1998 Chevrolet Truck dashboard problem

You didn't bother to list the truck model, engine size, type of transmission, type of fuel injection system, two or four-wheel-drive, or options, so there's no way to know what combination of Engine Computer, Air Bag Computer, Anti-lock Brake Computer, Body Computer, Remote Keyless Entry Computer, Anti-Theft Computer, or Automatic Load-Leveling Computer you have. All of them talk back and forth to each other on the data buss, and they all need information from the Engine Computer to do what they do. The instrument cluster is another computer module. Everything on it runs on digital signals that come from those other computers. For example, the vehicle speed sensor sends its signal to the Engine Computer, Air Bag Computer, and ABS Computer. It's usually the Engine Computer that sends the vehicle speed information to the instrument cluster. If you eliminate the Engine Computer, how will the instrument cluster know vehicle speed? On some models the speed signal goes to the instrument cluster, then that one tells vehicle speed to the other computers that need that information.

The Engine Computer also knows engine temperature. On most trucks that is another digital signal sent to the instrument cluster.

You didn't say why you're trying to do this modification. If it's just to bypass a problem with the fuel injection system, you're WAY better off having a professional diagnose and repair the problem. Even if you are able to bolt a carburetor to the engine, they can only be close to perfect fuel / air mixture at idle and at high speed. Electronic fuel injection fuel / air mixture is perfect at any engine speed, any load, any air temperature, any coolant temperature, and any barometric pressure. The only way a carburetor can give you better fuel mileage is to be set to run too lean. That will lead to increased emissions, potentially burned intake valves, and annoying hesitations and stumbles. In the mid-range, all carburetors run a little rich to avoid dropping into a lean condition and these running problems. Running rich wastes fuel as the trade-off for running smooth.

You also have to consider that carburetors can only handle about three to four pounds of fuel pressure from a mechanical pump. Most GM fuel injection systems run on more than 50 pounds of fuel pressure. That will blow a carburetor's float to kingdom come.

Your transmission will have a lock-up torque converter for better fuel mileage and reduced transmission temperature. That won't work with no signals coming from the Engine Computer. Even if you try to leave the Engine Computer connected, it is going to go wild with diagnostic fault codes. When codes are set, the computer suspends some of its operations and things it controls. You'll need to use a distributor for spark and ignition timing advance.

One more thing most of us forget about is gasoline today is really terrible. The ethanol in it is very corrosive and will usually destroy the rubber seals and diaphragms. Just ask Jay Leno about all the problems he's having with his fleet of cars with carburetors. You'll need to buy ethanol-free gas which is always more expensive, assuming you can find it.

Everyone here knows I really hate all the insane engineers at all the manufacturers for their ridiculous over-use of unnecessary, unreliable, complicated, and expensive technology for systems that have been around for decades but never needed a computer before, but as a hater of "inappropriate" use of technology, I would never want to go back to a carburetor. I don't mind being bothered to have to tap the accelerator pedal to prime a carburetor for starting, and I certainly would prefer to not need high-failure sensors to trigger injector and ignition coil timing, but when you consider that today you can suck on a tail pipe and live to tell about it, and in spite of the likelihood of an electrical failure, today's vehicles are pretty certain to get you back home, even I am happy to have an Engine Computer in most of my vehicles.

The only reasons I can surmise you want to perform this modification is you're building a truck to race or you don't understand how to diagnose a problem. As a former amateur racer myself, I can tell you I didn't have much time to study the gauges. You can install aftermarket gauges for that.

If you're doing this because you don't know how to diagnose a problem, join the crowd. None of us know everything about every system, but for every system, there are plenty of experts around who DO understand them. If you post a bunch of additional information related to the things I brought up, I might be able to provide a better answer for you. If you want to diagnose a problem you're having with the fuel injection system, there are a couple of other, really good experts here I can get involved on your behalf.

You aren't going to get the gauges to work. They are not like the mechanical gauges used in the past. Each gauge is a "stepper" motor that is run by the computer circuitry in the instrument cluster. A stepper motor has four electromagnet coils in it, and the pointer is attached to the armature. The computer positions the armature to the desired position by varying the polarity and strength of the four magnetic fields. The fuel gauge should still work because that isn't involved with any of the other computers and data related to that isn't broadcast on the data buss to the other computers. The problem is the cluster may need information from the Engine Computer to turn on.

On a '98 model there's a good chance there are two coolant temperature sensors. One is for the Engine Computer and is very accurate, and a second one is just for the dash gauge. If you have that, the temperature gauge should also work. If the instrument cluster gets coolant temperature information from the Engine Computer, that gauge won't work unless the Engine Computer is working.

The same applies to the tachometer. The Engine Computer needs signals from the crankshaft position sensor and / or camshaft position sensor to calculate engine speed. That data is converted to digital information that goes to the instrument cluster. The cluster takes that and uses it to know where to set the tachometer pointer. This is not a simple tachometer that needs one wire connected to the distributor. If you're using the Engine Computer to run the ignition system, the tach should still work.

I guessed at a '98 C1500 5.7L and looked at the wiring diagram. The speed sensor sends a signal to a module called the "vehicle speed sensor adapter", then that module sends the information to the Engine Computer. That computer interprets the signal, then sends its own version to the instrument cluster. If you leave the Engine Computer connected and working, it might still run the instrument cluster. The only things that won't be working are the circuits that run the injectors and the emissions components. That will set diagnostic fault codes and turn on the Check Engine light.

The only other thing I can suggest is my '88 Grand Caravan is the last year they used a speedometer cable. My '89 has a gauge cluster that looks exactly the same but the speedometer is electronic, like yours, and is run with a speed sensor. Inside the transmissions, the drive gears are the same meaning you could use the speedometer cable and older cluster in a newer van. That would get the speedometer working, but you'd have to modify the circuitry for the transmission which, starting in '89, shifts electronically through its own computer. You might be able to use a speedometer cable and drive gear from an older truck if the assembly will bolt to the transmission, but then you'd need the speedometer from that older truck too. At that point you're getting into something no one here has ever tried to do. What I would look into instead is if someone offers an aftermarket speedometer with a simple speed generator that bolts to your transmission. I seem to recall reading about those for people wanting to use a newer transmission in an older truck.