum shims or crush washers, or even an o-ring. The stock mechanical drive window will get plugged - probably with a freeze plug.
Alternatively, I may hack a chunk out of the NP233 housing (the part with the threaded sensor hole) and weld that sucker into the new 'window' in the NP208 housing.
Here's a NP233 outut housing with the reluctor and VSS in place:
Here's the NP233 from the outside - note the sensor points directly at the center of the output shaft versus the offset mechanical gear setup of the NP208 in the picture below.
See? Much more work and difficulty than required!