Why To prevent us from worrying about coolant temperature fluctuations and the service calls that would result Toyota designed the coolant temperature gauge to be numb to changes in temperature in a band centered near normal operating temperature. This makes for a gauge that moves towards the center during warm up and then just sits there unmoving under normal conditions. This is nice until the engine heads towards an overheat, the gauge will not tell you anything until you are well on your way. One way around this is to install an aftermarket temperature gauge, there are some down sides to this, they are expensive and use up space that could be used for other things. You also loose the stock look of the interior if that is important to you. The upside of an aftermarket gauge it that you get actual markings of the temperature instead of reletive needle position on a mostly blank background. Fortunately this numb zone is caused by just 2 components soldered on the circuit board of the gauge. All we need to do is replace a Zener diode and a 75 ohm resistor with 2 new resistors one 50 and the other 110 ohms. The amount of work to mod the gauge I would think is comparable to properly installing an aftermarket gauge and should take you 3 hours or less. Many Thanks to Semlin & David Dearborn ( dd113 ) for donating gauges, this mod would not have been possible without them. Also thanks to all that contributed to the development thread. this was a group effort. thsi is also a good place to go if you want to look "under the hood" on how all of this works. How Please note that like any work you can damage you truck by performing this modification, you are responsible to see that everything is done correctly, resulting damages rest solely on your shoulders. But anything can be repaired if you have enough time and money. Tools required #2 philips Screw driver #2 philips stubby or ratcheting angle screwdriver Small Pair of dikes Pick set Small soldering iron (~40 watt) .032” Flux/rosin core solder De-soldering braid and/or bulb Drill (smaller the better) array of drill bits And although a digital multimeter is not required to do the mod it is recommended. Parts edit 1/08, Fixed links thanks to NLXTACY 50 Ω 3 watt 1% wire wound mil spec resistor. Mouser P/N 71-RS2B-50 MFG P/N RS02B50R00FB12 RS02B50R00FB12 110 Ω 3 watt 1% wire wound mil spec resistor. Mouser P/N 71-RS2B-110 MFG P/N RS02B110R0FB12 RS02B110R0FB12 These resistors are 82 cents each, shipping will cost much more than the resistors them selves so get 2 each or more just in case, especially if you know other 80 series owners you will collectively will save on shipping. If you have any bulbs out in the cluster now is a good time to order them from Dan. from left to right the bulbs are air bag, ABS, AT/Oil temp, CDL, blank, check engine, seatbelt, cruise, high beam,__________ space________brake, low oil, A/TP (tecase neutral wile trans in park) blank, front locker, rear locker, air filter clogged (not lamped in the US), and finally door open there are also the gear indicatoor lights and the back lighting for the cluster. Accessing the gauge: For removing the cluster See the FSM it has some good pictures, here it is in my words. All directionals (left/right. foreword/aft up/down) down are trucks view. The dash can be damaged during disassembly, there are many push pull clips, if you are doing it right these only take light pressure to release, always pull the panel strait out as close as possible to the clip and never pry at the opposite end of the panel from where the clip is, if it feels like you are about to break something you probably are. 1. Remove the drivers knee panel by removing the 2 screws on either side of the hood/fuel release, one screw one the lower right side and one near where the hand throttle would be ). Gently Pull along the upper edge and release the 3 clips. Disconnect the speaker wire. You can let it hang by the fuel/hood cables and work around it or go through the trouble of removing them, to remove the releases remove the screws and pull the release all the way out then slide them forward 2. The next dash section up wraps around from the key cylinder over the steering column to the left hand buttons. Start at either end and gently pull working your way through the 8 clips, release the connectors to the locker switch, light dimmer and mirror control. Each one of the connecters releases in a different way, the dimmer is a side press, the mirror control a top press and the diff lock switch has a very small release on top of the connector. A pick is quite handy here. 3. Remove the upper half of the steering column cover by removing the two most aft screws on the bottom of the column then releasing two clips at the front of the cover. The bottom half can stay. 4. Remove the Cluster finish panel, this panel houses two vents on either side of cluster and is slightly tricky. Remove the two screws at the top center. It has two clips at either end just outside the vents but also has some interference to the dash pad, first release the clips by pulling but not too far then work the panel around the dash pad (see pic ) to not cock the panel at any time, only pull strait out. The clips that hold the vents to the panel cannot take much load 5. Now with the cluster exposed you will see 4 white legs with a screw in each, there are two above and two below the cluster. Remove these, carefully work the left side of the cluster out, you will have to pull the right side out some also, the right side interferes slightly with the upper center dash. After you get the left side out some you will be able to push the cluster outboard and the center dash inboard and work the cluster past. The first goal is to get the left side of the cluster out far enough to get you hand in (see pic) , you will release two wire bundle connectors there, with those two released you will have enough slack to be able to flip the top of the cluster aft and down to access the other two connectors and then pull the cluster out 6. Remove the gauge from the cluster. first the cluster splits in two, the gauges and a white backing are one half, the "glass" and black mask in the other. There are clips around the perimeter lift each one in turn by hand and the two halves will split, after