This is largely a cut-and-paste from a reply I posted in the general discussion group. Many of you have already done this swap, but for those considering it, I am posting some of my experiences.
If you want to add a tachometer to your instrument cluster, you can swap your current cluster for one from a Toyota truck that had a tach. You can use a cluster from a 4Runner of the same era, but will find that you lose the transmission gear indicator lights, as the 4Runner had a floor shifter and the indicators in a separate unit. So, you can get an instrument cluster out of a T100 (I used a 1995), but you have to make sure it has a mechanical speedometer connection rather than an electronic one. You will lose the ECT "Power" indicator light, but that is a very minor inconvenience, as you can visually and by feel tell the position of the ECT switch.
To remove the instrument cluster, you start by removing the lower left dash - one bolt at the bottom left corner and 4 screws - disconnect the dimmer switch and maybe the speaker. Pop off the cover around the ignition switch - it just pulls right off. Remove the plate around the heater/air conditioner controls - pull the four knobs, then carefully pry the plate from the right side and pull to the right, as it has tabs that hold it under on the left side. The a/c switch will come with it. Remove the upper left dash - one screw at the upper right of the a/c switch cover you just removed, two at the top of the instrument cluster, two more (I think) somewhere - I'm writing this from memory. There are two more electrical connections to unplug and you will have to move the transmission lever down into 1st gear, which means you have to turn on the ignition and hold the brake in. Then carefully pull it out from around the instrument cluster and from under the upper dash board - there are a couple clips that hold it in, but it is not very fragile. Finally, remove the four screws that hold the instrument cluster in and pull it out enough to unplug the 4 electrical connectors and the speedo cable. As to the latter, once you find the right place to push (toward the back end of the protrusion on one "side" of the housing), it comes off pretty easily.
With the help of my brother, I just finished an instrument cluster swap on my 1992 Winnebago Warrior. For the sake of others who do this and start at the same level of ignorance as I, here's what I found out.
You MUST swap the oil pressure sender on the block - the original sender is just an on/off switch and will not work with the new gauge. I have read, in fact, that it will burn it up. There was no problem swapping the oil pressure senders (we got ours from the same donor T100 as we took the instrument cluster from) except that the electrical pig tail didn't fit the new sender, so we cut the boot off of it and remodeled the connector a little so it would stay on, then we used some butyl sealant to replace the function on the boot. To get at the sender, you will have to remove the fan belt and swing the alternator out of the way.
We DID reset the odometer to the correct mileage. This was a lot of work and required a great deal of care and patience. It was painful, too, as I had to add about 10,000 miles to the donor odo to get to the 32k on my Toy. To do this, we used a punch to carefully open up the crimped "horns" that held the odo axle and we were able to remove the unit from speedo assembly. With some playing around, I was able to figure out how to adjust the wheels, which I did one at a a time starting with the left most wheel. I won't try to explain how it is done in detail, because you really just need to play with it. Basically, though, you need to hold the little tabs in a straight line while you turn the wheel to the right of the one you are trying to adjust. Turn that wheel back and forth through it's entire travel and it will advance the one to the left one digit for each cycle. When you get to the 1 mile digit, you adjust it using the far right wheel, which has no numbers on it but would be the 1/10th mile wheel if it showed tenths. Since it does not show tenths, I set the odo to the next whole mile above the mileage on my original odometer. Then, you have to put the whole thing back together, praying you don't break off those "horns" as you again crimp them around the axle.
With the odometer put back together (and, BTW, mine reads PERFECTLY correct - in 31 miles, the 1/10 mile digit was not perceptibly off AT ALL), it was then just a matter of installing the instrument cluster in the dash... or so you would think.
Turns out the tach was erratic as all get out and registered way high once we got it all put back together. If others have similar problems, here's what I found out: When we used a diagnostic tach (plugged in to the spade connector designed for that purpose right next to the fuse box under the hood), the tach in the instrument cluster worked fine. Using the screw on the potentiometer(accesible through the face of the tachometer - you may have to bend it up a tad to get a little screwdriver on it), we could get it adjusted correctly. Then, we would put the dash back together... and the tach would be way off and jumping around all over the place. We tried removing the small add-on printed circuit board (rectangular with 3 leads coming from it and attached to the main circuit board for the instrument cluster - not all instrument clusters with tachs will have this), but the results were the same. Finally, we realized that unplugging the diagnostic tach is what caused it to go haywire, so we scratched our heads and tried to figure out why. Finally, we checked what the diagnostic tach was doing electrically and found that there was about 1434 ohms of resistance between the pulse lead and the negative terminal of the battery. We then checked the add-on pc board and found the same 1434 ohms or resistance between the black wire and the light colored wire. Sooo.... we put that little pc board back on the instrument cluster but connected only those 2 wires, attaching the black wire to its original terminal, but attaching the light colored wire to the "P" (for pulse) terminal. Ta Da! It worked like a charm! The tach is now very steady and reads correctly at all RPMs.
There is no indicator light for the "Power" setting of the ECT. Here is a work-around I haven't actually tried: The "new" instrument cluster has a "4WD" light that is not being used. I should be able to figure out what lead was designed for the "Power" indicator and pull it from the connecter it is in and replace the "4WD" lead with that one. Then, when I put the ECT in Power mode, the "4WD" light should come on. I probably won't take it apart again to do this, but if anybody does and it works, let me know!
As to the speedometer - we had to pull the needle off to set the odometer and we had to experiment to get it set correctly. Every time we would put it where we wanted it, when we pushed it on to the shaft, it ended up too low. BE CAREFUL removing the needle, BTW - pry carefully with two small flat blade screw drivers so as not to bend or break the shaft. With the needle set just below the "0" mark, my speedo registers within 1 mph at speeds from 20 to 50 - close enough for me!
Finally, the temperature gauge works differently from my original. The original generally set at about 1/3 or the range of the gauge and would crawl up to the middle or just a hair above on the longest, hardest uphill pulls. The "new" one sit just about dead center when the engine is warmed up and doesn't move more than about a hair's breadth on the nastiest long climbs. I am convinced that it is working, but it sure isn't giving much information. I only hope that it will climb up if there is ever any serious overheating.
That's it! My "new" instrument cluster looks and functions beautifully. Good luck with yours!