the glass is off be careful not to set the cluster on its face, the cards and needles are kind of fragile. Try not to not touch the face cards of any of the gauges, fingerprints leave marks, there are 4 screws (see pic) back these screws out partially and then press them with moderate finger pressure. the gauge will seam stuck at first but then will pop up a little bit, if is fells wrong or partially attached stop and give it a good looking over, completely remove the screws and then work the gauge out, it had some slight interference with the tack card but it can be worked out of there. Be careful the tach needle does not receive any load. Modify the gauge If you do not know how to work with solder give these a read 7.1.1 Soldering Basics EPE "Basic Soldering Guide" First need to secure the gauge so that you will have two free hands but the gauge is not resting on the needles, there is an area of the PCB that can be clamped in a vice at a corner, when you need to apply even light force to the PCB also support it with your hand, do not bend the PCB. More secure clamping method provided by Rookie2 IH8MUD.com Forum - View Single Post - 93-97 Coolant Temperature Gauge Modification The two components we are interested in are a 75 ohm resistor and a Zener diode (see pic) both of these have two connections each for a total of 4 connections, the resistor looks somewhat like a peanut and is just over ¼” long on the back of the PCB it is marked with a zig-zag line , the zener diode is a tiny red bead it is marked on the PCB by a rectangle with a stripe at one end. First we need to remove both of these components, de-solder these 4 connections straighten out the wires and push them through, if you have any troubles with them hanging up on solidified solder use the tip of your soldering iron. Using the soldering iron as a tool is handy as it will keep the solder flowing wile you move stuff around. Then remove any excess solder as necessary to have the holes open to allow insertion of the new components, a small amount of solder left on the pad of the PCB is acceptable and actually desirable. The 110 Ohm resistor will replace the 75 ohm resistor; first bend the leads of the resistor to until they are parallel to each other and perpendicular to the resistor. You do not want to bend them tight right as it comes out of the resistor, this may form cracks in the wire that could cause the lead to break with vibration rendering the gauge useless. Set a ~1/8” drill bit against the resistor touching both the resistor and the lead, bend the lead 90* over the bit this will form a radius. You want the two legs spaced as close as possible to the width of the two holes in the PCB, trim the two leads just enough to get between the PCB and the face card of the gauge, This resistor can get hot in use so we want air flow around it, we also want to keep it away from the clear plastic light guide and the needle motor, I let it naturally hang with gravity and put it closer to the PCB (but not to close) than the face card and it seamed about right (see pic) solder it in place then trim the excess lead. The zener diode is replaced with the 50 ohm resistor. The holes for the zener diode were smaller than the leads of the resistor, chuck up a very small drill bit (same size as the lead of the resistor) in a drill and turn the chuck by hand to enlarge the holes. These holes are closer together than the width of the resistor so you will need to bend one of the legs into a joggle, make a180° bend at the end of the resistor then a 90°bend at about the middle of the resistor, bend the other one 90* at a location that will achieve proper spacing, trim the leads so that more than enough will go through the hole to support the resistor. this resistor does not get hot and due to its location is easier to install this one on the bottom of the PCB opposite of the other resistor. There is clearance in the cluster to do so just keep it tight against the PCB (see pic) If you have a meter check the connections. the resistance of the joints should no be more than 1 ohm higher than the resistance you get with your probes touched together, Check the resistance from the sense post (top post in the pictures) to the two close legs of the two resistors, the other leg of the 110 ohm resistor is checked to the + post (lower left in the pictures) , the other leg of the 50 ohm resistor is checked to the nut marked “U”, there is a thin coating on the nut you need to pierce to make a connection Extra credit: add these characters to the 4 existing characters on the bottom of the gauge 注：中温安定ではありません。 (may not display correctly on some computers) The original 4 say "middle temperature stable, the new text is "no longer middle temperature stable" Thanks Mot Reassembly is reverse of disassembly, one note wile pushing the cluster back into position look at the left side between the vent and cluster the bundle can get caught up on a vertical piece of the dash and pinched preventing the cluster from seating, gently help it around this piece with a screwdriver Results You now have a gauge that will move with all temperature changes within its sensitive range. Full sweep peg to peg is 94° Pegged hot 244° Top of red is 227° Bottom of red 217° Center is approximately 189° Cold line is 160° Pegged cold 150° Notes “Center” is pretty much horizontal. It is the middle between the bottom of red and the cold line, witch is not the same as center of the full range of the gauge. Some describe “center” as the just below the middle, also the needle sits high above the face card with the markings making it quite susceptible to parallax error, at any rate after warm up wherever your needle sat before mod is the “center” point for our purposes. Please give feedback on how this goes for you. And what changes can be made in the directions to make it clearer. If you find any tips or tricks along the way please post 'um up 93-94 trucks Thanks to the effort of Semlin a mod is in the works. Edit 3-15-05, the same mod also works for 93-94 trucks within a few degrees, accessing the gauge may be a little diffrent as the dash is diffrent on the 93-94 trucks, if anybody does one of these would you mind taking notes on accessing the guage. 91-92 trucks so far nobody has championed this one.